Polímeros: Ciência e Tecnologia 1st. issue, vol. 31, 2021

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Polímeros Charge transfer

VOLUME XXXI - Issue I - Jan./Mar., 2021

São São Paulo Paulo 994 994 St. St. São São Carlos, Carlos, SP, SP, Brazil, Brazil, 13560-340 13560-340 Phone: Phone: +55 +55 16 16 3374-3949 3374-3949 Email: Email: abpol@abpol.org.br abpol@abpol.org.br 2021 2020

Reduce Charge transfer



ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.03101

Prof. Adhemar is retiring from Polímeros Marco Aurelio De Paoli1*  President of ABPol, São Carlos, SP, Brasil

1

*madpaoli@unicamp.br

How to cite: De Paoli, M. A. (2021). Prof. Adhemar is retiring from Polímeros. Polímeros: Ciência e Tecnologia, 31(1), e2021000. https://doi.org/10.1590/0104-1428.03101

After a long period of contribution, and hard work, for our journal Polímeros: Ciência e Tecnologia Prof. Adhemar Collà Ruvolo Filho is retiring. The pathways of Polímeros and of Prof. Adhemar crossed during many years. He had active participation in the journal’s consolidation since its creation. He markedly contributed to maintain the scientific level and periodicity of the journal. Joined the Editorial Council and Editorial Committee in 2003, and always participated actively in the discussions and decisions taken by these commissions. His suggestions were well accepted and his participation in the debates was serene and conclusive. In 2006, he accepted the task to become Editor in Chief of the journal. At that time, the journal was setting feet in the national and international scientific scenario and he played an important role for these actions up to 2011. From 2015 to 2016, he acted as provisional Editor in Chief. After that, he was an active member of the Editorial Committee, with a strong

Polímeros, 31(1), e2021000, 2021

position favorable to the peer review system, participating himself in the system as a careful referee. In the period from 2006 to 2018 Prof Adhemar had 14 papers accepted for publication in Polímeros, the last one in volume 28[1]. The Brazilian Polymer Association (Associação Brasileira de Polímeros, ABPol), The Editorial Council of “Polímeros: Ciência e Tecnologia” and all the polymer Science community, acknowledge the contribution of Prof. Adhemar and wish him productive fishing expeditions in the future.

1. References 1. Gomes, A. C. O., Backes, E. H., Ruvolo Filho, A. C., Paranhos, C. M., Passador, F. R., & Pessan, L. A. (2018). Hybrids membranes with potential for fuel cells – Part 3: extruded films of nanocomposites based on sepiolite and PC/sulfonated PC blends. Polímeros: Ciência e Tecnologia, 28(2), 112-119. http://dx.doi.org/10.1590/0104-1428.02616.

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ISSN 0104-1428 (printed)

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A ss o ci at e E d i t o r s Alain Dufresne Bluma G. Soares César Liberato Petzhold José António C. Gomes Covas José Carlos C. S. Pinto Paula Moldenaers Richard G. Weiss Rodrigo Lambert Oréfice

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“Polímeros” is a publication of the Associação Brasileira de Polímeros São Paulo 994 St. São Carlos, SP, Brazil, 13560-340 Phone: +55 16 3374-3949 emails: abpol@abpol.org.br / revista@abpol.org.br http://www.abpol.org.br Date of publication: March 2021

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Polímeros / Associação Brasileira de Polímeros. vol. 1, nº 1 (1991) -.- São Carlos: ABPol, 1991Quarterly v. 31, nº 1 (Jan./Mar. 2021) ISSN 0104-1428 ISSN 1678-5169 (electronic version)

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1. Polímeros. l. Associação Brasileira de Polímeros. E2

Polímeros, 31(1), 2021


Editorial Section Editorial................................................................................................................................................................................................E1 News....................................................................................................................................................................................................E5 Agenda.................................................................................................................................................................................................E7 Funding Institutions.............................................................................................................................................................................E8

O r i g in a l A r t ic l e Use of biodegradable polymer for development of environmental tracers: a bibliometric review Adriana Marques and Sandra Maria da Luz ................................................................................................................................................. 1-14

10-(pyren-1-yl)-10h-phenothiazine and pyrene as organic catalysts for photoinitiated ATRP of 4-vinylpyridine Loc Tan Nguyen, Hung Quang Pham, Duc Anh Song Nguyen, Luan Thanh Nguyen, Ky Phuong Ha Huynh, Hai Le Tran, Phong Thanh Mai, Ha Tran Nguyen , Le-Thu Thi Nguyen, Thuy Thu Truong................................................................................................................................ 1-7

Polyaniline-based electrospun polycaprolactone nanofibers: preparation and characterization Juliana Donato de Almeida Cantalice, Edu Grieco Mazzini Júnior, Johnnatan Duarte de Freitas, Rosanny Christinny da Silva, Roselena Faez, Ligia Maria Manzine Costa and Adriana Santos Ribeiro....................................................................................................... 1-9

Tensile and structural properties of natural rubber vulcanizates with different mastication times Nabil Hayeemasae, Siriwat Soontaranon, Mohamad Syahmie Mohamad Rasidi and Abdulhakim Masa....................................................... 1-8

Experimental investigation on stacking sequence of Kevlar and natural fibres/epoxy polymer composites Murali Banu* , Vijaya Ramnath Bindu Madhavan , Dhanashekar Manickam and Chandramohan Devarajan ........................................... 1-9

Cationic polymerization of styrene using iron-containing ionic liquid catalysts in an aqueous dispersed medium Gabriel Victor Simões Dutra , Weslany Silvério Neto , Pedro Henrique Hermes de Araújo , Claudia Sayer , Brenno Amaro da Silveira Neto and Fabricio Machado.................................................................................................................................................................................... 1-14

Potential of calcium carbonate as secondary filler in eggshell powder filled recycled polystyrene composites Nabil Hayeemasae and Hanafi Ismail ............................................................................................................................................................. 1-7

New biodegradable composites from starch and fibers of the babassu coconut Carla Veronica Rodarte de Moura , Douglas da Cruz Sousa , Edmilson Miranda de Moura , Eugênio Celso Emérito de Araújo and Ilza Maria Sittolin .......................................................................................................................................................................................... 1-11

A foldable high transparent fluorinated polyimide (HFBAPP/6FDA) film material for transparent flexible substrate Chuanhao Cao , Lizhu Liu and Xiaorui Zhang.............................................................................................................................................. 1-10

The effects of residual organic solvent on epoxy: modeling of kinetic parameters by DSC and Borchardt-Daniels method Victor de Carvalho Rodrigues, Denise Hirayama and Antonio Carlos Ancelotti Junior ............................................................................... 1-8

Induction of defense in apples by sulfated and deacetylated chichá gum Carlos Pinheiro Chagas de Lima , Andréia Hansen Oster , Fábio Rossi Cavalcanti , Regina Célia Monteiro de Paula and Judith Pessoa Andrade Feitosa ........................................................................................................................................................................ 1-8

Accurate measurement of pitch-based carbon fiber electrical resistivity Caroline Jovine Bouças Guimarães, Alcino Palermo de Aguiar and Alexandre Taschetto de Castro ........................................................... 1-6 Cover: Charge transfer reaction diagram, by Cao & Liu. Selected to honor Prof. Ruvolo, a chemist by heart. Arts by Editora Cubo.

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I I I I I I I I I I I I I I I I I



Building a better biosensor polymer

Vaccine Packaging

A new organic (carbon-based) semiconducting material has been developed that outperforms existing options for building the next generation of biosensors. An international research team led by KAUST is the first to overcome some critical challenges in developing this polymer. Much research effort is currently expended into novel types of biosensors that interact directly with the body to detect key biochemicals and serve as indicators of health and disease. “For a sensor to be compatible with the body, we need to use soft organic materials with mechanical properties that match those of biological tissues,” says Rawad Hallani, a former research scientist in the KAUST team, who developed the polymer along with researchers at several universities in the U.S. and the U.K. Hallani explains that the polymer is designed for use in devices called organic electrochemical transistors (OECTs). For these types of devices, the polymer should allow specific ions and biochemical compounds to permeate into the polymer and dope it, which in turn can modulate its electrochemical semiconducting properties. “The fluctuation in the electrochemical properties is what we are actually measuring as an output signal of the OECT,” he says. The team had to confront several chemical challenges because even minor changes in the polymer’s structure can have a significant impact on performance. Many other research groups have tried to make this particular polymer, but the KAUST team is the first to succeed. Their innovation is based on polymers called polythiophenes with chemical groups called glycols attached in precisely controlled positions. Learning how to control the locations of the glycol groups in ways not previously achieved was a key aspect of the breakthrough. “Identifying the right polymer design to fit all the criteria that you are looking for is the tough part,” says Hallani. “Sometimes what can optimize the performance of the material can negatively affect its stability, so we need to keep in mind the energetic as well as the electronic properties of the polymer.” Sophisticated computational chemistry modeling was used to help achieve the right design. The team was also aided by specialized x-ray scattering analysis and scanning tunneling electron microscopy to monitor the structure of their polymers. These techniques revealed how the location of the glycol groups affected the material’s microstructure and electronic properties. “We are excited by the progress Rawad made on the polymer synthesis, and we are now looking forward to testing our new polymer in specific biosensor devices.” says Iain McCulloch of the KAUST team, who is also attached to the University of Oxford in the U.K. McCulloch says that the research group is now trying to improve the stability of their polymers and the sensors built from them, as they move from laboratory demonstrations toward real world applications. Source: EurekAlert - www.eurekalert.org

Following the approval of promising vaccines and the beginning of immunization distribution, there is general hope for relief of the current situation. To transport and administer these vaccines, there is a need for large quantities of primary packaging goods, including syringes and medicine vials. To ensure patient safety, these are subject to strict regulations. The ZwickRoell portfolio includes scalable solutions for all required tests. Mandatory tests include, among others, measurement of the residual seal force on vials, which is used to evaluate the integrity of the seal and thereby avoid damages during transport. Single use syringes with attached needle are used for withdrawal and application of the vaccine: Luer lock connections tested according to ISO 80369 ensure a secure connection between the injection needle and the syringe by means of rotating spin lock. To make certain that the needle is not damaged when penetrating the rubber seal, a further test is performed to determine the required puncture force. The syringe itself must also accurately meet the requirements. Parameters such as breakaway force and glide force, which are tested to ISO 7886-1 must fall between precisely specified limits to guarantee safe administration of the vaccine. In addition to the system composed of vials, single use syringes and injection needles, which allows for on-site injections of a variety of different medications and vaccines, prefilled syringes are also available. These are pre-filled with the correct dose of the respective active ingredient and are individually packaged and quickly administered. ZwickRoell also provides standardized and modular testing solutions for the ten mechanical tests specified in ISO 11040. Given the amount of vaccine doses needed, the number of required tests for quality assurance purposes is also vast. ZwickRoell offers X-Y tables for simplified performance of a complete test cycle or automation via robots. roboTest N is the perfect flexible aid in the testing lab. The quick and easy to set up testing assistant based on a lightweight robot can be used with different testing machines for any pick and place applications. In this way, both small and large test series can be automated as required and at short notice. Source: ZwickRoell Ltda. - www.zwickroell.com

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N N N N N N N N N N N N N


Massa Molar Absoluta Distribuição de Tamanho Conformação e Ramificação Viscosidade Intrínseca Plot de Mark-Houwink

18 Ângulos A maior sensibilidade da categoria Massa molar de 200 Da até 1GDa Raio de giração (Rg) a partir de 10nm Raio hidrodinâmico (Rh) a partir de 0,5nm

Flowscience Instruments Comércio Ltda R. Adib Auada, 354 sala 21 - Cotia, SP | 06710-700 Fone: +55 (11) 4702-0422

www.flowscience.com.br


October Polymer Sourcing & Distribution Europe Date: October 4-6, 2021 Location: Hamburg, Germany Website: www.ami.international/events/event?Code=C1138 4th International Conference on Materials Engineering & Science Date: October 6-7, 2021 Location: Duhok, Iraq Website: iconeas.com International Conference on Materials Science and Engineering Date: October 11-14, 2021 Location: Brisbane, Australia Website: www.materialsconferenceaustralia.com Sustainable Polymers Date: October 17-20, 2021 Location: Safety Harbor, United States Website: www.polyacs.net/21sustainablepolymers 16th Brazilian Polymer Conference – (16º CBPol) Date: October 24-28, 2021 Location: Ouro Preto, Brazil Website: www.cbpol.com.br

November Performance Polyamides USA Date: November 2, 2021 Location: Cleveland, United States Website: www.ami.international/events/event?Code=C1149 Plástico Brasil Date: November 8-12, 2021 Location: São Paulo, Brazil Website: www.plasticobrasil.com.br Controlled Radical Polymerization Date: November 14-17, 2021 Location: Charleston, United States Website: www.polyacs.net/crp2021 7th International Conference and Exhibition on Polymer Chemistry & Biopolymers Date: November 19-20, 2021 Location: Paris, France Website: https://polymerchemistry.insightconferences.com Multilayer Flexible Packaging Date: November 23-25, 2021 Location: Barcelona, Spain Website: www.ami.international/events/event?Code=C1147 16th European Bioplastics Conference Date: November 30 – December 1, 2021 Location: Berlin, Germany Website: www.european-bioplastics.org/events/eubp-conference

December Silicon-Containing Polymers and Composites Date: December 1-4, 2021 Location: San Diego, United States Website: www.polyacs.net/2021siliconc 2nd Plenareno Material Science and Nanotechnology Conference Date: December 3-4, 2021 Location: Barcelona, Spain Website: materialscience-nanotech.plenareno.com 11th International Colloids Conference Date: December 5-8, 2021 Location: Mallorca, Spain Website: www.colloidsconference.com

Polymers in Flooring Date: December 9-10, 2021 Location: Berlin, Germany Website: www.ami.international/events/event?Code=C1150

January 13th International Conference & Expo on Chromatography Techniques and Spectrometry Date: January 28-29, 2022 Location: Singapore, Singapore Website: chromatographicspectrometry.conferenceseries.com/

February World Congress on Carbon and Advanced Energy Materials Date: February 07-08, 2022 Location: Webinar Website: global.materialsconferences.com Polymer Colloids Date: February 19-22, 2022 Location: San Diego, United States Website: www.polyacs.net/22polycolloids 18th International Plastics and Petrochemicals Trade Exhibitions Date: February 21-24, 2022 Location: Riyadh, Saudi Arabia Website: saudi-pppp.com/saudi-plastics-petrochem International Conference on Material Science and Engineering Date: February 24-25, 2022 Location: Prague, Czech Republic Website: materialsscience.conferenceseries.com

April 37th International Conference of the Polymer Processing Society (PPS-37) Date: April 11-15, 2022 Location: Fukuoka, Japan Website: www.pps-37.org

May Polymers for Fuel Cells, Energy Storage and Conversion Date: May 15-18, 2022 Location: Napa, United States Website: www.polyacs.net/2022fuelcells 23rd World Congress on Materials Science and Engineering Date: May 18-19, 2022 Location: London, United Kingdom Website: materialsscience.insightconferences.com

June Polymers and Fire Date: June 5-8, 2022 Location: Napa, United States Website: www.polyacs.net/22fipo

July 49th World Polymer Congress – MACRO2022 Date: July 17-21, 2022 Location: Winnipeg, Canada Website: iupac.org/event/macro2022

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ABPol Associates Sponsoring Partners

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ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.00221

Use of biodegradable polymer for development of environmental tracers: a bibliometric review Adriana Marques1*  and Sandra Maria da Luz2  Instituto Federal de Educação, Ciência e Tecnologia de São Paulo – IFSP, Itapetininga, SP, Brasil 2 Faculdade do Gama, Universidade de Brasília – UnB, Brasília, DF, Brasil

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*adrimarks@ifsp.edu.br

Abstract Qualitative and quantitative measuring in water bodies, nuclear medicine, agriculture, and world oil production use tracers to monitor, evaluate and continuously improve their processes. The bibliometric information about the past and the future of artificial tracers, to monitor surface and groundwater by using sustainable biodegradable materials it will be important for future generation. To fulfil this purpose, bibliometric literature analysis has been considered as a solution to identify research areas that need to be improved. The results of this paper showed that even with the increase in research in ​​biopolymers, and the use of artificial tracers, academic development is still not significant. The United States, China, and Germany are the top publishers in this field however, there is no country that constantly develops research in these areas concomitantly using biodegradable polymers. Because of that, this field could be further explored, globally using innovative techniques and materials for new tracers. Keywords: polyhydroxybutyrate, polymeric membranes, nanocellulose, biodegradable materials, tracers. How to cite: Marques, A., & Luz, S. M. (2021). Use of biodegradable polymer for development of environmental tracers: a bibliometric review. Polímeros: Ciência e Tecnologia, 31(1), e2021012. https://doi.org/10.1590/0104-1428.00221

1. Introduction Contamination of surface water and groundwater is an invisible threat to global development goals[1]. Over time, drinking water has become an indicator of life quality in contemporary society and a factor of social and economic development[2]. Waterborne diseases, lack of water quality for communities and contamination in watercourses are challenges that have been addressed by the development of research regarding new materials and technologies in the fields of nuclear medicine and pharmacology[3]. It would be necessary to develop new clinical protocols, new drugs and environmental evaluation to monitor changes in aquatic ecosystems[4]. This problem arises because of the need to monitor possible sources of water pollution[5]. One of the tools used in the hydrogeological environment for monitoring surface and groundwater are artificial tracers[4,6]. The term ‘tracer’ refers to any product or substance that, when incorporating the mass of another substance, allows analysis of its behaviour relating to physical or chemical processes, in addition to being able to identify the dynamic behaviour of a flow system[7,8]. The tracers must be conservative and innocuous and need to have a low impact on the environment, in addition to having a relevant cost-benefit[9,10]. Meanwhile environmental tracers are defined as ambient, natural or anthropogenic compounds or materials that are widely distributed in the environment, the artificial tracer are defined as a substances injected in the site with the scale of application limited to the space and time such as

Polímeros, 31(1), e2021012, 2021

fluorescence, salt, drifting particles (dye) and radioactive tracers[4,8]. Because of the various types of artificial and environmental tracers mentioned, they can provide hydrological, ecological, and biogeochemical information that allow an understanding of the interactions of surface water and groundwater[6]. Tracers allow hydrometric observations of the course of diffuse water contamination, based on the scale of the site studied and the type of tracer chosen[6,9]. Tracers can be made by using different materials with different chemical, optical, colour, fluorescence, gaseous, radioactive, saline, and biological properties[7]. Currently, tracers are used in several areas of knowledge, with a wide range of technological and scientific applications. In nuclear medicine, they are the contrasting products used in magnetic resonance imaging (MRI) and are used to improve the image quality for staining and identifying tumours, etc. The basis of this contrast is a rare earth element called Gadolinium, chelated with diethylenetriaminepentaacetic acid (DTPA)[10,11]. In the biological sciences, tracers are used to measure variations in nutrients received by plant sap and they appear in agriculture, increasing productivity by determining the relevant parameters for irrigation and application of pesticides[12]. In geoscience, geology and hydrology, tracers are widely used for environmental monitoring, being one of the most relevant applications for the monitoring of surface and groundwater[13-16].

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Marques, A., & Luz, S. M. In engineering, tracers are used in hydraulics, to characterize reservoir recharge and refilling[6,17]. Also they were used in this field to integrate hydrogeochemical, hydrogeological data to understand groundwater flow for a karstified aquifer system[18-21]. In innovative engineering research areas multiple-tracer data are tools that can bring information by using parameter models regarding the dispersion of fractured and carbonated rocks and also they can show how ages could be used to calibrate groundwater models[22-25]; Not only, traditional stable isotopic tracers as H, O, N and C are used but also Li, Zn and Cu in the last 20 years have emerged as a new tracers[26,27]. In civil engineering, tracers are used to assess soil contamination by diesel, gasoline, and kerosene[28-30]; Besides that, Hillebrand et al.[31] states that pesticides transportation in wetland, groundwater ad surface water interfaces could be investigated by artificial tracers. In electronics and aeronautics, fluorescent tracers work as sensors or detectors of emissions[32]. In chemistry, fluorescence detectors are used in chromatographic systems[33-35].In the field of oil and gas production, they use tracers in various exploration processes, throughout the life of a field[36-38]. However, there is a shortage of economically viable and commercially accessible tracers for applications where there is biphasic flow (where water predominates over oil) with large volumes of injection and production[30,39]. The application of tracers to different areas of knowledge via numerous ways and forms also raises questions about the toxicity and impact on the environment[40,41]. Regarding to the injection of artificial tracers in the environment, many studies[42,43] shown the harmless of Uranine, Eosine, Pyranine and Naphthiolate and also the toxicity of Rhodamine group even though they have been used in grams or kilograms they still live the residual contamination[44]. Because of that, firstly some tracers have already been banned due to potential harm to human health and the environment and secondly the planning of tracers experiment with tolerable daily intake (TDI) it must be considering for the future regarding their possibility of bio-accumulative or carcinogenic to nature[14,45]. Biodegradable tracers could be an alternative for overcoming such environmental concerns. In order to be clear about the information regarding BCM in this paper, some definitions are necessary. The definition of a composite membrane (CM) is one which consists of at least two structural elements made from different materials[46]. Usually in a CM, there is an asymmetric structure with layers that could be made of porous or non-porous materials[47]. These layers could have different properties depending on the structural elements and the functionalized or doped process[48]. Besides this, CM can be prepared by numerous processes such as blending[49], coating[50], casting[51], or electrospinning[52,53]. Regardless of the production model of the membranes, nanocomposite membrane (NCM) is formed with nanostructure and polymer composite membrane (PCM), that could be made with different polymers. It is also important for this discussion because of its functionality as a biodegradable composite membrane that could be made for the conjunction of NCM and PCM[17,54]. Biodegradable composite membranes (BCM), especially those based on natural and biodegradable polymers, are widely 2/14

used in different fields. For a reverse osmosis desalination product[55], prepared a polymeric matrix composed of a membrane doped with fumed silica particles. In the same year[56], developed biodegradable and recyclable cellulose nanofibre composite membranes for use as ultrafiltration (UF) membranes. In addition, Lin et al.[57] developed a biodegradable polyhydroxybutyrate/ poly-e-caprolactone fibrous membrane, modified with a silica composite and hydrolysed for superhydrophobic and outstanding antibacterial applications. The use of biodegradable polymer from the polyhydroxyalkanoates (PHA) family has been similarly mentioned in all of the research previously cited. For this reason, polyhydroxybutyrate (PHB), which is a natural polymer synthesised by a variety of microorganisms, is commonly used. This polymer is biocompatible, biodegradable and widely studied for medical applications[58,59]. Although PHB shows its hydrophobicity as an advantage, in some cases, its hydrophilicity and crystallinity, which promotes interactions of some materials, could be reached by the functionalisation or addition of other elements (such as nanocellulose or chitosan)[50,60]. On the other hand, to improve the PHB performance for bone regeneration in regeneration medicine (RM) and tissue engineering (TE), an alkaline NaOH-based treatment was used and the results showed that rapid, simple, and inexpensive sodium hydroxide treatment modifies the morphology and chemistry of the PHB membrane’s nanostructured surface, inducing hydrolysis of ester bonds and creating carboxylic surface functional groups, which increases the surface hydrophilicity[61]. In tissue engineering, a promising use has arisen within regenerative medicine (RM) by considering the context of the biomaterial scaffold made, mainly, by PHB[62]. The antibacterial and antifungal properties of the PHB oligomer proved effective, with some degree of polymerisation. Besides this, the oligomer is also a great candidate for antimicrobial modification in biomedical applications because of its low production cost[63]. The surface modification of PHB, poly (lactic acid) -PLA, poly (methyl methacrylate) -PMMA and polyurethane (PU)/ poly (dimethylsiloxane) (PDMS) films can be accomplished by several techniques and applied to different areas as biodegradable polymers. The thermal evaporation method[64] for surface modification that occurs by automatic structuring of membranes with fibers, PHB, octadecyl acrylate copolymer and hydrophilic vinylpyrifine of zwitterionics, zP (4VP-ODA)[60], is a mechanism that is able to control the interactions between fibers and bio-encrustation. In this sense, PHB is a material of interest for bio-scaffolding even though its hydrophobicity generates unwanted interactions with proteins and bacteria, so it is necessary to use an electrospun technique for changing the composite membrane surface with fibers and PHB[65]. In this way, spreading the blend of the membrane surface improved the PHB and fiber hydrophilicity, being able to reduce the serum album adsorption to 92%, lysozyme to 73% and fibrinogen to 50%. This new composite material is promising for tissue engineering (TE) and other applications in the field, for monitoring environmental sites[66,67]. Polímeros, 31(1), e2021012, 2021


Use of biodegradable polymer for development of environmental tracers: a bibliometric review As could be verified in the literature cited, PHB has been used in the preparation of membranes for various uses, especially in the healthcare industry[58]. One of the most important examples of these uses is the prefabricated stainless-steel masks with different mesh sizes, reinforced with different biodegradable polymers due to surface laser treatment[68]. Studies indicate that surface modification can be achieved with a high degree of success and precision[60,69]. The surface topographic features created by laser modification appear to improve the binding and growth of human fibroblastic cells in some of the films[68]. Notwithstanding this, regarding the energy and electricity areas, some conductive polymers and nanocomposites have been extensively studied and applied in the field of organic photovoltaic elements and flexible organic electronics. As an example, flexible conductive biopolymer nanocomposites (made with silver nanowires and PHB) have also emerged as a transcutaneous, stimulated sensor[70]. This was developed by Tematio et al.[70] in the Laboratory of Applied Nano Sciences at the University of Applied Sciences, Western Switzerland. Finally, another environmental example was a biopolymer used to produce a nanofiber composite nanofiltration membrane (NCNM) to remove dye from contaminated water. In this research, applied PHB and calcium alginate[71]. The membrane pores were formed as nano tubes by using the electrospinning technique. The NCNM (CaAlg-c-PHB/ CNT) was tested to remove dyes with a molecular weight between 400 g/mol and 900 g/mol and it worked better in a system with high pressure (0.1 to 0.7 MPa)[72,73]. We sought to explore the research from the last 46 years that dealt with these materials, as well as which countries have published most papers regarding biodegradable tracers for hydrological modelling, identifying emerging pollutants and the ways in which systematic research can inform areas of study that still need to be developed further, regarding new materials and new technologies for the production of biodegradable tracers. In summary, the main purpose of this section is to explore how environmental tracers (ETS), biodegradable composite membranes (BCM), and functionalised biodegradable polymers (FBP) have been used, especially when applied to environmental monitoring purposes. Furthermore, an overview of the data source, using similar visualisations, will be explained to elucidate how it will be possible to interconnect the ETS, BCM, and FBP with the keywords used to find the papers published and indexed on the Scopus database, relating to the environmental and biodegradable tracers for monitoring surface water and groundwater.

2. Materials and Methods The methodology used in this paper was qualitative and used a bibliographic method for map and cluster development, based on the Visualisation of Similarities (VOS). The free software known as VOS viewer uses the VOS technique. The software is made available by Leiden University for the creation of bibliographic networks. The text mining features allow the building and visualisation of the co-occurrence of networks including the terms extracted from the search results. Polímeros, 31(1), e2021012, 2021

This study aims to present a systematic bibliometric literature review of publications (from 1973 to 2020) that present the state of the art of environmental and artificial tracers with low levels of contamination, used for monitoring surface and groundwater. To find these tracers in the publications indexed in the Scopus Database, the bibliometric methodology used was to identify the co-occurrence of nine keywords that could be linked to the production, use, or synthesis of the tracers. The keywords used were: composite membrane, cellulose, silane, microfiltration, nanofiltration, PHB, environmental tracer, membrane, and functionalization. However, these keywords could be combined as a way to better search the database and, because of that, some groups of keywords will appear to showing agglutinated words. The motivation for using these words is because they could reveal the combination of natural and biodegradable products with low cost benefit and high added value that could made innovative tracers.

2.1 Data sources and visualisation of similarities using the VOS viewer program Functional forms to identify similarities in publications related to the same issues are important when determining the status quo of a specific study area. However, when the number of publications, authors, and study year starts to become huge, it is difficult to systematise without a computer tool. Some of these tools were used for a long time by bibliometric and scientometric areas in the library and information sciences[74]. Bibliometrics is a method of associating statistical and econometric laws and principles to map scientific productivity comprising journals, keywords, and authors[75]. The laws used are Bradford’s Law (to inventory journal productivity), Lotka’s Law (to evaluate authors’ productivity in the scientific sphere) and Zipf’s Law (to locate the frequency of keywords used by researchers)[76]. The tool used to display bibliometric data in an easy and organised way is map projections. These projections can be created by using lines that represent the distances between the items of interest or they can use graphs referencing the items. One of the most-used, open-source tools for visualising bibliometric networks is the VOS viewer software[77,78]. VOS viewer constructs a map based on a co-occurrence matrix system. To start, the VOS viewer needs a similar matrix that will work as an input, relating to associations of linked strength data, as shown in Equation 1[79,80]. sij =

cij

( wi * wj )

(1)

Where sij is the similarity of i and j, from the terms inside the matrix system, c means the co-occurrence existing in the same indexes i and j, and the weights wi and wj are related to the items correlated in the matrix system[81,82]. Then, the maps are created using the VOS layout and the cluster technique. The program has three forms of view: network, overlay, and density. In the network map, the label size and circle size depend on the weight of the author or keywords. The overlay map has the same dynamics as the network map but the items are coloured differently[79,82]. 3/14


Marques, A., & Luz, S. M. The main principle of density map identifies items by a label and each one has a colour, depending on the number of items in the vicinity and the importance of neighbouring items[79].VOS viewer also uses the Gaussian kernel function for the density maps, where k means the dot that will be the product transformed exponentially in the infinite dimension space, and t is the entrance vector data, as shown in Equation 2[79].

( )

k= ( t ) exp −t 2 (2)

The other way to see the information could be by cluster density. In this view, all points are calculated separately and, therefore, the density could be calculated using a point x, a cluster p, and a density D by the Equation 3[79]. n

= Dp ( x ) ∑ [ I p ( i ) wi ]K ( x − xi / (d h ) (3) i =1

Ip = a function 1 if the item i belongs to cluster p but 0 otherwise; K = Gaussian kernel function. As a result, all clusters calculated will receive a color that´s depend on the total item density of a point[83]. For the research in this database[84], the strategy of using the nine keywords cited above could be viewed in Figure 1. combined in seven groups, always based on the three words retained out of the nine chosen. In the first six groups, PHB and membrane were fixed and a word was added. The seventh group worked with environmental tracer and membrane. In the research, the “AND” connector was used to bring results that included all terms according to the constructed groups. Thus, it was possible to obtain global results, allowing a comprehensive view of the topic of interest. The search found 203 publications related to the seven groups. Then, the results were downloaded in the Research Information Systems (.ris), which is the file format used to transfer and store bibliographic citation data. The file in “.ris” with the final version, was extracted from Mendeley and uploaded to VOS viewer to create the bibliometric network that identifies the recurrence of the

most common terms in this research. A network was created, following the settings within the software: the option to create a map based on bibliographic data was selected. The type of analysis was co-occurrence and co-authorship (the relationship between items is determined by the number of documents in which they occur together). The units of analysis were keywords and authors. The counting method was fractioned, where the weight of each co-authorship or co-occurrence link is fractioned, and ultimately, the minimum number of occurrences of keywords chosen for the search was three. Windows 10, Excel® and SAS® software were also used to analyse the data such as years of publication, papers, publication by country and groupings by keywords.

3. Results and Discussions 3.1 The scenario of artificial and natural tracers’ publications per year The result of the five decades of publications presented in this paper picked up the scenario of artificial and natural tracers for monitoring the aquatic environment. The beginning of the studies of natural tracers was in 1973, in the medicine field in Germany. The study was related to phosphate transfer through natural membranes, as measured by tracer exchange properties[85]. Firstly, in this research, it was possible to infer that there was an ascending number of publications related to the nine keywords used in this research (see Figure 2). Secondly, it was possible to figure out that in two decades, between the 1990s and 2000s, there was a significant growth rate related to the sustainable approach all over the world. In the year 2000 in Mexico, groundwater flow was monitored downfield from wastewater systems using artificial tracer techniques on St. George Island, located in the North-East Gulf of Mexico[86]. In the 1990s, research about artificial tracer tests contributed to the knowledge of the transfer time for a pollutant between the inlets and the outlets[87]. Figure 2 also indicates that there is a crescent tendency of the number of publications in the years 1997 to 2002 and 2002 to 2008. This was probably related to the concept of

Figure 1. The seven groups encompassing the keywords searched using the Scopus databased. 4/14

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Use of biodegradable polymer for development of environmental tracers: a bibliometric review sustainable development which was represented by some important international conferences such as Rio-92 and the Ministerial Conference of Environmental[88]. In the following decade, the Agenda 21 and Millennium Development Goals (MDG) international events constituted an exciting emerging area regarding responsible consumers and sustainable development[89]. Even with the annual growth, statistical data analyses show there is still a lag concerning the subject, with the number of papers published annually from 1990 to 2000 being incipient, which totals no more than 3.3 per year in a decade of global publication. In the following decade, even with the tripling of the annual average of publications, it was still not possible to reach 10 publications per year, which shows relevant but not high enough annual growth. Moreover, most of the publications do not mention any information about the approach to decrease environmental risks related to the permanence of artificial tracers in the environment.

Following the potential and exponential growth of contaminated aquatic areas, the growth of the research concerning environmental tracers does not follow the trends. The papers presented in the indexed journals in Scopus databases still work with old types of artificial tracers like radioactive or color tracers, as shown by Hillebrand et al.[90] and Zhang et al.[91].

3.2 Distribution by journals, publications, and country to identify the tendencies The journals database and the number of publications that used the nine keywords chosen for this paper over the last 47 years, found 94 journals published in different areas and subjects of study. For the most part, the average journal publication was 2, which represented only 8% of the total annual publication in those journals. Alternatively, Figure 3 shows the main 12 journals with more publications. These journals use an impact

Figure 2. Number of publications from 1973 to February 2020, using the keywords: PHB, membrane, cellulose, silane, nanofiltration, microfiltration, functionalization, membrane composite and environmental tracer, and Mean number of publications for every six year.

Figure 3. Journals with the most papers published (from 1973 to 2020) that used the keywords research. Polímeros, 31(1), e2021012, 2021

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Marques, A., & Luz, S. M. factor as an index of quality and they have the 3 highest impact factors in the environmental field in 2019. i.e. Water Research Journal (9,130), the Environmental Science and Technology (7,864) and Science of the Total Environment (6,551). The other journals also use impact factor and none of them have impact factors below 1.0[92]. The remaining 82 journals were completely spread among all over the world in areas such as medicine, biology, microbiology, chemistry, physics, the environment, water, veterinary medicine, polymeric materials, oil, and energy. The Environmental Science and Technology journal represented most of the publications. An examination of the findings of the results shown in Figure 4 provides relevant information about the countries that lead research outputs, by groups of keywords and by study area. The first country with the most publications regarding environmental tracers is the United States of America (28.12%), the second is Germany (14.06%) and the third, France (10.16%). However, the country with the most publications on biodegradable materials, that could be used for biodegradable tracers, is China (31.95%), followed by Brazil (14.28%) and Germany (11.90%). Concerning microfiltration and nanofiltration membranes using the PHB, the ranking of the countries is led by China (77.77%) and then France and Malaysia, with the same proportion (11.11%). Furthermore, the general ranking concerning publications by country showed that the countries with the highest research numbers in the summary area

(within all nine keywords used) are the United States, China, Germany, and France. It may be inferred that the ranking is linked to investment in research and development (R&D). Since 2000, the USA, Germany, the United Kingdom, China, and France have been among the twenty countries that have invested the most in R&D, according to OECD data[41]. The USA has been targeting more than 2.6% since 2000 and has remained almost constant over time. In the same year, Germany invested more than 2.4% and this percentage went up to 3.1% in 2018. China had an increase in its investment, going from 0.9% in 2000 to 2.1% in 2018. France has remained at around 2.1% and the United Kingdom invested 1.61% in 2000 and had a 9% increase in 2018[93]. These countries concentrated their important research in Universities and Research Centres, as shown in Table 1. This presents the distribution of the papers by affiliation and the countries in which they were published. Most of the publications were conducted by the two or three main universities in the country with the referenced research areas. There are some exceptions, e.g. the University of Wrocław in Poland and the University of Rouen in France. To sum up, there are four countries and eleven universities in three continents putting effort into these areas of study. They have been responsible for 68.07% of all publications in the last 47 years, with an average of 2.35 papers published per year (less than one paper per continent per year). Using this information, it is possible to infer that there is a gap in publications in this field.

Figure 4. The world map with the number of publications by country, relating to groups of three keywords (PHB, membrane, cellulose, silane, nanofiltration, microfiltration, functionalisation, and environmental tracer) using the Scopus Database. 6/14

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Use of biodegradable polymer for development of environmental tracers: a bibliometric review 3.3 Author and co-authorship The relationship between the authors and co-authorship was visualized by VOS viewer because of our study, there are 798 authors involved in at least one paper with co-authorship. This presents 52 clusters in an overlay visualization. In the next scenario (Figure 5), the arrangement was defined by two conditions: firstly, that each author must have 3 papers published and, secondly, there should be a connection between them. The way that was chosen to show the clusters was ‘overlay visualization’ and, because of that, Figure 5 shows all of the clusters, with a stronger linkage among them. This type of data is important because it shows an overview of all authors. On the other hand, some of the authors seen in Figure 5 will disappear if the connection is narrowed, and sometimes important information is lost. When the clusters are narrowed or when there are too many restrictions, this results in Figure 6.

The three researchers with the most publications and co-authorship appear with the highest circle in the clusters shown in Figure 6. These researchers are described below. Zhijian Cai (an associate professor and Ph.D. supervisor at Tianjin University) works with nanofiltration membranes and PHB. Fei Liu (a professor in the Key Laboratory of Water Resources and Environmental Engineering and works with the groundwater monitoring system); some of his research has studied artificial tracers for monitoring groundwater and considered the presence of emerging contaminants, in his publishing group. Ziheng Zhang (from the Research Centre for Smart Wearable Technology at the Institute of Textile and Clothing) who works with PHB and its blends, showing studies based on the kinetic and mechanical properties of these blends. The identification of authors separated by affiliation and country is shown in Figure 6 and the clusters are separated by main authors and co-authors. The results indicate the

Table 1. Distribution of the intellectual production by affiliation and country. Related clusters 5 5 5 3 5 6 2 2 2 2 2 1 3 3 6

Affiliation State Key Laboratory of Separation Membranes and Membrane Processes, School of Textiles, Tianjin Polytechnic University, Key Laboratory of Eco-textiles of Ministry of Education, College of Textiles Jiangsu Engineering Technology Research Center for Functional Textiles School of Material Science and Engineering, Tianjin Polytechnic University School of Textiles, Tianjin Polytechnic University, Tianjin Department of Biology, University of Rouen Technical University of Dresden Ernst Moritz Arndt University of Greifswald Silesian Piast Medical Academy Wroclaw University in Wrocław Academia Médica Estadual de Nizhny Novgorod University of Moscow Department of Poultry Science, Auburn Department of Chemistry, University of Massachusetts, Lowel Department of Physiology, Michigan State University

Country China China China China China France Germany Germany Poland Poland Russia Russia USA USA USA

Figure 5. Clusters with authors that have 3 papers or more and with a linkage between them. Polímeros, 31(1), e2021012, 2021

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Marques, A., & Luz, S. M. existence of 6 clusters. Figure 6b identifies how the cluster appears in the VOS viewer programme and how the author and authorship could be studied by their linked strength correlation. Figure 6a shows the groups of authors. Cluster 1 is from the research group at Moscow University and its main research concerns poly(3-hydroxybutyrate) and polymer film production[94]. In cluster 2, the main research objectives are biocomposites made by flax fibres for testing the proliferation of bacteria[95]. Cluster 3 shows the authors and co-authors that research composite hydrogels reinforced by cellulose[96]. Cluster 4 researches polymers and Cluster 5 shows the authors that research in the field of membrane and filtration systems[44]. Artificial and natural tracers are the main field of research for Cluster 6[86].

3.4 Keywords and co-occurrence scenarios By using keywords as the unit of analysis with the full counting method, the chosen threshold found 3,148 keywords with a minimum of 5 occurrences. The network visualisation per occurrence and co-occurrence indicates that there are four clusters, as presented in Figure 7 and 8. The first deals with the use of membranes in sewage treatment plant bioreactors. The second cluster talks about groundwater and environmental monitoring, traditionally performed with conventional tracers. The third cluster refers to the use of polymers, such as PHB, composite membranes, and artificial membranes, and their use in areas such as medicine and biology (in terms

Figure 6. Authors and co-authors linked with the highest total linked strength by publications (a) list of author and co-authorship related to affiliation on Table 2; (b) network visualisation of clusters by authors and co-authorship.

Figure 7. Overview visualisation on time-based trends in academic studies from 1995 to 2020. 8/14

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Use of biodegradable polymer for development of environmental tracers: a bibliometric review

Figure 8. Occurrence of keywords during the years of investigation, from 1973 to 2020. Table 2. The relations between keywords and their occurrence; a linked strength of the Top 10. Keyword Membrane/artificial membrane/ Composite membranes Water pollutant/ Wastewater treatment plant (WWTP)/ sewage Environmental monitoring/ water monitoring Tracer: radioisotope/ environmental PHB/ polymer Unclassified drugs Bioreactor Biodegradation Groundwater Cellulose/nanocellulose

of biocompatibility). The fourth cluster deals with water pollution by chemicals, their concentrations, occurrences, and bioaccumulation problems in humans and the high loads present in nature. From 2010, the research continued in the environmental area, incorporating the medicine area to better develop the issue of biocompatibility of biopolymers and membranes with drugs and, therefore, possibilities for improvement and effectiveness in the quality of human life. At the same time, the words ‘fibre-reinforced membranes’, ‘adsorption’, ‘hydrophobicity’, ‘biodegradable polymers’, and ‘tracers’ are present in several papers concerning sewage, groundwater and surface water treatment. Nonetheless, the major concern that motivated and directed the research was the need to improve the quality of environmental monitoring so that prior knowledge of a possible problem could be mitigated before more expense was needed to solve it[59,95]. Under different circumstances and by analyzing the specific keywords directly related to the words that were researched, but from a temporal point of view, it is possible to determine that, by 2010, research in the area of biodegradable polymers using PHB, membranes, tracers, and cellulose was focused on the analysis of environmental issues using Polímeros, 31(1), e2021012, 2021

Occurrence 33 29 27 18 11 9 7 6 6 3

Total linked strength 181 290 163 128 30 73 45 104 36 10

tracers as a tool (isotopes) in surface and groundwater, as shown in Table 2. Also, regarding the use of membranes, most of the publications were concerned with the implantation of filters and reactors in water and wastewater treatment plants. However, the most important information in Table 2 is the relationship between linked keywords and their occurrence in the papers. Correlation between the published articles can be made by assigning values to the relationship between the repeated keywords inside them[81]. This correlation is identified and quantified in the VOSviewer program, according to its manual, using numerical values referring to the strength of the keywords[97]. This binding force is linked to the number of occurrences and the highest value gives the strongest link. Table 2 shows the top 10 keywords which gave the highest value of linked strength in the research[82]. Numbered lists can be added a Finally, Table 3 presents a scatter plot for each group of relevant words by country. This information gives a quick view about the preferences of publications by country and shows the tendencies of academic research works. China is a unique country that works in all fields mentioned in the Table 3 and also has 9/14


Marques, A., & Luz, S. M. Table 3. Word group by publication and country. Country

TP

United States of America China Germany France United Kingdom Australia Brazil Canada Japan Poland Taiwan Netherlands Turkey

44 28 25 16 13 10 9 9 6 6 6 5 5

ETS 35 7 18 13 13 9 2 7 5 1 2 4 1

PCM 1 2 2 -

PMS 1 2 -

Word Group (WG) PMN PMM 3 4 1 -

PMF 5 1 2 2 1 2 1 2 3 3

PMC 3 10 3 1 4 3 1 1 1

TP: total publication; ETS: environmental tracers – membrane; PCM: Polymer and Composite Membrane; PMS: Polymer, membrane, and Silane; PMN: Polymer, membrane and nanofiltration; PMM: polymer membrane, and microfiltration; PMF: Polymer, membrane and functionalization; PMC: Polymer membrane cellulose.

the highest publication in the field of PCM (10) followed by Brazil (4), Germany, and Poland (3). The United States of America has most of the publications on the monitoring of surface water and groundwater using environmental tracers - ETS (35) followed by Germany (18), France and the United Kingdom (13). Moreover, none of the countries used biodegradable tracers. Most of the tracers used were colored, isotopic or luminescent tracers. The data also showed that the minority of publications are in the field of polymeric membranes, functionalized with silanes and nanofiltration polymeric membranes. In addition to making large financial investments, these countries have funding institutions for research and development: in China, it is the National Foundation for Natural Sciences; in Europe, it is the European Commission and, in the United States, it is the National Science Foundation. All of these investments create returns for the countries involved, and it is worth noting that these are the countries with the greatest economic growth in recent years, being in the G20 group and with the largest investment of GDP in R&D in the world[98].

4. Conclusions Undoubtedly, in times of great challenges and socioeconomic and environmental risks, there is a need for academic research to be ahead of the times. The need for quick and direct responses to new threats of contamination of the water, soil and the global population itself raises the question of planning and directing academic research. This paper provides a bibliometric investigation of research on a biodegradable tracers for monitoring surface water and groundwater, from 1973 to February. The research findings gave three important answers. Firstly, in the last 47 years, tracers have been used in different ways and for different purposes in medicine, agriculture, and hydrological engineering. Secondly, most of the tracers used worldwide are either radioisotope, fluorescent, or chemical, and all of them leave a residue of contaminants in the aquatic environment after being used. 10/14

Environmental concerns have increased over the past 20 years, notwithstanding the fact that the artificial tracers that have been developed still carry a large polluting load and there has been no new research for biodegradable tracers with low ecotoxicity for environmental sites. According to the prior study, artificial tracers made with biodegradable materials with low toxicity should be an important research area for the future.

5. Acknowledgements The authors would like to thank CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior), DPG/ UnB (Decanato de Pós Graduação/ University of Brasília), FAPDF (Fundação de Apoio à Pesquisa do Distrito Federal) and CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) for their financial support to this Project.

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Polímeros, 31(1), e2021012, 2021


ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.08120

10-(pyren-1-yl)-10h-phenothiazine and pyrene as organic catalysts for photoinitiated ATRP of 4-vinylpyridine Loc Tan Nguyen1, Hung Quang Pham1, Duc Anh Song Nguyen1, Luan Thanh Nguyen1, Ky Phuong Ha Huynh2, Hai Le Tran2, Phong Thanh Mai2, Ha Tran Nguyen1,3* , Le-Thu Thi Nguyen1,3, Thuy Thu Truong1,3** National Key Laboratory of Polymer and Composite Materials, Ho Chi Minh City University of Technology – HCMUT, Vietnam National University–Ho Chi Minh City – VNU–HCM, Ho Chi Minh City, Vietnam 2 Faculty of Chemical Engineering, Ho Chi Minh City University of Technology – HCMUT, Vietnam National University - Ho Chi Minh City - VNU–HCM, 268 Ly Thuong Kiet, District 10, Ho Chi Minh City, Vietnam 3 Faculty of Materials Technology, Ho Chi Minh City University of Technology - HCMUT, Vietnam National University – Ho Chi Minh City - VNU–HCM, Ho Chi Minh City, Vietnam

1

*nguyentranha@hcmut.edu.vn; **trtthuy@hcmut.edu.vn

Abstract The UV light-mediated metal-free polymerizations of 4-vinylpyridine (4VP) have been successfully performed by using 10-(pyren-1-yl)-10H-phenothiazine (PPTh) and pyrene as photocatalysts. The preparation of narrow polydispersity poly (4-vinyl pyridine) (P4VP) with high conversions was enabled in both protic as well as unprotic reaction media at ambient temperature. Additionally, copolymerizations of 4VP with acrylate and methacrylate monomers were also demonstrated, affording metal-free copolymer products. Keywords: organic photocatalyst, metal-free atom transfer radical polymerization, 4-vinylpyridine, pyrene. How to cite: Nguyen, L. T., Pham, H. Q., Nguyen, D. A. S., Nguyen, L. T., Huynh, K. P. H., Tran, H. L., Mai, P. T., Nguyen, H. T., Nguyen, L. T., & Truong, T. T. (2021). 10-(pyren-1-yl)-10h-phenothiazine and pyrene as organic catalysts for photoinitiated ATRP of 4-vinylpyridine. Polímeros: Ciência e Tecnologia, 31(1), e2021001. https://doi. org/10.1590/0104-1428.08120

Graphical Abstract

1. Introduction Pyridine-ring is an attractive chemical structure which possesses not only an annular enclosed conjugated system but also one sp2 hybrid orbital on the nitrogen atom, occupied by one lone pair of electron. Because this pair of electrons is excluded from the aromaticity, it can bind with the protons; therefore, pyridine is alkaline and could also coordinate with transition metal ions to form complexes[1,2]. With these special properties above, pyridine contained polymers have caught much attention of scientists for various applications, for example dye sensitized solar cell[3], pH-responsive materials[4], block copolymer micelles used in drug delivery[5], moreover, they are capable of forming

Polímeros, 31(1), e2021001, 2021

self-assembly supramolecular structures via nitrogen atoms on the pyridine ring[1]. Materials use for applications above are typically based on poly (4-vinylpyridine) (P4VP) or copolymers of 4-vinylpyridine (4VP) and other monomers depending on specific purposes. Typical methodologies to synthesize polymers from 4VP are free radical polymerization[6,7], living anionic polymerization[8], and controlled radical polymerizations (CRPs) including nitroxide-mediated polymerization (NMP)[9], reversible addition fragmentation chain transfer polymerization (RAFT)[10,11] and atom transfer radical

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Nguyen, L. T., Pham, H. Q., Nguyen, D. A. S., Nguyen, L. T., Huynh, K. P. H., Tran, H. L., Mai, P. T., Nguyen, H. T., Nguyen, L. T., & Truong, T. T. polymerization (ATRP)[2,3,12]. Nowadays, polymer chemistry has been significantly developing and the polymerization methods are required to produce polymers with high quality such as narrow polydispersity index (PDI) as well as control molecular weight. Therefore, CRPs have recently become the most common routes to gain polymers with mentioned criteria though there are some difficulties which scientists have to deal with[13], such as the high-temperature requirement of NMP[14] or unavoidable radical termination events of RAFT[15]. Regarding ATRP, although it has been successfully employed for the synthesis of various polymers with well-defined structures[16]. Polymerization of polar monomers, especially of 4VP, via traditional ATRP method is challenging due to the strong coordination between pyridine rings and ligands that can compete with the binding of the metal catalysts in these systems[2]. Thus the selection of solvents as well as catalyst complexes is very tricky. Additionally, ATRP has the drawback of metal contamination in products[17] leading to the limitation in electronic and biomedical applications. In recent years, the development of organic photo-catalysts (o-PC) has emerged as a powerful tool to solve the issue of residual metal in the products[18,19]. In 2014, Hawker et al. first presented a metal-free ATRP of methyl methacrylate using 10-phenylphenothiazine as an o-PC[20]. After that, K. Matyjaszewski’s group reported three phenothiazine derivatives as catalysts used for photo-induced polymerization of acrylonitrile in 2015[21]. Very recently, in April 2018, our group developed (10-(pyren-1-yl)-10H-phenothiazine (PPTh) as the effective o-PC for the metal-free ATRP of several monomers including methyl methacrylate, N,N-dimethylamino2-ethyl methacrylate, 2-([4,6- dichlorotriazin-2-yl]oxy) ethyl methacrylate as well as a copolymerization of methyl methacrylate and 1-pyrenemethyl methacrylate[18]. In 2018, Miyake’s research group reported the novel photocatalyst of N,N-diaryl dihydrophenazines can be applied for both metal-free ATRP and PET-RAFT polymerization to obtain the diblock copolymers of PMA-b-PMMA via both PET-RAFT and metal free-ATRP in sequence[22,23]. Especially, o-PC is expected to be used for the synthesis of pyridine polymers, because of the absence of metal complex catalyst system. ATRP using o-PC does not require the use of ligand so that choice of solvent is much more facile. To our knowledge, despite the potential of o-PC, this method has not been applied to the preparation of pyridine polymers yet. Thus, in this work, we demonstrate the novel metal-free photo-induced polymerization as the facile methodology for polymer preparations from 4VP, which could solve the issue of residual metal in the product of previous ATRP systems. In particular, phenothiazine derivative (10-(pyren1-yl)-10H-phenothiazine (PPTh)) and pyrene were used as o-PCs for the polymerizations of 4VP. We also employed these o-PC to synthesis copolymers of 4VP with stearyl methacrylate and stearyl acrylate as the models of acrylate and methacrylate monomers. We chose these two monomer because of the challenging polymerization as well as interesting applications based on their self-assembly ability. We believe this study will remove the difficulties of traditional ATRP and contribute the new facile methodology for polymer preparation from 4VP. 2/7

2. Materials and Methods 2.1 Materials 4-vinylpyridine (4VP, 95%), stearyl acrylate (SA, 97%), stearyl methacrylate (SMA, 97%) were purchased from Sigma Aldrich and passed through an alumina column to remove inhibitor; pyrene (Sigma Aldrich, 98%), ethyl 2-bromo-2-methylpropionate (Initiator) (Sigma Aldrich, 98%); tetrahydrofuran (THF, 99.8%, no stabilizer) was purchased from Acros Organics, methanol (99.9%), ethanol (99.5%), diethyl ether (99%) were purchased from Fisher and used as received; 10-(pyren-1-yl)-10H-phenothiazine (PPTh) was prepared via the route provided by Nguyen et al.[18].

2.2 Polymer Preparation Polymerization reactions were carried out in UV box at room temperature (RT) in two-neck flasks, they were flamed three times and filled with N2 before chemicals were added into. Monomer(s), solvent(s), catalyst and initiator were respectively added into the flask under nitrogen. The mixture was stirred at RT for 15 minutes and degassed with three freeze-pump-thaw cycles. The reaction system was flushed with nitrogen then the polymerization was carried out in UV box (a 365 nm UV light, 12 x 9 watt bulbs, intensity of 2.2 mW cm-2 determined by a VLX365 radiometer) while the mixture was kept continuously stirred. After the time indicated, UV light was turned off and the mixture was stirred in the dark for 30 minutes for stabilization. Then, the polymer was precipitated twice in cold diethyl ether and dried under vacuum until remain weight. After being dried under vacuum, the yellowish powder was received as the final product of the synthesis of P4VP.

2.3 Characterizations 1 H NMR spectra were recorded in deuterated chloroform (CDCl3) with TMS as an internal reference, on a Bruker Avance 500 MHz spectrometer. Gel permeation chromatography (GPC) measurements were performed on a Polymer PL-GPC 50 gel permeation chromatograph system equipped with an RI detector; DMF containing 0.1 mol/L of lithium bromide as the eluent at a flow rate of 1.0 mL/min, molecular weights and molecular weight distributions were calculated with reference to polystyrene standards. FT-IR spectra, collected as the average of 64 scans with a resolution of 4 cm-1, were recorded from KBr disk on a FT-IR Bruker Tensor 27. Differential scanning calorimetry (DSC) measurements were carried out with a DSC Q20 V24.4 Build 116 calorimeter under nitrogen flow, at a heating rate of 10 oC/min.

3. Results and Discussions The photoinitiated ATRP has been performed under nitrogen environment under UV irradiation system at 365 nm using suppercure 352S, SAN-EI ELECTRIC CO., Osaka, Japan. Figure 1 presents a proposed general photoinitiated ATRP mechanism[13,19,20,22]: under UV light irradiation, o-PC is activated to excited state (o-PC*) which is able reduce the alkyl bromide initiator or subsequent polymer chain ends. This process is capable to form o-PC+/Br- complex Polímeros, 31(1), e2021001, 2021


10-(pyren-1-yl)-10h-phenothiazine and pyrene as organic catalysts for photoinitiated ATRP of 4-vinylpyridine as well as a carbon centered propagating radical species allowing polymerization in the presence of monomers. The deactivation of o-PC+/Br- complex could oxidize the propagating radical to regenerate the alkyl bromide and the original state of o-PC which temporarily ends a propagation cycle. Subsequently, the photoexcitation of the o-PC to o-PC* reinitiates the new propagation cycles contributing to the formation of long-chain and well-defined polymers. The synthetic routes of polymers in this study were summarizes as Scheme 1.

Figure 1 A proposed general photoinitiated ATRP mechanism (Pn = polymer chain)

3.1 Preparation of P4VP 3.1.1 Effect of solvents Table 1 (entry 1-7) summarizes the results of the polymerization of 4VP in different reaction media. Three solvents, with a dielectric constants are nearly the same, including ethanol, methanol, THF are used as the sample of protic and aprotic polar reaction media. After 24-hour reaction, PPTh showed the high catalyst efficiency in ethanol and methanol, which results in high monomer conversions (Table 1, entry 1 -2) compared to traditional ATRP[2]. These results could be explained by the good solubility of both 4VP and P4VP in alcohol solvents. Additionally, polymer products gave PDI below 1.2 which indicated good controllable polymerizations. Furthermore, to examine whether the polymerization using o-PC could improve the strict solvent selection or not, the mixed solvent of THF (as the unprotic solvent) and ethanol with different fractions was used (Table 1, entry 3-5). The conversion almost remained constant with the ratio of THF and ethanol was 1:1 (v/v) but it began decreasing when we used more THF in the mixture and this value was just 65% when absolute THF was used. However, conversions could reach 92% after polymerizing in THF for 48h (Table 1, entry 6). So that it could be concluded that the unprotic solvent were possibly used as the reaction media although the reaction rate might fall. The same effect was received in pyrene catalyst systems though the conversions were lower in the same reaction time (Table 1, entry 7-9). 3.1.2 Effect of photo-catalyst

Figure 2. Conversion of polymerizations of 4VP during time catalyzed by Pyrene and PPTh

The reactions using PPTh or pyrene were conducted in ethanol to compare the effect of catalysts on the polymerization and the results are illustrated in Figure 2. The new phenothiazine derivative - PPTh showed better catalyst efficiency. The monomer conversion of reaction using PPTh gradually increased to 91% after 24h compare to 60% of the one catalyzing by commercial catalyst – pyrene. When expanding the reaction time, because of the increase of reaction mixture viscosity, conversions slowly grew to 94% and 80% for PPTh and pyrene system respectively then level off. GPC traces of P4VP synthesized using PPTh as photocatalyst (Figure 3a) showed the shift in elution time vs monomer conversion indicating the

Figure 3. (a)) GPC traces vs conversions of P4VP synthesized using PPTh as photocatalyst in ethanol (b) Dependence of molecular weight (Mn, GPC) and polydispersity (Mw/Mn) on monomer conversion. Polímeros, 31(1), e2021001, 2021

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Nguyen, L. T., Pham, H. Q., Nguyen, D. A. S., Nguyen, L. T., Huynh, K. P. H., Tran, H. L., Mai, P. T., Nguyen, H. T., Nguyen, L. T., & Truong, T. T.

Scheme 1. Synthetic pathways of P4VP (a), P(4VP-r-SA) (b), P(4VP-r-SMA) using o-PCs. Table 1. Results of photo-induced metal-free ATRP of 4VP*. Entry

Monomer

[4VP]0:[I]0:[C]0

Solvent

Catalyst

Time (h)

Mnb (g/mol)

PDIb

Conva (%)

1 2 3 4 5 6 7 8 9 10 11

4VP 4VP 4VP 4VP 4VP 4VP 4VP 4VP 4VP 4VP, SA (2:1) 4VP, SMA (2:1)

150:1:0.1 150:1:0.1 150:1:0.1 150:1:0.1 150:1:0.1 150:1:0.1 150:1:1 150:1:0.1 150:1:0.1 100:1:0.1 100:1:0.1

Ethanol Methanol THF: Ethanol (1/1 v/v) THF: Ethanol (7/2 v/v) THF THF Ethanol Methanol THF THF THF

PPTh PPTh PPTh PPTh PPTh PPTh Pyrene Pyrene Pyrene PPTh PPTh

24 24 24 24 24 48 24 24 24 48 48

14,186 14,260 14,450 12,323 10,750 14,225 9,257 9,442 7,201 6,804 11,670

1.12 1.09 1.12 1.19 1.18 1.11 1.15 1.16 1.12 1.77 1.59

91 90 90 75 65 92 60 58 45 71 78

*Polymerizations was carried out under a 365 nm UV irradiation; a Conversion was determined gravimetrically: conversion = (m - mI - mPC)/ mM where m denotes the weight of product, and mI, mPC, mM are the weights of the initiator, photocatalyst and monomers, respectively. b PDI, Mn were determined by GPC.

increase in polymer molecular weight. Kinetic plot for the PPTh-catalyzed polymerization (Figure 3b) showed linear increase in molecular weight with conversion indicating the “living” nature of the chain growth process. In addition, the polydispersity of the obtained polymers maintained a low level at around 1.2 which is the important factor of CRPs. 3.1.3 Photo-controllable polymerization To illustrate the temporal control of polymerization by UV light, the ON/OFF experiment was carried out (Figure 4). The model polymerization experiments using a ratio of [4VP]/ 4/7

[I]/[PPTh] or [4VP]/[I]/[Pyrene] = 150/1/0.1 were conducted in ethanol. After the reaction mixtures was repaired in a flasks (2.2), they were taken into the UV box then UV light system was operated to repeated circles of 3-hour UV-ON and 1-hour UV-OFF. At each interval, equivalent volumes of samples were syringed out from the polymerization system and precipitated into excess cold diethyl ether to gravimetrically determine the conversions. Results indicated the polymerizations were ultimately irradiation dependent and there was almost no monomer consumption occurred in UV-OFF periods either PPTh or pyrene was used as o-PC. Polímeros, 31(1), e2021001, 2021


10-(pyren-1-yl)-10h-phenothiazine and pyrene as organic catalysts for photoinitiated ATRP of 4-vinylpyridine 3.2 Preparation of random copolymers P(4VP–r-SA) and P(4VP–r-SMA)

might lead to an unstable propagation and that could be the cause of this inconvenience.

As can be seen from the Table 1 (entry 10, 11), polymerizations occurred with the yields of 71% for P(4VP–rSA) and 78% for P(4VP–r-SMA) after 48h. As the result of GPC (Table 1, entry 10-11), P(4VP–rSA) and P(4VP–r-SMA) were afforded with Mw values of 12,032 as 18,556 as well as PDI of 1.77 and 1.59 respectively. The polydispersity was much higher than the one of P4VP homopolymer. The use of two monomers in the systems

The chemical structure of these copolymers were characterized by 1H-NMR measurement (Figure 5). In both 1 H-NMR spectra, two broad proton signals at δ 8.15 – 8.6 and δ 6.27 – 7.07 ppm which contributed to the meta- and orthoprotons of pyridine rings, respectively. In addition, signals in the range of δ 2.87 – 4.08 ppm that assigned to methylene protons (-O-CH2-) from stearyl groups. The resonance signals observed in the regions between 1.00 – 2.61 ppm indicated the protons in copolymer backbones and -CH2-[CH2]15- in stearyl groups. Methyl protons (CH3) of stearyl groups in P(4VP-r-SA) or of stearyl and methacrylate groups in P(4VP-r-SMA) contributed to the peaks below 1.00 ppm in the spectra. Moreover, by measuring the relative intensities of meta-protons from pyridine ring (d, 2H) and of methylene protons (e, 2H) from stearyl group, molar ratio of two units in copolymers could be calculated, particularly, the fraction of 4VP/SA in P(4VP–r-SA) was 1.12 and the value of 4VP/ SMA in P(4VP–r-SMA) was 1.37, whereas the feeding ratio of 4VP over SA (or SMA) was 2/1.

Figure 4. Monomer conversion vs. time in ON-OFF experiments demonstrating photo-controlled polymerization propagation using PPTh (blue triangle line) and pyrene (red dot line)

P4VP received was analyzed by FTIR (Figure 6) illustrating the characteristic vibration of pyridine ring which evidenced by the absorption bands at 1596.99 cm-1. Moreover, the absorption bands at 1068.52 and 993.30 cm-1 were observed to confirm the in-plane and out-of-plane rings C–H bending, respectively[24]. Absorption band of C=O at 1730 cm-1 in of poly (stearyl acrylate) and poly (stearyl methacrylate) were slightly broadened to the lower wavenumber might cause of the hydrogen bonding of C=O and pyridine in the copolymer. Moreover, the formation of hydrogen bonding

Figure 5. 1H-NMR spectra of (a) P(4VP-r-SA) and (b) P(4VP-r-SMA). Polímeros, 31(1), e2021001, 2021

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Nguyen, L. T., Pham, H. Q., Nguyen, D. A. S., Nguyen, L. T., Huynh, K. P. H., Tran, H. L., Mai, P. T., Nguyen, H. T., Nguyen, L. T., & Truong, T. T.

Figure 6. FTIR spectrum of P(4-vinylpyridine) (blue line), Poly (stearyl methacrylate) (orange line), Poly (4-Vinylpyridine-r-Steary acrylate) (black line) and Poly (4-Vinylpyridine-r-Stearylmethacrylate) (green line).

Figure 7. Differential scanning calorimetry (DSC) curve of P4VP product.

also slightly shifted the deformation band of pyridine at 1068.52 cm-1 to 1070.45 cm-1. Thermal properties was also testified by DSC (Figure 7) demonstrating the melting point of stearyl groups at 15 oC and 20 oC of P(4VP-r-SMA) and P(4VP-r-SA), respectively. In addition, DSC curve also showed the glass transition of P4VP at a temperature between 140 and 160 oC[25].

4. Conclusions Metal-free polymerization of 4VP as well as copolymerization with acrylate and methacrylate monomers have been successfully constructed by using PPTh and pyrene as organic photocatalysts. The preparations of P4VP could be controlled under irradiation which produce products with high conversion and very narrow PDI (<1.2) with no metal contamination. In addition, the reactions were not only possibly carried out in protic media but also unprotic solvent like THF, which is the limitation of traditional ATRP.

5. Acknowledgement This research is funded by Vietnam National University Hochiminh City (VNU-HCM) under grant number C201920-19. 6/7

6. References 1. Chen, Y., Zhao, W., & Zhang, J. (2017). Preparation of 4-vinylpyridine (4VP) resin and its adsorption performance for heavy metal ions. RSC Advances, 7(8), 4226-4236. http:// dx.doi.org/10.1039/C6RA26813G. 2. Xia, J., Zhang, X., & Matyjaszewski, K. (1999). Atom transfer radical polymerization of 4-vinylpyridine. Macromolecules, 32(10), 3531-3533. http://dx.doi.org/10.1021/ma9816968. 3. Gopinath, A., Sathiyaraj, S., & Nasar, A. S. (2017). Star poly (4-vinylpyridine) s using dendritic ATRP initiators: Synthesis, electrolyte property and performance in dye sensitized solar cell. Journal of Polymer Research, 24(8), 116. http://dx.doi. org/10.1007/s10965-017-1274-8. 4. Zhang, Z., Sèbe, G., Wang, X., & Tam, K. C. (2018). Gold nanoparticles stabilized by poly (4-vinylpyridine) grafted cellulose nanocrystals as efficient and recyclable catalysts. Carbohydrate Polymers, 182, 61-68. http://dx.doi.org/10.1016/j. carbpol.2017.10.094. PMid:29279126. 5. Tanum, J., Han, U., Shin, J. W., & Hong, J. (2018). Preparation of multifunctional micelles from two different amphiphilic block copolymers. Colloids and Surfaces. A, Physicochemical and Engineering Aspects, 537, 566-571. http://dx.doi.org/10.1016/j. colsurfa.2017.10.042. 6. Luo, S., Liu, S., Xu, J., Liu, H., Zhu, Z., Jiang, M., & Wu, C. (2006). A stopped-flow kinetic study of the assembly of interpolymer complexes via hydrogen-bonding interactions. Polímeros, 31(1), e2021001, 2021


10-(pyren-1-yl)-10h-phenothiazine and pyrene as organic catalysts for photoinitiated ATRP of 4-vinylpyridine Macromolecules, 39(13), 4517-4525. http://dx.doi.org/10.1021/ ma060581y. 7. Xiang, M., Jiang, M., Zhang, Y., Wu, C., & Feng, L. (1997). Intermacromolecular complexation due to specific interactions 4. The hydrogen-bonding complex of vinylphenol-containing copolymer and vinylpyridine-containing copolymer. Macromolecules, 30(8), 2313-2319. http://dx.doi.org/10.1021/ ma9614611. 8. Liu, F., & Eisenberg, A. (2003). Synthesis of Poly (tert-butyl acrylate)-block-Polystyrene-block-Poly(4-vinylpyridine) by Living Anionic Polymerization. Angewandte Chemie International Edition, 42(12), 1404-1407. http://dx.doi. org/10.1002/anie.200390361. PMid:12671981. 9. Matsuno, R., Yamamoto, K., Otsuka, H., & Takahara, A. (2004). Polystyrene-and poly (3-vinylpyridine)-grafted magnetite nanoparticles prepared through surface-initiated nitroxidemediated radical polymerization. Macromolecules, 37(6), 2203-2209. http://dx.doi.org/10.1021/ma035523g. 10. Convertine, A. J., Sumerlin, B. S., Thomas, D. B., Lowe, A. B., & McCormick, C. L. (2003). Synthesis of block copolymers of 2-and 4-vinylpyridine by RAFT polymerization. Macromolecules, 36(13), 4679-4681. http://dx.doi.org/10.1021/ma034361l. 11. Qi, Y., Perepichka, I. I., Song, Z., &Varshney, S. K. (2018). Synthesis and thermal properties of poly (vinylcyclohexane)b-poly (4-vinylpyridine) diblock copolymers prepared via RAFT polymerization. e-Polymers, 18(2), 197-203. https:// doi.org/10.1515/epoly-2017-0102 12. Yang, R., Wang, Y., Wang, X., He, W., & Pan, C. (2003). Synthesis of poly(4-vinylpyridine) and block copoly (4-vinylpyridine–bstyrene) by atom transfer radical polymerization using 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazamacrocyclotetradecane as ligan. European Polymer Journal, 39(10), 2029-2033. http:// dx.doi.org/10.1016/S0014-3057(03)00070-3. 13. Corrigan, N., Jung, K., Moad, G., Hawker, C. J., Matyjaszewski, K., & Boyer, C. (2020). Reversible-Deactivation Radical Polymerization (Controlled/Living Radical Polymerization): From Discovery to Materials Design and Applications. Progress in Polymer Science, 111, 101311. http://dx.doi.org/10.1016/j. progpolymsci.2020.101311. 14. Grubbs, R. B. (2011). Nitroxide-mediated radical polymerization: limitations and versatility. Polymer Reviews (Philadelphia, Pa.), 51(2), 104-137. http://dx.doi.org/10.1080/15583724.2 011.566405. 15. Hill, M. R., Carmean, R. N., & Sumerlin, B. S. (2015). Expanding the scope of RAFT polymerization: recent advances and new horizons. Macromolecules, 48(16), 5459-5469. http://dx.doi. org/10.1021/acs.macromol.5b00342.

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16. Matyjaszewski, K. (2018). Advanced Materials by Atom Transfer Radical Polymerization. Advanced Materials, 30(23), 1706441. http://dx.doi.org/10.1002/adma.201706441. PMid:29582478. 17. Lou, Q., & Shipp, D. A. (2012). Recent developments in atom transfer radical polymerization (ATRP): methods to reduce metal catalyst concentrations. ChemPhysChem, 13(14), 3257-3261. http://dx.doi.org/10.1002/cphc.201200166. PMid:22539367. 18. Nguyen, T. H., Nguyen, L.-T. T., Nguyen, V. Q., Phan, L. N. T., Zhang, G., Yokozawa, T., Phung, D. T. T., & Nguyen, H. T. (2018). Synthesis of poly (3-hexylthiophene) based rod–coil conjugated block copolymers via photoinduced metal-free atom transfer radical polymerization. Polymer Chemistry, 9(18), 2484-2493. http://dx.doi.org/10.1039/C8PY00361K. 19. Corrigan, N., Shanmugam, S., Xu, J., & Boyer, C. (2016). Photocatalysis in organic and polymer synthesis. Chemical Society Reviews, 45(22), 6165-6212. http://dx.doi.org/10.1039/ C6CS00185H. PMid:27819094. 20. Treat, N. J., Sprafke, H., Kramer, J. W., Clark, P. G., Barton, B. E., Read de Alaniz, J., Fors, B. P., & Hawker, C. J. (2014). Metal-free atom transfer radical polymerization. Journal of the American Chemical Society, 136(45), 16096-16101. http:// dx.doi.org/10.1021/ja510389m. PMid:25360628. 21. Pan, X., Lamson, M., Yan, J., & Matyjaszewski, K. (2015). Photoinduced metal-free atom transfer radical polymerization of acrylonitrile. ACS Macro Letters, 4(2), 192-196. http:// dx.doi.org/10.1021/mz500834g. 22. Theriot, J. C., Miyake, G. M., & Boyer, C. A. (2018). N,N-Diaryl Dihydrophenazines as Photoredox Catalysts for PET-RAFT and Sequential PET-RAFT/O-ATRP. ACS Macro Letters, 7(6), 662-666. http://dx.doi.org/10.1021/acsmacrolett.8b00281. PMid:30705782. 23. Pearson, R. M., Lim, C.-H., McCarthy, B. G., Musgrave, C. B., & Miyake, G. M. (2016). Organocatalyzed atom transfer radical polymerization using N-aryl phenoxazines as photoredox catalysts. Journal of the American Chemical Society, 138(35), 11399-11407. http://dx.doi.org/10.1021/ jacs.6b08068. PMid:27554292. 24. Wu, K., Wang, Y., & Hwu, W. (2003). FTIR and TGA studies of poly (4-vinylpyridine-co-divinylbenzene)–Cu (II) complex. Polymer Degradation & Stability, 79(2), 195-200. http://dx.doi. org/10.1016/S0141-3910(02)00261-6. 25. Orwoll, R. A., & Chong, Y. S. (1999). Polyacrylamide. In J. E. Mark (Eds.), Polymer data handbook (pp. 247-51). United Kingdom: Oxford University Press. Received: Sep. 26, 2020 Revised: Feb. 09, 2021 Accepted: Feb. 10, 2021

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ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.09320

Polyaniline-based electrospun polycaprolactone nanofibers: preparation and characterization Juliana Donato de Almeida Cantalice1, Edu Grieco Mazzini Júnior1, Johnnatan Duarte de Freitas2, Rosanny Christinny da Silva3, Roselena Faez4, Ligia Maria Manzine Costa1 and Adriana Santos Ribeiro1*  1

Laboratório de Polímeros Condutores Eletrocrômicos e Fluorescentes, Centro de Tecnologia (Materiais), Universidade Federal de Alagoas – UFAL, Maceió, AL, Brasil 2 Instituto Federal de Alagoas – IFAL, Campus Maceió, Maceió, AL, Brasil 3 Instituto Federal de Alagoas – IFAL, Campus Penedo, Penedo, AL, Brasil 4 Laboratório de Materiais Poliméricos e Biossorventes, Universidade Federal de São Carlos – UFSCar, Campus Araras, Araras, SP, Brasil *aribeiro@qui.ufal.br

Abstract This work provides a convenient strategy for preparation of conducting polycaprolactone (PCL)/polyaniline (PAni) nanofibers, useful for development of optoelectronic sensors and devices. PCL/PAni nanofibers were obtained by electrospinning technique and characterized by SEM, FTIR, thermal analysis, and DC electrical conductivity. The influence of the experimental conditions in the electrospinning process, such as the applied voltage, on the nanofiber morphology was discussed in detail. Incorporation of PAni into PCL nanofibers significantly increased the electrical conductivity from a non-detectable level for the neat PCL to 0.032 ± 0.022 S/cm for the nanofibers containing 7.5 wt.% PAni. Therefore, electrospun PCL/PAni nanofiber mats presented optical and electrical properties, that awaken the possibility of applications for these materials as acid-base sensors and electrochromic devices. Keywords: electrospinning, nanofibers, polyaniline, polycaprolactone. How to cite: Cantalice, J. D. A., Mazzini Júnior, E. G., Freitas, J. D., Silva, R. C., Faez, R., Costa, L. M. M., & Ribeiro, A. S. (2021). Polyaniline-based electrospun polycaprolactone nanofibers: preparation and characterization. Polímeros: Ciência e Tecnologia, 31(1), e2021002. https://doi.org/10.1590/0104-1428.09320

1. Introduction Conjugated polymers which can be synthesized into one dimensional nanostructures, such as nanotubes, nanowires and nanofibers, have recently attracted attention in the areas of nanoscience and nanotechnology, due to their processability, chemical and electrochemical properties[1]. Among the various classes of conjugated polymers, polyaniline (PAni) is considered one of the most technologically promising material, owing to its high stability, easy of synthesis, excellent optical and magnetic properties, feasibility of electrical conductivity control by changing either the protonation state or the oxidation state, and the low cost of the aniline monomer[1-3]. PAni is typically synthesized by the oxidation of aniline with ammonium peroxydisulfate in an acidic aqueous medium[4], and its morphology and mechanical/electrical properties are strongly affected by different experimental parameters, such as type of dopant used, pH, temperature, and reaction time[5,6]. However, the processability of PAni to produce films or fibers is rather difficult because it is infusible and insoluble in common solvents, and thus, its practical use is limited. Significant efforts have been made in order to overcome such drawbacks and to improve the solubility, processability and mechanical properties of

Polímeros, 31(1), e2021002, 2021

PAni by changing the dopant agent[7], blending it with other polymers[8] or preparing hybrid organic/inorganic composites[8-10]. Recently, the electrospinning process has emerged as an efficient and promising technique for the preparation of polymer nanofibers with controlled diameters on the order of tens of nanometers to micrometers. It requires a simple and inexpensive setup, which can be further developed to scale up into a continuous production of nanofibers from various polymers[2,6,11]. Due to their large surface area, PAni nanofibers show enhanced water processability, improved acid-base sensitivity and response time when they are exposed to chemical vapors. Therefore, these nanofibers provide a wealth of possibilities to obtain materials with new features, including high specific surface areas, and improved properties when compared to bulk materials[12]. The high electrical conductivity of PAni associated with the high surface area and very high porosity of mats, make these materials potentially interesting for a variety of applications, for example, as multifunctional textiles, sensors, electronic devices, scaffolds for tissue engineering, and substrates for surface functionalization and modification[11,12].

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O O O O O O O O O O O O O O O O


Cantalice, J. D. A., Mazzini Júnior, E. G., Freitas, J. D., Silva, R. C., Faez, R., Costa, L. M. M., & Ribeiro, A. S. However, the preparation of PAni nanofibers by electrospinning technique is still a great challenge due to its low solubility in common solvents, low molecular weight and rigid backbone structure[12,13]. To circumvent this problem, one of the main strategies includes blending PAni with high molecular weight polymers that serve as processing aids, such as poly(ethylene oxide) (PEO)[14,15], poly(styrene)[16], poly(methyl metacrylate) (PMMA)[11,17], poly(acrylonitrile) [18] , poly(vinyl alcohol) (PVA)[19], poly(lactic acid) (PLA)[2,6] and poly(caprolactone) (PCL)[20-22]. Among polymer matrices, PCL has stimulated extensive research due to its good solubility in common solvents, low degradation rate, with non-toxic products of degradation, low melting point, superior mechanical properties, exceptional ability to form blends with various polymers in a wide range of compositions, and superior rheological and viscoelastic properties over many of its aliphatic polyester counterparts, coupled with relatively inexpensive production routes[20,23-26]. Preparation of PCL/PAni blends aimed at applications in the biological field has been reported in the literature. Chen et al.[21] studied the incorporation of PAni nanoparticles into PCL and gelatin blends for thermal ablation of tumor cells in vivo, Chen et al.[22] and Ku et al.[27] used electrospun PCL/PAni nanofiber scaffolds as materials for skeletal muscle regeneration in tissue engineering. Recently, Garrudo et al. [28] prepared PCL/PAni scaffolds for cultivation of neural stem cells. They showed that the nanofiber mats were biocompatible, which make this material an ideal candidate for in vitro neural differentiation studies under electrical stimulation. Although most of the works described in the literature are towards the application of PCL/PAni blends in tissue engineering, the possibilities of their use are widespread, because there is a synergy between the biodegradable characteristics of PCL and the inherent conductivity of PAni. For example, such features may be useful for application in optical devices and sensors. In recent years, the research field of green electronics with focus in the development of biodegradable substrates and electrodes has demonstrating important results, since these biodegradable devices may enable the use of electronic products that can decompose after their useful lifetime[29]. Thus, PCL is a good choice regarding the fabrication of such electronic devices, as reported by Jürgensen et al.[29] who showed that PCL reduces film roughness, provides a broad electrochemical stability window and reduction of operating voltages, and by Dierckx et al.[30] who applied PCL/poly(3-alkylthiophene) nanofibers for optoelectronic devices. Our research group has synthesized and characterized a series of PAni-based hybrid materials for application as electrochromic materials and optical sensors, and found interesting results, such as enhanced electrochromic properties with an intense color variation from yellow to green and blue in function of the applied potential, from blending PAni with chitosan with the incorporation of nanoclay fillers[9,10]. Therefore, the use of biopolymers along with PAni aroused considerable interest in the preparation of electrospun nanofibers for several applications in addition to the biological ones. Herein, electrospun PCL nanofibers based on PAni were prepared and characterized by structural (FTIR), morphological (SEM) and thermal analysis (TGA and DSC), aiming at applications in acid-base optoelctronic sensors. 2/9

2. Materials and Methods 2.1 Materials Polycaprolactone (PCL, Mw ~ 70000-90000 g mol-1), ammonium persulfate ((NH4)2S2O8), hydrochloric acid (HCl), sodium hydroxide (NaOH), dimethylsulfoxide (DMSO), chloroform (CHCl3) and methanol (CH3OH) were purchased from Sigma-Aldrich and used as received without further purification. Aniline (Synth) was destilled before use.

2.2 PAni synthesis PAni was synthesized according to the method described by Silva et al.[10], in which 1.0 mL of freshly distilled aniline was dissolved in 10.0 mL of 1.0 mol L−1 HCl and the solution was cooled to −10 °C in an ice, NaCl and ethanol bath. Then, a solution containing 3.0 g of (NH4)2S2O8 dissolved in 40 mL of 1.0 mol L−1 HCl was slowly poured into the monomer solution, under vigorous stirring. The reaction temperature was maintained at ca. −10 °C for 2 h. After this time, a dark green precipitate was recovered from the reaction mixture by filtration under reduced pressure, washed thoroughly with 50 mL of 1.0 mol L−1 HCl, deprotonataded and dried under vacuum.

2.3 Preparation of electrospun nanofibers PCL solution was prepared by dissolving 1.25 g (12.5 wt%) PCL in 5.0 mL CHCl3 and then stirring for 20 min at 60 °C. Meanwhile, 0.09 g (7.5 wt% with respect to the PCL) of PAni previously sifted in a 60 mesh sieve was slowly dissolved in 5.0 mL DMSO, filtered, and then the suspension was stirred for 10 min at 60 °C. The PCL and PAni solutions were mixed together under stirring and sonicated for 10 min at 60 °C in order to ensure their homogeneity. The electrospinning apparatus was set up inside a fume hood. It consisted of a support for a glass syringe and a static collector, wherein the polymer solution flowed through the syringe by gravity at a constant flow rate of 0.08 mL min-1. Both the syringe and needle were heated to ca. 80 °C before use. A high-voltage power supply was used to generate DC voltage up to 30 kV. The working distance between the syringe tip and collector was 12 cm and nanofibers were collected on aluminum foil with a collection time of 10-15 s. All samples were prepared using a needle diameter of 1.2 mm, at temperature 25-29 °C and relative humidity of 62-69%. The optimization of the electrospinning parameters was performed according to the applied voltages (15, 17, 18 and 20 kV).

2.4 Characterization The CIE (Commission Internationale de l’Eclairage) 1931 xy color coordinates and chromaticity diagram of the PCL/PAni mats were obtained by colorimeter app (version 3.7.7, developed by Loomatix Team, Israel) installed on an android smartphone. The measurements were based on CIE standard illuminant D65 ans standard observer (2º). The track of the CIE 1931 xy chromaticity coordinates in the CIE chromaticity diagram was obtained using a Spectra Lux Software v.2.0 Beta[31]. The morphology of the samples was analyzed by Scanning Electron Microscopy (SEM) using a Vega LM TESCAN Orsay Holdin microscope, in which the samples were sputtered with gold before the analysis by a Sputter Coater (Q150T, Quorum Technologies, Darmstadt, Germany). Fiber average diameters were calculated using ImageJ (NIH, Polímeros, 31(1), e2021002, 2021


Polyaniline-based electrospun polycaprolactone nanofibers: preparation and characterization

Figure 1. Images of electrospun PCL/PAni mats according to the doping state (EB or ES) and applied voltage: (a-d) EB 15, 17, 18 and 20 kV; and (e-h) ES 15,17, 18 and 20kV, respectively.

Bethesda, MD, USA), after the measurement of at least 100 fibers from each sample. FTIR spectra of the samples were obtained using an FTIR spectrometer (Nicolet 6700, Thermo Scientific). Thermogravimetric analyses (TGA) were performed on a Shimadzu thermoanalyzer TA-50, under nitrogen atmosphere with a flow of 50 mL min-1 and a heating rate of 10 °C min-1. The electrical conductivity was measured by a fourpoint method using a Jandel Multi Height Probe with Jandel cylindrical probe head (25.4 nm diameter × 48.5 nm high), tip spacing of 1.591 nm controlled by the RM3-AR Test Unit. The mats (0.05 mm thickness) used in this measurement were prepared by applying 18 kV. Electrical conductivity of the doped and undoped mats was an average of six values of each case, measured from two sides and in different places.

3. Results and Discussions The experimental conditions for electrospinning affect the morphology and the properties of the polymer nanofibers. Therefore, the results will be discussed taking into account the optimization of the electrospinning parameters and the characterization of the samples obtained in the optimized conditions.

3.1 Morphological analysis The PCL/PAni nanofiber mats formed very flexible (Figure S1, Supplementary material) and stable macroscopic structures. Figure 1 shows the photographic images of undoped (emeraldine base, EB) and doped (emeraldine salt, ES) PAni based PCL mats, which displayed characteristic blue and green color, respectively. When the electrospun PCL/PAni (EB) mats were treated with 1.0 mol L-1 HCl solution, their color changed from blue to green. In a similar way, when the mats were treated with 1.0 mol L-1 NH4OH solution, their color changed from green to blue, indicating that a reversible acid-base transition occurred. Furthermore, with the increase of the applied voltage during the electrospinning process, the color of the mats became more pronounced, except for PCL/PAni mats obtained at 15 kV, since they were macroscopically heterogeneous, presenting droplets and fibers. In this case, due to the presence of droplets in the Polímeros, 31(1), e2021002, 2021

Figure 2. CIE 1931 chromaticity diagram showing the colors of PCL/PAni mats according to their oxidation (undoped/doped, blue/ green) state and the electrospinning applied potential (17,18 and 20 kV). The color of mats prepared at 15 kV were not measured because they presented darker and lighter regions due to the presence of droplets on the surface.

mats surface, it was observed darker and lighter regions in the mats, and the color seen could not be accurately interpreted. Considering the application of electrospun PCL/PAni mats as an optical pH sensor device, in-situ colorimetry measurements of the samples prepared at different applied potentials were performed in order to show quantitative results in terms of the color of the mats, giving a numerical description of the color stimulus, and thus providing a more precise way to define color[32]. CIE 1931 (x, y) chromaticity coordinates changes as a function of the oxidation (undoped/ doped) state and the electrospinning applied potential are shown in Figure 2 (and Table S1, Supplementary material). 3/9


Cantalice, J. D. A., Mazzini Júnior, E. G., Freitas, J. D., Silva, R. C., Faez, R., Costa, L. M. M., & Ribeiro, A. S. For all samples the color reversibly changes from blue (undoped) to green (doped) in presence of HCl, wherein this change was more significant for the PCL/PAni mat prepared at 20 kV. The results obtained for these mats show the importance of a suitable choice of the applied potential for electrospinning and further arouses the possibility of application of the material in an optical device for acidbase sensor.

The influence of the applied voltage on the formation of PCL/PAni nanofibers was assessed from the SEM images, along with their diameter distribution, as shown in Figure 3 and Table 1. It was possible to notice that defectsand beads-free electrospun fiber mats with uniform fiber diameter distributions were obtained at 18 kV. For the lowest values of the applied voltage, the fiber diameter was less homogeneous and presented some defects. For mats

Figure 3. SEM micrographs of electrospun PCL/PAni nanofibers according to the applied voltage: (a) 15 kV; (b) 17 kV; (c) 18 kV; and (d) 20 kV. 4/9

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Polyaniline-based electrospun polycaprolactone nanofibers: preparation and characterization produced at 20 kV, it was possible to observe the formation of beads in the nanofiber morphological structure. This behavior can be explained by taking into account that at relatively low voltages, a drop is typically suspended at the needle tip, and a jet will originate from the Taylor cone producing bead-free spinning (assuming that the force of the electric field is sufficient to overcome the surface tension). As the voltage increases, the volume of the drop at the tip decreases, causing the Taylor cone to recede. The jet originates from the liquid surface within the tip, and more beading is seen[33]. Upon further increasing the voltage, the jet eventually moves around the edge of the tip, with no visible Taylor cone; at these conditions, the presence of many beads can be observed[20]. Indeed, the SEM image of the electrospun PCL/PAni nanofibers prepared at 20 kV exhibits a web of nanofibers with many sub-micrometer-sized spheres over the whole substrate (Figure 3d). This indicates that there are many ‘nanoknots’ in the hierarchical structure that link the nanofibers together, as well as many beads. A similar behavior was reported by Zhu et al.[16] for electrospun PAni doped with azobenzene sulfonic acid/polystyrene (3.72 wt.%) films prepared by using DMF as solvent. Thus, the best experimental conditions for smooth, homogeneous and bead-free fiber formation were found by applying 18 kV.

3.2 Structural characterization The FTIR spectra of neat Pani, electrospun PCL, and PCL/PAni mats are presented in Figure 4. The PAni FTIR spectrum shows the characteristic bands at 1584 cm-1 and 1488 cm-1, which are assigned to the stretching vibration of quinoid rings and the stretching mode of benzonoid rings[9,10,34], respectively. The bands at 1321 cm-1 refer to C-N stretching bonds of secondary amine group[35], whilst the bands at 1158 and 829 cm-1 are attributed to the in-plane C-H bending of quinoid structure and the out-of-plane bending of C-H bonds in the aromatic ring. For the electrospun PCL mats, the FTIR spectrum displays bands at 2942 cm-1 and 2864 cm-1, assigned to asymmetric and symmetric C-H stretching, respectively. The bands at 1721 cm-1, 1293 cm-1, 1240 cm-1 and 1186 cm-1 are due to C=O, C-O and C-C stretching[36,37]. The FTIR spectrum Table 1. Average fiber diameter of PCL/PAni nanofibers prepared with different applied voltage. Applied voltage (kV) 15 17 18 20

Average fiber diameter (nm) 371 ± 130 383 ± 115 417 ± 118 252 ± 82

of the electrospun PCL/PAni mat presents characteristic peaks of PCL. However, due to the prevalence of PCL with respect to the amount of the PAni present in the blend, the bands related to the PAni are not clear. Magnification of the PCL/PAni FTIR spectrum (Figure 4 inset) shows the absorption peaks corresponding to the stretching modes of quinoid and benzonoid rings of the PAni. However, when compared with neat PAni, these peaks shifted to higher wavenumbers (1594 and 1506 cm-1). This may be attributed to the interaction between PAni and PCL[17,38].

3.3 Thermal analysis Thermal analysis of the neat PAni, electrospun PCL and PCL/PAni mats by TG/DTG is discussed on basis of the results shown in Figure 5. TG analysis of the doped PAni exhibits two decomposition steps besides the water loss at temperatures lower than 100 °C, which are related to the release of dopant anion (~ 300 °C) and to its decomposition (280-630 °C)[9,39]. PCL nanofibers were found to be stable up to 380 °C, whereas complete degradation took place at about 460 °C, with a single step trend[23,36]. Comparing the thermal behavior of the PCL/PAni with that of neat PCL, the onset of polymer decomposition occurs at a lower temperature for the hybrid material, presenting a shift of the endothermic peak from 438 to 398 °C, due to the presence of PAni. This may be related to the infuence of the volatiles from PAni[40], which accelerate the degradation process of the mixture in a non-synergistic manner.

3.4 Electrical conductivity Room temperature DC electrical conductivity values of PCL/PAni mats were measured in doped and undoped states were measured. According to Silva et al.[10], the conductivity of PAni doped with HCl (pressed pellets) was (0.70 ± 0.20) S/cm. Neat PCL nanofibers did not show detectable conductivity (~ 1.1 x 10-11 S/cm), as reported by Wu et al.[41] The electrical conductivity of the PCL/PAni (EB) nanofibers was (9.09 ± 0.22 x 10-6) S/cm, while after doping with HCl, and the occurrence of the base-acid transition, the conductivity significantly increased to (0.032 ± 0.022) S/cm. These results indicate homogeneous incorporation of PAni into the PCL nanofibers and they are in agreement with the conductivity values found in the literature for electrospun polymer/PAni blends, Table 2. Concerning HCl doped PAni blends, the electrical conductivity values found for the as prepared PCL/Pani mats in this work was higher than the reported by Wu et al.[41] Such behavior may be attributed to the experimental conditions used for preparation of the PCL/PAni nanofibers, since these authors have electrospun a dispersion of PAni into PCL matrix without melt them.

Table 2. Electrical conductivities of PCL/PAni nanofibers. Blend PCL/PAni PCL/PAni PCL/PAnia PCL/PAni

PAni (%) 1.0-3.0 5-12 1.0-20 7.5

Dopant CSA CSA HCl HCl

Conductivity (S/cm) (0.016 ± 0.001) – (0.064 ± 0.006) 0.042 – 0.077 2.8 x 10-9 – 2.0 x 10-7 (0.032 ± 0.022)

Reference 22 28 39 This work

CSA: camphor sulfonic acid. adispersions were prepared by mechanical mixing of PAni in PCL solution.

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Cantalice, J. D. A., Mazzini Júnior, E. G., Freitas, J. D., Silva, R. C., Faez, R., Costa, L. M. M., & Ribeiro, A. S.

Figure 4. (a) PAni; (b) electrospun PCL mat; (c) electrospun PCL/PAni mat.

Figure 5. TG and DTG curves for (a) Pani; (b) PCL mat; and (c) PCL/PAni mat.

The changes in the electrical conductivities for the doped/dedoped PCL/PAni mats may be explained on the basis of the acid-base transition of the material. Upon exposure to NH4OH, the diffusion of such molecules into the PCL/PAni mats destroys the conduction paths by swelling the polymer matrix and deprotonating PAni. The transferred electrons recombines with holes as a majority of carriers are reduced in number, thus decreasing the conductivity of PAni[15]. When an acid is employed as dopant, it may turn an insulating PAni (EB) into a conducting PAni salt as a result of the addition of a proton (H+) and a counterion for nitrogen atoms on the imine. Therefore, such PCL/PAni mats are effective materials for applying in optical and electrical acid-base sensors.

4. Conclusions PCL/PAni nanofibers were successfully prepared by using a simple and versatile electrospinning technique, and they were characterized by morphological, structural, thermal, and DC conductivity analysis. The best experimental conditions for smooth, homogeneous, and bead-free fiber formation 6/9

were found with 7.5 wt.% PAni, a working distance of 12 cm between the syringe tip and collector, and by applying 18 kV, which resulted in a nanofiber diameter of 417 ± 118 nm. The electrospun mats from PCL solution containing PAni (EB) displayed blue color, whilst after doping with HCl it became green, which is typical of PAni (ES). The PCL/PAni (EB) mats presented a low value of electrical conductivity ((9.09 ± 0.22 x 10-6) S/cm), behaving as an insulating material. After doping with HCl, and the occurrence of the base-acid transition, the conductivity significantly increased to (0.032 ± 0.022) S/cm. This combination of a conducting PAni and electrospinnable PCL enabled the production of nanofibers that may be applied to develop new smart materials for a variety of applications that exploit optical and electrical stimulation, such as sensors and optoelectronic devices, owing to the changes in the chromaticity diagram and electrical conductivity in function of the PAni oxidation states.

5. Acknowledgements The authors wish to thank the research funding agencies Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Coordenação de Aperfeiçoamento de Polímeros, 31(1), e2021002, 2021


Polyaniline-based electrospun polycaprolactone nanofibers: preparation and characterization Pessoal de Nível Superior (CAPES) and Fundação de Amparo à Pesquisa de Alagoas (FAPEAL) for financial support.

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Polyaniline-based electrospun polycaprolactone nanofibers: preparation and characterization

Supplementary Material Supplementary material accompanies this paper. Figure S1. Image of electrospun PCL/PAni flexible mat. Table S1. Colorimetry data (CIE 1931 xy color coordinates) of PCL/PAni mats prepared at different applied potential and in their undoped and doped states. This material is available as part of the online article from http://www.scielo.br/polimeros

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ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.09120

Tensile and structural properties of natural rubber vulcanizates with different mastication times Nabil Hayeemasae1,2 , Siriwat Soontaranon3, Mohamad Syahmie Mohamad Rasidi4,5 and Abdulhakim Masa2,6*  Department of Rubber Technology and Polymer Science, Faculty of Science and Technology, Prince of Songkla University, Pattani Campus, Pattani, Thailand 2 Research Unit of Advanced Elastomeric Materials and Innovations for BCG Economy (AEMI), Faculty of Science and Technology, Prince of Songkla University, Pattani Campus, Pattani, Thailand 3 Synchrotron Light Research Institute, Muang District, Nakhon Ratchasima,Thailand 4 Faculty of Chemical Engineering Technology, Universiti Malaysia Perlis (UniMAP), Perlis, Malaysia. 5 Geopolymer & Green Technology, Centre of Excellence (CEGeoGTech), Universiti Malaysia Perlis (UniMAP), Perlis, Malaysia 6 Rubber Engineering and Technology Program, International College, Prince of Songkla University, Hat Yai, Songkhla, Thailand 1

*abdulhakim.m@psu.ac.th

Abstract Mastication reduced the molecular weight of natural rubber (NR). This would affect the tensile properties and straininduced crystallization of the rubber vulcanizates due to the structural changes of the rubber molecules. In this study, influences of mastication time on tensile response, deformation-induced crystallization, and structural effects of crosslinked NR were investigated. The crystallization behavior and structural changes during stretching were studied by means of wide angle X-ray scattering (WAXS) and small angle X-ray scattering (SAXS). Increased mastication time significantly affected modulus at specified strain and upturn point of strain-induced crystallization of the crosslinked samples while the tensile strength was influenced slightly by mastication. During stretching, degree of crystallinity at given strain was found to decrease with increasing mastication time, while the crystallite size was reduced. Moreover, the size of crosslinked network structures induced by crosslinking also decreased slightly with increasing mastication time, as suggested by SAXS measurement. Keywords: mastication, natural rubber, strain-induced crystallization, tensile properties. How to cite: Hayeemasae, N., Soontaranon, S., Rasidi, M. S. M., & Masa, A. (2021). Tensile and Structural Properties of Natural Rubber Vulcanizates with Different Mastication Times. Polímeros: Ciência e Tecnologia, 31(1), e2021003. https://doi.org/10.1590/0104-1428.09120

1. Introduction Although natural rubber (NR) has many attractive properties such as high tensile strength and extensibility, good crack growth resistance, and low heat build-up[1], it is never used in its pure form without vulcanization[2]. During the compounding, NR requires proper mastication to reduce its molecular weight before incorporating the other chemical ingredients, because excessive molecular weight would prevent dispersion of the added ingredients[3]. Since mastication is usually required, the effects of molecular weight reduction through the mastication on tensile properties and on micro-structure are of great interest for understanding the final properties of vulcanizates. Dependence on molecular weight of the properties of rubber was initially investigated by Flory[4] in butyl rubber, and he found that the tensile strength was directly proportional to the weight fraction of the active network structure. Later on, Kok[5] also reached

Polímeros, 31(1), e2021003, 2021

a similar conclusion for NR. That is, the tensile strength increased with molecular weight. However, in this report, the tensile strength tended to be constant when the molecular weight reached 500,000. Ono et al.[6] investigated the stress-strain properties and strain-induced crystallization of NR vulcanizates with different molecular weights, and reported that a higher molecular weight provided a higher level of stress at a given strain and better strain-induced crystallization ability due to the homogeneous network structures of high molecular weight rubbers. However, no evidence of a network structure was offered. Up to now, the effects of different parameters such as crosslink density, filler content, temperature and deformation rate on the final properties, crystallization behavior and microstructure of NR have been extensively investigated and are well discussed in the report[7], but how the molecular

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Hayeemasae, N., Soontaranon, S., Rasidi, M. S. M., & Masa, A. weight affects strain-induced crystallization and the network structure is still largely unexplored. Thus, further investigation of the mechanical properties and changes of network structure remains interesting, in order to verify the effects of molecular weight. It is well accepted that mastication affects NR viscosity due to molecular weight reduction[8,9]. Previously, the effect of mastication time on the physical properties and microstructure of uncrosslinked NR was reported[9]. Unfortunately, the contributions of mastication time to the network structure and the deformation-induced crystallization behavior of NR after vulcanization were not included. Thus, this work aimed to highlight the effects of molecular weight reduction by mastication on microstructure and final properties of the NR vulcanizates. The NR was initially masticated for various times before addition of the other compounding chemicals. The processing properties of NR were investigated by means of the moving die rheometer. The tensile properties, strain-induced crystallization and network structures of the crosslinked NR were analyzed by means of tensile test, wide angle X-ray diffraction (WAXD) and small angle X-ray scattering (SAXS), respectively.

2. Materials and Methods 2.1 Materials Details of rubber and the various chemical ingredients used in the rubber formulation are summarized in Table 1. The quantities of all ingredients are indicated as part(s) per hundred parts of rubber (phr).

2.2 Sample preparation NR was initially masticated for different mastication times (0, 5 and 10 min) prior to adding the other chemicals. The mastication and compounding were performed in a laboratory-sized internal mixer (Brabender® GmbH & Co. KG, Duisburg, Germany) with an initial mixing temperature of 40 °C, a fill factor of 0.8, and a rotor speed of 60 rpm. After the initial mastication, the rubber was further mixed for another 30 sec before incorporating stearic acid and ZnO. When the mixing time reached 1.5 min, an accelerator, CBS, was added and the mixing was continued for another 1 min. Finally, sulfur was incorporated and the compound was further mixed for another 2.5 min. The semi-efficiency system was chosen in this study. The total mixing time after the mastication was kept constant at 5 min for all compounds. The compounds were left at room temperature for 24 h prior to the tests. The compounds were pressed in a compression mold at 160 °C to obtain 1 mm thick sheets, following their respective curing times (Tc90).

2.3 Characterization 2.3.1 Curing characteristics A moving die rheometer (Montech MDR 3000 BASIC, Buchen, Germany) was used to assess the curing characteristics of the NR compounds. With the testing temperature set at 160 °C, the curing parameters scorch time (Ts1), cure time (Tc90), minimum torque (ML), maximum torque (MH) and torque difference (MH-ML) were recorded. Further information on the rheometer can be found elsewhere[10]. The percentage reversion degree was calculated at 300 sec after reaching the maximum curing torque (Maximum torque), in order to estimate the aging resistance of the vulcanizates at an elevated temperature (R300). The R300 is defined as follows[11]: = R300

M H − M 300 × 100 (1) MH

where MH is the maximum torque on the curing curve and M300 is the torque at 300 s after MH. 2.3.2 Equilibrium Swelling The equilibrium swelling test was performed in order to estimate the crosslink density (ν). The rubber samples were swollen in toluene solvent for 168 h at room temperature, followed by drying at 70 °C until constant weight. The ν was estimated based on the Flory-Rehner equation[12]. ν= −

ln(1 − Vr ) + Vr + χVr2 V  (2)  2 ρVs  Vr1/ 3 − r  2 

where Vr is the volume fraction of rubber in the swollen mass, χ is the polymer-solvent interaction parameter which is equal to 0.39 for the NR-toluene system, ρ is the density of the polymer and Vs is molar volume of the solvent (106.3 cm3/mol). 2.3.3 Tensile properties The tensile properties of crosslinked NR were investigated by means of a universal tensile testing machine (LLOYD Instruments, LR5K Plus, UK). The test was performed according to ISO37 (type 2). Extensometer was used to measure the strain during tensile testing. Five specimens were used and the results reported are averages. 2.3.4 Crystallization behavior and structural changes during deformation Variations of crystallinity and crystallite size in the vulcanized NR samples during the tensile stretching were investigated by

Table 1. Formulation of the NR compounds. Ingredient NR (STR 5L) N-cyclohexyl-benzothiazyl-sulphenamide (CBS) Sulfur Zinc oxide (ZnO) Stearic acid

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Supplier Chalong Concentrated Natural Rubber Latex Industry Co., Ltd., Songkhla, Thailand Flexsys America L.P., West Virginia, USA Siam Chemical Co., Ltd., Samut Prakan, Thailand Imperial Chemical Co. Ltd., Pathumthani, Thailand Global Chemical Co. Ltd., Samut Prakarn, Thailand

Quantity (phr) 100 1 2 3 1

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Tensile and structural properties of natural rubber vulcanizates with different mastication times means of WAXD. The structural parameters such as size and dispersion of crosslinked network structures were estimated by means of SAXS measurements. Both WAXD and SAXS were performed at the Siam Photon Laboratory, Synchrotron Light Research Institute (SLRI), Nakhon-Ratchasima, Thailand. The X-ray energy was 9 keV and the measurements were conducted at room temperature (25 °C) with 500 mm/ min extension rate during testing. The tensile machine was in-house developed and a laser system was used to measure the extension between gauges. The exposure time at a fixed strain was 30 seconds and the WAXD and SAXS data were corrected and analyzed by using SAXSIT4.41 software. From WAXD data, the degree of crystallinity (%) corresponding to the (200) and (120) planes during stretching were estimated for samples with different mastication times by using the following equation[13]: Xc =

Ac × 100 Ac + Aa

(3)

where Xc is the degree of crystallinity, Ac is the area under crystalline peaks of the (200) or (120) planes, and Aa are the area of the amorphous halo. The average crystallite sizes corresponding to (200) and (120) planes, (L200 and L120) were estimated from the Scherrer equation[13,14]: Kλ Lhkl = β cos θ

(4)

where, Lhkl is the average crystallite size in the (200) or (120) planes, K is equal to 0.89, λ is the wavelength, β is the half-width at half-height, and θ is the Bragg angle. Herman’s orientation function (OP) was used to estimate the degree of chain orientation during stretching, and it can be defined as follows[15,16]:

Figure 1. Cure curves of NR vulcanizates prepared with different mastication times.

OP =

3(cos 2 θ ) − 1 2

(5)

π /2 2

2 ∫ I (φ ) cos φ sin φ dφ

(cos θ ) =

0

π /2

∫ I (φ ) sin φ dφ

(6)

0

Here ϕ is the azimuthal angle and I(ϕ) is the scattered intensity along the angle ϕ. Structural changes, e.g., size of crosslinked network structures, with varied mastication times were analyzed by fitting the SAXS profiles with Guinier approximation defined as follows[17,18]:  q 2 Rg2   = I (q ) I0 exp  −  3   

[ I (q )] Ln [ I0 ] − Ln =

(7)

q 2 Rg2 3

(8)

where Rg is Guinier’s radius, a measure of the size of the scattering object.

3. Results and Discussions 3.1 Curing characteristics Torque-time curves during vulcanization of the NR vulcanizates prepared with varied mastication times are shown in Figure 1. Curing characteristics such as Ts1, Tc90, ML, MH, and MH-ML are listed in Table 2. The ML, an indication of viscosity and processability of the uncured rubber, decreased with mastication time. The ML is usually proportional to the uncured physical crosslinking or chain entanglement[10] and to molecular weight of rubber. It is well-known that mastication mechanically breaks and shortens long rubber molecular chains[8,9,19]. A longer mastication then corresponds to greater molecular weight reduction. In contrast, the MH is a measure of the cured compound’s stiffness, and it tended to increase with mastication time. Also, the MH - ML torque difference represents the total crosslinking density of the vulcanizate, and it also increased over mastication time. This was simply due to the radicals formed by chain scissions. Shortening the molecular chains may facilitate the reactions of curing precursors with rubber chains during vulcanization, due to a reduction of steric hindrances from large molecular chains. Consequently, the crosslink density of the samples increased with mastication time. That the crosslink density increased with mastication time was later

Table 2. Curing characteristics of NR vulcanizates. Sample

ML (dN.m)

MH (dN.m)

MH-ML (dN.m)

Ts1 (min)

Tc90 (min)

R300s

ν (× 10-7 mol/cm-3)

0 min 5 min 10 min

3.81 ± 0.04 3.52 ± 0.02 3.27 ± 0.10

21.73 ± 0.18 22.50 ± 0.10 22.52 ± 0.04

17.92 ± 0.14 18.98 ± 0.02 19.25 ± 0.14

1.01 ± 0.01 0.99 ± 0.01 1.02 ± 0.02

2.94 ± 0.03 2.71 ± 0.05 2.89 ± 0.02

6.62 ± 0.03 7.99 ± 0.02 8.50 ± 0.07

1.10 ± 0.01 1.28 ± 0.01 1.41 ± 0.01

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Hayeemasae, N., Soontaranon, S., Rasidi, M. S. M., & Masa, A. confirmed by using equilibrium swelling test, and the results agreed well with the variation of the MH - ML.

Considering Ts1 and Tc90, it was found that the Ts1 appeared to be independent of the mastication time, but the shortest curing time (Tc90) was observed when the rubber was initially masticated for 5 min. Previously, it was demonstrated that non-rubber components in the masticated rubber were more homogeneously distributed throughout the rubber matrix when the rubber was masticated for a comparatively short time (e.g., for less than 5 min)[9]. These impurities may be involved in the crosslinking reactions, possibly acting as natural accelerators[20,21]. Better chemical crosslinking is expected when the dispersion of these impurities is homogeneous. The effects of mastication time on aging behavior of the rubber were assessed from the percentage reversion of the vulcanizates at elevated temperatures, by determining R300[11], and the results are included in Table 2. A larger R300 usually indicates lesser reversion resistance. As the mastication time increased, R300 increased accordingly, suggesting that the ability of rubber molecules to withstand reversion at an elevated temperature diminished. This was clearly due to molecular chain scission. The lower molecular weight rubber was more susceptible to heat degradation[22,23].

3.3 Crystallization behavior and structural changes during deformation Figure 3 shows combination of WAXD images and real-time stress responses as functions of strain, recorded during the WAXD measurement. It can be seen from Figure 3 that the simultaneous stress response during the WAXD test showed similar trend to the stress response obtained from the tensile test (Figure 2). This indicates that the molecular chains responded similarly during tensile testing and WAXD measurement. This provides scientific proof for the similarity of stress response during WAXD and tensile measurements. It is also noted that the stress response during stretching decreased with mastication time, as this caused molecular chain breakdown and lowered molecular weight of the rubber; hence the tensile properties deteriorated. The appearance in inserted WAXD images

3.2 Tensile properties The dependence of stress response during the tensile tests on the mastication time is shown in Figure 2, and the tensile properties in terms of modulus at 100% (M100), 300% (M300) and 500% (M500) strains, tensile strength (TS) and elongation at break (EB) are listed in Table 3. Apparently, the stresses in the vulcanizate sample without mastication (0 min) at all given strains (M100, M300 and M500) were higher than those in rubbers masticated for 5 and 10 min, suggesting that the rubber was stronger without mastication, even though a lower total crosslink density was found for the sample without mastication. The tensile testing results clearly confirm that the effect of molecular weight is more pronounced than that of the total crosslink density. High stress response in the sample without mastication was tentatively attributed to a large proportion of chain entanglements in high molecular weight NR as suggested by the ML value (Table 2). These entanglements are capable of hindering the mobility of rubber chains during deformation, resulting in a higher stress. A reduction of chain entanglements with decreasing rubber molecular weight should be responsible for the decreased tensile stress response[24]. It was also found that the EB tended to increase with mastication time. Since the molecular weight decreased with mastication, a longer mastication time resulted in a lesser molecular weight. For longer mastication, some molecular chains turned to short chains with low molecular weight. Such chains may act as a plasticizer, facilitating extension of the vulcanized sample.

Figure 2. Plot of tensile stress versus tensile strain for NR vulcanizates prepared with different mastication times.

Figure 3. Coupled WAXD images with real time stress responses during WAXD measurement.

Table 3. Tensile properties in terms of M100, M300, M500, TS and EB for NR vulcanizates prepared with different mastication times. Sample 0 min 5 min 10 min

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M100 (MPa) 0.71 ± 0.04 0.75 ± 0.01 0.71 ± 0.02

M300 (MPa) 1.51 ± 0.03 1.50 ± 0.06 1.44 ± 0.02

M500 (MPa) 3.31 ± 0.01 3.29 ± 0.12 3.04 ± 0.11

TS (MPa) 18.75 ± 0.21 21.81 ± 0.12 20.48 ± 0.59

EB (%) 770 ± 14 812 ± 19 818 ± 17

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Tensile and structural properties of natural rubber vulcanizates with different mastication times depends strongly on the strain applied. The image without any reflection spot is for the sample without deformation (0%), suggesting that no crystallization occurs. When the sample was stretched to 330% strain, various reflection spots corresponding to different crystallographic planes are seen. However, the (200) and (120) planes are of main interest in this study. These crystallographic planes were more intense with further increase in deformation, implying that the molecular chain orientation and crystallization increased with strain. Figure 4 shows the variation of crystallinity corresponding to the (200) and (120) planes for the rubber samples prepared with different mastication times. The crystallinity increased with strain in all cases, because stretching caused chain orientation and reduced the degree of disorder of the NR chains[25]. These crystallites provided self-reinforcement to the rubber due to their ability to act as virtual filler or as crosslink[7,26]. As a result, the tensile stress steeply increased after strain-induced crystallization began. It is also seen that the degree of crystallinity of both crystallographic planes had rank order 0 min > 5 min > 10 min. The result clearly suggests that strain-induced crystallization decreased with reduction of molecular weight, achieved via mastication. The decreased crystallinity with increasing mastication time can be explained by increased total crosslink density, as previously shown in Figure 1 and Table 2. Enhanced crosslinking could reduce the mobility or NR chains and delay the orientation of the crystallites during stretching. It is well accepted that the ability for strain-induced crystallization decreases when crosslink density increases, because the molecular chain spans between crosslinks (Mc) are shortened according to υ = 1/Mc, where υ is crosslink density. Consequently, the ability to produce large crystallites is lowered[27,28]. In this case, increased mastication time reduced the molecular chain weight and increased crosslink density of the NR. As a result, the Mc was smallish and limited the crystallization during stretching. One may argue that ploting the crystallinity data as a function of stress might be useful. However, the reinforcement of rubber is often defined by an improvement in properties at a given strain, i.e., modulus at a certain strain, and tensile strength. Futhermore, the changes of crystallization during stretching are involved with the rubber deformation particularly at large strains, and reflect the reinforcement of rubber, appearing as a function of stretching level. Thus, plotting the crystallization data as a function of stress might be inappropiate due to the different phenomena in the progression of strain-induced crystallization. Figure 5 shows variation of the average crystallite size in NR vulcanizates. The size of crystallites appeared to decrease with strain due to the increased number of crystallites with strain (Figure 4), and a reduction of mean distance between the stretched chains which acted as crystallite precursors[29]. It is also observed that the average size of crystallite during stretching of the sample without mastication (0 min) was slightly smaller than in the samples with 5 and 10 min of mastication. This is attributed to the large number of crystallites formed in the samples with mastication, resulting in a small average size of crystallite. To gain a deeper understanding regarding the orientation of crystallites during stretching, Herman’s orientation Polímeros, 31(1), e2021003, 2021

Figure 4. Degree of crystallinity at various strains in the NR vulcanizates prepared with different mastication times.

Figure 5. Variation of crystallite size at various strains in the NR vulcanizates prepared with different mastication times.

function (OP) was used to estimate the degree of crystallite orientation. The degree of crystallite orientation was calculated from the azimuthal intensity distribution of the (120) plane. The OP approaches 1 when the crystallites are completely aligned along the stretching direction, while 5/8


Hayeemasae, N., Soontaranon, S., Rasidi, M. S. M., & Masa, A. the value is 0 for crystals that are randomly oriented, and 0.5 when the crystals are aligned perpendicular to the stretching direction[13]. Figure 6 shows the OP for NR vulcanizates prepared with different mastication times, as functions of strain. The OP slightly increased with strain and approached 1, implying that the alignment of crystallites was almost parallel to the stretching direction. The results obtained in recent studies align well with these observations[30,31]. Since the SAXS intensity patterns of the vulcanized rubber samples were attributed to the presence of crosslinked networks[32,33], the variations of crosslinked networks were investigated. Effects of the mastication time on dispersion and size of crosslinked network structures were investigated by using the SAXS measurement, and the results are shown in Figures 7 and 8. Figure 7 shows a plot of the scattering intensity I(q) against scattering vector q for the NR vulcanizates prepared with different mastication times. The q is defined as: q = (4πsinθ)/λ; λ is the wavelength and 2θ is the scattering angle. From Figure 7, a reduction of molecular weight with increasing mastication time slightly affected the SAXS profile of the vulcanizate. This was attributed to the fact that there was only reorientation of scattering bodies such as a crosslinked network[34]. The I(q) in all cases decreased continuously with increasing q and the I(q) was almost constant when the value of q approached 0.2. However, the lowest intensity was observed for the sample with 5 min of mastication, suggesting a homogeneous distribution of the crosslinked networks throughout the rubber matrix[9]. The homogeneity of crosslinked networks clearly contributed to the tensile properties and thus the highest tensile strength was obtained for the sample with 5 min mastication. This clearly confirms the importance of crosslinked networks to tensile properties. To gain further details of network structural differences in vulcanizate samples prepared with varied mastication times, the SAXS profiles were then fitted with the Guinier equation. Figure 8 shows typical Guinier plots (Ln I(q) versus q2) for the crosslinked NR samples. The data in Figure 8 were fitted with straight lines with R-squared of 0.99. It can be seen that the slope decreased with mastication time. From the slopes obtained from Figure 8, the Rg can be estimated, and the results are shown in Figure 9. Figure 9 shows changes in size of crosslinked network structures in the NR vulcanizates prepared with different mastication times. The Rg slightly decreased with mastication time, implying that the size of crosslinked networks in the vulcanizates was reduced with decreasing rubber molecular weight. A reduction of Rg would be attributed to the decreased number of high molecular weight chains that participated in crosslinking. This study suggests not only the variation of crosslinked structures with decreasing molecular weight caused by mastication, but also suggests reduction in size of crosslinked structures with decreasing molecular weight. As compared to NR without vulcanization[9], the size of Rg for the vulcanized rubber was larger. This is due to the fact that the Rg in unvulcanized NR was attributed to the aggregation of 6/8

impurities (non-rubber components)[35], but the Rg in the vulcanized sample was due to crosslinking of clusters by all impurities, activating agents and crosslinking agents[36]. Therefore, a larger Rg was achieved for samples after vulcanization.

Figure 6. Orientation of crystallinity at various strains in the NR vulcanizates prepared with different mastication times.

Figure 7. Correlation of scattering intensity I(q) with scattering vector q for NR vulcanizates prepared with different mastication times.

Figure 8. Typical Guinier plots for NR vulcanizates prepared with different mastication times. Polímeros, 31(1), e2021003, 2021


Tensile and structural properties of natural rubber vulcanizates with different mastication times

Figure 9. Radius of gyration (Rg) for the NR vulcanizates prepared with different mastication times.

4. Conclusions In this study, influences of mastication times on the tensile response, the deformation-induced crystallization and the molecular structure of NR vulcanizates were investigated. Increased mastication time slightly enhanced the crosslink density in rubber vulcanizates. The stress response to tensile deformation tended to decrease with increasing mastication time, which agrees well with the deformation-induced crystallization behavior. Increased mastication time significantly affected modulus at specified strain and upturn point of strain induced-crystallization of the crosslinked samples. Prolonged mastication decreased chain crystallization during stretching and size of crosslinked networks due to shorter rubber polymer chains. Based on the results obtained in this study, it is suggested that prolonged mastication influenced the deformation induced crystallization and structural homogeneity of the crosslinked NR, which in turn affected the tensile properties.

5. Acknowledgements Financial support from the Prince of Songkla University (Grant No. RDO6202102S) to the first author is gratefully acknowledged. Research and Development Office (RDO), Prince of Songkla University and Assoc. Prof. Dr. Seppo Karrila are acknowledged for assistance in editing the English language in this manuscript. The SCG chemicals is acknowledged for in-house developed extensometer during WAXD and SAXS measurements.

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Hayeemasae, N., Soontaranon, S., Rasidi, M. S. M., & Masa, A. non-rubber components on properties of sulphur crosslinked natural rubbers. Advanced Materials Research, 844, 345-348. http://dx.doi.org/10.4028/www.scientific.net/AMR.844.345. 21. Kongkaew, C., Intiya, W., Loykulnant, S., & Sae-oui, P. (2017). Effect of protein crosslinking by maillard reaction on natural rubber properties. KGK. Kautschuk, Gummi, Kunststoffe, 5, 37-41. 22. Karaagac, B., Cengiz, S. C., Bayram, T., & Sen, M. (2018). Identification of temperature scanning stress relaxation behaviors of new grade ethylene propylene diene elastomers. Advances in Polymer Technology, 37(8), 3027-3037. http:// dx.doi.org/10.1002/adv.21973. 23. Ray, S., & Cooney, R. P. (2018). Thermal degradation of polymer and polymer composites. In M. Kutz (Ed.), Handbook of environmental degradation of materials (pp. 185-206). Oxford: William Andrew Publishing. http://dx.doi.org/10.1016/ B978-0-323-52472-8.00009-5. 24. Bueche, F. (1958). Mechanical properties of natural and synthetic rubbers. Rubber Chemistry and Technology, 31(1), 1-18. http://dx.doi.org/10.5254/1.3542259. 25. Tosaka, M. (2007). Strain-induced crystallization of crosslinked natural rubber as revealed by x-ray diffraction using synchrotron radiation. Polymer Journal, 39(12), 1207-1220. http://dx.doi. org/10.1295/polymj.PJ2007059. 26. Tosaka, M., Senoo, K., Kohjiya, S., & Ikeda, Y. (2007). Crystallization of stretched network chains in cross-linked natural rubber. Journal of Applied Physics, 101(8), 084909. http://dx.doi.org/10.1063/1.2716382. 27. Trabelsi, S., Albouy, P. A., & Rault, J. (2003). Crystallization and melting processes in vulcanized stretched natural rubber. Macromolecules, 36(20), 7624-7639. http://dx.doi.org/10.1021/ ma030224c. 28. Chenal, M., Chazeau, L., Guy, L., Bomal, Y., & Gauthier, C. (2007). Molecular weight between physical entanglements in natural rubber: A critical parameter during strain-induced crystallization. Polymer, 48(4), 1042-1046. http://dx.doi. org/10.1016/j.polymer.2006.12.031. 29. Klug, H. P., & Alexander, L. E. (1974). X-ray diffraction procedures: For polycrystalline and amorphous materials. New York: Wiley.

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30. Che, J., Burger, C., Toki, S., Rong, L., Hsiao, B. S., Amnuaypornsri, S., & Sakdapipanich, J. (2013). Crystal and crystallites structure of natural rubber and synthetic cis-1,4-polyisoprene by a new two dimensional wide angle x-ray diffraction simulation method, I, strain-induced crystallization. Macromolecules, 46(11), 4520-4528. http://dx.doi.org/10.1021/ma400420k. 31. Che, J., Burger, C., Toki, S., Rong, L., Hsiao, B. S., Amnuaypornsri, S., & Sakdapipanich, J. (2013). Crystal and crystallites structure of natural rubber and peroxide-vulcanized natural rubber by a two-dimensional wide-angle x-ray diffraction simulation method, II, strain-induced crystallization versus temperatureInduced crystallization. Macromolecules, 46(24), 9712-9721. http://dx.doi.org/10.1021/ma401812s. 32. Salgueiro, W., Somoza, A., Torriani, I. L., & Marzocca, A. J. (2007). Cure temperature influence on natural rubber - a small angle x-ray scattering study. Journal of Polymer Science. Part B, Polymer Physics, 45(21), 2966-2971. http://dx.doi. org/10.1002/polb.21293. 33. Masa, A., Soontaranon, S., & Hayeemasae, N. (2020). Influence of sulfur/accelerator ratio on tensile properties and structural inhomogeneity of natural rubber. Polymer, 44(4), 519-526. http://dx.doi.org/10.7317/pk.2020.44.4.519. 34. Weng, G., Huang, G., Lei, H., Qu, L., Nie, Y., & Wu, J. (2011). Crack initiation and evolution in vulcanized natural rubber under high temperature fatigue. Polymer Degradation & Stability, 96(12), 2221-2228. http://dx.doi.org/10.1016/j. polymdegradstab.2011.09.004. 35. Karino, T., Ikeda, Y., Yasuda, Y., Kohjiya, S., & Shibayama, M. (2007). Nonuniformity in natural rubber as revealed by small-angle neutron scattering, small-angle X-ray scattering, and atomic force microscopy. Biomacromolecules, 8(2), 693699. http://dx.doi.org/10.1021/bm060983d. PMid:17243766. 36. Ikeda, Y., Higashitani, N., Hijikata, K., Kokubo, Y., Morita, Y., Shibayama, M., Osaka, N., Suzuki, T., Endo, H., & Kohjiya, S. (2009). Vulcanization: new focus on a traditional technology by small-angle neutron scattering. Macromolecules, 42(7), 2741-2748. http://dx.doi.org/10.1021/ma802730z. Received: Oct. 07, 2020 Revised: Jan. 25, 2021 Accepted: Feb. 15, 2021

Polímeros, 31(1), e2021003, 2021


ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.04320

Experimental investigation on stacking sequence of Kevlar and natural fibres/epoxy polymer composites Murali Banu1* , Vijaya Ramnath Bindu Madhavan2 , Dhanashekar Manickam3  and Chandramohan Devarajan3  Department of Mechanical Engineering, Anna University, Chennai, Tamil Nadu, India Department of Mechanical Engineering, Sri Sairam Engineering College, Chennai, Tamil Nadu, India 3 Department of Mechanical Engineering, St. Peter’s Institute of Higher Education and Research, Chennai, Tamil Nadu, India 1

2

*bmprojectss@gmail.com

Abstract This paper investigates the stacking sequence of combined natural and synthetic fibres reinforced epoxy composites for better mechanical properties. The hybrid composites fabricated using vacuum assisted compression molding process with the natural and synthetic fibres layered in three different sequences such as type I, type II and type III where the synthetic fibers were placed alternatively. The ultimate tensile strength of composite type III was increased by 12% and 30% when compared to composite type I and type II respectively. The flexural test results showed that composite type III have better flexural strength 223 MPa which is 13% and 11% greater than composite type I and type II respectively. Overall, it can be declared that the composite type III shows better tensile, and flexural properties i.e., the composite with aloe vera and palmyra palm fibres have better wettability with the matrix when compared to bamboo fibre. Keywords: aloe vera fibre, bamboo fibre, epoxy resin, Kevlar fibre, palmyra palm fibre. How to cite: Banu, M., Madhavan, V. R. B., Manickam, D., & Devarajan, C. (2021). Experimental investigation on stacking sequence of Kevlar and natural fibres/epoxy polymer composites. Polímeros: Ciência e Tecnologia, 31(1), e2021004. https://doi.org/10.1590/0104-1428.04320

1. Introduction In this world, natural fibre-reinforced polymer composites are being most widely considered as a potential area for researchers and research opportunities for producing advanced polymer composites. Natural fibres[1] such as coconut coir, sisal, banana, kenaf, hemp, pineapple leaf fibre, bamboo, bagasse and jute are widely used as reinforcements in polymer composites and the detailed applications of each fibres in automobile and other industries are listed by Mohammed et al. [2] . Renewable resource consumptions afford the progressive ways for sustainability of ‘green’ environment conditions[3,4]. Composites having multi reinforcements are termed as hybrid composites and mostly the multi reinforcements are either natural or synthetic fibres else a combination of both. The hybrid composites that take the most advantage of the best properties of the constituents, and thereby an ideal, greater but economical composite can be increased[5]. The elements of a composite, such as fibres and matrix, affect the mechanism working in the composites during loading, failure modes, damage progression, and finally the strength[6]. Effective hybridization of woven natural fibre fabrics with carbon, E glass, aramids, basalt and Kevlar fibres are the vital way to characterize the best bio-composites[7,8]. Chandramohan et al.[9] investigated that the relative hybrid Sisal/ Rosell/ Banana fibre reinforced epoxy composites and illustrated that composites are greater for successful tensile

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loading and flexural loading. Srinivasan et al.[10]. Concluded that hybrid composite has potential properties than single glass fibre reinforced composite beneath collision and flexural loads and also fibre strength is significantly linked with the positioning and orientation of fibres. Vijaya Ramnath et al.[11] have studied the effects of twisting the natural fibres (kenaf and neem) and fibre orientation on mechanical properties of composites. The twisted fibre with 45° orientation produced better mechanical properties. Mohammed et al.[2] have reviewed the effect of chemical treatment on natural fibres and concluded that it improved adhesion between the fibre surface and the matrix which ultimately enhanced physicomechanical and thermochemical properties of the natural fibre reinforced polymer composites (NFRPCs). Yan et al.[12] reported that chemical treatment of natural fibre reduces the stiffness and toughness but when treated with Silane (Si) an significant improvement in tensile strength of flax fibres was observed and this was due to silane being grafted with carbonyl chain between microfibrils. Increase in ratio of cellulose in the natural fibre enhances the mechanical properties. Synthetic and natural fibres are combined to increase the tensile strength of the high density polyethylene (HDPE) composites[13]. Combined effects of chemical treatment of fibres and inclusion nano-fillers have improved the dynamic mechanical properties of natural

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Banu, M., Madhavan, V. R. B., Manickam, D., & Devarajan, C. fibre reinforced polymer composites[14]. Chaitanya et al.[15] investigated the effect of alkaline treatment (5% concentration of NaOH) on surface modifications of aloe vera fibres and concluded that there was an improvement in thermal stability, tensile, compression and flexural properties while impact strength decreased as the alkaline treatment removed the lignin and waxes from the fibre surface. Sampath et al.[16] have concluded that inclusion of nano-fillers increases the mechanical properties of the composites by strengthening the matrix yield strength. Karaduman et al.[17] have investigated the effect of stacking sequence on chemically treated flax/jute fibre reinforced hybrid composites by means of tensile and flexural tests. Nunes et al.[18] stated that fibre orientation, laminate sequence and surface waviness have significant effect on the flexural strength of the composites. Li et al.[19] suggested the hybrid polymer composites with windmill palm fibre and a strong-brittle fibre (such as flax or jute), can produce high elongation properties which allows better stress transfer, and increase the effective fibrematrix adhesion. Dynamic mechanical analysis of natural fibre reinforced polymer composites were studied to understand the viscoelastic behaviour. Weight/volume percentages, fibre size, fibre shape, fibre orientation and stacking sequence of hybrid laminates influence the dynamic mechanical properties[14]. The basic dynamic mechanical analysis output parameters such as storage modulus, loss modulus and damping factor (tan δ) with respect to temperature enunciate about matrix-fibre interfacial bonding[20]. Gupta et al.[21] studied and concluded that the dynamic mechanical performance of hybrid epoxy composite (jute + sisal fibres) was better than plain jute and sisal composite, which shows the effect of hybridization of natural fibre reinforced polymer composites as an advantage. To the best of the author’s knowledge, no peer reviewed research articles were made to experimentally study the effect of Kevlar with natural fibre reinforced epoxy composites. Only a few researchers have focused on stacking arrangements of hybrid natural fibres influence on the mechanical and dynamic properties of the polymer composites. The goal

of this research was to investigate the optimal stacking sequence of dual natural fibres in kevlar/epoxy composites.

2. Materials and Methods Aramid is abbreviated from the word aromatic polyamide (poly-p-phenylene terephthalamide) which is also called as Kevlar and it is the most widely used organic fibre in the aerospace applications. Researchers made efforts to utilize many compostable and biodegradable fibres extracted from the plants as reinforcement materials in the polymer for the development of advanced composites. In this research work, the natural fibres such as aloe vera, bamboo and palmyra palm along with the Kevlar fibre with the density of 1.4, 0.9, 1.3 and 1.43 g/cm3 respectively, were used in a defined stacking sequence in fabricating the hybrid composites. The thickness of Kevlar fibre mat and all natural fibres was 0.3 mm and 0.35 mm respectively. Epoxy resin (LY556) and hardener (HY951) of density 1.15-1.20 g/cm3 and 0.970.99 g/cm3 respectively were used as the matrix. Epoxy has excellent moisture repellent qualities when utilized in polymer composites. The natural fibres were alkaline treated in 2.5 ml NaOH solution for 6 hr at room temperature and dried to remove the wax and oils available on the external surface of natural fiber. Then natural fibres were again treated with 2.5 ml NaOH solution, boiled at 75 °C for 3 hr to remove a certain rate of lignin and hemicellulose which enhances the matrix-fiber interface and ensures better adhesion. Increase in NaOH solution concentration causes defibrillation and pore formation on the fiber surface. Now the natural fibers are washed with distilled water 3-4 times for the deletion of NaOH solution. Later the natural fibres neutralization was achieved by treating in 2% HCl solution boiled at 75 °C for 4 hr which removes residual hydroxyl. Finally fibres were immersed in 1% ethanol and dried at 110 °C for 10 hr [16]. The hybrid composites were fabricated using vacuum assisted compression molding process, which completely eliminates the defects produced in hand lay-up technique and it is reliable when compared to other techniques namely resin transfer molding and Pultrusion[11]. The diagrammatic representation of the vacuum assisted compression moulding process is shown in Figure 1a-c. Three types of composites were fabricated based on the sequence of layering the synthetic

Figure 1. Sequence involved in Vacuum assisted compression moulding; (a) placement of mold and pattern; (b) placement of fibres in mold; and (c) external application of compression pressure. 2/9

Polímeros, 31(1), e2021004, 2021


Experimental investigation on stacking sequence of Kevlar and natural fibres/epoxy polymer composites Kevlar fibre and dual natural fibres where the number of layers were fixed to be 5 layers in common and the matrixreinforcement ratio to be 60:40. The resin hardener ratio of 10:1 was followed to enhance the interfacial bonding between the fibres. Initially the mould is vacuumed prior to layering the fibre mat in the given sequence as mentioned in the Figure 2a. The matrix is introduced into the mould and the preform is compressed by applying external pressure. The mold is left idle for 8 hours for complete drying and then the fabricated composites are ejected from the mould. The photographs of fabricated composites shown in Figure 2b.

2.1 Testing methods The tensile test is carried out using a Universal Testing machine for the fabricated composite laminate blanked as per ASTM D638 standards as shown in Figure 3a[22]. The

test samples were firmly clamped to the fixture to ensure there is no slip during the application of load and the traverse speed was maintained at 2.5 mm/min. The tensile results reveal the bond strength of different fibre layers and the load bearing capacity of the composite material is examined. The ASTM D790 standards was followed for Flexural analysis of the composite sample using the Universal Testing machine with the specimen dimensions as given in Figure 3b. The flexural test results aids to identify the interlaminar shear strength as the concave shape formation induces more shear in between the lamina. The fracture surface analysis highlights the failure mechanisms and the reasons. All the testing were carried out at standard atmospheric temperature. The dynamic mechanical analysis was performed using DMA Q800 V20.6 build 24 at Institute of Plastic Technology, Chennai, India according to ASTM D4065-01[23]. The test specimen was prepared with the

Figure 2. (a). 3D design of layering the fibres in three different types; (b) Photographs of fabricated composites.

Figure 3. 2D diagrams of composite dimensions machined for various testing according to the ASTM standards (All Dimensions are in mm). Polímeros, 31(1), e2021004, 2021

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Banu, M., Madhavan, V. R. B., Manickam, D., & Devarajan, C. dimensions of 65 mm x 12.7 mm x 5 mm. All the tests were performed under bending mode at a frequency of 1 Hz and at specific temperatures range from 28 °C to 200 °C with a heating rate of 4 °C/min.

3. Results and Discussion 3.1 Tensile strength The tensile test results for the fabricated three types of hybrid composites are given in Table 1 and the values are averaged from 3 readings for each type of composite. Figure 4 shows the type III (Palm + Aloe vera fibres) composite having a higher tensile strength of 128 MPa with an elongation of 18%. This is due to the increase in wettability of the natural and synthetic fibre- matrix interface. The alkaline treatment of palm and aloe vera fibre, removes the lignin and waxes off the fibre surface[15]. Simultaneously the fibre surface is roughened enabling the strong interface bond strength between the fibres and the matrix. In case of bamboo fibres, the material properties are directional based (compression force applied perpendicular to the longitudinal direction, undergoes frequent random break of bond in tangential direction because of the weaker bond between fibres)[24]. The density of the bamboo fibre is lesser when compared to aloe vera and palm fibre, this is one of the reasons for the composites reinforced with bamboo fibre exhibited a variation in load distribution when subjected to tensile test. The bamboo reinforced composites failed due to the higher concentration of inter laminar shear stress at the fibre-matrix interface and the initial failure of bamboo fibre is observed in the stress vs. strain plot (as shown in Figure 4), with a deflection. As a result, the bamboo fibre reinforced composites experienced failure earlier to the other composites.

3.2 Flexural strength Flexural strength of the fabricated composites is tested to understand the strengthening mechanism between the fibre and the matrix interface. Due to the mixed combinations of stresses and the different elastic properties of the stacked fibre laminate, estimation of the flexural strength is important in deciding the end application. Similar to the ultimate tensile strength, the flexural behaviour of the composite depends on the orientation of the fibre and the fibre laminate stacking sequence. Application of load in the three point bending test, the anisotropic nature of fabricated composite experience bending failure due to tensile, compressive, shear or combination of these stresses. The flexural strength and energy absorption properties of the fabricated composites are provided in Table 2[25]. For each type of composite three samples were tested and the average value is given in the tabulation. From the Figure 5, it is evident that type III composite have ultimate flexural strength of 223 MPa, which is 13% and 11% higher than type I and type II composites respectively. This is due to the better adhesive bond between the palmyra palm fibre surface and the matrix. The formation of cusps and scallops are observed in the relatively brittle systems as they are formed from the micro-crack nucleation ahead of the crack tip[26]. For the interlaminar fracture, initial delamination will propagate when local interlaminar stress produced by local bending makes the strain energy release rate G of stratified edge reach or exceed the critical value Gc (interlaminar fracture toughness)[27,28]. The palmyra palm fibres exhibit a high elongation at break[19]. The high density palmyra palm and aloe vera fibres in the type III composite have better wettability with the resin and the load applied is uniformly distributed within the composite layers. The presence of low

Figure 4. Stress vs. strain for the fabricated composites. Table 1. Tensile Strength of the fabricated composite specimen. Composite specimen

Break load (KN)

(TYPE -I) (TYPE -II) (TYPE -III)

3.68 4.7 5.5

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Displacement at break load (mm) 4.5 4.6 6.4

Elongation (%) 16.38 14.54 17.26

Ultimate tensile strength (MPa) 102 114 127

Tensile modulus (MPa) 238.67 261.82 298.38

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Experimental investigation on stacking sequence of Kevlar and natural fibres/epoxy polymer composites Table 2. The flexural test results for three types of composite. Composite specimen

Break load (KN)

(TYPE -I) (TYPE -II) (TYPE -III)

0.49 0.44 0.62

Displacement at break load (mm) 13.6 16.2 19.7

Elongation (%) 9.67 9.93 11.46

Ultimate flexural strength (MPa) 196.53 201.64 223.48

Figure 5. Type of composite vs. ultimate flexural strength (MPa).

density bamboo fibres in the composite type I and type II have failed in distributing the load applied. The moderate wettability nature of the bamboo fibre is the prime reason for the failure of the composites. The type of failure observed in type I and type II composites were pure delamination at the bamboo and Kevlar fibre interface which was due to weak bond strength.

3.3 Dynamic mechanical analysis 3.3.1 Storage modulus (E’) In the dynamic mechanical analysis, the applied energy is stored in the material to a certain limit and it is displayed in the form of storage modulus (E’). With the increase in temperature a significant transition is observed in the storage modulus and termed as glass transition temperature (Tg), beyond which rubbery state occurs as shown in Figure 6. In the rubbery state, the epoxy resin becomes unstable and the molecular movement occurs as the molecular bond becomes weak, ultimately resulting in the reduction of stiffness[28]. E’ of composite type III (palmyra palm + aloe vera) have the higher stiffness and increased bond strength with the epoxy resin. The composite type I (bamboo + aloe vera fibres) showed least storage modulus ascribed due to weak fibre-matrix adhesion and lower density of bamboo fibre. It’s clear from Figure 6, that the E’ tends to become broader in the glassy region, as the components are in close, tightly packed and in frozen state resulting in high storage modulus value below glass transition temperature (Tg). Moreover, the E’ curves suffer an intense drop around 85-105 °C indicating glass/rubbery state transition. But when the temperature increases the composites components tend to show increased molecular mobility, hence, lose their tight packing arrangement, this then gradually decreases the E’ values in the rubbery region. However no considerable Polímeros, 31(1), e2021004, 2021

Figure 6. Storage modulus vs. temperature.

changes in the rubbery region were observed for palmyra palm fibre/epoxy composites[29]. 3.3.2 Loss modulus (E”) The viscous response of the hybrid composite material is represented by loss modulus (E”). Figure 7 shows the plot between loss modulus and temperature and it can be observed that irrespective of the type of composite, the loss modulus increases initially until a peak and then decreases with the increase in temperature. The fibre type and the fibre wt. % have a greater influence on the loss modulus of the composites[30]. Broadening the curve of the polymer matrix in the composite type III (palmyra palm + aloe vera) indicated that natural fibres played an important role above Tg. The broadening in E” is due to enhancement in chain segment[31]. The position and height of the loss modulus 5/9


Banu, M., Madhavan, V. R. B., Manickam, D., & Devarajan, C. curve are indicative of the structure and the extent to which the polymer is reinforced, reflecting changes in molecular dynamics in this region[32]. From Figure 7, it is evident that the incorporation of palmyra palm and aloe vera fibres in the type III composite increases the chain segment and causes the widening of the loss modulus peak. The position and height of the loss modulus curve are indicative of the structure and the extent to which the polymer is reinforced, reflecting changes in molecular dynamics in this region[32]. The loss modulus peak height was higher for type III composite because of the increased matrix-fibre bond strength and increase in internal friction. This phenomena agrees well with results observed and reported by Saba et al.[29]. The type I composite had lowered loss modulus peak height whereas, the composite type III (palmyra palm and aloe vera) displays higher peak height. The maximum value of the loss modulus can be used to determine the maximum rate of heat dissipation, which represents the glass transition temperature (Tg) and this behaviour represents the free movement of the polymer chain at high temperatures[28]. Hybridization of the palmyra palm and aloe vera fibres along with the Kevlar fibres results in a close arrangement between the laminate layers of the composite. The composite type III exhibits improved interfacial adhesion between the fibre and the matrix, which accordingly enriches the structural mobility of the polymer chains. Similar results were observed by Ramesh et al.[33], that combined effect of hybridization and close arrangement between the Kenaf and PET fibres improves the fibre/matrix interfacial adhesion of the composites. The heat dissipated at the interface is due to the frictional resistance generated at the molecular movement. The inclusion of palmyra fibre resulted in an increase in the amount of heat dissipated and results in the transition peak shift to a higher temperature and increases the loss modulus. The enhanced palmyra fibre bond strength with the epoxy matrix restricted the molecular movement[28]. 3.3.3 Damping factor (Tan δ) Damping factors of the composites are dependent on the type of fibres reinforced as shear stress concentration produced at the matrix interface and the dissipation of viscoelastic energy. The presence of laminate fibre in the polymer composite acts as an obstacle, restricting the movement of the polymer chains as a result reducing the molecular motion and overall damping factor of the hybrid composites. If the fibre matrix interface bond is weak, it results in greater values of tan δ. Damping is reduced for composites type III as the fibre-matrix have a better adhesion. From Figure 8, it is clear that composite type III (palmyra palm + aloe vera) had a lower tan δ value. Therefore, when the fibre/matrix interfacial bonding is enhanced, the molecular mobility of the polymer decreases; this consequently results in a reduction in the damping factor. When the degree of stiffness of a composite is increased, the degree of freedom of the polymer chain is limited; this restricts the molecular mobility, resulting in a decrease in the damping value of the material[28]. A positive shift in the damping factor peak values with the increase in temperature was due to the enhanced interfacial adhesion between the palmyra palm + aloe vera 6/9

Figure 7. Loss modulus vs. temperature.

Figure 8. Tan δ (Damping factor) vs. temperature.

fibres and matrix. The tan δ peak was narrow for the bamboo reinforced composites compared to palmyra palm fibre reinforced composites. The narrow peak confirms the weak interfacial bond strength similarly the wider peak was due to the formation of higher crosslinking density for the composite type III composites. The results are in accordance with Saba et al. [29]. The damping factor is related to molecular movements, viscoelasticity besides the certain defects that contribute towards damping such as dislocations, grain boundaries, phase boundaries and various interfaces[34]. 3.3.4 Morphological analysis using Scanning Electron Microscope (SEM) From the scanning electron microscopy images observed for the tensile and flexural test composites to study the fracture surfaces. The hybrid composites are examined for matrix fibre adhesion and understand the failure modes[18]. Possible failure that occurs can be either matrix cracking, fibre breaking, matrix-fibre debonding or other possible damages and these damages can be confirmed during the fracture surface detection process[35]. Figures 9 and 10 shows the SEM images interfaced with Image J software[36,37] of tensile and flexural test sample surfaces. It is clear from Figure 9a, the fibres with poor wettability are stressed at Polímeros, 31(1), e2021004, 2021


Experimental investigation on stacking sequence of Kevlar and natural fibres/epoxy polymer composites

Figure 9. SEM images of tensile test composite (a) type I; (b) type II; (c) type III.

Figure 10. SEM images of flexural test composite (a) type I; (b) type II; (c) type III.

higher tensile loads, resulting in fibre breakage and fibre pullouts. The presence of voids is higher in composite type I when compared to others composites and these voids greatly diminishes the mechanical properties. Similar results have been reported by Arthanarieswaran et al.[37]. The Figure 9b shows the voids in the matrix are reduced but not completely eliminated. The addition of palmyra palm fibres has enhanced tight interface bonds with the matrix and the bamboo fibres have poor wettability which is seen in the Figure 9b. The composite type III (palmyra palm + aloe vera) fibres have better wettability and reduced voids. The palmyra palm fibres have a tight interfacial bond with the matrix with no gap is shown in Figure 9c. From the Figure 10a, it is clearly seen that there is a clear separation between the fibre and the matrix and this effect is due to the abnormalities in the fibre-matrix bond. Polímeros, 31(1), e2021004, 2021

The presence of bamboo fibre in the composite type I, have undergone non-uniform distribution of flexural load. Figure 10b, shows the fractured surface of composite type II (Bamboo + palmyra palm) fibre. The presence of palmyra palm fibre nullified the degrading effect of bamboo fibre and there is no clear separation between the matrix-fibre interfaces. It is clear from the Figure 10c, there is no separation between the fibre-matrix interfaces. The crack propagation in the matrix has been minimized when compared to composite type I (Aloe vera + Bamboo) fibre. The increase in wettability of the natural fibres (Palmyra palm + Aloe vera) in composite type III, increases the interfacial adhesion contributing to the highest flexural strength. The crack propagation is minimized in the composite type III materials. 7/9


Banu, M., Madhavan, V. R. B., Manickam, D., & Devarajan, C.

4. Conclusion The natural fibre reinforced hybrid composites were successfully fabricated using Vacuum assisted compression moulding along with Kevlar synthetic fibre. The effect of chemically treated aloe vera, bamboo and palmyra palm fibres are studied in detail for their mechanical and dynamic analysis property. The following conclusions have been drawn from the experimental investigation: • The ultimate tensile strength of composite type III was increased by 12% and 30% when compared to composite type I and type II respectively; • The flexural test results show that composite type III have better flexural strength 223 MPa which 13% and 11% greater than composite type I and type II respectively; • Dynamic mechanical analysis results reveal that composite type III (Palmyra Palm + Aloe vera) had better damping factor due to the enhanced interfacial bond with the matrix; • Overall, it can be declared that hybridization of composite materials having natural fibres in a well-defined stacking sequence produces better mechanical and flexural properties; • The composite type III shows better tensile, and flexural properties i.e., the composite with natural fibres such as aloe vera and palmyra palm have better wettability with the matrix and synthetic fibre when compared to bamboo fibre.

5. References 1. Jeyapragash, R., Srinivasan, V., & Sathiyamurthy, S. (2019). Mechanical properties of natural fiber/particulate reinforced epoxy composites: a review of the literature. Materials Today: Proceedings, 22(3), 1223-1227. http://dx.doi.org/10.1016/j. matpr.2019.06.655. 2. Mohammed, L., Ansari, M. N. M., Pua, G., Jawaid, M., & Islam, M. S. (2015). A review on natural fiber reinforced polymer composite and its applications. International Journal of Polymer Science, 2015, 243947. http://dx.doi.org/10.1155/2015/243947. 3. Saba, N., Jawaid, M., Hakeem, K., Paridah, M., Khalina, A., & Alothman, O. (2015). Potential of bioenergy production from industrial kenaf (Hibiscus cannabinus L.) based on Malaysian perspective. Renewable & Sustainable Energy Reviews, 42, 446-459. http://dx.doi.org/10.1016/j.rser.2014.10.029. 4. Saba, N., Jawaid, M., Paridah, M., & Al-othman, O. (2016). A review on flammability of epoxy polymer, cellulosic and non‐cellulosic fiber reinforced epoxy composites. Polymers for Advanced Technologies, 27(5), 577-590. http://dx.doi. org/10.1002/pat.3739. 5. Elanchezhian, C., Vijaya Ramnath, B., Ramakrishnan, G., Rajendrakumar, M., Naveenkumar, V., & Saravanakumar, M. K. (2018). Review on mechanical properties of natural fiber composites. Materials Today: Proceedings, 5, 1785-1790. http://dx.doi.org/10.1016/j.matpr.2017.11.276. 8/9

6. Vijaya Ramnath, B., Elanchezhian, C., Nirmal, P. V., Prem Kumar, G., Santhosh Kumar, V., Karthick, S., Rajesh, S., & Suresh, K. (2014). Experimental investigation of mechanical behavior of Jute-Flax based glass fiber reinforced composite. Fibers and Polymers, 15(6), 1251-1262. http://dx.doi.org/10.1007/ s12221-014-1251-3. 7. Vijaya Ramnath, B., Manickavasagam, V. M., Elanchezhian, C., Vinodh Krishna, C., Karthik, S., & Saravanan, K. (2014). Determination of mechanical properties of intra-layer abacajute-glass fiber reinforced composite. Materials & Design, 60, 643-652. http://dx.doi.org/10.1016/j.matdes.2014.03.061. 8. Reddy, A. C. (2015). Evaluation of curing process for kevlar 49-epoxy composites by mechanical characterization designed for brake liners. International Journal of Scientific Research, 4, 2365-2371. Retrieved in 2020, June 24, from https://www. ijsr.net/archive/v4i4/SUB153699.pdf 9. Chandramohan, D., & Bharanichandar, J. (2013). Natural fiber reinforced polymer composites for automobile accessories. American Journal of Environmental Sciences, 9(6), 494-504. http://dx.doi.org/10.3844/ajessp.2013.494.504. 10. Srinivasan, V. S., Rajendra Boopathy, S., Sangeetha, D., & Vijaya Ramnath, B. (2014). Evaluation of mechanical and thermal properties of banana-flax based natural fibre composite. Materials & Design, 60, 620-627. http://dx.doi.org/10.1016/j. matdes.2014.03.014. 11. Vijaya Ramnath, B., Rajesh, S., Elanchezhian, C., Santosh Shankar, A., Pithchai Pandian, S., Vickneshwaran, S., & Sundar Rajan, R. (2016). Investigation on mechanical behaviour of twisted natural fiber hybrid composite fabricated by vacuum assisted compression molding technique. Fibers and Polymers, 17(1), 80-87. http://dx.doi.org/10.1007/s12221-016-5276-7. 12. Yan, L., Chouw, N., & Jayaraman, K. (2014). Flax fibre and its composites: a review. Composites. Part B, Engineering, 56, 296-317. http://dx.doi.org/10.1016/j.compositesb.2013.08.014. 13. Aldousiri, B., Alajmi, M., & Shalwan, A. (2013). Mechanical properties of palm fibre reinforced recycled HDPE. Advances in Materials Science and Engineering, 2013, 508179. http:// dx.doi.org/10.1155/2013/508179. 14. Das, P. P., & Vijay Chaudhary, V. (2019). Tribological and dynamic mechanical analysis of bio-composites: a review. Materials Today: Proceedings, 25(4), 729-734. http://dx.doi. org/10.1016/j.matpr.2019.08.233. 15. Chaitanya, S., & Singh, I. (2016). Novel Aloe Vera fiber reinforced biodegradable composites: development and characterization. Journal of Reinforced Plastics and Composites, 35(19), 14111423. http://dx.doi.org/10.1177/0731684416652739. 16. Sampath, P., & Santhanam, S. K. V. (2019). Effect of moringa and bagasse ash filler particles on basalt/epoxy composites. Polímeros: Ciência e Tecnologia, 29(3), e2019034. http:// dx.doi.org/10.1590/0104-1428.01219. 17. Karaduman, Y., Onal, L., & Rawal, A. (2015). Effect of stacking sequence on mechanical properties of hybrid flax/jute fibers reinforced thermoplastic composites. Polymer Composites, 36(12), 2167-2173. http://dx.doi.org/10.1002/pc.23127. 18. Nunes, J. P., Pouzada, A. S., & Bernardo, C. A. (2002). The use of a three-point support flexural test to predict the stiffness of anisotropic composite plates in bending. Polymer Testing, 21(1), 27-33. http://dx.doi.org/10.1016/S0142-9418(01)00040-X. 19. Li, J., Zhang, X., Zhu, J., Yu, Y., & Wang, H. (2020). Structural, chemical, and multi-scale mechanical characterization of waste windmill palm fiber (Trachycarpus fortunei). Journal of Wood Science, 66(1), 8. http://dx.doi.org/10.1186/s10086-020-1851-z. 20. Saba, N., Jawaid, M., Alothman, O. Y., & Paridah, M. T. (2016). A review on dynamic mechanical properties of natural fibre reinforced polymer composites. Construction & Polímeros, 31(1), e2021004, 2021


Experimental investigation on stacking sequence of Kevlar and natural fibres/epoxy polymer composites Building Materials, 106, 149-159. http://dx.doi.org/10.1016/j. conbuildmat.2015.12.075. 21. Gupta, M. K., & Srivastava, R. K. (2016). Mechanical Properties of Hybrid Fibers-Reinforced Polymer Composite: A Review. Polymer-Plastics Technology and Engineering, 55(6), 626-642. http://dx.doi.org/10.1080/03602559.2015.1098694. 22. Niranjan, R. R., Junaid Kokan, S., Sathya Narayanan, R., Rajesh, S., Manickavasagam, V. M., & Ramnath, B. V. (2013). Fabrication and Testing of Abaca Fibre Reinforced Epoxy Composites for Automotive Applications. Advanced Materials Research, 718-720, 63-68. http://dx.doi.org/10.4028/www. scientific.net/AMR.718-720.63. 23. Gheith, M. H., Aziz, M. A., Ghori, W., Saba, N., Asim, M., Jawaid, M., & Alothman, O. Y. (2019). Flexural, thermal and dynamic mechanical properties of date palm fibres reinforced epoxy composites. Journal of Materials Research and Technology, 8(1), 853-860. http://dx.doi.org/10.1016/j.jmrt.2018.06.013. 24. Li, X. (2004). Physical, chemical, and mechanical properties of bamboo and its utilization potential for fibre board manufacturing (Master’s theses). Louisiana State University and Agricultural and Mechanical College, USA. Retrieved in 2020, June 24, from https://digitalcommons.lsu.edu/gradschool_theses/866 25. Li, S., Zheng, T., Li, Q., Hu, Y., & Wang, B. (2019). Flexural and energy absorption properties of natural-fiber reinforced composites with a novel fabrication technique. Composite Communications, 16, 124-131. http://dx.doi.org/10.1016/j. coco.2019.09.005. 26. Gilchrist, M. D., & Svensson, N. (1995). A fractographic analysis of delamination within multidirectional carbon/epoxy laminates. Composites Science and Technology, 55(2), 195-207. http://dx.doi.org/10.1016/0266-3538(95)00099-2. 27. Xie, X., Zhou, Z., & Yan, Y. (2019). Flexural properties and impact behaviour analysis of bamboo cellulosic fibers filled cement based composites. Construction & Building Materials, 220, 403-414. http://dx.doi.org/10.1016/j.conbuildmat.2019.06.029. 28. Ahmad, M. A. A., Abdul Majid, M. S., Ridzuan, M. J. M., Mazlee, M. N., & Gibson, A. G. (2018). Dynamic mechanical analysis and effects of moisture on mechanical properties of interwoven hemp/polyethylene terephthalate (PET) hybrid composites. Construction & Building Materials, 179, 265-276. http://dx.doi.org/10.1016/j.conbuildmat.2018.05.227. 29. Saba, N., Paridah, M. T., Abdan, K., & Ibrahim, N. A. (2016). Dynamic mechanical properties of oil palm nano filler/kenaf/epoxy hybrid nanocomposites. Construction & Building Materials, 124, 133-138. http://dx.doi.org/10.1016/j. conbuildmat.2016.07.059.

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30. Shanmugam, D., & Thiruchitrambalam, M. (2013). Static and dynamic mechanical properties of alkali treated unidirectional continuous Palmyra Palm Leaf Stalk Fiber/jute fiber reinforced hybrid polyester composites. Materials & Design, 50, 533-542. http://dx.doi.org/10.1016/j.matdes.2013.03.048. 31. Ornaghi, H. L., Jr., Silva, H. S. P., Zattera, A. J., & Amico, S. C. (2011). Hybridization effect on the mechanical and dynamic mechanical properties of curaua composites. Materials Science and Engineering A, 528(24), 7285-7289. http://dx.doi. org/10.1016/j.msea.2011.05.078. 32. Dan-Mallam, Y., Hong, T. W., & Abdul Majid, M. S. (2015). Mechanical characterization and water absorption behaviour of interwoven Kenaf/PET fibre reinforced epoxy hybrid composite. International Journal of Polymer Science, 2015, 371958. http://dx.doi.org/10.1155/2015/371958. 33. Ramesh, M., Palanikumar, K., & Hemachandra Reddy, K. (2017). Plant fibre based bio-composites: sustainable and renewable green materials. Renewable & Sustainable Energy Reviews, 79, 558-584. http://dx.doi.org/10.1016/j.rser.2017.05.094. 34. Safri, S. N. A., Sultan, M. T. H., Jawaid, M., & Abdul Majid, M. S. (2019). Analysis of dynamic mechanical, low-velocity impact and compression after impact behaviour of benzoyl treated sugar palm/glass/epoxy composites. Composite Structures, 226, 111308. http://dx.doi.org/10.1016/j.compstruct.2019.111308. 35. Schindelin, J., Arganda-Carreras, I., Frise, E., Kaynig, V., Longair, M., Pietzsch, T., Preibisch, S., Rueden, C., Saalfeld, S., Schmid, B., Tinevez, J. Y., White, D. J., Hartenstein, V., Eliceiri, K., Tomancak, P., & Cardona, A. (2012). Fiji: an open-source platform for biological-image analysis. Nature Methods, 9(7), 676-682. http://dx.doi.org/10.1038/nmeth.2019. PMid:22743772. 36. Dhanashekar, M., & Kumar, V. S. (2018). Tribological behaviour of squeeze cast AL-SI7MG/SIC/GR hybrid composites. Journal of the Balkan Tribological Association, 24(1), 106121. Retrieved in 2020, June 24, from https://scibulcom.net/ en/article/HuylZtXN2S7ndz9HQyTb 37. Arthanarieswaran, V. P., Kumaravel, A., & Kathirselvam, M. (2014). Evaluation of mechanical properties of banana and sisal fiber reinforced epoxy composites: influence of glass fiber hybridization. Materials & Design, 64, 194-202. http:// dx.doi.org/10.1016/j.matdes.2014.07.058. Received: June 24, 2020 Revised: Jan. 28, 2021 Accepted: Feb. 18, 2021

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ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.04620

Cationic polymerization of styrene using iron-containing ionic liquid catalysts in an aqueous dispersed medium† Gabriel Victor Simões Dutra1 , Weslany Silvério Neto1 , Pedro Henrique Hermes de Araújo3 , Claudia Sayer3 , Brenno Amaro da Silveira Neto2*  and Fabricio Machado1*  Laboratório de Desenvolvimento de Processos Químicos, Instituto de Química, Universidade de Brasília – UnB, Brasília, DF, Brasil 2 Laboratório de Química Medicinal e Tecnológica, Instituto de Química, Universidade de Brasília – UnB, Brasília, DF, Brasil 3 Laboratório de Controle de Processos e Polimerização, Departamento de Engenharia Química e Engenharia de Alimentos, Universidade Federal de Santa Catarina – UFSC, Florianópolis, SC, Brasil † Selected paper presented at the 15th Brazilian Polymer Conference – (15thCBPol) held in Bento Gonçalves, Brazil, on 27–31 October, 2019. 1

*brenno.ipi@gmail.com, fmachado@unb.br

Abstract This work aims to study the cationic miniemulsion polymerization of styrene catalyzed by iron-containing imidazoliumbased ionic liquids. The polystyrenes had very high number-average molar mass around 1300 kg mol-1 at 85 °C, molarmass dispersity close to 2.0 and glass transition temperature higher than 102 °C with average particle diameter that remained practically unchanged during the reaction, indicating that the monomer droplets correspond to the polymerization locus. First-order kinetics up to a limit conversion, along with the increase in molar mass as the temperature decreases, styrene polymerization at low temperatures and catalyst inability to polymerize monomers that react exclusively via free radical and/or anionic polymerization, indicate the cationic nature of polymerization. 1H-NMR and 13C-NMR spectra suggested the formation of polystyrene, allowing for tacticity distribution quantification: 10% isotactic, 20% atactic and 70% syndiotactic configurations. TEM micrographs confirmed the formation of spherical polymer nanoparticles and the presence of catalysts in the polymer matrix. Keywords: high molar mass, ionic liquid catalysts, cationic polymerization, miniemulsion polymerization, styrene. How to cite: Dutra, G. V. S., Silvério Neto, W., Araújo, P. H. H., Sayer, C., Silveira Neto, B. A., & Machado, F. (2021). Cationic polymerization of styrene using iron-containing ionic liquid catalysts in an aqueous dispersed medium. Polímeros: Ciência Tecnologia, 31(1), e2021005. https://doi.org/10.1590/0104-1428.04620

1. Introduction Polymerizations performed in heterogeneous medium have many advantages, such as high heat removal capacity, low viscosity of the end product, ease of homogenization and manipulation, among others. For these reasons, polymerizations performed in water as a continuous phase are among the most widely used methods for large scale polymer synthesis[1]. However, conventional ionic polymerizations are often performed under anhydrous conditions and traces of water should be avoided as catalysts react with water and become inactive[2]. In recent decades several efforts have been made in order to develop new catalysts very attractive to the ionic polymerization of different monomers in aqueous dispersed medium[3-12]. However, these works use an excessive catalyst load (3 to 10 mol%) and, except for the work of Vasilenko et al. [12] , there was the formation of polymers with low molar masses (less than 40 kg mol-1), similar to that obtained via free radical bulk polymerization. In the literature, there are few reports on cationic polymerization using the miniemulsion technique. Generally, polymerizations occur slowly and at

Polímeros, 31(1), e2021005, 2021

the monomeric/water droplets interface resulting in low molar mass polymers and narrow molar mass distribution[8,9]. In relation to cationic polymerization, recent researches in this area are focused on the development of new catalysts with reusability possibilities, the investigation of new methods using mild experimental conditions, avoiding the use of excess organic solvents, and which allow the more effective control of the final properties of the polymeric materials, such as: molecular structure and molar mass distribution, aiming its use in specific applications. In this direction, it is worth highlighting the controlled cationic polymerizations of styrene in the presence of water[3,13] and the polymerizations of other cationically polymerized monomers in the presence of ionic liquids, such as p-methyl styrene[14], vinyl ether and its derivatives[15,16], and isobutylene[17-19]. We recently described the efficient encapsulation of hexadecane in high molar mass polystyrene nanoparticles obtained through cationic miniemulsion polymerization[20], and the synthesis of several ILs catalysts

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Dutra, G. V. S., Silvério Neto, W., Araújo, P. H. H., Sayer, C., Silveira Neto, B. A., & Machado, F. intended to produce tailored polystyrene through bulk polymerization process[21]. Alves et al.[22] performed cationic miniemulsion polymerization of styrene using 1-butyl-3-methylimidazolium heptachlorodiferrate ionic liquid (BMI⋅Fe2Cl7) as cationic catalyst. Using a low molar ratio of 1:1000 of catalyst/ monomer, the reactions showed colloidal stability and high conversion, 88%, using the temperature of 90 °C and 6 h of reaction. The particle size remained practically unchanged until the end of the polymerization process. The viscosimetric-average molar mass obtained at this temperature range, equal to 2231 kg mol-1, was higher than those usually found for cationic polymerization. Ayat et al.[23] used a modified natural clay initiator for the cationic copolymerization of vinylidene chloride (VDC) and α-methylstyrene (α-MS). The initiator was obtained by treating montmorillonite clay with sulfuric acid and proved to be an efficient, non-toxic, inexpensive, stable, and noncorrosive catalyst for cationic polymerization. Sang et al. [24] synthesized homopolymers derived from vinyl ether and p-substituted styrene by electro-controlled living cationic polymerization. The new method used an organocatalyst, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), which through an electroredox process promoted the oxidation of the chain transfer agent, S-1-isobutoxylethyl S′-ethyl trithiocarbonate to form carbocations. The resulting polymers exhibited narrow and well-defined molar mass distribution and predictability of the functional groups present at the ends of the polymer chain. As a differential, the proposed method allows stopping the polymerization by removing the applied external potential. In view of this, we extend here the use of different types of iron-containing imidazolium-based ionic liquids: 1-butyl3-methylimidazolium heptachlorodiferrate (BMI⋅Fe2Cl7), 1-methyl-3-carboxymethylimidazolium heptachlorodiferrate (MAI⋅Fe2Cl7) and 1,2-Bis(methylimidazolium)ethane bis(heptachlorodiferrate) (bMIE⋅2Fe2Cl7), which efficiently acted as catalysts for styrene bulk polymerization[25,26], expanding its applications as catalysts in a water-phase dispersed polymerization process. The application of these catalysts in this type of polymerization has the following advantages: i) the incorporation of iron into the anionic group of the ILs, which makes them relatively cheaper and more attractive from an economic point of view when compared to the Lewis acids commonly used in cationic polymerization; ii) the use of ILs, the low catalyst/monomer molar ratio (quite low loading) and the absence of traditional organic solvents favor more sustainable processes; iii) synthesis of polystyrene with very high molar mass; iv) and possibility of undergoing cationic polymerization[22].

2. Materials and Methods 2.1 Materials Monomer (styrene, Merck, 99%) was purified before use by washing with a 10% (w/v) aqueous NaOH solution (Vetec, 99%). It was then allowed to stand with anhydrous sodium sulfate (Na2SO4) (Dynamic, 99%) for 24 h, filtered, vacuum distilled and stored in the refrigerator[27]. The reagents 1-methylimidazole (Sigma-Aldrich, 99%), ethyl acetate (Dynamic, 99.5%) and dichloromethane (Dynamic, 99.5%) were previously vacuum distilled. The other 2/14

reagents used in the development of the experimental part were of analytical grade and without previous purification, 1-chlorobutane (99%), chloroacetic acid (99%), iron (III) chloride anhydrous (97%), hexadecyltrimethylammonium bromide (CTAB) (cationic surfactant, 99%) and hexadecane (co-stabilizer, 99%) were purchased from Sigma-Aldrich.

2.2 Synthesis 2.2.1. Synthesis of ionic liquids The 1-butyl-3-methylimidazolium chloride (BMI⋅Cl) was synthesized as described by Dupont et al.[28]. In a twoneck round-bottom flask under an inert atmosphere of N2 was added 1.3 equiv (2.40 mol) of 1-chlorobutane, keeping the system under reflux, magnetic stirring and heating. Slowly, 1.0 equiv (1.85 mol) of 1-methylimidazole was added. The solution was heated at reflux at 80 °C for 48 h. The product was washed with ethyl acetate and a portion of acetonitrile and then dried under reduced pressure at 60 °C and crystallized to form a white solid. Figure S1 (Supporting Information) shows the 1H-NMR spectrum. Yield: 82.1%. 1H-NMR (CDCl3, δ in ppm): 0.95 (3H, t, N(CH2)3CH3), 1.38 (2H, m, N(CH2)2CH2CH3), 1.90 (2H, m, NCH2CH2CH2CH3), 4.12 (3H, s, NCH3), 4.34 (2H, t, NCH2(CH2)2CH3), 7.58 (1H, t, CH3NCHCHN), 7.73 (1H, m, CH3NCHCHN) and 10.32 (1H, s, NCHN). 1-methyl-3-carboxymethylimidazolium chloride (MAI⋅Cl) was synthesized as it follows[29]: In a round bottom flask was added 1.0 equiv (0.20 mol) of 1-methylimidazole, 50 mL of acetonitrile and 1.3 equiv (0.26 mol) of chloroacetic acid. The solution was heated at reflux at 80 °C under magnetic stirring and in an inert atmosphere for 48 h. The solid was washed with ethyl acetate until the filtrate was colorless, and then washed with aliquots of acetonitrile. The obtained white solid was vacuum dried at 80 °C. 1H-NMR and 13C-NMR spectra are shown in Figures S2 and S3, Supporting Information. Yield: 50.4%. 1H-NMR (D2O, δ in ppm): 3.93 (3H, s, NCH3), 5.08 (2H, s, NCH2COOH), 7.48 (2H, m, CH3NCHCHN) and 8.78 (1H, s, NCHN). 13C-NMR (D2O, δ in ppm): 38.84 (NCH3), 53.10 (NCH2COOH), 126.43 (CH3NCHCHN), 140.25 (NCHN) and 173.26 (NCH2COOH). 1,2-Bis(methylimidazolium)ethane dichloride (bMIE⋅2Cl) was obtained as described by Ahrens et al.[30]. In a round bottom flask was added 1.5 equiv (0.3 mol) of 1-methylimidazole, 1.0 equiv (0.2 mol) of 1,2-dichloroethane and 100 mL of acetonitrile. The solution was heated at reflux at 80 °C for 48 h under magnetic stirring and in an inert atmosphere. The solid was washed with ethyl acetate and small portions of acetonitrile and vacuum dried at 80 °C, resulting in a pale yellow solid. NMR spectra are shown in Figures S4 and S5 in the Supporting Information. Yield: 67.8%. 1H-NMR (D2O, δ in ppm): 3.91 (6H, s, NCH3), 4.77 (4H, s, NCH2), 7.45 (2H, d, CH3NCHCHN) and 7.53 (2H, d, CH3NCHCHN). 13C-NMR (D2O, δ in ppm): 38.83 (NCH3), 51.59 (NCH2), 124.97 (CH3NCHCHN), 127.41 (CH3NCHCHN) and 139.46 (NCHN). 2.2.2. Synthesis of iron-containing ionic liquid catalysts Previously synthesized ILs were mixed with anhydrous FeCl3 to form the iron-containing ILs catalysts: 1-butyl3-methylimidazolium heptachlorodiferrate (BMI⋅Fe2Cl7), Polímeros, 31(1), e2021005, 2021


Cationic polymerization of styrene using iron-containing ionic liquid catalysts in an aqueous dispersed medium 1-methyl-3-carboxymethylimidazolium heptachlorodiferrate (MAI⋅Fe2Cl7) and 1,2-Bis(methylimidazolium)ethane bis(heptachlorodiferrate) (bMIE⋅2Fe2Cl7), keeping the following ratio: 2.0 equiv. FeCl3/1.0 equiv. IL-chloride. Initially, each IL was added to a schlenk and then a defined amount of FeCl3 was added. The reactions were kept under heating, magnetic stirring and inert atmosphere, as shown in Scheme 1. The catalysts were not purified, obtaining a quantitative yield, where BMI⋅Fe2Cl7 is a dark liquid and the others are dark solids at room temperature. 2.2.3. Miniemulsion polymerization The polymerizations were carried in a 150 mL jacketed glass reactor, integrated with a thermostatic bath at the desired temperature (85, 70 or 55 °C), remaining for 8 h under constant mechanical stirring of 400 rpm and N2 bubbling. Polymerizations were performed in triplicates, maintaining a catalyst/styrene molar ratio of 1:1000. The formulations used were adapted from Alves et al.[22]. Initially, the aqueous phase, made up of 0.36 g of the cationic surfactant CTAB, and 66 g of deionized water, and the organic phase, 0.90 g of hexadecane and 18 g of styrene, were prepared separately under magnetic stirring at 300 rpm for 20 min and at room temperature. Then the organic phase was added to the aqueous phase, while maintaining magnetic stirring at 800 rpm. After 20 min, the coarse emulsion was miniemulsified using an ultrasonic homogenizer for 1 min in a 70% amplitude ice bath (10 s on/ 5 s off). Subsequently, a solution consisting of catalyst and 6 g of water was prepared, leaving it to stir until complete solubilization of the catalyst. First, the miniemulsion and, later the catalytic solution were transferred, in a single step, to the reactor, kept under stirring for 5 min and N2 bubbling. After the mixing time, the thermostatic bath was integrated with the reactor and a condenser was adapted at one end and the system was kept under N2 bubbling. Aliquots of latex were collected during the reaction. The latex obtained has a light yellow color and the dry polymers are yellow color (Figure S6a, Supporting Information). The dry polymers

were purified by solubilization in dichloromethane and then submitted to extraction with distilled water. The organic phase was allowed to stand with anhydrous Na2SO4 and then filtered and the purified polymer was obtained after evaporation of the solvent (Figure S6b, Supporting Information). 1H-NMR (600 MHz, CDCl3) (Figure S7, Supporting Information) δ (ppm): 0.88 (6H, CH3(CH2)14CH3 of hexadecane), 1.26 (28H, CH3(CH2)14CH3 of hexadecane), 1.30-1.55 (2H, -CH2CH(Ph)-), 1.83 (1H, -CH2CH(Ph)-), 6.30‑7.08 (m, 5H, Ar) - Polystyrene obtained using BMI⋅Fe2Cl7/styrene molar ratio of 1:1000 at 85 °C and 8 h of synthesis; M n = 1266 kg mol-1 and ĐM = 1.88.

2.3 Material Characterization The mass-average molar mass ( M w ) and the numberaverage molar mass ( M n ) and the dispersity (ĐM) of the polymers were determined using a gel permeation chromatograph (Malvern, model Viscotek RImax) equipped with a refractive index detector with a set of three columns of 300 x 8 mm mounted in series (KF-802.5, KF-804L and KF-805L) operating at 40 °C. The system was calibrated using polystyrene standards with molar mass ranging from 1.20 kg mol-1 to 4500 kg mol-1 and monodisperse (ĐM close to 1.0). Tetrahydrofuran solvent (THF), HPLC grade, was used as the mobile phase with a flow rate of 1 mL⋅min-1. Prior to injection, previously prepared solutions (1.5 mg sample/1.0 mL THF) were filtered through hydrophobic polytetrafluoroethylene (PTFE) membranes with 0.45 μm pore size and the injection volume was 100 μL. The differential scanning calorimetry (DSC) curves were obtained using a Shimadzu model DSC-60 equipment. The initial masses used were approximately 5.0 mg and aluminum crucibles were used. The measurements were made under a helium atmosphere at a flow rate of 30 mL min−1, with heating rate of 10 °C min−1 and two heating ramps (−40 to 180 °C). The second heating cycle was used to determine the TG of the polymers. Nuclear magnetic resonance (1H-NMR and 13C-NMR) spectra were obtained using a Bruker 600 Ascend spectrometer,

Scheme 1. Schematic representation of synthesis route of iron-containing ILs catalysts. Polímeros, 31(1), e2021005, 2021

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Dutra, G. V. S., Silvério Neto, W., Araújo, P. H. H., Sayer, C., Silveira Neto, B. A., & Machado, F. equipped with a 5 mm probe operating at 600 MHz. About 20 mg of the samples were dissolved in 0.5 mL of deuterated solvent and the spectra were acquired expressing chemical shifts in parts per million (ppm) and tetramethylsilane (TMS) was used as internal standard. The average sizes of the monomer droplets (initial Dp) and latex polymer particles (final Dp) and their respective polydispersity indexes (PdI) were determined using the dynamic light scattering technique (DLS) using a Malvern equipament, Zetasizer Nano ZS model. Measurements were made by direct dispersion of a drop in approximately 2.5 mL of water. Transmission electron micrographs (TEM) were obtained using a JEOL JEM-2100 microscope operating at 200 kV. The samples were initially dispersed in distilled water and sonicated for 15 min. Then a drop of the suspension was applied to a 200 mesh copper containing a supported carbon film and dried at room temperature. The conversions were obtained by gravimetric data. At predetermined time intervals, aliquots of the reaction medium, about 1.0 g, were removed and added to preweighed aluminum foil capsules treated with 5 drops of NaOH solution (4 g L-1)[8]. The capsules were dried in an oven at 60 °C for approximately 48 h. The conversion was obtained by the ratio of the dry polymer mass to the monomer mass used. The polymer mass was obtained by subtracting the added NaOH fraction and the non-polymer species (initiator, surfactant and stabilizer).

3. Results and Discussion The polymerizations were performed in accordance with the experimental procedure previously described and

aliquots were collected throughout the polymerization process in order to evaluate performance of iron-containing ILs catalysts incorporated in their anionic structure, BMI⋅Fe2Cl7, MAI⋅Fe2Cl7 and bMIE⋅2Fe2Cl7, and the effect of the reaction temperature on final polymer properties, nanoparticle size and conversion. Table 1 shows the conversion and diameter values of monomer droplets (initial Dp) and polymeric particles (final Dp), including polydispersity index (PdI), corresponding to the mean and standard deviation of the essays performed in triplicate, and the values obtained for each synthesis are presented in Table S1 in the Supporting Information. The polymerizations were carried out in triplicate in order to evaluate the reproducibility of the polymerization process and minimize experimental scattering due to gravimetry measurements. Figure 1 shows the evolution of conversion, mean particle diameter and the semilogarithmic plot of monomer concentration for the different synthesis conditions. The high conversion values indicate that all ILs catalysts were efficiently capable of producing polystyrene. The highest conversions, generally under the same experimental conditions, were achieved using the bMIE⋅2Fe2Cl7 catalyst (comparing Entries 2, 5 and 8 and Entries 4, 7 and 10 of Table 1). This behavior was not observed at 70 °C, as Entries 3 and 6 of Table 1 showed the largest standard deviations. This difference in reactivity of the catalysts is due to the higher proportion of the anionic species Fe2Cl7 in the bMIE⋅2Fe2Cl7 than in the others, since according to Rodrigues et al.[31] these species are responsible for initiating styrene polymerization. In addition, these polymerizations showed a limit conversion of 80 to 90%, related to monomer loss due to nitrogen drag in the bubbling system, besides the occurrence of the glass effect which is associated with the

Table 1. Conversion, average diameter of the monomer droplets (initial Dp) and polymeric particles (final Dp) and polydispersity index for polymerizations after 8 h of reaction. Entry 1a 2 3 4 5 6 7

Catalyst Blank BMI⋅Fe2Cl7

8b 9 10 11b, c 12b, d 13b, d 14a, d 15b, e, f 16b, e, g 17b, e, h 18b, e, i 19b, e, j 20b, e, k

bMIE⋅2Fe2Cl7

MAI⋅Fe2Cl7

BMI⋅Fe2Cl7 Blank BMI⋅Fe2Cl7

T (°C) 85 85 70 55 85 70 55

Conversion (%) 29.1 ± 10.2 80.8 ± 4.0 23.3 ± 8.2 7.0 ± 1.9 77.3 ± 2.5 21.8 ± 7.4 7.7 ± 1.4

Initial Dp (nm) 138.4 ± 0.99 138.6 ± 10.3 162.3 ± 19.5 141.1 ± 11.0 175.2 ± 20.4 157.1 ± 11.1 168.0 ± 3.0

Initial PdI 0.135 ± 0.042 0.086 ± 0.007 0.075 ± 0.020 0.109 ± 0.025 0.039 ± 0.019 0.098 ± 0.037 0.063 ± 0.022

Final Dp (nm) 172.6 ± 11.6 168.5 ± 7.3 152.9 ± 17.4 147.3 ± 10.2 187.9 ± 9.48 135.9 ± 19.8 160.0 ± 3.2

Final PdI 0.039 ± 0.051 0.030 ± 0.012 0.099 ±0.029 0.077 ± 0.039 0.009 ± 0.006 0.094 ± 0.015 0.090 ± 0.034

85 70 55 85 60 50 60 70 70 70 70 30 0

90.8 21.1 ± 4.8 14.9 ± 1.6 63.9 71.6 33.2 17.9 ± 4.0 0.4 1.7 0.0 71 68.0 9.9

136.5 159.1 ± 18.2 171.2 ± 2.9 138.5 137.5 144.5 147.4 ± 5.0 — — — — — —

0.145 0.102 ± 0.037 0.076 ± 0.014 0.099 0.037 0.093 0.121 ± 0.001 — — — — — —

136.7 158.7 ± 7.6 158.8 ± 1.41 190.3 182.2 162.7 166.5 ± 3.6 — — — — — —

0.037 0.095 ± 0.022 0.104 ± 0.038 0.021 0.057 0.017 0.023 ± 0.016 — — — — — —

Duplicate syntheses and b only 1 experimental run. c Synthesis using unpurified styrene. d 24 h of reaction. e Bulk polymerization - experimental conditions: BMI⋅Fe2Cl7/monomer molar ratio of 1:1000 and 6 h of synthesis, using f methyl methacrylate, g vinyl pivalate, h butyl acrylate and styrene at i 15 min[31], j 3 h and k 6 h of reactions. a

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Polímeros, 31(1), e2021005, 2021


Cationic polymerization of styrene using iron-containing ionic liquid catalysts in an aqueous dispersed medium

Figure 1. Evolution of (a) conversion; (b) mean diameter of latex particles during reactions at 85 and 70 °C and; (c) dependence of semilogarithmic of the monomer concentration with synthesis time at 85 °C.

increase of viscosity inside the monomer droplets, leading to a reduction of the reaction rates[32]. The polymerizations were performed at different temperatures of 55, 70 and 85 °C with catalyst/monomer molar ratio of 1:1000. For BMI⋅Fe2Cl7, the conversions increased from 7% to 81% (Table 1 and Figure 1a) when synthesis temperatures were increased from 55 to 85 °C. Similar behavior was observed for MAI⋅Fe2Cl7 with an increase from 8% to 77%, whereas for bMIE⋅2Fe2Cl7, it was observed an increase in the conversion from 15% to 90%. This is because catalytic species become more reactive at higher temperatures[33]. In order to evaluate if competitive thermal polymerization via radical initiation is favored by the reaction temperature (85 °C), a blank reaction was performed under the same experimental conditions, but without the addition of ILs or any other initiator (Entry 1, Table 1). This blank reaction presented a much lower conversion, about 30%. In addition, average molar masses are relatively different from those obtained by using ILs catalysts. Also, under the same experimental conditions, polymerization was carried out in the presence of BMI⋅Fe2Cl7 and radical polymerization inhibitors, as the synthesis was performed with unpurified styrene containing approximately 50 ppm of 4-tert-butylcatechol as stabilizer (Entry 11, Table 1). This reaction presented a relatively high conversion, about 64% and molar masses very close Polímeros, 31(1), e2021005, 2021

to those obtained using BMI⋅Fe2Cl7 with purified styrene. Although these reactions were conducted at relatively high temperatures, which might favor the self-initiation of styrene, the polymerizations carried out in presence of ILs catalysts are favorably governed by the cationic route due to the extremely high reactivity of the ILs catalysts. The effect of temperature was also investigated at 24 h of reaction, and the results can be observed in Entries 12-14 of Table 1. Again, a low monomer conversion is observed for the blank reactions, around 18% at 60 °C (Entry 14, Table 1). On the other hand, the polymerization carried out in the presence of BMI⋅Fe2Cl7 (Entry 12, Table 1) showed conversion of 72% and even when reactions where conducted at lower temperature of 50 °C, polystyrene is formed, with conversion of 33% (Entry 13, Table 1) and molar masses quite different from that obtained in the blank reaction. In addition, the decrease in temperature favored the formation of polystyrene with higher number-average molar mass and with narrower dispersity. As a matter of fact, the above-mentioned results suggest that cationic polymerization is undoubtedly taking place. Additionally, experiments were carried out to verify the ability of other vinyl monomers [methyl methacrylate (MMA), vinyl pivalate (VPi) and butyl acrylate (BuA)] to polymerize via bulk polymerization using BMI⋅Fe2Cl7 at 70 °C (Entries 15-17, Table 1). It is well-known that these 5/14


Dutra, G. V. S., Silvério Neto, W., Araújo, P. H. H., Sayer, C., Silveira Neto, B. A., & Machado, F. monomers cannot be polymerized in accordance with the cationic mechanism due to the instability of the formed carbocation and, therefore, the absence of polymerization gives more evidence that the reaction mechanism is purely cationic. Figure S8 (Supporting Information) presents the 1 H-NMR spectrum of the experiment in which VPi was added (Entry 16, Table 1) and the absence of a signal at 1.7 ppm, attributed to methylene (–CH2–) protons in the polymer backbone[34], indicating that poly(vinyl pivalate) was not synthesized. In comparison, experiment using only styrene (Entry 18, Table 1), led to a rapid polymerization, forming a highly viscous solution in only 15 min, which is highly unlikely to happen through self-initiated thermal polymerization of styrene. Polymerization also succeeded at lower temperatures, as for instance, i) 30 °C (Entry 19, Table 1), occurring more slowly and achieving similar conversion after 3 h and ii) 0 °C, achieving a 9.9% conversion in 6 h of reaction (Entry 20, Table 1), which clearly indicates that the polymerization of styrene mediated by this kind of catalyst follows predominantly a cationic mechanism. These experimental results agree very well with the one described by Alves and coauthors[22], in which replacing ILs catalysts with benzoyl peroxide, conversion around 30% has been achieved, after 5 h, under the same operating conditions. One should also bear in mind that in the particular case of the ILs catalysts used here, styrene homopolymerization takes place due to the formation of a key styrene chloronium cation, which is believed to be stabilized by the catalyst through ion-pairing effects[31]. Bulk polymerizations (Entry 18, Table 1)[31] achieved limit conversions in much shorter reaction times than miniemulsion polymerizations. This difference can be attributed to the high water content of the miniemulsion system, leading to a reduced reaction rate and thus decreasing conversion. Generally, one of the main obstacles of cationic polymerization has been the high reaction rates that make polymerization control difficult. Thus, the water effect resulted in lower rates, allowing better control of polymerization[22]. It was observed that the mean particle diameter (Dp) remained practically unchanged, as shown in Figure 1b and Table S1 of the Supporting Information, that is, the average diameter of the polymeric particles (final Dp) shows values close to the monomer droplets (initial Dp), indicating that nucleation occurs preferentially within the monomeric droplets, and that both effects of coalescence and diffusional degradation were properly minimized during polymerization. Thus, the polymeric latexes were obtained with high colloidal stability and particle diameter between 113 nm and 197 nm, presenting low polydispersity indexes (PdI) (Table S1, Supporting Information), indicating the formation of polymer nanoparticles with narrow size distribution. The semilogarithmic plot of monomer concentration versus synthesis time using the ILs catalysts shown in Figure 1c, indicates that polymerization does not follow first-order kinetics throughout the reaction. Kostjuk et al.[35] observed the same behavior when studying cationic polymerization of styrene using AlCl3-based initiators. The authors explained that this curvature (similar to the one depicted in 6/14

Figure 1c indicates that the concentration of propagating and reversibly terminated chains decreases more rapidly than the formation of new chains by slow initiation. Theoretically, for a truly living polymerization system, this plot should be linear. Thus, a more detailed analysis shows a linear dependence of the semilogarithmic plot up to a certain polymerization period, indicating that the reactions follow a first-order kinetic up to a limit conversion of about 51% (BMI⋅Fe2Cl7), 73% (MAI⋅Fe2Cl7) and 77% (bMIE⋅2Fe2Cl7). Semilogarithmic linear dependence has been reported up to a 51% conversion[36] and a 23% conversion[37] in the cationic polymerization of α-methylstyrene using FeCl3based initiators and in the reaction of a peptide-poly(methyl methacrylate) (peptide-PMMA) hybrid bioconjugates by atom transfer radical polymerization (ATRP), respectively. Therefore, polymerization systems using ILs catalysts are possibly cationic in nature, following a first-order kinetics to a limit conversion. Figure 2 shows the molar mass distribution curves of the polystyrenes obtained as a function of conversion using the different ILs catalysts at 85 °C, these values are shown in Table S2 in the Supporting Information. It was possible to synthesize polystyrene with high number-average molar mass, around 1300 kg mol-1, much higher than those obtained by Cauvin et al.[8,11] and Touchard et al.[9] who studied the cationic miniemulsion polymerization of p-methoxystyrene, obtaining M n between 1.0-39.4 kg mol-1. These results are indicative that the polymerization mechanism occurs preferably inside monomer droplets rather than at the monomer droplets/water interface, commonly obtained in miniemulsion ionic polymerizations[5,8,9,11]. Table S2 in the Supporting Information shows the average molar masses and ĐM of the polystyrenes obtained in bulk polymerization (shown in Entries 18-20, Table 1). These polymers had number-average molar mass, M n around 15 kg mol-1, much lower than those achieved by miniemulsion polymerization. These differences can be attributed to the characteristics of the two synthesis processes, since in the bulk polymerization there is an increase in viscosity as the conversion increases, causing the interruption of the homogenization of the system and delaying the mobility of the monomer to the growing chains. In addition, there is a much higher concentration of reactive species at the bulk polymerization locus, resulting in lower molar masses[22]. The values of the average molar masses of polystyrene obtained after 8 h of synthesis showed significant differences when using different ILs catalysts, and the polymers obtained in the presence of bMIE⋅2Fe2Cl7 showed the lowest numberaverage molar mass. This difference is attributed to the higher concentration of the catalytic species, Fe2Cl7, in the polymerization locus. As a result, the initiation step is favored, resulting in the formation of several polymer chains with smaller molar mass (less than 800 kg mol-1) and with different sizes, consequently, increasing the molar-mass dispersity (higher than 2.1). The samples obtained with the other catalysts, BMI⋅Fe2Cl7 and MAI⋅Fe2Cl7, presented similar number-average molar mass and molar-mass dispersity, since they have relatively equal concentrations of the catalytic species. These results support the assumption that cationic polymerization takes place, since the increase in the concentration of catalytic species, using bMIE⋅2Fe2Cl7, led a Polímeros, 31(1), e2021005, 2021


Cationic polymerization of styrene using iron-containing ionic liquid catalysts in an aqueous dispersed medium

Figure 2. Molar mass distribution curves of the polystyrenes synthesized by using ILs catalysts at 85 °C (a) BMI⋅Fe2Cl7; (b) MAI⋅Fe2Cl7; (c) bMIE⋅2Fe2Cl7.

decrease in the molar masses. Rodrigues et al.[31] had already observed a reduction in the average molar masses with the increase in the concentration of the catalyst BMI⋅Fe2Cl7 in the styrene bulk polymerization. The molar mass distribution curves of the obtained polymers, Figure 2, are unimodal and a slight displacement of the curves is observed for higher average molar masses values as the conversion increases. Nevertheless, under these conditions, molar-mass dispersity remained relatively high (ĐM ≅ 1.9) throughout the reaction, suggesting that polymerization is not as well controlled as in other studies[38]. However, the samples synthesized here have narrower distribution compared to other studies that exemplified the formation of polymer with high molar mass. The molar mass distribution curves of the blank reaction performed at 85 °C (Entry 1, Table 1), and of the polymerizations in the presence of BMI⋅Fe2Cl7 at 85 °C with purified (Entry 2, Table 1) and unpurified styrene (Entry 11, Table 1) are shown in Figure 3a. The values of average molar masses and ĐM are shown in Table S2 (Supporting Information). The blank reaction showed lower molar masses than those obtained using ILs catalysts and the profile of the molar mass distribution curves are relatively different. On the other hand, the molar mass distribution, as well as, the average molar masses using BMI⋅Fe2Cl7 with purified and non-purified styrene are very similar. Polímeros, 31(1), e2021005, 2021

Figure 3b shows the molar mass distribution curves of reactions performed at lower temperatures and 24 h of reaction, including polymerizations in the presence of BMI⋅Fe2Cl7 at 60 °C (Entry 12, Table 1) and 50 °C (Entry 13, Table 1) and the blank reaction at 60 °C (Entry 14, Table 1). Table S2 (Supporting Information) shows the values of average molar masses and ĐM. Again, the blank reaction showed lower molar masses than the one observed for the reaction conducted in the presence of the catalyst BMI⋅Fe2Cl7 and the decrease in the synthesis temperature caused a slight increase in the number-average molar mass and a decrease in the molar-mass dispersity. This behavior is characteristic of cationic polymerization. Figure 4 shows the molar mass distribution of the polystyrenes obtained at different synthesis temperatures. It is noticed that higher average molar masses and narrow dispersions were obtained as temperature is decreased (from M n = 1266 kg mol–1 and ĐM = 1.88 at 85 °C to M n = 1940 kg mol–1 and ĐM = 1.73 at 55° C using BMI⋅Fe2Cl7 catalyst; see Figure 3a). This behavior is typical of cationic polymerization, since they are better controlled at low temperatures, which favors the propagation reactions over the termination reactions, leading to the formation of polymers with higher average molar masses[33]. The observed values for the molar-mass dispersity close to 2 is characteristic of cationic chain polymerizations where 7/14


Dutra, G. V. S., Silvério Neto, W., Araújo, P. H. H., Sayer, C., Silveira Neto, B. A., & Machado, F.

Figure 3. Molar mass distribution curves of the polystyrenes obtained by (a) blank reaction and using BMI⋅Fe2Cl7 in the presence of purified and unpurified styrene at 85 °C and (b) reactions conducted at 60 and 50 °C for 24 h.

Figure 4. Molar mass distribution curves of polystyrene as a function of the synthesis temperature line and symbol black (85 °C) and red (55 °C) using catalysts (a) BMI⋅Fe2Cl7 and (B) bMIE⋅2Fe2Cl7.

termination and transfer reactions are highly expected to take place, mainly when the reaction achieves elevated conversions. Based on this reaction behavior, it is strongly expected to produce polystyrenes exhibiting broad molar mass distributions with ĐM relatively higher than 2, reflecting the effect of the reaction operation conditions. As an additional effect, the increase of the local viscosity may also contribute to the occurrence of transfer reactions, accounting for the reduction of the reaction rates, as a result of the decreased migration of the monomer molecules to the active species. The molar-mass dispersity, in all syntheses, remained higher than 1.8 throughout polymerization. ĐM values obtained here were relatively larger than those reported by Touchard et al.[9], Satoh et al.[3], Kostjuk & Ganachaud[6] and Biedrón & Kubisa[39] who polymerized p-methoxystyrene by miniemulsion, p-alkoxystyrenes by emulsion, and styrene by suspension and solution polymerization, respectively, and obtained ĐM of less than 1.5. However, the molar mass obtained in these studies ( M n ≤ 4.5 kg mol-1) were much lower than those reported here. There are reports in the literature of styrene-derived polymers with higher number-average 8/14

molar mass, 40[11], 117[12], and 550 kg mol-1[40], in which, the distributions are relatively high, from 2.0 to 3.8, because it is very difficult to control these polymerization mechanism as large polymer chains are formed. These results are very promising because the other authors who synthesized polystyrene or its derivatives via cationic polymerization in water-based systems (miniemulsion, emulsion or suspension) generally obtained low molar mass and used high synthesis times and excessive catalyst concentration and/or activators[3,6,7,11]. For example, Satoh et al.[3] performed cationic emulsion polymerization of p-methoxystyrene (pMOS) using pMOS–HCl adduct/lanthanide triflates initiation system. The authors used [pMOS] = 3.0 M; [pMOS–HCl] = 60 mM; [Yb(OTf)3] = 300 mM and obtained a conversion of 78% after 50 h of synthesis at 30 °C and achieved M n = 2.38 kg mol-1 and ĐM = 1.38. Cauvin et al.[11] used [pMOS] = 1.5 M; [pentachlorophenol (PCP)] = 30 mM; [Yb(OTf)3] = 150 mM and obtained conversion of 67% in miniemulsion after 400 h at 60 °C and reached M n = 39.4 kg mol-1 and ĐM = 3.8. More recently, Zhang et al.[10] obtained conversion of 51% and M n = 3.2 kg mol-1 in cationic suspension polymerization Polímeros, 31(1), e2021005, 2021


Cationic polymerization of styrene using iron-containing ionic liquid catalysts in an aqueous dispersed medium of styrene initiated by cumyl alcohol (CumOH)/B(C6F5)3, using the following experimental conditions: [St] = 1.75 M; [CumOH] = 0.05 M; [B(C6F5)3] = 0.05 M at 20 °C and 50 h of synthesis. Therefore, our process enables cost savings, leading to reagent savings, low catalyst concentration, while also avoiding the use of rare earth catalysts such as ytterbium triftalates, and relatively shorter synthesis time. In order to verify the living nature of these polymerizations, a monomer feeding was performed in the miniemulsion previously prepared with BMI⋅Fe2Cl7 at 85 °C (Xp = 83.0% Table S1, Supporting Information, M n = 989 kg mol-1 and ĐM = 1.65). In this experiment 1% (w/w) styrene was added to the total, and the reaction was kept under magnetic stirring at 800 rpm at 70 °C for 4 h. The molar mass distribution curves are shown in Figure 5. This styrene feeding caused a system destabilization, being observed the formation of two phases, a dark clot in less quantity (Xp = 84.7%) and a stable latex phase (Xp = 82.5%). The molar mass distribution curve of the clot exhibited a broad signal, with marked displacement towards higher molar masses, showing M n = 1163 kg mol-1 and ĐM = 1.82. According to Banerjee et al.[41] the increase in molar mass distribution after monomer feeding is due to the slow initiation and slow exchange between reversibly terminated and propagating species. Regarding the obtained stable latex phase (Figure 5c), a change in the distribution curves was observed, leading to a reduction in the average molar mass and a narrow distribution, indicating that the polymeric chains present in dispersion were more monodisperse. This behavior and experimental data from Figure 1c may suggest a certain livingness of the reactive

Figure 5. Molar mass distribution curves of polystyrene from the monomer feeding experiment (A) before feeding and after feeding: (B) clot and (C) stable latex phase.

species, however the styrene polymerization with the IL catalyst evaluated here cannot be considered as a living polymerization in its strict sense, due to destabilization of cationic propagating species at the reaction temperature. The glass transition temperature (Tg) values of the purified and unpurified polymers were determined through the DSC curves and are presented in Table 2. In this work, Tg values of unpurified polymers were determined in the range from 87 °C to 95 °C, depending on the type of catalyst used. Samples with higher molar mass presented higher Tg values, due to reduced mobility of polymeric chains. The blank reaction (performed in the absence of catalyst) showed a Tg value close to the unpurified polymer, around 96 °C. On the other hand, the Tg values of purified polymers were determined in the range from 102 °C to 108 °C, depending on the amount of hexadecane present in the polymeric materials. Tg values obtained for the purified polymers are similar to the polystyrene samples reported in the literature (107 ± 2 °C)[42,43] that presented high average molar mass. The increase in Tg values above 15 °C after purification of the polymers is due to the hexadecane removal occurred during the drying process with dichloromethane and cannot be associated to very small amount of ILs catalysts used in the polymerizations. The evidences supporting this hypothesis are: i) very low molar concentration of catalysts, around 0.4%; ii) blank reaction (without the addition of ILs) with low Tg, around 96 °C; and iii) reduction of the hexadecane content in the polymeric structure calculated by relative integration method of the spectrum of 1H-NMR[44] (Figure S9 of the Supporting Information). The co-stabilizer, hexadecane, used to prevent Ostwald ripening, may act as a plasticizer due to its extremely low volatility, remaining in the polymeric structure, causing a reduction in intermolecular forces between chains, increasing molecular mobility and consequently reducing Tg. Recently, polystyrene with Tg = 90.1 °C was obtained by miniemulsion polymerization containing 4 wt.% of hexadecane[45]. Christie et al.[46] observed the same effect using glycerol suspended polystyrene films and Shen et al. [47] reported that hexadecane dispersed in the polyacrylate matrix caused a significant reduction in Tg, decreasing from 6.38 °C (emulsion) to 3.92 °C (miniemulsion). In addition, the plasticizing effect of residual catalyst can be ruled out, since the molar ratio used is very low in relation to the polymeric fraction obtained and in a previous study low Tg values (up to 88 °C with M n = 183 kg mol-1) were obtained after the polystyrene purification[31]. The formation of branched polystyrene can also be phased out, because these polymers have Tg and molar masses much lower than those obtained here[39-42] [48-51] and the formation of branching favors a significant increase of M w and molar-

Table 2. Average molar masses, molar-mass dispersity and glass transition temperature (g) of the polymers synthesized at 85 °C using ILs catalysts and blank polymerization. Entry

Catalyst

T (°C)

M w (kg mol-1)

M n (kg mol-1)

ĐM

Tg (°C)a

Tg (°C)b

HD (%)c

1 2 3 4

Blank BMI⋅Fe2Cl7 MAI⋅Fe2Cl7 bMIE⋅2Fe2Cl7

85 85 85 85

1821 2375 2526 1749

891 1266 1306 770

2.04 1.88 1.93 2.27

95.8 93.6 94.5 87.7

108.1 102.8 106.6

1.6 3.1 2.0

a

before and b after purification. c hexadecane content after purification of the polymers, calculated by the relative integration method.

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Dutra, G. V. S., Silvério Neto, W., Araújo, P. H. H., Sayer, C., Silveira Neto, B. A., & Machado, F. mass dispersity with an increase in conversion[48,51], which did not occur in our work (Table S2, Supporting Information). In addition, the polystyrene tacticity after purification was determined by integrating the signals between 144147 ppm of the 13C-NMR spectra (Figure S10, Supporting Information). On the basis of isotactic (mm), atactic (mr) and syndiotactic (rr) triads assignments, syndiotactic polystyrenes were obtained, consisting essentially of around 10% isotactic, 20% atactic and 70% syndiotactic configurations. The micrographs (Figure 6) and the size distribution histograms (Figure 7) show the formation of particles of nanometer size, relatively uniform in size and shape, being spherical and having an average size of 119.6 ± 23.3 nm (BMI⋅Fe2Cl7), 153.5 ± 37.3 nm (MAI⋅Fe2Cl7) and 116.7 ± 15.2 nm (bMIE⋅2Fe2Cl7). The average size estimated

by TEM is slightly smaller than the hydrodynamic size obtained by DLS, approximately 30 nm smaller (Entries 2, 5 and 8 of Table 1). This difference is explained by the fact that the techniques are performed under completely different conditions. The DLS measures the Brownian motion of aqueous dispersion and relates them to the diameter of the particles, which may be affected by the swelling of the polymeric shell, causing an increase in the average particle size. While TEM analysis the sample is dried on a film and exposed to electron beam, probably promoting shrinking phenomena[52]. In addition, the average particle size computed based on TEM images refers to the number-average, whereas DLS determines the intensity average particle size that provides a higher weight to the bigger particles. It is also observed, in all micrographs, the presence of some smaller nanoparticles, with undefined shapes and

Figure 6. TEM micrographs of unpurified polystyrene samples synthesized with catalysts (a) BMI⋅Fe2Cl7; (b) MAI⋅Fe2Cl7 and (c) bMIE⋅2Fe2Cl7 at 85 °C; (d) presence of bMIE⋅2Fe2Cl7 catalyst and (e) crystalline plane observed in the sample with BMI⋅Fe2Cl7. 10/14

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Cationic polymerization of styrene using iron-containing ionic liquid catalysts in an aqueous dispersed medium

Figure 7. Size distribution histograms of polystyrene synthesized with ILs catalysts at 85 °C obtained from TEM analysis.

sizes below 10 nm, being also present inside the spherical polystyrene particles (Figure 6d). The presence of crystalline planes (Figure 6e) indicates that ILs catalysts are contained in the polymeric matrix. This corroborates that polymerization occurs within the monomer droplets and with the reduction in Tg previously reported.

4. Conclusions The use of iron-containing ILs catalysts has been shown to be very effective in styrene miniemulsion polymerization, achieving high conversions even at low catalyst concentrations. The largest conversions were obtained using the bMIE⋅2Fe2Cl7 catalyst and the largest molar masses were achieved in BMI⋅Fe2Cl7 and MAI⋅Fe2Cl7. The size of the polymeric nanoparticles remained practically unchanged during the reactions and high number-average molar masses were obtained (around 1300 kg mol-1) with molar-mass dispersity of 2.0, indicating that the polymerization mechanism occurred preferentially within the monomer droplets. The cationic polymerization behavior was confirmed by the reduction of the average molar masses with increased concentration of the catalytic species, Fe2Cl7, and with increasing temperature, by the first-order kinetics until a limit conversion, successful monomer feeding evaluation, obtaining polystyrene at low temperatures and inability to polymerize methyl methacrylate and other monomers not able to be polymerized through a cationic mechanism. Finally, these results are very promising and future work may focus on the application of these catalysts in cationic polymerization in miniemulsion of other vinyl monomers or via ring opening, as well as studying the encapsulation of different materials inside polystyrene nanoparticles.

5. Acknowledgements This work has been supported by CNPq, CAPES – Finance code 001, FINEP, FAPDF and PROCAD – Process nº 88881.068432/2014-01. The authors thank the Laboratório Multiusuário de Microscopia de Alta Resolução (LabMic) for the TEM micrographs.

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Supplementary Material Supplementary material accompanies this paper. Figure S1. 1H-NMR spectrum of BMI⋅Cl (CDCl3, 600 MHz). Figure S2. 1H-NMR spectrum of MAI⋅Cl (D2O, 600 MHz). Figure S3. 13C-NMR spectrum of MAI⋅Cl (D2O, 600 MHz). Figure S4. 1H-NMR spectrum of bMIE⋅2Cl (D2O, 600 MHz). Figure S5. 13C-NMR spectrum of bMIE⋅2Cl (D2O, 600 MHz). Figure S6. Photograph of the synthesized polymers using the catalysts BMI⋅Fe2Cl7, MAI⋅Fe2Cl7 and bMIE⋅2Fe2Cl7 (a) before and (b) after purification. Figure S7. 1H-NMR spectrum of unpurified polystyrene sample synthesized using BMI⋅Fe2Cl7 catalyst at 85 °C (CDCl3, 600 MHz). Table S1. Conversion, average diameter of the monomer droplets (initial Dp) and polymeric particles (final Dp) and polydispersity index for all polymerizations after 8 h of reaction. Figure S8. 1H-NMR spectrum of the vinyl pivalate polymerization test in the presence of BMI⋅Fe2Cl7 (CDCl3, 600 MHz). Table S2. Average molar masses and molar-mass dispersity as a function of conversion (Xp) of the polymers synthesized at 85 °C using ILs catalysts. Figure S9. 1H-NMR spectra of purified polystyrene samples synthesized using (a) BMI⋅Fe2Cl7; (b) MAI⋅Fe2Cl7 and (c) bMIE⋅2Fe2Cl7 at 85 °C (CDCl3, 600 MHz). Figure S10. 13C-NMR spectra of purified polystyrene samples synthesized using (a) BMI⋅Fe2Cl7; (b) MAI⋅Fe2Cl7 and (c) bMIE⋅2Fe2Cl7 at 85 °C (CDCl3, 600 MHz). This material is available as part of the online article from http://www.scielo.br/po

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ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.09720

Potential of calcium carbonate as secondary filler in eggshell powder filled recycled polystyrene composites Nabil Hayeemasae1*  and Hanafi Ismail2  1

Department of Rubber Technology and Polymer Science, Faculty of Science and Technology, Prince of Songkla University, Pattani Campus, Pattani, Thailand 2 School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, Nibong Tebal, Penang, Malaysia *nabil.h@psu.ac.th

Abstract Recycling of plastic waste is considered a key intention in regards to the continuous growth of plastic industry. In this work, new composite based on recycled polystyrene (R-PS) was prepared in the presence of eggshell powder (ESP). This was to make a value-added plastic material based on polystyrene. To further extend its performance, calcium carbonate (CaCO3) was used as secondary filler to optimize its performance. It is observed that the stabilization torque of composites decreased with increasing the weight percent of CaCO3. Increasing the amount of CaCO3 has increased the mechanical properties such as Young’s modulus, tensile strength, elongation at break and impact strength of the composites. These findings corresponded well to SEM images. It revealed homogenous dispersions of CaCO3 throughout R-PS matrix in comparison to ESP alone which formed agglomerations in R-PS matrix. Further evidence on thermal stability has confirmed that CaCO3 provided better heat resistance over the ESP. Keywords: polystyrene, calcium carbonate, eggshell powder, composites. How to cite: Hayeemasae, N., & Ismail, H. (2021). Potential of Calcium Carbonate as Secondary Filler in Eggshell Powder Filled Recycled Polystyrene Composites. Polímeros: Ciência e Tecnologia, 31(1), e2021006. https://doi. org/10.1590/0104-1428.09720

1. Introduction Polystyrene is one of the most widely used plastics in many applications i.e., food packaging, home appliance, consumer goods and so forth. Polystyrene is similar to other types of plastic which is non-degradable after use. The abundant of these products has brought to several environmental issues. Therefore, special attention is devoted to recycling, reuse, and making them biodegradable by various approaches[1,2]. The first approach is considered to be most possible way to reduce the discarded polystyrene. To date, the recycling of polystyrene mainly includes mechanical, thermal and chemical methods. The mechanical recycling is one among the methods that has a cost-effective and good environmental option. This technique deals with grinding the polystyrene waste and reprocesses into a new raw material. However, the products obtained from this process still possesses low mechanical properties due to degradation aspect[3-5]. Hence, recycled polystyrene with remarkable performance was to develop to overcome this situation while prevailing sustainability. Introducing fillers to the recycled plastics is another route to increase some properties to the composite. There are two main types of fillers available in the plastic industry which are reinforcing and non-reinforcing fillers. Reinforcing fillers are used to improve the strength and abrasion resistance to the composites[6]. However, for some cases where cost

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and processability are essential, the use of non-reinforcing fillers are suggested. Calcium carbonate (CaCO3) is a good example of non-reinforcing fillers that has been widely used in plastics. For instance, polypropylene[7], polyethylene[8], polystyrene[9] and so forth. It can be said that CaCO3 is still a filler of choice in plastic industry. However, the advancement of plastic technology introduces a new era in the environmental and industrial field of green research. Therefore, searching for an alternative source of filler that is chemically similar to CaCO3 is challenging in the near future. Traditionally, most eggshell waste is discarded to landfills without further processing. For economic aspects, eggshell waste can be used to convert biomaterials into commercial products and creates new values from these waste materials. It is known that eggshell waste contains lots of valuable organic and inorganic components which can be used in many applications. The chemical composition (by weight) of eggshell has been reported to consist mainly of a mineral part (95%) of calcite crystals and a pervading organic matrix (1–3.5% of the remaining material)[10-13]. Thus, it can be considered a good source of CaCO3. Since CaCO3 is by far an important and the most widely used filler because of its whiteness, low abrasion, availability in wide-size ranges and low cost. Consequently, If the eggshell is prepared properly, it is not only a filler added to reduce costs but today it is

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Hayeemasae, N., & Ismail, H. the material engineering for the different requirements of modern products Despite replacing the CaCO3, the point of application of chicken eggshell waste was to ensure that hybrid fillers based on these two fillers would be able to provide synergistic properties. To date, there has been a limited report on the properties of ESP/CaCO3 hybrid filled R-PS composites. Due to the above reasons, the effect of ESP/CaCO3 hybrid fillers on the properties of R-PS composites is thoroughly investigated.

2. Materials and Methods 2.1 Materials The materials used for compounding the composites is listed in Table 1. All the materials were used as received except for the eggshell powder (ESP) which was ground until desired particle size.

2.2 Preparation of Eggshell Powder (ESP) The collected chicken eggshell waste was cleaned, washed and dried prior to proceed to the grinding step. The mechanical grinding supplied by Rong Tsong Precision Technology Co. Ltd. was used in the grinding process. The obtained powder form of ESP was sieved using a sieve No. 200 where the size ranging from 30 – 150 μm was used for compounding.

2.3 Preparation of Composites The formulation used for preparing ESP/CaCO3 hybrid filled R-PS composites is listed in Table 2. ESP and CaCO3 were dried in an oven at 105 °C for 2 h before mixing. All the compounding ingredients were melt-mixed in a Thermo HAAKE PolyDrive internal mixer with co-rotating blades. The compounding was carried out at 180 °C for a total of 8 min with rotor speed of 60 rpm. First, R-PS was charged and ESP and/or CaCO3 was added after 4 min of mixing. The mixing was continued and left for another 4 min. The mixing torques were recorded until dumping out from the internal mixer. Lastly, resultant compounds were weighed at approximately 30 g and placed inside the mold prior to undergo the compression-molding process using a hydraulic press from GoTech Testing Machines Model KT-7014 A. The compounds were preheated first for 6 min, followed by

compression molding for 2 min at a temperature of 180°C. The samples were then cooled under pressure for 3 min at ambient temperature.

2.4 Measurement of mechanical properties Tensile tests were performed using a Universal Testing Machine (UTM) Instron Model 3366 according to the ASTM D638. The samples were pulled at a crosshead speed of 1 mm/min with a constant gauge length of 100 mm. The results of the Young’s modulus, tensile strength and elongation at break were discussed. As for the impact test, a sample with the dimension of 64 x 12.7 x 3.2 mm was prepared for un-notched Izod impact test. It was carried out using a Zwick Impact Tester Model 5101 according to ASTM D256.

2.5 Scanning electron microscopy The morphological observation was carried out using a scanning electron microscope (SEM) model Zeiss Supra35VP. The samples from the tensile and impact fractures were used to captured the image. It was coated with a layer of gold to prevent an electrostatic charge formation prior to scanning.

2.6 Thermogravimetric Analysis Thermogravimetric analysis of the composites were investigated by a Perkin-Elmer Pyris 6 TGA Analyzer. The sample was heated from 25 °C to 600 °C with a heating rate of 10 °C/min under a nitrogen atmosphere.

3. Results and Discussions 3.1 Mixing and Stabilization Torque Figure 1 shows the processing torque of the R-PS composites at various weight percent of ESP/CaCO3 hybrid fillers. At initial stage of mixing, the processing torque increased rapidly due to the shearing action from the solid R-PS pellets. Afterwards, the processing torque gradually reduced, indicating a decrease of viscosity as R-PS pellets melt by continuous mechanical shearing and high processing temperature. Then processing torque increase again after 4 min of mixing. This caused by the addition of the ESP and CaCO3 into molten R-PS. The melt viscosity increased in the presence of fillers, which reduced the polymer chain mobility in the blend and consequently raised the processing

Table 1. List of materials used in this study. Materials Recycled Polystyrene (R-PS)

Role Polymer matrix

Eggshell powder (ESP) Calcium Carbonate (CaCO3)

Filler Filler

Grade/Trade name rePS-5, the melt flow index of R-PS is 6.29 g/10 min (200˚C/5 Kg) Waste with range of size 30 – 150 µm Ultra-fine precipitated CaCO3/ MICROMAC UFC with range of size 1 – 10 µm

Supplier Total Petrochemicals Sdn. Bhd., Selangor, Malaysia Local Restaurant in Penang, Malaysia Macri Chemicals s.r.l., Milan, Italy

Table 2. Composition and mixing sequence of neat R-PS and ESP/CaCO3 hybrid filled R-PS composites. Materials Neat R-PS RPS/ESP/CaCO3

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Composition (wt%) 100 80/20/0, 80/15/5, 80/10/10, 80/5/15, 80/0/20

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Potential of calcium carbonate as secondary filler in eggshell powder filled recycled polystyrene composites torque value[14]. The processing torque decreased gradually and the stabilization torque achieved after the composite in homogenized mixture. The similar trend on processing torque was also observed from other literatures with different composites[15,16]. Figure 2 shows the stabilization torque of the R-PS composites at various weight percent of ESP/CaCO3 hybrid fillers. The results clearly indicate that stabilization torque of composites is decreasing with the increasing the weight percent of CaCO3. Such a decrease in stabilization torque could be due to the fine CaCO3 particles that possesses high dispersibility as compared to ESP. Therefore, CaCO3 was well-dispersed in molten R-PS, resulting lower viscosity found in the composites containing CaCO3. The coarser ESP forms a large network and agglomerates, which required high shear force to disperse the ESP in molten R-PS. Thus, high stabilization torque is observed in the composite based on the ESP alone.

3.2 Mechanical Properties Figure 3 shows the tensile strength of the R-PS composites at various weight percent of ESP/CaCO3 hybrid fillers. From the result obtained, the tensile strength gradually increased upon increasing the weight percent of CaCO3. The highest tensile strength was observed for the composite containing 20 wt% of CaCO3. The above results indicate that the

Figure 1. Processing torque as a function of mixing time of ESP/ CaCO3 hybrid filled R-PS composites.

Figure 2. Stabilization torque of ESP/CaCO3 hybrid filled R-PS composites (Red dash-line is for stabilization torque of neat R-PS). Polímeros, 31(1), e2021006, 2021

use of CaCO3 led to improve the tensile strength of the composites. This is simply related to the well dispersion of CaCO3 throughout R-PS matrix as compared to ESP which is shown later in SEM images. Strong adhesion between CaCO3 and polymer interface can cause better stress transfer from one to each other. This has led to a higher tensile strength. Toro et al.[17], reported that slightly lower tensile strength of the composite containing solely ESP compared to CaCO3 counterpart can be attributed to its particle size. ESP is normally coaser and bigger size compared with the CaCO3. It is easily agglomerated each other, causing to lower the tensile strength eventually. CaCO3 used in this composite is considered ultra-fine precipitated grade (see Table 1) which has smaller particle size as compared to ESP. The smaller particle size of the filler could produce a maximal interface contact with polymer matrix because of larger specific surface area[18,19]. This will improve the wettability of CaCO3 by the R-PS matrix. As a result, a high tensile strength is obtained in composites containing more CaCO3. The location and behaviour of CaCO3 throughout the R-PS matrix can be proved in the SEM micrographs of tensile fractured surfaces in the next section. Elongation at break of the R-PS composites at various weight percent of ESP/CaCO3 hybrid fillers is also shown in Figure 3. It can be seen that the elongation at break was increased with the increasing the weight percent of CaCO3. According to literature[17], the increment in elongation at break of the composite can be attributed to the formation of stronger filler aggregates. In this case, it is CaCO3 which is not similar to ESP. ESP is rather found in agglomerated form. The CaCO3 aggregate has better interfacial adhesions with R-PS matrix, resulting in efficient stress transfer from the matrix to filler, thus increasing the elongation at break. The lower flexibility found for the composite containing more ESP can be attributed to extensive ESP agglomeration as mentioned earlier, which transformed into multiple stress concentration sites. Therefore, the crack propagation across the composite could occur on a large scale, prohibiting the composite from dissipating stress in the form of matrix deformations. Figure 4 shows the Young’s modulus of R-PS composites at various weight percent of ESP/CaCO3 hybrid fillers. Young’s modulus of the composites increased as a

Figure 3. Tensile strength and elongation at break of ESP/CaCO3 hybrid filled R-PS composites (Red and green dash-lines are for the tensile strength and elongation at break of neat R-PS). 3/7


Hayeemasae, N., & Ismail, H. function of weight percent of CaCO3 due to the increase in the composite’s stiffness. According to Fu et al.[20], Young’s modulus is known to be less sensitive variation of interfacial adhesion than the tensile strength which is strongly associated with interfacial failure behaviour. This is due to the fact that Young’s modulus is measured before any significant plastic deformation takes place. Increase in tensile modulus of some composite samples is attributed

Figure 4. Young’s Modulus of ESP/CaCO3 hybrid filled R-PS composites (Red dash-line is the Young’s Modulus of neat R-PS).

to better distribution of small particle size of CaCO3 in the matrix. A smaller particle size of the CaCO3 could produce a maximal interface contact because of larger specific surface area. The higher surface area of filler gives better adhesion in composite, thus forming a stiffer material which is attributed to higher modulus. Therefore, small particles and homogeneous distribution are the main contribution to originate a more rigid structure. As for the impact strength (see Figure 5), similar trend was observed. There was an enhancement in impact strength with the addition of CaCO3 to the hybrid composite. This behaviour is similar to a result reported by Bashir et al.[21], considerable improvement in the impact strength of the composite containing CaCO3 as compared to ESP is mainly due to a well-dispersed CaCO3. This has brought to an increase in the interfacial adhesions between filler and polymer matrix. It is then required higher impact force to overcome the strong interfacial adhesions between CaCO3 within the R-PS matrix. The lower impact strength of the ESP filled composite is simply due to the effect posed by agglomerations of ESP which may act as stress concentration points or points of discontinuity in composites thereby promoting crack initiation and propagation.

3.3 Morphology Figure 6A presents the SEM micrograph of the CaCO3. The shape of the CaCO3 was irregular with various particle sizes. The average particle size of CaCO3 is ranging from 1 – 10 µm. It can be seen that there is smaller than the ESP which is in range of 30 – 150 µm (see Figure 6B). This has shown that CaCO3 possesses high possibility to disperse throughout the matrix when compared to ESP.

Figure 5. Impact strength of ESP/CaCO3 hybrid filled R-PS (Red dash-line is the impact strength of neat R-PS).

Figure 7 represents the SEM images obtained from tensile fractured surfaces of ESP/CaCO3 hybrid filled R-PS composites. Some agglomeration is seen in the composite containing ESP alone (see Figure 7A) and its size was much bigger than the CaCO3 found in Figures 7B and 7C. The agglomeration of ESP indicates poor dispersion of ESP throughout the R-PS matrix. The agglomerates and bigger particle of ESP may act as stress concentration points or points of discontinuity in composites thereby inhibiting stress

Figure 6. SEM images of raw CaCO3 and ESP. 4/7

Polímeros, 31(1), e2021006, 2021


Potential of calcium carbonate as secondary filler in eggshell powder filled recycled polystyrene composites

Figure 7. Tensile fractured surfaces of ESP/CaCO3 hybrid filled R-PS composites at 300× magnification. 80/20/0 wt% (A), 82/10/10 wt% (B) and 80/0/20 wt% of R-PS/ESP/CaCO3 respectively.

Figure 8. Impact fractured surfaces of ESP/CaCO3 hybrid filled R-PS composites.at 300× magnification. 80/20/0 wt% (A), 82/10/10 wt% (B) and 80/0/20 wt% of R-PS/ESP/CaCO3 respectively.

transfer and reduced the tensile strength of ESP composite. Furthermore, the surface was found to be rougher especially at higher weight ratio of CaCO3, this reflects less brittle deformation and higher energy absorption is generated. Therefore, a higher tensile strength is obtained in composite containing more CaCO3. The SEM results for the tensile fractured surfaces were in good agreement with the results obtained by Siriwardena et al.[22], who reported that an increase in energy was responsible for the roughness and the matrix tearing line of the fractured surface. Figure 8 demonstrates the SEM images obtained from impact fractured surfaces of ESP/CaCO3 hybrid filled R-PS composites. Figure 8A shows the SEM image of the R-PS composite containing 20 wt% of ESP. It exhibits a rough surface with bigger particle of ESP, indicating its poor dispersion in R-PS matrix. This may have brought to a stress concentration points or points of discontinuity in composites thereby promoting crack initiation and propagation. Thus the impact strength of composites is reduced. The image also displays several occurrences of voids due to pullingout of the ESP. Comparing with the composites containing higher weight percent of CaCO3 (see Figures 8B and 8C), it exhibited less void formation, better dispersion and adhesion of CaCO3 throughout the R-PS matrix. Such remarkable interfacial adhesions of CaCO3 to the R-PS matrix allowed impact force to be absorbed and transferred more uniformly throughout the composite system, thus providing higher impact strength compared to the composite with ESP alone.

3.4 Thermogravimetric Analysis Thermogravimetric analysis or TGA is a method to determine the rate of the weight changes as a function of Polímeros, 31(1), e2021006, 2021

temperature at a controlled atmosphere. TGA is primarily used to analyse the compositions and thermal stability of materials[23]. The thermograms (TG) and derivative thermograms (DTG) of ESP/CaCO3 hybrid filled R-PS composites are shown in Figure 9. The decomposition temperature at 50% (T50%) mass loss, maximum mass loss (Tmax) and char residue are also listed in Table 3. From the results obtained, there is only one region of the thermogram observed. The degradation started at the temperature around 350oC and was then completed at about 450oC. The decomposition was mainly due to the degradation of polystyrene segment where it corresponds to the major peak observed off the DTG curve (see the DTG peaks embedded in Figure 9). To compare the results, the temperature for 50% mass loss was determined as degradation temperature of the composite. The R-PS composites with 20 wt% of CaCO3 had a highest degradation temperature of T50% (425.93 ˚C) as compared to the other composite. This is simply due to the presence of well-dispersed CaCO3 in R-PS matrix, which is expected to provide a barrier to the diffusion of degradation products, suppressing R-PS mass loss in the composite. The balance set of thermal stability is served for the composite with 10wt% of ESP and CaCO3 respectively. This is interesting to highlight that the incorporation of ESP with the assistance of CaCO3 can promote the resistance to heat of R-PS matrix. In addition to that, lowest degradation temperature of T50% (424.11 ˚C) found in the composite with 20 wt% of ESP may be due to poor dispersion of ESP in R-PS matrix, arising from the formation of agglomerated ESP which reduced thermal stability of the composites. The amount of residue of R-PS composites with 20 wt% of CaCO3 is highest (approximately 20.79%) among the 5/7


Hayeemasae, N., & Ismail, H. Table 3. Degradation temperatures and weight residue of ESP/CaCO3 hybrid filled R-PS composites. Sample Designation

20 wt% of ESP 10/10 wt% of ESP/ CaCO3 20 wt% of CaCO3

T50% (˚C)

Tmax/DTG Peak (˚C)

Residue (%)

424.03 424.11 425.93

422.57 423.48 424.11

18.33 19.49 20.79

Figure 9. TG and DTG curves of ESP/CaCO3 hybrid filled R-PS composites.

other composite. This residual value of composite is higher than its filler content because the R-PS matrix may contain thermally stable impurities. R-PS composite with 20 wt% of ESP has lower residue content (roughly 19.49%) than 20 wt% of CaCO3 composite. This is attributed to the loss of organic composition during or after testing.

4. Conclusions The processability, mechanical, morphological, and thermal properties of ESP/CaCO3 hybrid filled R-PS composites were studied with respect to different weight percent of ESP/CaCO3 hybrid fillers. It is found that the stabilization torque of composites was decreased with the increased of the amount of CaCO3. Increasing CaCO3 content has increased tensile strength, elongation at break, Young’s modulus, and impact strength of the composites. The mechanical properties obtained clearly corresponded to SEM images observed. Further evidence on the thermal stability has confirmed that CaCO3 provided better heat resistance over the ESP alone. Even the CaCO3 seemed to play important role in the properties of ESP/CaCO3 hybrid filled R-PS composites. However, the hybridization of ESP and CaCO3 does show balance set of properties especially the mechanical and thermal properties of ESP/CaCO3 hybrid filled R-PS composites. It can be concluded that the ESP hybridized CaCO3 is applicable to prepare the composite based on the R-PS where the ratio of ESP and CaCO3 at 10/10 (wt%) is highly suggested.

5. References 1. Pimentel, T. A. P. F., Durães, J. A., Drummond, A. L., Schlemmer, D., Falcão, R., & Sales, M. J. A. (2007). Preparation and characterization of blends of recycled polystyrene with cassava starch. Journal of Materials Science, 42(17), 7530-7536. http:// dx.doi.org/10.1007/s10853-007-1622-x. 6/7

2. Gutiérrez, C., García, M. T., Gracia, I., de Lucas, A., & Rodríguez, J. F. (2012). Recycling of extruded polystyrene wastes by dissolution and supercritical CO2 technology. Journal of Material Cycles and Waste Management, 14, 308316. http://dx.doi.org/10.1007/s10163-012-0074-9. 3. Borsoi, C., Scienza, L. C., & Zattera, A. J. (2013). Characterization of composites based on recycled expanded polystyrene reinforced with curaua fibers. Journal of Applied Polymer Science, 128(1), 653-659. http://dx.doi.org/10.1002/app.38236. 4. Lisperguer, J., Bustos, X., & Saravia, Y. (2011). Thermal and mechanical properties of wood flour-polystyrene blends from postconsumer plastic waste. Journal of Applied Polymer Science, 119(1), 443-451. http://dx.doi.org/10.1002/app.32638. 5. Poletto, M., Dettenborn, J., Zeni, M., & Zattera, A. J. (2011). Characterization of composites based on expanded polystyrene wastes and wood flour. Waste Management (New York, N.Y.), 31(4), 779-784. http://dx.doi.org/10.1016/j.wasman.2010.10.027. PMid:21172732. 6. Kourki, H., Famili, M. H. N., Mortezaei, M., & Malekipirbazari, M. (2018). Mixing challenges for SiO2/polystyrene nanocomposites. Journal of Thermoplastic Composite Materials, 31(5), 709-726. http://dx.doi.org/10.1177/0892705717718599. 7. Chan, C.-M., Wu, J., Li, J.-X., & Cheung, Y.-K. (2002). Polypropylene/calcium carbonate nanocomposites. Polymer, 43(10), 2981-2992. http://dx.doi.org/10.1016/S00323861(02)00120-9. 8. Bartczak, Z., Argon, A. S., Cohen, R. E., & Weinberg, M. (1999). Toughness mechanism in semi-crystalline polymer blends: II. High-density polyethylene toughened with calcium carbonate filler particles. Polymer, 40(9), 2347-2365. http:// dx.doi.org/10.1016/S0032-3861(98)00444-3. 9. Suetsugu, Y., & White, J. L. (1983). The influence of particle size and surface coating of calcium carbonate on the rheological properties of its suspensions in molten polystyrene. Journal of Applied Polymer Science, 28(4), 1481-1501. http://dx.doi. org/10.1002/app.1983.070280421. 10. Ghabeer, T., Dweiri, R., & Al-Khateeb, S. (2013). Thermal and mechanical characterization of polypropylene/eggshell biocomposites. Journal of Reinforced Plastics and Composites, 32(6), 402-409. http://dx.doi.org/10.1177/0731684412470015. 11. Sutapun, W., Pakdeechote, P., Suppakarn, N., & Ruksakulpiwat, Y. (2013). Application of Calcined Eggshell Powder as Functional Filler for High Density Polyethylene. PolymerPlastics Technology and Engineering, 52(10), 1025-1033. http://dx.doi.org/10.1080/03602559.2013.769578. 12. Toro, P., Quijada, R., Arias, J. L., & Yazdani‐Pedram, M. (2007). Mechanical and morphological studies of poly(propylene)filled eggshell composites. Macromolecular Materials and Engineering, 292(9), 1027-1034. http://dx.doi.org/10.1002/ mame.200700147. 13. Feng, Y., Ashok, B., Madhukar, K., Zhang, J., Zhang, J., Reddy, K. O., & Rajulu, A. V. (2014). Preparation and Characterization of Polypropylene Carbonate Bio-Filler (Eggshell Powder) Composite Films. International Journal of Polymer Analysis and Characterization, 19(7), 637-647. http://dx.doi.org/10.1 080/1023666X.2014.953747. 14. Halimatudahliana, A., Ismail, H., & Nasir, M. (2002). Morphological studies of uncompatibilized and compatibilized Polímeros, 31(1), e2021006, 2021


Potential of calcium carbonate as secondary filler in eggshell powder filled recycled polystyrene composites polystyrene/polypropylene blend. Polymer Testing, 21(3), 263-267. http://dx.doi.org/10.1016/S0142-9418(01)00079-4. 15. Gallagher, L. W., & McDonald, A. G. (2013). The effect of micron sized wood fibers in wood plastic composites. Maderas. Ciencia y Tecnología, 15(ahead), 357-374. http:// dx.doi.org/10.4067/S0718-221X2013005000028. 16. Sarifuddin, N., & Ismail, H. (2013). Comparative study on the effect of Bentonite or Feldspar Filled Low-Density Polyethylene/Thermoplastic Sago Starch/Kenaf Core Fiber Composites. BioResources, 8(3), 4238-4257. http://dx.doi. org/10.15376/biores.8.3.4238-4257. 17. Toro, P., Quijada, R., Yazdani-Pedram, M., & Arias, J. L. (2007). Eggshell, a new bio-filler for polypropylene composites. Materials Letters, 61(22), 4347-4350. http://dx.doi.org/10.1016/j. matlet.2007.01.102. 18. Tanaka, H., & White, J. L. (1980). Experimental investigations of shear and elongational flow properties of polystyrene melts reinforced with calcium carbonate, titanium dioxide, and carbon black. Polymer Engineering and Science, 20(14), 949-956. http://dx.doi.org/10.1002/pen.760201406. 19. Ismail, H., Awang, M., & Hazizan, M. A. (2006). Effect of waste tire dust (WTD) size on the mechanical and morphological properties of polypropylene/waste tire dust (PP/WTD) blends. Polymer-Plastics Technology and Engineering, 45(4), 463-468. http://dx.doi.org/10.1080/03602550600553739.

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20. Fu, S. Y., Feng, X. Q., Lauke, B., & Mai, Y.-W. (2008). Effects of particle size, particle/matrix interface adhesion and particle loading on mechanical properties of particulate-polymer composites. Composites. Part B, Engineering, 39(6), 933-961. http://dx.doi.org/10.1016/j.compositesb.2008.01.002. 21. Bashir, A. S. M., & Manusamy, Y. (2015). Recent Developments in Biocomposites Reinforced with Natural Biofillers from Food Waste. Polymer-Plastics Technology and Engineering, 54(1), 87-99. http://dx.doi.org/10.1080/03602559.2014.9354 19. 22. Siriwardena, S., Ismail, H., & Ishiaku, U. S. (2000). Effect of mixing sequence in the preparation of white rice husk ash filled polypropylene/ethylene-propylene-diene monomer blend. Polymer Testing, 20(1), 105-113. http://dx.doi.org/10.1016/ S0142-9418(00)00008-8. 23. Nabil, H., & Ismail, H. (2014). Enhancing the thermal stability of natural rubber/recycled ethylene-propylene-diene rubber blends by means of introducing pre-vulcanised ethylenepropylene-diene rubber and electron beam irradiation. Materials & Design, 56, 1057-1067. http://dx.doi.org/10.1016/j. matdes.2013.12.020. Received: Nov. 01, 2020 Revised: Jan. 19, 2021 Accepted: Feb. 24, 2021

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ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.09519

New biodegradable composites from starch and fibers of the babassu coconut Carla Veronica Rodarte de Moura1* , Douglas da Cruz Sousa1 , Edmilson Miranda de Moura1 , Eugênio Celso Emérito de Araújo2  and Ilza Maria Sittolin2  Departamento de Química, Universidade Federal do Piauí – UFPI, Teresina, PI, Brasil Empresa Brasileira de Pesquisa Agropecuária – EMBRAPA Meio-Norte, Teresina, PI, Brasil 1

2

*carla@ufpi.edu.br

Abstract This work aimed to obtain thermoplastic starch composites (TPS) derived from starch and fibers of babassu coconut. The (TPS) was prepared with 40% plasticizer (glycerol). The fibers underwent chemical treatment of alkalinization and bleaching. SEM images and infrared spectra showed that wax, lignin, and hemicellulose were removed from the fiber surface. SEM images of TPS starch showed a smooth and uniform surface, whereas images of the TPSWF composite (washed fiber) showed voids between the fiber and the TPS. This phenomenon was not observed in the SEM images of the composites TPSAF (alkalized fiber) and TPSBF (bleached fiber). The tensile strength and elastic modulus of the composites were higher than the pure TPS matrix. Concerning elongation, composites underwent less elongation than TPS. The mechanical properties found for the TPSWF and TPSAF composites do not differ. However, the mechanical properties of the TPSBF composite were better than the properties of the other composites. Keywords: TPS, Babassu, composites, fiber, starch. How cite: Moura, C. V. R., Sousa, D. C., Moura, E. M., Araújo, E. C. E., & Sittolin, I. M. (2021). New biodegradable composites from starch and fibers of the babassu coconut. Polímeros: Ciência e Tecnologia, 31(1), e2021007. https:// doi.org/10.1590/0104-1428.09519

1. Introduction Plastics are very versatile, malleable, lightweight, and low-cost polymeric materials that confer numerous advantages over other materials in several applications. However, due to environmental issues with the pollution caused by these materials, there is an urgent need to reduce long-lasting plastics, especially in disposable items, which have increased in recent years. This environmental issue has led to a global interest in replacing petroleum-derived polymers, which are not biodegradable, with biodegradable polymers derived from renewable sources[1,2]. Among the various possibilities of using biodegradable materials, starch was widely used, as it is abundant in nature, and the cost is low. This biopolymer is produced by many plants and stored in the cells as an energy source. Also, agricultural products can be considered a way to reduce environmental pollution and consolidate such products for other purposes[3]. Starch is formed by glucose units, which occur naturally, have hydroxyl froups and are composed of partially crystalline microscopic granules. The polysaccharide chains that form the starch are interconnected through strong inter and intramolecular hydrogen bonds. These strong bonds hinder the movement of the polymer bonds resulting in low plasticity. A solution to improve the processability of starch is the addition of plasticizing agents such as glycerol sorbitol, xylitol, ethylene glycol, and water, heating and shear stress so that it becomes a workable plastic

Polímeros, 31(1), e2021007, 2021

material, called thermoplastic starch (TPS)[4,5]. Among the plasticizers mentioned, glycerol is the classic plasticizer for starch because it forms compounds wich are colorless, transparent, odorless, and non-toxic properties[6]. Thermoplastic starch (TPS) has properties that allow its processing using extrusion, injection, and compression molding[7]. However, TPS shows weak resistance to water absorption, i.e., it is hydrophilic and has low mechanical properties compared to conventional synthetic polymers, and these are limiting factors for its industrial application[8,9]. Some studies described in the literature show that TPS films need modifications to improve the mechanical properties and their low water absorption and, at the same time, not altering their biodegradability[10-12]. Different strategies to improve such weaknesses can be employed, for example, by mixing TPS with another polymeric material as well as by adding fillers[13,14]. The literature shows that adding loads such as natural fibers, a more environmentally friendly material, would be a correct way to correct the minimize problems presented by TPS. However, it is necessary to promote the adhesion of the fiber to the TPS matrix. Authors have shown that if adhesion is not adequate, the mechanical properties tend to be reduced compared to the original product. Therefore, the challenge is to improve this adhesion and consequently obtain improvements in the generated composite properties[13,15].

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O O O O O O O O O O O O O O O O


Moura, C. V. R., Sousa, D. C., Moura, E. M., Araújo, E. C. E., & Sittolin, I. M. Natural fibers have been used to reinforce polymer matrices to manufacture environmentally friendlily composites. These fibers have excellent mechanical performance, low density, and are easy to handle due to their non-abrasive nature. Also, they are renewable, biodegradable, and inexpensive[16-18]. Composites made with natural fiber and TPS are used for various industrial purposes, including automotive industry, civil construction, products for sports use, as surfboards and fishing rods[19,20]. Natural fibers are mainly composed of lignin, hemicellulose, and cellulose. Such structures do not have good adhesion with the structure of the TPS and can affect the composites’ process of preparation. Besides, natural fibers are hygroscopic and contain waxes in their composition that can hinder adhesion with TPS. Therefore, modifying the starch, using a plasticizer, and pre-treating the fibers with a chemical agent can result in composites with modified mechanical properties[21]. Several studies report the effects of lignocellulosic fibers on starch films from different sources, such as rice[22], cassava[23], peas[24] and corn[25]. Brazil has enormous potential for lignocellulosic fiber production, such as the Babassu (Orbignya speciosa) biomass. Babaçu is a palm tree found naturally in the North, Northeast, and Midwest regions of Brazil. The Babassu coconut has four parts: epicarp, mesocarp, endocarp, and almonds. Edible oil is extracted from the almonds. The other three components can be used to produce alcohol, fertilizer, charcoal, flour, and animal feed. The mesocarp is incorporated as a ground meal that serves as food for humans and animals[26]. Depending on the origin, between 60 and 70% of the flour is starch and is exploited to produce thermoplastic starch. Therefore, this work aimed to improve thermoplastic starch’s mechanical properties (TPS), reinforced with babassu coconut fibers. The effects of chemically treated fibers on the resulting composites, thermal, and spectroscopic properties were evaluated.

2. Materials and Methods The Babassu starch (BS) and epicarp fibers were kindly provided by Embrapa Meio Norte (Piauí, Brazil). Glycerol and sodium hydroxide were purchased from Vetec. Sodium chloride was purchased from Sigma-Aldrich, and acetic acid was purchased from Synth. All reagents came with the analytical grade and were used without further purification.

2.1 Processing of the Babassu Starch (BS) and Fibers 100 g of babassu starch (BS) was immersed in distilled water (1 L) and kept under mechanical stirring for 30 min. This system then has settled overnight, and the supernatant was removed by siphoning. The BS obtained was dried in an oven for 12 h at 60 °C, ground, and sifted in a 53 μm (270 mesh) sieve. The Babassu fibers were washed with tap water, ground in a knife mill, and dried in an oven at 90 °C for 12 h. The dyed fibers were immersed in NaOH (4%) solution and mechanically stirred for two hours. Then, they have been dried in an oven for 12 h at 90 °C. The washed fibers were immersed in a bleaching solution composed of equal parts of acetate buffer solution (2.7 g of sodium 2/11

hydroxide and 7.5 ml of glacial acetic acid in each 100 mL of solution) and aqueous sodium chloride solution (1.7% w/v). They were kept in this solution and mechanically agitated for four h at 70 °C[14]. They have been then oven dried for 12 h at 90 °C. The fibers thus obtained fibers were labelled, respectively, as washed fibers (WF), alkaline fibers (AF), and bleached fibers (BF).

2.2 Synthesis of Thermoplastic Starch (TPS) and Composites 12.00 g of glycerol was dissolved in 50 mL of water and then 30.00 g of BS (corresponding to 60 BS and 40% glycerol) were slowly added, and the system was placed under mechanical stirring for 5 min. Stirring was sustained, and the mixture was heated to 100 °C for 8 min to complete gelatinization. The gel formed was deposited on a glass plate, and the water evaporated in an oven at 60 °C[27]. The TPS was obtained (molded) by thermopressure, using a hydraulic press under a load of 3 t at 130 ºC for 30 min. The composites have been obtained using 90% of TPS and 10% of fibers (washed, alkalized, or bleached). The quantity of TPS and fibers was mixed with 50 mL of water and placed under mechanical stirring for 5 min, and then it was heated to 100 ºC for 8 min. The composites have been dried in an oven at 60 °C for 4 h.[14]. The dried composites were wrought by thermopressure, using a hydraulic press under a load of 3 T at 130 ºC for 30 min. The TPS and composites were designated as the following: Thermoplastic starch matrix (TPS), composites with washed fibers (TPSWF), composites with alkaline fibers (TPSAF), and composites with bleached fibers (TPSBF).

2.3 Characterization Moisture and ash contents were analyzed by the methodology described by AOAC Nº 925.09 and AOAC Nº 923.03, respectively. The AOAC Nº 991.15 methodology mesasures protein content; a nitrogen conversion factor of 5.75 was applied. The lipids measurements have been carried out following the procedure described by AOAC Nº 963.15. Food Fiber was measured by AOCS Nº 985.29. The amylose content was analyzed by the colorimetric iodine method, proposed by Perez and Juliano[28]. The TPS and the composites’ moisture absorption tests were carried out following the guidelines of ASTM E104-02. Three specimens have been tested for each sample. The percentage of water absorbed was calculated using Equation: WA%= [Mt-M0/M0]x100. M0 = initial mass of the test specimen, Mt = mass after a particular exposure time. Compression molded TPS and composites were laser cut as ASTM D638 type I samples. The tensile tests were performed on a Shimadzu AG-X 250 KN mechanical test machine following ASTM D638. FTIR analyzes were performed on a Perkin Elmer spectrometer, model Spectrum 100, the tablets being pressed in KBr, and the spectra obtained with 32 scans in the range of 4000 to 400 cm-1. TG and DSC analyses were performed using a Shimadzu thermogravimetric analyzer, model TGA-60, and DSC-60. The samples were packed in Platinum samples, and the experiments have been carried out under nitrogen atmosphere, with a flow rate of 50 mL min-1. The temperature Polímeros, 31(1), e2021007, 2021


New biodegradable composites from starch and fibers of the babassu coconut was raised to 600 °C (TG) and 500 °C (DSC), with a heating rate of 10 °C min-1. X-ray diffraction analyzes were performed on a PANalytical Empyrean X-ray diffractometer, using CoKα radiation with 40 kV/40 mA. The scans were made in the range of 5º- 60º at a speed of 2º min-1. A scanning electron microscope (FEI Quanta FEG 250) was used to obtain SEM images. The samples were placed on carbon tape and covered with a thin layer of gold in a Q150R ES quorum metallizer.

2.4 Statistical analysis The chemical analysis of babassu starch and the composites’ mechanical properties were analyzed using Analysis of Variance (ANOVA), using the STATISTICA® software (Version 10.0, StatSoft Inc., Tulsa, USA). All analyzes were performed in triplicate, and the results were expressed in terms of mean and standard deviation.

3. Results and Discussions 3.1 Characterization of babassu starch and fibers The data from the centesimal analysis for BS are presented in Table 1. The chemical composition results found in our study were not different from those found in the literature. Except for humidity and purity, which Maniglia et al.[29] found (15.1%) and Ferreira et al.[30] found 66.5%, respectively. In our work, we found 6.35% humidity and 94% purity. This

difference in humidity and purity may be due to drying or storing the material after processing. The FTIR of BS (Figure 1A) presented bands attributed to OH groups at 3600 to 3100 cm-1. The CH2 and CH3 stretching bands appeared from 2910 to 2850 cm-1. At 1639 cm-1 corresponds to δ(O-H) of the hydroxyl groups in the cellulose structure. The band in the 1245 cm-1 region is related to the OH group’s flexural vibration of the glucose units. The bands range from 1161 to 1079 cm-1 have been attributed to α1-4 C-O-C linkage elongation, and 1026 and 1006 cm-1 have been attributed to ν-C-O-H. For the fibers (Figure 1B), the most significant differences are at 1742 cm-1, ν(C=O) for carboxylic esters, at 1650 cm-1, ν(C-OH) for alcohol groups, and 1249 cm-1, (ν-C-O-C) also due to the presence of lignin and hemicellulose[31-35]. Those band can be seen in the IR spectra of WF and AF fibers. The bands around 1464 cm-1 refer to ν(C=C) of aromatic rings, aromatic vibrations, aromatic ring deformation, and –CHO out of plane vibrations. At 1172 and 895 cm-1, bands related to δ(C-O-C) of polysaccharides and (β1-4) linkage of glucose ring[35,36]. In 1042 cm-1, a band attributed to the Si-O bond stretching was verified. The treatments applied to the fibers in this work were not enough to remove the fibers’ silica. Campos et al. also observed this fact regarding composites made using palm oil mesocarp fibers and cassava starch TPS[37]. The SEM images of BS, WF, AF, and BF are shown in Figure 2. The starch granules’ morphology is related to their botanical origin and how they were isolated or processed.[38,13]

Table 1. Chemical Composition of Babassu Starch (BS). Parameter Moisture content* (g/100 g of total material) Ash (g/100 g of dry material) Lipids (g/100 g of dry material) Protein (g/100 g of dry material) Food Fiber (g/100 g of dry material) Amylose (g/100 g of starch) Starch (g/100 g of dry material)

(%) 6.35 ± 0.14 0.31 ± 0.01 0.72 ± 0.03 1.02 ± 0.06 4.25 ± 0.27 36.51 ± 1.02 93.70 ± 0.98

Literature[29] 15.1 ± 1.6 1.1 ± 0.1 1.8 ± 0.4 1.4 ± 0.1 3.7 ± 0.2 36.6 ± 0.5 92.0 ± 0.4

Literature[30] 5.0 ± 0.1 0.30 ± 0.0 1.2 ± 0.1 8.6 ± 0.1 66.5 ± 0.2

*Expressed on moisture basis, Starch (obtained by difference) = 100 – (Ash (%) + Lipids (%) + protein (%) + Food Fiber (%).

Figure 1. FTIR spectra of the BS (A), and WF, AF, and BF (B). Polímeros, 31(1), e2021007, 2021

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Moura, C. V. R., Sousa, D. C., Moura, E. M., Araújo, E. C. E., & Sittolin, I. M. The surface of the granules was predominantly smooth, without any grooves. Some granules’ residues surface was noticed due to proteins, lipids, and fibers[39,40]. The micrography of WF showed rough structures adhered to the surface, i.e., a layer formed by lignin, hemicellulose, and waxes[41]. Some untreated fibers may have holes, and these are filled with silica. Treatments can remove the silicon and leave the holes open[42]. Silica is present in longitudinal linear structures such as “tyloses,” which can act as adhesion points between the resin and the matrix[43,44]. The FTIR spectra of fibers confirmed the presence of silica in WF fibers. The AF fibers presented a smoother surface than the WF fibers showing the interfibrillar structure. It was evidenced that the alkaline treatment removed part of the hemicellulose, lignin, and waxes[45,46]. The BF fibers showed more significant surface changes due to eliminating the residual lignin. Consequently, the fiber’s internal structure was exposed, exhibiting a bundle of continuous oriented microfibrils[44]. The micrography of WF showed rough structures. The diffractogram of BS (Figure 3) showed peaks at 2θ = 5.91º, 11.45º, 13.12º, 17.52°, 20.03°, 23.33°, and 26.63°, typical of A and B type starches[47,48]. Then, it is classified as type C, with characteristics closer to type A starches[40,49]. The starch so-called starch C was described in leguminous seeds and banana[50]. The crystallinity index (CI) found for the BS, WF, AF, and BF fibers are listed in Table 2. The crystallinity of BM (27.65%) was lower than the one found by Maniglia et al. (29.00%)[29]. Amylopectin has a branched structure and linear segments arranged in double helices stabilized by hydrogen bonds. This structure is associated with starch crystallinity. However, it is essential to note that the amount

of amylose is high; the starch’s crystallinity has decreased because amylose represents the starch’s amorphous part. In our study, the amount of amylose (36.45%) was higher than that found in the literature[29]; therefore, the crystallinity value was lower[50]. The crystallinity of starch can also be influenced by other components present in starch, such as ash, protein, lipids, and crude fiber, or by the starch extraction method[51]. The epicarp fibers (Figure 3) exhibited similar diffraction patterns to those observed in other studies on Babassu epicarp fibers[31], and fibers of other botanical origins. In the present work, the fiber diffraction profile was the same as cellulose type I, which is commonly found in lignocellulosic materials[52]. Fibers (WF, AF, and BF) crystallinity increased progressively with the chemical treatments (Table 2). Furthermore, it is a consequence of the progressive removal of amorphous components (hemicellulose and lignin) promoted by the treatments, which allowed a higher ordering of the cellulose chains[53]. There is no difference between the crystallinity of TPS and TPSWF, TPSAF, and TPSBF composites. Although the results show a progressive increase when using fibers that have undergone chemical treatment, as seen in the fibers. Table 2. Crystallinity Index. Sample BS WF AF BF TPS TPSWF TPSAF TPSBF

CI% 27.65 23.60 41.81 43.34 19.61 18.51 20.15 21.99

Figure 2. Micrography of (A) BS, (B) WF, (C) AF, (D) BF.

Figure 3. XRD of BS and Fibers. 4/11

Polímeros, 31(1), e2021007, 2021


New biodegradable composites from starch and fibers of the babassu coconut Figure 4 shows the TG/DTG and DSC curves of BS, WF, AF, and BF. Two events were observed in the TG/DTG curve of babassu starch (BS), and they were attributed to the removal of physically absorbed water (67 °C) and the degradation of hemicellulose (359.5 °C)[54,55]. BS has thermal stability up to 290 °C. On the other hand, the TG/DTG curve of the WF fiber presented three events, attributed to the removal of physically absorbed water (49 °C), degradation of hemicellulose (300 °C), and cellulose (390 °C)[54]. TG/DTG curves of the alkaline fibers (AF) show a discrete event at 290 °C, attributed to the decomposition of lignin and hemicellulose. This event indicates that such compounds were not completely eliminated with the alkalinization treatment. Kabir et al. studied hemp fibers and found that treating the fibers with 4% NaOH was insufficient to eliminate lignin and hemicellulose[42]. In this work, the same alkalinization treatment was carried out; i.e., the babassu fibers were treated with 4% NaOH solution. TG/DTG curves of BF fibers, show just two events, one of them at 45 °C and other at 322.7 °C related to water absorption and cellulose decomposition. In this case, the treatment was sufficinte to eliminate lignin and hemicellulose. The DSC curves of all fibers show an endothermic event from 30 °C to 150 °C, which corresponds to the loss of adsorbed water[37]. This event is more accentuated to washed fibers (WF), being the maximum at 80.2 ºC. The event attributed

to water absorption was observed at 96.2 °C to alcanilized fibers (AF) and 70.6 ºC to bleached fibers. An accentuated exotérmic event was seen at 420.5 °C (WF), which correspond to hemicellulose and lignin decomposition[37,43]. The event was observed to alcanized fibers at 394.5 ºC; however, it was less accentuated than washed fibers indicating that the hemicellulose and lignin were partially removed from the fibers. This result is following the SEM images, infrared and TG/DTG results. The event attributed to hemicellulose and lignina was not observed in DSC curve of the BF. All the fiber showed a peak at 474.3 ºC (WF), 445.3 °C (AF), and 420.4 °C (BF) attributed to cellulose degradation, which is the predominant constituent of the fiber. Bhaduri et al.[56] and Shafizadeh et al.[57] observed that this endotherm is a consequence of dehydration and depolymerization of the cellulose component of the fiber, leading to the formation of flammable volatile products.

3.2 Characterization of TPS and Composites SEM micrographs of TPS, TPSWF, TPSAF, and TPSBF are shown in Figure 5. TPS has a relatively smooth and even surface. No starch granules were observed, indicating that the method applied in TPS synthesis was suitable for restructuring of the granules[58,59]. SEM image of TPSWF clearly shows voids at the interface between the fiber and the thermoplastic matrix. The TPSAF and TPSBF composites’ micrographs showed that the treatment applied to the fibers

Figure 4. TG, DTG and DSC curves of WF, AF and BF. Polímeros, 31(1), e2021007, 2021

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Moura, C. V. R., Sousa, D. C., Moura, E. M., Araújo, E. C. E., & Sittolin, I. M.

Figure 5. SEM Micrographs of the TPS, TPSWF, TPSAF, and TPSBF. Table 3. Results of Mechanical Properties of Babassu TPS and Composites. Starch type

Plasticizer (%)

Cassava Cassava Babassu Kefiran Raw oil palm Horse chestnut Corn

Glycerol (40) Glycerol (45) Glycerol (19) Glycerol (25) Glycerol (43) Glycerol (30) Glycerol (25)

Babassu TPSWF TPSAF TPSBF

Glycerol (40)

Tensile Strength Elongation (%) (MPa) 2.2 ± 0.1 108.3 ± 5.0 1.8 ± 0.03 50 ± 4.2 63 ± 4.1 0.7 ± 0,2 7.2 ± 1.7 135 ± 1.4 0.3 ± 0.1 79.0 ± 27 1.5 ± 0.3 1.02 ± 0.04 1.24 ± 0.5 48.7 ± 0.01 Our Results 31.67 ± 1.07 10.63 ± 1.6 45.38 ± 1.15 7.54 ± 1.7 50.58 ± 1.13 6.87 ± 1.5 58.15 ± 1.16 5.37 ± 1.4

Elastic Modulus (MPa) 174.6 ± 1.8 10.5 ± 2.6 4285 ± 208 198 ± 4.6 8.5 ± 2 3.0 ± 0.3 19.93 ± 0.07

Reference 61 23 26 67 37 40 63

120.65 ± 2.55 222.36 ± 3.27 231.65 ± 3.19 254.81 ± 3.31

provided better matrix adhesion on the fiber surface, as there were no voids at the interface between the components. The bleached treatment promoted the removal of the surface layer of the fibers, composed of hemicellulose and lignin, leaving the cellulose more exposed. As glucose units form both cellulose and starch, there is a strong interaction between them, especially hydrogen bonds[37]. Also, silica (SiO2), present on the fiber surface (it was not completely removed by the treatments) can strongly interact with OH group from TPS[37]. Silica can interact with the unsaturated bonds of polymer molecules (TPS) through electrons of the fiber’s unsaturated bonds[59]. Figure 6 shows the XRD of TPS and composites. XDR of the thermoplastic starch (TPS) does not have the type C starches’ characteristic crystallograph profile. Type C starch was observed in natural babassu starch (BS). Peaks observed a new pattern at 15.8° and 24.3°, typical of thermoplastic starches with a crystalline V-type pattern[60,61]. As expected, and seen in Table 3, the composites’ crystallinity increased according to the chemical treatment received by the fibers. The higher crystallinity was observed for the composite made with the bleached fibers. TPS and composites TG/DTG curves (Figure 7) are very similar, virtually overlapping. The materials have lost mass relatively gradually. However, DTG curves helped identify the three most notable mass-loss events. The first ranged from 30° C to 130° C and corresponded to the loss of physisorbed water[62]. This event appears on the DSC curve (Figure 6) as an endothermic peak reaching about 180 °C. The second mass loss event ranged from 140 °C to 250 °C and mainly corresponded to the volatilization of glycerol 6/11

Figure 6. XRD of TPS, TPSWF, TPSAF, TPSBF.

molecules[37]. This event appears on the DSC curve as an endothermic peak. The third and most significant mass loss event ranged from 250 °C to 430 °C and corresponded to the degradation of TPS and fibers and the boiling of the remaining glycerol (PE = 290 °C)[63,64]. This event appears on the DSC curve as an exothermic peak. As found in nature, granular starch is a polysaccharide with polyhydroxyl groups and strong inter and intramolecular interactions of hydrogen, limiting the mobility of its molecules. It means that the starch has no melt flowability and cannot be directly processed by melting[65]. However, starch can have the granule structure destroyed if processed in the presence of a plasticizer substance (such as glycerol), under the action of heat and shear. In this material, the hydrogen bonds between the amylose and amylopectin molecules are replaced by bonds with the plasticizer molecules, causing the Polímeros, 31(1), e2021007, 2021


New biodegradable composites from starch and fibers of the babassu coconut destruction of the granular crystallinity and increasing the mobility between the polymeric starch chains[37,60]. However, there is an optimum amount of plasticizer for starch, which varies with the starch’s origin and makes TPS more fluid and industrially processable. Liu et al.[65] have studied the variation of glycerol (30-50%) added to cassava starch and found an optimum amount of glycerol that must be added to starch to make it a plasticizer. They observed that the best extrusion characteristics were achieved when 40% of glycerol was added to the starch. It is worth mentioning that we also use a 40% plasticizer (glycerol). Table 3 and Figure 8 show the results of tensile strength, elongation at break, and elastic modulus found in this work, as well as results found in the literature for comparison purposes. The results of the TPS’s mechanical properties in our study differ slightly from those found in the literature. Note that the tensile strength was more remarkable than many studies (Table 3), except for the result found by Maniglia et al.[29,40]

The mechanical behavior of matrials depends on several parameters, such as the particle distribution, dispersion, and adhesion of the matrix in the dispersed phase [66]. Plastic deformation decreases as the tensile strength and modulus increase and the elongation at break decreases. The TPS elongation (ductility) was lower than most of the results mentioned in the literature[66] except for the values found by Maniglia et al. and Castaño et al.[29,48]. The ductility is related to the material’s softness, and the higher the value, the more ductile, the softer the material will be. The modulus of elasticity that measures the stiffness of materials (hardness) is directly related to the forces of intermolecular and intermolecular connections. For this parameter, the results found were superior to the results shown by Campos et al.[37], Castaño et al.[48], and Fazeli et al.[67]. The described composites were obtained using TPS with 40% glycerol with plasticizer and without chemical

Figure 7. TG/DTG and DSC curves of TPS, TPSWF, TPSAF, and TPSBF.

Figure 8. Mechanical Properties of TPS and Composites. Polímeros, 31(1), e2021007, 2021

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Moura, C. V. R., Sousa, D. C., Moura, E. M., Araújo, E. C. E., & Sittolin, I. M. treatment. Chemical treatment was carried out in order to eliminate waxes, lignin, and hemicellulose. Also, the fibers have a great attraction for water due to their structure, and the removal of these components from the fibers implies a more significant interaction between the fibers and the polymeric matrix, resulting in increased stress transfer, improving the mechanical properties, as well as reducing the water absorption [68]. In general, the mechanical properties analyzed (tensile strength, modulus of elasticity, and elongation) of the composites showed improvements if compared to neat TPS Chemical treatment with NaOH (mercerization) improves the mechanical properties, mainly the tensile strength and elongation. In bleaching, this treatment improves the fiber/ TPS adhesion, giving a more significant interaction between the fiber and the TPS components. This interaction occurs mainly via hydrogen bonding between the hydroxyl groups of the fibers and the TPS[37]. The removal of lignin and hemicellulose can be seen by FTIR analysis, where the band at 1742 cm-1, attributed to the stretching of the ester C=O bond, disappeared for TPSBF composite. It appears that the band at 1249 cm-1, related to α-C-O-C, was reduced for the treated fibers. The composites where the fiber was chemically treated (TPSAF, TPSBF) showed better mechanical properties than the composite made with the untreated fiber (TPSWF), showing that while hydrophilic materials were removed from the fibers, adhesion with the TPS (hydrophobic) was more effective. The mechanical properties of composites are shown in Table 3. The tensile strength of TPSWF (45.38 ± 1.07 MPa), TPSAF (50.58 ± 1.13 MPa), and TPSBF (58.15 ± 1.16 MPa) are higher than the pure TPS matrix, which presented a maximum tension of 31.67 ± 1.07 MPa. The maximum tensions were 43.3%, 59.7%, and 83.6% higher than the pure TPS matrix, respectively. The TPSWF composite has a lower tensile strength than the other composites that may have been caused by void formations at the interface between the fibers and the TPS, resulting in a discontinuous matrix, as shown by the SEM images. The results of tensile strength presented in this work have been better than the ones described by Grylewicz et al. (5.7 MPa), that used TPS from starch potato and wood fiber in a rate 88.12%, respectively[69]. The elongation at break of the composites, TPSWF (7.54 ± 1.6%), TPSAF (6.87 ± 1.7%), TPSBF (5.37 ± 1.4%), were smaller than the ones found by Campos et al.[37], Fazelli et al. [67], and Grylewicz et al.[69] The results of elastic modulus found for the composites were 222.36 ± 3.27 MPa (TPSWF), 231.65 ± 3.19 MPa (TPSAF), and 254.81 ± 3.31 MPa (TPSBF). This improvement related to TPS is due to the strong interfacial interaction between the starch and the fibers[65,70]. The elasticity modulus was 84.30%, 91.99%, and 111.19%, more significant than the pure TPS, respectively. Fontelles et al.[68] prepared composites using babassu epicarp fibers with and without chemical treatment and as a matrix using unsaturated polyester and orthopthalic resin. The results showed for the composite made with fibers without chemical treatment, 15 Mpa for tensile strength, 1.8% for elongation, and 1600 MPa for 8/11

Figure 9. Water Absorption of TPS, TPSWF, TPSAF, and TPSBF.

elastic modulus, and with chemical treatment, 18 MPa, 2.2%, and 1500 MPa, respectively. When comparing the mechanical properties found for the TPSWF and TPSAF composites, it appears that they do not differ much from each other. This fact is probably due to the O-H and Si-O interactions between TPS and fibers. However, as the bleached fibers’ changes were more pronounced, the results of the mechanical properties of this composite (TPSBF) were much better than the properties of the other composites (TPSWF and TPSAF). The results of mechanical properties found by Fontelles et al. [68] do not differ between treated and untreated fibers. The same result was observed in our work. The results of water absorption (75% humidity) are shown in Figure 9. It can be seen that the largest mass gain (water) for both TPS and composites was after 194 h. Also, it is noted that the composites had less moisture absorption than pure TPS. Theoretically, composites should have greater moisture absorption than the original matrix due to the fibers’ hydrophilic character[68] . However, in this case, the matrix is thermoplastic starch, a compound also hydrophilic, with great water absorption capacity[23]. The amount of water absorbed by the TPS and the compounds gradually increased up to 150 h. From that time on, the amount of water absorbed was small and remained practically the same until the end of the test, reaching the maximum absorption at 194 h. The maximum water absorption (300 h) was 29.7%, 28.6%, 27.9% and 27.5% (TPS, TPSWF, TPSAF and TPSBF), respectively. The literature reports similar results with 25.37% TPS/Sisal fiber [71,72], and 27.1% TPS/ Luffa fiber[59]. The TPSBF compound had the least amount of water absorption, which is 7.4% less than the amount of pure TPS absorption. The TPSWF and TPSAF composites showed a water absorption amount of 3.7% and 6.1% less than that of the pure matrix. With the addition of fibers, this problem can be alleviated.[73]. The moisture absorption results showed that the composites absorbed less water than the TPS and improved the composite properties.

4. Conclusion Many studies show the use of TPS of several starches as a matrix to obtain composites with fibers. However, the use of TPS derived from babassu starch and composites reinforced with babassu fiber is being described for the first time. The textures of the babassu epicarp applied as reinforcement in composites presented thermal properties Polímeros, 31(1), e2021007, 2021


New biodegradable composites from starch and fibers of the babassu coconut and X-ray diffraction patterns, like other botanical sources. The chemical treatments applied to the fibers promoted the removal of wax, hemicellulose and lignin, revealing its internal fibrillar structure. The bleaching process developed the most significant modifications, causing a reduction in moisture absorption. X-ray results showed an increase in the crystallinity of the fibers. SEM images and infrared spectra showed that compounds such as wax, lignin, and hemicellulose were removed from the fiber surface. The SEM images of the TPS starch showed a smooth and uniform surface. In the case of composites, SEM images show voids between the fiber and the TPS for the composite TPSWF (washed fiber) and better adhesion for the composites TPSAF (alkalinized fiber) and TPSBF (bleached fiber). The fiber treatments resulted in more significant interfacial interaction between the TPS and the fibers. There was a substantial improvement in the mechanical properties of the composites compared to the pure TPS, i.e., higher tensile strength and modulus of elasticity, making the composites more rigid. The mechanical properties found for the TPSWF and TPSBF composites, do not differ much from each other. However, as the bleached fibers’ changes were more pronounced, the results of the mechanical properties of this composite (TPSBF) were better than the properties of the other composites (TPSWF and TPSAF).

5. Acknowledgements The authors would like to thank Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – CAPES and Conselho Nacional de Desenvolvimento Científico e Tecnológico – CNPQ (for their financial support).

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ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.10520

A foldable high transparent fluorinated polyimide (HFBAPP/6FDA) film material for transparent flexible substrate Chuanhao Cao1 , Lizhu Liu1,2*  and Xiaorui Zhang1,2 1

School of Materials Science and Engineering, Harbin University of Science and Technology, Harbin, China 2 Key Laboratory of Engineering Dielectric and Its Application, Harbin University of Science and Technology, Ministry of Education, Harbin, China *Corresponding author: mrliu_hust@163.com

Abstract Flexible transparent substrate materials, which was able to withstand high dynamic strain, in contrast to traditional substrate materials. A flexible and transparent material with advantages including transparency, stable size, and excellent corrosion and electrical resistance was provided. The polyimide(PI) film was prepared by introducing a structure with a high content of F atom and a fine optimization process to enhance the various properties of the film. However, the properties of the films were optimized effectively by gradient vacuum and secondary dissolution so that the film had a transmittance at 400 nm of 82%. The films with low dielectric constant and low dielectric loss represent the breakdown strength of 202 kV/mm. The glass transition temperature of the film was 267 °C, and the thermal expansion coefficient was 35ppm/k (30 °C~270 °C), indicated outstanding thermal dimensional stability. Therefore, this polyimide film was an optoelectronic device with extremely high application potential on the folding mobile phone and the PI film is the finest materials of screen. Keywords: transparent PI, transmittance, insulation, thermal performance, dielectric properties. How to cite: Cao, C., Liu, L., & Zhang, X. (2021). A foldable high transparente fluorinated polyimide (HFBAPP/6FDA) film material for transparent flexible substrate. Polímeros: Ciência e Tecnologia, 31(1), e2021008. https://doi. org/10.1590/0104-1428.10520

1. Introduction Transparent flexible substrate materials were capable of bending deformation, compared with traditional glass substrate materials. In recent years, the demand and technical requirements of the transparent flexible substrate in the flexible display screen and other fields were increasing[1-4]. A new replacement of glass substrate material was provided by PI film, which was good at transparency, mechanical strength, corrosion resistance, heat resistance. With the creasing of technology, enterprises were not satisfied with the status quo that grow a large-scale market for transparent flexible materials[5,6]. Hitherto, the key issue of the high transparent polyimide included high optical transmittance(TR)[7-9] and low yellow value[10,11]. It was well known that polyimide was generally shown to be brown yellow and caused TR to be lower. A high yellow value led to the bad vision and the film unable to be used on a large scale. Stretchability, electrical and thermal stability under mechanical deformation were new essential criteria because transparent films need to be applicable to next-generation material. Thus far, thin transparent polyimide film had served as an ideal choice for because of good resistance to shockresistant to many chemicals and good heat resistance[12-15].

Polímeros, 31(1), e2021008, 2021

In the past few decades, new PI had been grown by excellent performance as film material, such as aliphatic PI[16,17], aromatic PI[18,19], and other PI[20,21].Imposing the conjugate plane structure or polar structure on the PI film was an essential method in principle. The aliphatic PI was a better compared with aromatic PI that was attributed to the molecular weight. Transparent properties were considered the fatal flaw that limited the PI film qualities’ s application in the flexible screen application field. The traditional aromatic benzene type PI was usually brownish yellow and low light transmittance, which was mainly due to the strong charge-transfer complex(CTC)[22] formed between aromatic dianhydride and aromatic diamine and some dark functional groups were produced during thermoimination. Therefore, the surface of PI film had a certain orientation structure by introducing F group[23-26], alicyclic polyimides[27-29]and so on into the PI system. Studies had shown that the introduction of orientation structure could effectively eliminate the CTC effect within the molecule, and the introduction of macrogroup side groups could effectively eliminate the CTC effect between molecules. [30,31] The solvent contains amides, which

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O O O O O O O O O O O O O O O O


Cao, C., Liu, L., & Zhang, X. were prone to oxidation in the heating process to make the film darker. A gradient vacuum method was developed, which could effectively remove the influence of solvent without affecting the flowability of PAA glue soluting and the performance of PI film.

was purchased from Shanghai Macklin Biochemical Co., Ltd. All the chemicals were analytical grade reagents that were commercially available and were used without further purification.

In this paper, a transparent material was provided by PI-HFBAPP/6FDA film. This transparent film would not be corroded and deformed by chemical agents and had good transparency. The orientation of the folding screen would not affect the visual experience of the reader. By adjusting the polymer structure and optimizing the process especially, the film with high visible light transmittance was prepared on the premise of ensuring the film performance. Thus, this film had great development space in the market potential in the future.

1.2 Experimental Procedure

2 Experimental Section

By using ice-water mixing bath and nitrogen protect reaction process, HFBAPP(1.985 g, 0.03828mol) was added to DMAC(15 ml) solvent and stirred slowly until completely dissolved. 6FDA(1.701 g, 0.003829mol) was added into the reaction system six times. The time interval for each addition of monomers was 15 minutes and added a monomer in an amount half as small as it was left. Stirring continued for 2 hours after the average viscous molecular weight climbed.

2.1 Experiment materials Dimethylacetamide(DMAC) was purchased from Tianjin Fuyu Fine Chemical Co., Ltd. 4,4’-(Hexafluoroisopropylidene) diphthalic anhydride(6FDA) was purchased from Wuhan lullaby pharmaceutical chemical Co., Ltd, the purify was 99%. 2,2-Bis[4(4-aminophenoxy)phenyl]-hexafluoropropanane(HFBAPP) was purchased from Shanghai Macklin Biochemical Co., Ltd, the purify was 99%. 4,4-oxydianiline(ODA) was purchased from Sinopharm Group Chemical Reagent Co., Ltd. 3,3’,4,4’-biphenyl tetracarboxylic dianhydride(PMDA)

The reaction equation of HFBAPP as diamine monomer, 6FDA as dianhydride monomer was taken as an example. The reaction equation of the polymerization was shown in Figure 1. Polyimide films were synthesized using the dianhydride 6FDA or PMDA, the diamines HFBAPP or ODA in the polar aprotic DMAc, as shown in Figure 1. The film preparation of PI-HFBAPP/6FDA was taken as a representative to illustrate the detailed procedures. The reaction flow chart of PI-HFBAPP/6FDA Film was shown in Figure 2.

The polymer was transfer into a small beaker and the beaker was placed in a vacuum box. Then the air was extracted from the vacuum box so that the gel did not contain bubbles.

Figure 1. Preparation of PI-HFBAPP/6FDA Film. 2/10

Polímeros, 31(1), e2021008, 2021


A foldable high transparent fluorinated polyimide (HFBAPP/6FDA) film material for transparent flexible substrate After cleaning the glass plate, the standing polyamic acid(PAA) was poured onto the glass plate and spread with a scraper of a certain thickness. The size of the glue film was 20 cm × 25 cm and the final formed thickness of the film was 10 μm ~50 μm. The film was put into the oven, and the removal of solvents from the film was carried out by a gradient vacuum. As a consequence, the treated film would avoid oxidation. The specific process was shown in Table 1. The film was put into the oven and the thermal imidization of the film was carried out by gradient heating. The specific process was shown in Table 2.

The Bruker-EQUINOX 55-type Fourier transform infrared spectrometer manufactured by Germany was used to attenuate the total reflection infrared attachment. The polyimide film was characterized. The test condition was that the resolution was 4 cm-1, the scanning time was 16 times of the sample to be tested, the background was scanned 16 times, the delay time was 10s, and the test range was 500-4000 cm-1. The dimensional stability of the films wares using by L75 Pt vs/VD vertical mode thermal dilatometer. Heating interval 0~270 °C, heating rate 5 °C/min. The glass transition temperature and storage modulus of the PI films were measured by the dynamic mechanical analysis(DMA). All test samples were rectangular, with a length of 20 mm and a width of 8 mm. The dielectric constant and dielectric loss of the PI film was tested by an Alpha-a-type broadband dielectric spectrometer manufactured by Novocontrol Company of Germany. The breakdown strength of the PI film was measured by using the HT-5/20 breakdown voltage tester developed by the Guilin Electrical Equipment Scientific Research Institute.

3. Measurement The transmittance of the film and the different PI film proportions were analyzed by the UV-visible spectrophotometer of Shimadzu UV-2450. Test conditions: the wavelength was 600-250 nm and the sample size was 20-30 mm.

Figure 2. Reaction flow chart of PI-HFBAPP/6FDA Film. Table 1. Gradient vacuum process. Stage Time/min Vaccum/1atm Temperature/°C

A 10 0.4 80

B 20 0 80

C 10 0.5 80

D 10 0.6 80

E 20 0 80

F 20 0 100

G 20 0.7 100

H 20 0.7 120

I 30 0 120

Table 2. Thermoimidization gradient heating schedule. Temperature/°C Time/min

Climb to 80 15

80 45

Polímeros, 31(1), e2021008, 2021

80 to 120 10

120 20

120 to 160 10

160 20

160 to 200 10

200 20

200 to 250 10

250 20

250 to 300 10

300 20

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Cao, C., Liu, L., & Zhang, X.

4. Result of discussion 4.1 FTIR characterization The chemical structures of the pure PI film and PI-vacuum treatment/secondary film formation were confirmed by FTIR spectroscopy, and it was shown in Figure 3. As a consequence, there were 6 characteristic absorption peaks of polyimide films at 1787 cm-1,1729 cm-1, 1600 cm-1,

1500 cm-1, 1380 cm-1, 850 cm-1, where 1787 cm-1 corresponded to the asymmetric expansion vibration of C=O, and the 1729 cm-1 corresponded to the C=O symmetric expansion vibration. Compared with ordinary PI, the carbonyl stretching vibrations on the fluorinated PI rings moved toward higher wavenumbers. It was due to the inducing effect of the trifluoromethyl group that made the carbonyl double bond enhance and the frequency of stretching vibration increase. The peaks at 1600 cm-1 corresponded to N-H flexural expansion vibration. The peaks at 1500 cm-1 and 850cm-1 corresponded to the vibration of the benzene ring skeleton. The absorption peak at 1380 cm-1 represented the symmetric contraction vibration of the C-N, and the 725 cm-1 corresponded to the bending vibration of the C=O. These characteristic peaks were consistent with the standard peak positions of the PI in Figure 3, indicating the presence of PI structure and characterizing the PI structure. From Figure 3 that the change of process does not change the structure of the polymer, which could be proved by the fact that there was no difference between line 4 (PI-HFBAPP/6FDA) and line 5 (PI-HFBAPP/ 6FDA-remade).

4.2 Characterization and optoelectronic properties

Figure 3. Infrared spectrum of pure PI film and PI film after Process Optimization.

The optical performance of CPI films was shown in Figure 3. Because of the flatness of coating and PAA glue soluting could be naturally leveled. The CPI films were uniformly distributed on the glass substrate and separated breezily. Four kinds of films to meet different production requirements were prepared, and the transmittances were characterized. As shown in Figure 4a, several kinds of CPI

Figure 4. Transparent of PI composite films with different preparation process. (a)Transmittance of different monomers PI (b)Different thickness transmittance of PI-HFBAPP/6FDA (c)Effect of technology on the transmittance of PI (d)Optimization of thin film transmittance. 4/10

Polímeros, 31(1), e2021008, 2021


A foldable high transparent fluorinated polyimide (HFBAPP/6FDA) film material for transparent flexible substrate films exhibit a high transmittance. The 341 nm was UV cutoff wavelength of pure PI-HFBAPP/6FDA, it was the least of PI films prepared from different monomers and the transmittance was also the best. As shown in Figure 4b, the lower the thickness, the better the transparency of the films. The reason was that the thicker films were easy to clip impurities or tiny bubbles in the process, and there were not conducive to solvent volatilization. The higher the yellow value of the film thickness also reduces the transparency of PI to a certain extent. The transparency of pure PI films at 400 nm was 67%(CPI-3 15 μm), 37%(CPI-4 30 μm), 3%(50 μm), and 500 nm was 94%(15 μm), 93%(30 μm) and 87%(50 μm). The technology of gradient vacuum could improve the light transmittance, because it could avoid the discoloration caused by the solvent oxidation during the thermal imidization process and could deal with the bubble problem during the synthesis process, it was shown in Figure 4c that the technology of gradient vacuum could improve the light transmittance by about 2%. After the secondary film-forming process after dissolution(CPI-2 30 μm) was prepared, the free monomer and colored impurity in the polymer could be effectively removed. It would improve the wavelength of the UV stage and reduce the transmittance of 420 nm band, which was due to the introduction of a new solvent(NMP) to produce a certain residue in the process of heating secondary film formation. It was shown in Figure 4d that if reducing the thickness of the film, it would greatly reduce the residual solvent NMP. In the film, the transmittance(CPI-1 15 μm) could reach 82% at 400 nm and the yellowness index is 5.5. Yellowness index (YI) is used to characterize the yellowing degree of colorless, transparent, translucent or near white polymer materials, which is one of the important optical indexes in resin plastics industry. YI =

100 ( CxX − CzZ ) Y

The YI of the PI-(CPI-1 15 μm) is 5.5 and the PI-HFBAPP/ PMDA is 78.6. The YI of the film is greatly decreased a lot by structural design and process optimization. As you can see from Figure 5, the yellow effect of the film is not visible at all. As could be seen from Figure 6, the transparency of the same series of PI films increased with the increase of the F group ratio. We attribute the reason for the introduction of the side group in the PI chain that destroyed the charge transfer complex(CTC). Increasing the high electronegative group was an effective way to reduce the CTC effect by weakening the intermolecular and intramolecular interactions, reducing the number of free monomers, and introducing large resistance side groups.

4.3 Thermal performance characterization Dynamic thermo-mechanical analysis(DMA) was used to measure the modulus of the material along with the change of temperature, the glass transition temperature of the material in the characterization of molecular chain motion, and molecular inter-atomic forces. The change of loss modulus(E”) represented the intensity of the movement of molecular chains. The change of storage modulus(E′) represented the strength of intermolecular forces. From the curves of loss modulus(E′′) versus temperature in Figure 7, it could be seen that the peak temperature of the respective loss modulus represented the glass transition temperature, the Tg of PI-HFBAPP/6FDA was 267 °C, and energy storage density up to 1170 MPa. The larger the molecular weight of the material, the more tangles between the molecules. The more heat generated by friction, the greater the storage modulus. After the tangent of the curve, the temperature continued to increase, the value of the storage modulus momentarily dropped to zero, and the modulus decayed completely in the temperature range where the temperature increase was small.

Figure 5. The actual effect of PI film. Polímeros, 31(1), e2021008, 2021

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Cao, C., Liu, L., & Zhang, X.

Figure 6. Schematic diagram of the charge transfer of CTC.

Figure 7. Glass transition temperature and storage modulus of PI composite films.

Thermo-mechanical analysis(TMA) characterized in the case of a certain temperature program, the load force was close to zero. The dimensional variation of the measured sample was functional with temperature or time in isotropic polymers, the molecular chains were promiscuously oriented, and their thermal expansion coefficient strongly depended on the weak intermolecular interactions. In Figure 8, the thermal expansion coefficient of the films prepared by different processes was different, and the film without doping the filler achieved a more ideal effect. The thermal expansion coefficient of PI-HFBAPP/6FDA 6/10

cure film was 67 ppm/k. The presence of CF3 resulted in a decrease in the packing density of PI macromolecules, an increase in the free volume of the molecular chain, and a smaller interaction between the molecular chains. Therefore, by adding less F element would decrease the coefficient of linear thermal expansion. However, the structure with a high content of F atom was more stable and could be used to prepare films with higher molecular weight. It increased the packing density of molecules and reduces the free space in the same volume. Under the same conditions, the dimensional stability of the film was better. After the process optimization, the thermal expansion coefficient of the PI film increased. The reason was that the molecular weight of the film was lost after the secondary dissolution process, the intermolecular force was reduced, and the heat required to expand the distance between molecules was reduced. For this problem, we could only optimize the process as far as possible to reduce the loss caused by repeated coating. Theoretically, if the process was optimized enough, the thermal expansion coefficient of the second film was expected to be lower than that of the pure film.

4.4 Characterization of electrical properties The dielectric property of the PI films was characterized from 100 Hz to 107 Hz and obtained the dielectric constant. Since the dielectric loss was related to the energy dissipation due to the oscillating electric field, it was necessary to select the low dielectric constant and low dielectric loss materials. Since the dielectric loss was related to the energy dissipation due to the oscillating electric field, it was necessary to Polímeros, 31(1), e2021008, 2021


A foldable high transparent fluorinated polyimide (HFBAPP/6FDA) film material for transparent flexible substrate

Figure 8. Thermal expansion coefficient diagram of PI film.

Figure 9. Dielectric properties of PI different films.

select the low dielectric constant and low-dielectric loss materials and longer service life. Accounting to Figure 9, PI-HFBAPP/6FDA had a lower dielectric constant that defeat traditional PI films. The low-k and the low dielectric were attributed to increasing the spatial group of molecules by adding a C-F bond. The C-F bond had less dipole polarization. Consequently, the increase of C-F bond could effectively reduce the dielectric constant and dielectric loss. The PIHFBAPP/6FDA film with a large number of CF3 groups had an excellent dielectric ability. The dielectric constant of the PI films was 2.1 and the dielectric loss had been below 10-2 at the full frequency band. The breakdown field strength of the dielectric was the ultimate ability of the dielectric to maintain insulation performance under the action of an electric field. Figure 10 showed the volume resistivity and electrical field strength of PI. Figure 10 reported the electric field and volume resistivity of the different PI. The prepared F-containing PI film reached 171.3 kV/ mm and the volume resistivity of the four films was characterized. The volume resistivity Polímeros, 31(1), e2021008, 2021

of PI-HFBAPP/6FDA film was reached 3.3×1015 Ω/m. In general, the dielectric breakdown occurred mainly in the weakest part of the dielectric properties of materials, and the regular structure results in fewer leakage pathways. For pure films, some of the films with the structure with a high content of F atom would distort the structure to some extent and form weak points, so the breakdown strength would be weaker than that of PI films with benzene structure to some extent. The PI(HFBAPP/6FDA) film with the structure with a high content of F atom also had a regular structure and the anti-wear loss rate was low, and the charge transfer effect inside the film was very weak. The voltage needs to consume more energy and converted it into heat energy to lose the structure of the film before it could break down at the weak point. The volume resistivity of PI film was mainly related to the intramolecular or intermolecular phase resistance. With the decrease of the internal charge of the polymer, the volume resistivity of the PI film increased, and the insulation performance of the PI film was better. The volume resistivity of the PI composite film showed the same trend as the breakdown field strength. In addition, due to the long distance between the two adjacent particles, the 7/10


Cao, C., Liu, L., & Zhang, X.

Figure 10. Volume resistivity and Electrical Field strength of PI by different monomers.

Figure 11. Electric Field of PI films with different process.

barrier which hindered the carrier migration decreases, and the volume resistivity increased. Figure 11 reported the field strength of the PI obtained by the gradient vacuum treatment and the secondary film formation could reach 202 kV/ mm. The reason was that the film defects and the impurities could be effectively reduced during the process treatment process. Gradient vacuum technology could effectively reduce the physical defects in the preparation process of PI films, which made PI films have a smoother surface and a higher degree of the electric field. After dissolution, the residual monomer impurities and other influencing factors could be removed by secondary film formation, and the capacity of the film properties could be further improved. Compared with the film after the gradient vacuum, the PI film after dissolution increased more insulation performance.

5. Conclusion Two series of PI were synthesized by the two-step method from HFBAPP, 6FDA, ODA, and PMDA. Various 8/10

optimization attempts were made to determine the effect of gradient vacuum and secondary dissolution on the research direction. With the increase of F content in the film, the effect of CTC was greatly weakened, the transmittance was obviously improved, the dielectric constant and dielectric loss were reduced, and the coefficient of thermal expansion was reduced. The process-optimized PI-HFBAPP/6FDA film reached 82% at 400 nm and total penetration at 425 nm. The glass transition temperature was 267 °C and the storage modulus was 1170 MPa. The coefficient of thermal expansion was 67 ppm/k. The film had excellent electrochemical performance, which breakdowns resistance voltage was 5.3 kV and the breakdown field strength was 202 kV/mm. The dielectric constant was 2.1, the dielectric loss was less than 10-2 and bulk resistance was 3.3×1015 Ω•m. In summary, it effectively inhibits that the CTC action in the polyimide film structure and ensures the other properties of PI by optimizing the process. The PI transparent film had high permeability, excellent heat resistance, excellent dimensional stability, high breakdown field strength, low dielectric constant, and low dielectric loss. The OLED Polímeros, 31(1), e2021008, 2021


A foldable high transparent fluorinated polyimide (HFBAPP/6FDA) film material for transparent flexible substrate commonly used in folding screens have the problems of low melting point and short service life - unlike the PI films. However, this PI film had considerable progress in the transparent film and a good application prospect for the flexible display market. We have achieved good performance in the transparent properties of films and it can be effectively applied to practical improvements.

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Polímeros, 31(1), e2021008, 2021


ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.09820

The effects of residual organic solvent on epoxy: modeling of kinetic parameters by DSC and Borchardt-Daniels method Victor de Carvalho Rodrigues1* , Denise Hirayama2  and Antonio Carlos Ancelotti Junior3  Núcleo de Inovação em Moldagem e Manufatura Aditiva – NIMMA, Universidade Federal de Santa Catarina – UFSC, Florianópolis, SC, Brasil 2 Laboratório de Materiais Poliméricos, Escola de Engenharia Industrial e Metalúrgica de Volta Redonda – EEIMVR, Universidade Federal Fluminense – UFF, Volta Redonda, RJ, Brasil 3 Instituto de Engenharia Mecânica – IEM, Universidade Federal de Itajubá – UNIFEI, Itajubá, MG, Brasil 1

*victor_crodrigues@yahoo.com.br

Abstract The curing reactions of epoxy resins are a complex process that defines thermosets final properties and are affected by any additive present on its formulation. Considering this, the aim of this study was to analyze the influence of the solvent addition on the curing kinetics of an epoxy system. The epoxy samples were prepared using different percentages by weight of acetone: 0, 2, 5 and 10 wt.%. From DSC and DMA tests, followed by the Borchardt-Daniels kinetic analysis it was reported that the addition of acetone can decrease the reactions rate, activation energy, Tg and elastic modulus. The presence of solvent, even in small amounts, can affect the curing mechanisms of epoxy resins. The changes on the curing behavior and the low quality of the final properties for the sample with 10 wt.% of solvent indicates that this may be a limit for acetone addition on the epoxy formulations. Keywords: Acetone, Borchardt-Daniels, curing kinetics, epoxy. How to cite: Rodrigues, V. C., Hirayama, D., & Ancelotti Junior, A. C. (2021). The effects of residual organic solvent on epoxy: modeling of kinetic parameters by DSC and Borchardt-Daniels method. Polímeros: Ciência e Tecnologia, 31(1), e2021009. https://doi.org/10.1590/0104-1428.09820

1. Introduction Epoxy resins are defined as a low-molecular-weight prepolymer based on epoxide groups capable to be converted on a thermoset or a three-dimensional network structure[1]. This transformation, also known as curing process, occurs due to the reaction between the free epoxide groups and curing agents which causes irreversible changes on the polymer network and will define the final properties of the material[1,2]. Due to its great chemical and mechanical properties and versatility, epoxy resins have been deeply studied and are widely used in different industrial fields as adhesives, electronics compounds, coatings and highperformance composite[3,4]. Different formulations are used to improve the epoxy resins properties or to facilitate the manufacturing processes[5]. For instance, for composite materials the solvents are generally used to reduce the matrix viscosity and to ease the inclusion of other materials, like fibers or fillers, or even to smooth the processing[6]. Nowadays, solvents are also used to recycle fiber composites in order to remove impregnated resins[7,8]. The presence of resins must be avoided on recycled fibers since this contamination can reduce the adhesion between fibers and matrix in a later production of other composites[7,9].

Polímeros, 31(1), e2021009, 2021

Despite the benefits of the solvents in manufacturing processes of epoxy, the presence of this additives, when still present after the curing process, may causes negative changes on thermal and mechanical properties of epoxy systems[10]. Loos[11] has studied the influence of acetone on epoxy samples and showed that although its presence reduces the viscosity and facilitates the addition of fillers and nanoparticles, the residual presence of this solvent after the curing process can also decrease some important properties of the material, for instance, the Young’s modulus, tensile strength and elongation at break[11]. Other studies concluded that the higher the boiling point of the solvent, the greater will be the effect on the curing process. It was also found that the presence of this additives may input barriers to resin/ hardener reaction, decreasing the crosslinking density[6]. In order to achieve suitable properties and also to improve the curing process, it is essential that the formulation and the curing cycle of the material are well defined[12]. Using thermal analysis, it is possible to understand the behavior of the curing process and analyze the kinetics of the reaction[13,14]. There are several different methods to study curing kinetics using both dynamic or isothermal tests each with its advantages and limitations. Cole[15] suggested a new

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Rodrigues, V. C., Hirayama, D., & Ancelotti Junior, A. C. approach to modeling the curing kinetics of epoxy-amine resins, based on Horie and Kamal methods, but considering the etherification reactions and also the diffusion effects. As a result, the degree of conversion could be calculated with excellent precision over the whole range of cure[15,16]. Despite its advantages, this method is based on a complex mathematical approach that still needs to be validated for other epoxy systems as it is less empirical than other conventional models[16]. Other studies have also investigated this subject in order to improve the accuracy of the cure kinetic analysis and presented several methods and adaptations that can be used, depending on the purpose of the work[17-19]. Isoconversional methods are capable to evaluate the activation energy (Ea) evolution throughout the curing process, while the methods previously mentioned produces only a single value. Hardis et al.[14] have used this model to monitor the curing kinetics of epoxy resins and obtained accurate values for the kinetic parameters. However, this approach is more complex and also require several DSC tests, which can make it difficult to reproduce[14]. The Borchardt-Daniels approach was the first single scan method used to calculate the curing kinetic parameters of epoxy resins, it is one of the simplest methods for modeling the curing reactions and its great advantage is the rapidity to perform the analysis[20,21]. This model is based on the relation between the degree of conversion (α), the enthalpy changes and can be applied for nth order reactions. Then, based on the enthalpy obtained by dynamic DSC analysis, it is possible to calculate α following the Equation 1, where ΔHp is the partial enthalpy and ΔHo is the total enthalpy (Area under the exothermic peak)[22].

α=

∆H p ∆H o

(1)

The analysis is based on the Equation 2, where dα/dt is the reactions rate (s-1), α is the conversion degree, k(T) is the velocity constant at temperature T and n is the reaction order[21]. dα n = k (T ) . ( 1 − α ) dt

(2)

With this method it is possible to simulate and evaluate some important parameters of the curing process such as activation energy (Ea), reaction order (n), and degree of conversion (α)[22,23]. The Borchardt-Daniels method considers the curing reaction as a single step process and that the Ea does not change with temperature[20]. As a result, the values obtained for the activation energy are dependent on the heating rate and are generally overestimated compared to other kinetic models, such as Ozawa and Kissinger[13,24]. Some studies have shown that for higher heating rates, the values of kinetic parameters tend to increase[4,24]. However, this method can be very useful as it provides a great overview of the curing kinetics behavior and is recommended when it is necessary to quickly evaluate approximate values of the curing kinetic parameters, since it requires just a single DSC scan[24]. The literature about epoxy-based composites has mainly explored the changes on the curing mechanisms 2/8

and epoxy’s properties caused by nanoparticles and fillers, with a little regard for the presence of residual solvent as a potential cause of the changes on the materials behavior[25]. In addition, it is known that an incomplete curing cycle can affect and reduce the materials final properties[10,25]. For these reasons, the aim of this study was to investigate the efficiency of the Borchardt-Daniels methodology to investigate the influence of different amounts of an organic solvent on the curing kinetics of an epoxy resin, generally used for composite materials, to analyze the effects of this additive on the reactions rate and also to evaluate the changes on the material properties, in order to aware about the possible impacts of residual solvent on epoxy composites formulation. A comparison between the curing kinetics parameters obtained by this model and the changes on thermal and mechanical properties of this epoxy system provided a preliminary result that should help to define a critical amount of solvent contamination in these systems and also to propose a useful method to evaluate the curing kinetics of epoxy resins.

2. Materials and Methods 2.1 Materials The resin used in this work was the Araldite® LY 5052, supplied by Huntsman®, which is a cold curing low-viscosity resin composed by 1,4-butanediol diglycidyl ether (C10H18O4) with 55-68 wt.% composition and an epoxy phenol novolac resin (C35H32O4) with 38-42 wt.% composition. The phenol novolac resin has four epoxide groups, and its molecular weight is 345 g/mol. The molecular weight of 1,4-butanediol diglycidyl is 202.3 g/mol and its functionality is two[26,27]. This epoxy system was combined with Aradur® 5052, used as a hardener, which is a mixture of polyamines containing IPDA (C10H22N2) with 30-60 wt.% and cycloaliphatic diamine (C15H30N2) with 30-60 wt.%[26]. Acetone (Neon®, 99,5% purity) was used as organic solvent.

2.2 Methods 2.2.1 Sample preparation The samples were mixed through mechanical stirring with the help of a glass rod for 2 minutes at room temperature using the listed epoxy system with different amounts of acetone. The weight ratio of the resin to hardener was 100:38 according to the manufacturer’s recommendation[28]. Four samples were prepared to each of the tests containing just the resin (0 wt.%) and also a mixing with 2%, 5% and 10% in weight of solvent. The solvent amounts were defined based on the quantities used on other works from literature and also on the percentage generally used in the processing of epoxy-based composites[11,29,30]. 2.2.2 Dynamic Tests For dynamic analysis the samples with approximately 5 mg were sealed on aluminum hermetic pans. The equipment (DSC Q20 2151 from TA Instrument®) was set to run from 0°C to 250°C using a 5°C/min heating rate and synthetic air atmosphere with 50 mL/min flow. Polímeros, 31(1), e2021009, 2021


The effects of residual organic solvent on epoxy: modeling of kinetic parameters by DSC and Borchardt-Daniels method 2.2.3 Curing Kinetics Modeling The kinetic parameters of the curing process were determined using the DSC dynamic data. Based on the Costa (1999) review about curing kinetic methods for epoxy resins, the mathematical approach chosen to analyze the kinetics involved on the curing reactions of the epoxy samples was the Borchardt-Daniels method, that is simpler than other kinetic models, as it allows to calculate the reaction order (n), pre-exponential factor (A) and activation energy (Ea) from one single dynamic DSC run, following the relations described on the ASTM E2041[22,31]. Based on this method, it was possible to understand the curing behavior and evaluate the curing degree (α) of the samples and also the kinetic parameters describe above. For the enthalpy (ΔH) evaluation it was considered that the solvent completely evaporated and only the weight of the resin and the hardener were taken into account. These results were also used to simulate and define the subsequent isothermal cycles conditions. 2.2.4 Isothermal Tests For the isothermal analysis samples with average 3 mg were carried out to the equipment previously settled at 90°C for a cycle during 90 minutes. A subsequent cycle was performed in order to verify the curing degree and glass transition temperature (Tg) of the samples. The equipment was cooled to 0°C and a second cycle starts with 5°C/min of heating rate until 200°C. 2.2.5 DMA The samples used on this test were prepared with the solvent content mentioned above and were placed on a silicon mold to shape the specimens with dimensions approximately (40 x 12 x 2.5) mm. Based on the BorchardtDaniels kinetic analysis results, a simulation of the curing cycle was performed to define the samples preparation conditions. Then, in order to cure the pieces, they were carried to an oven at 80°C for 3 hours, to later be used on tests. The curing cycle was different from that performed on the DSC analysis, in order to ensure that the samples were totally cured and verify the effect of different curing cycles on the behavior of the samples. The Dynamic-Mechanical Analysis experiments were carried on the DMA SII Seiko® Exstar 6000 using a dual cantilever assembly and bending operation mode with atmosphere air. The frequency adopted was constant 1 Hz, the temperature range was 25°C to 150°C using 5°C/min as heating rate and 4 N as maximum stress. The oscillation amplitude was held constant at 25 µm. The tests were performed in triplicate and the results were used to analyze the glass transition temperature (Tg) by the elastic modulus main drop and tangent delta peak.

if they can be analyzed by the same kinetic model. It can be noticed that the sample without solvent has presented just one exothermic peak due to the crosslinking process caused by the reactions between the oxirane molecules and the hardener. On the other hand, samples containing acetone have shown an endothermic peak between 45°C and 60°C, which corresponds to the boiling temperature region of acetone (approximately 55°C)[32]. Therefore, these endothermic events are related to the evaporation of solvent during the heating, retarding the curing reactions that started at higher temperatures for the samples containing 2 wt.% and 5 wt.% of solvent. Another important information is that, for the samples with 0 wt.%, 2 wt.% and 5 wt.% of solvent, the total enthalpy was similar, but the sample containing 10 wt.% of solvent has shown a different behavior and its total enthalpy of the curing reaction was really lower than the others (72% lower than the sample without solvent) which indicates that, for this amount of solvent, the curing process was deeply affected and the reactions involved may not be the same. In order to compare the reactions rate and the kinetics involved on the curing process it’s important to work with systems that presents the same reaction behavior. Therefore, the kinetics involved on the curing process for the sample with 10 wt.% of solvent cannot be compared with the other samples. Based on these considerations the kinetics analysis of this sample was disregarded. From data obtained by DSC dynamic tests, the BorchardtDaniels method were used to analyze the kinetics involved on the curing process since this model can be performed from only one DSC dynamic run[33]. In order to study kinetics parameters of the curing reactions some considerations must be taken into account. At first, this model can only be applied for dynamic analysis of a nth order reaction[22]. Based on the Raponi (2017) study about the same epoxy system which has concluded that this resin has a n-order behavior, this method can be used to analyze the kinetics involved on the curing process of this resin[34]. Another consideration is that the velocity constant k(T) is temperature dependent and follows the Arrhenius expression. Therefore, the Equation 2 can also be adapted follow the relation described on the Equation 3, which relates the reactions rate (dα/dt) with the curing degree (α), reaction order (n), pre-exponential factor

3. Results and Discussions 3.1 DSC In this section, the effects of the solvent addition on epoxy resins were evaluated by thermal analysis. Figure 1 presents the results obtained by dynamic tests on DSC in order to verify curing behavior of the samples and determine Polímeros, 31(1), e2021009, 2021

Figure 1. DSC dynamic run for Araldite®LY5052/Aradur®5052 epoxy system with 0, 2, 5 and 10 wt.% of acetone. 3/8


Rodrigues, V. C., Hirayama, D., & Ancelotti Junior, A. C. (A), activation energy (Ea), the ideal gas constant (R) and the temperature (T)[22,33]. − Ea

dα n = A.e RT . ( 1 − α ) dt

concentrations as exposed in Figure 2. It can be noticed that the maximum value of the conversion rate has decreased with the addition of solvent and also that the curing reactions started at higher temperatures in presence of solvent. These effects were due to the presence of residual solvent which difficult the interactions between the epoxy groups and hardener, reducing the curing rate. The acetone has changed the curing mechanisms in a manner that the reactions rate decreased, this result reveals that the changes on the reaction order and pre-exponential factor have affected the curing process more than the activation energy. The kinetic parameters described above were used to forecast the curing degree (α), according to the Equation 5[35], for the subsequent isothermal tests. From this relation, it is possible to set a temperature (T) and analyze the evolution of the curing degree (α) as function of the reaction time (t). Another approach is to choose the cycle time (t) of the isothermal run and observe the variation of the curing degree (α) with the temperature (T)[36]. Therefore, since many composites are processed under a constant temperature, the first approach was used for the simulation and an isothermal cycle with supposed complete curing degree for all the samples was chosen.

(3)

As an alternative for the Equation 3, the kinetic parameters can be expressed according to the relations described on the Equation 4 which follows the structure z = a + bx + cy, where z ≡ ln(dα/dt), x ≡ ln(1-α), y ≡ 1/T, a ≡ lnA, b ≡ n and c ≡ -Ea/R[31]. The results from DSC dynamic analysis were used to evaluate the curing rate (dα/dt) and the conversion degree (α) as function of the temperature (T). Using this data, the Equation 4 can be solved using multiple linear regression in order to calculate some important curing kinetics parameters as reaction order (n), activation energy (Ea) and pre-exponential factor (A).  dα ln   dt

Ea   = lnA + n.ln. ( 1 − α ) − RT 

(4)

The results obtained from the Borchardt-Daniels analysis for the kinetic parameters (A, Ea, n) are described in Table 1. It is important to notice that all these curing parameters have decreased with the presence of solvent and this reduction was greater for the samples with higher amounts of acetone. It indicates that the volume of these solvents on the resin formulation influences the velocity of the curing process. This behavior is similar to that reported on other works from literature which have concluded that the addition of solvent on epoxy systems can affect the crosslinking mechanisms due to the presence of residual solvent that may difficulty the curing process and decrease the crosslink density, affecting some kinetic parameters such as the reaction order (n) and activation energy (Ea)[6,29,30]. The reaction order (n) is mainly affected by the chemical behavior of the resin, so the reduction of reaction order observed by the addition of the solvent shows changes on the reaction mechanisms of the epoxy system during the curing process, caused by alterations in its formulation[11]. Furthermore, the reduction on activation energy (Ea) indicates that the acetone can work like a catalytic for curing processing, which does not mean that the reaction rate is higher, as the reaction rate does not depend only on this parameter. From the Equation 3 is expected that a reduction on the reaction order (n) and the pre-exponential factor (A) decreases the conversion rate (dα/dt) whereas the activation energy (Ea) has the opposite influence. Due to the contradictions in relation of conversion rate, the influence of theses parameters and the solvent addition on the curing rate were evaluated. Simulations of the evolution of the curing rate as function of the temperature were performed for the different acetone

 1   

 − Ea    1− n    RT  

α =1 −  1 − ( 1 − n ) . A.t.exp  

(5)

In DSC dynamic analysis, at the beginning of the curing process, the reactions are especially fast and disordered, which may difficult to measure the kinetic parameters. On the other hand, at the end of the curing process there are fewer molecular groups available to react, which may

Figure 2. The reaction rate (dα/dt) evolution with the temperature for the dynamic analysis of Araldite®LY5052/Aradur®5052 epoxy system with 0, 2, 5 and 10 wt.% of acetone.

Table 1. Kinetics parameters of the Araldite®LY5052/Aradur®5052 epoxy system with 0, 2, 5 and 10 wt.% of acetone obtained by DSC dynamic tests and Borchardt-Daniels analysis. Sample 1 2 3 4

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Solvent

Tonset

Tmáx

(wt.%) 0 2 5 10

(°C) 45.16 60.16 57.34 63.08

(°C) 94.47 106.91 108.07 93.88

Tendset

(°C) 147.85 180.70 177.10 156.06

-ΔHt

(J/g) 353.60 321.84 395.79 97.54

Ea

(kJ/mol) 61.10 37.29 42.75 24.31

lnA (s-1) 13.93 5.34 7.06 4.43

n 1.03 0.84 0.91 0.68

Polímeros, 31(1), e2021009, 2021


The effects of residual organic solvent on epoxy: modeling of kinetic parameters by DSC and Borchardt-Daniels method not be enough to analyze the kinetics involved. Therefore, isothermal cycle should be defined between the Tonset and the Tmáx of the curing reactions (exothermic peak) in order to improve the kinetic analysis[23]. For these reasons, the isothermal run temperature was defined as 90°C. From Figure 3, which contains the results for the simulation described above, it can be noticed that the resin without solvent is the first sample to reach the total curing degree (reaction conversion equal 1), followed by the sample with 2 wt.% of solvent and finally, the sample containing 5 wt.% of acetone. Therefore, in case of contamination of the resin by solvent, the curing time will take longer as the solvent amount present in the resin increases. According to the simulation, another important consideration is that, after 90 minutes of curing processing at 90°C, all the samples were completely cured. As previously mentioned, results of kinetics parameters can only be used to estimate the behavior of resin if the choice of the kinetic method has been made correctly. Through isothermal cycles graphs it is possible to evaluate the reaction order (n) and check if the employed method was appropriated for studied epoxy system. The model, proposed by Borchardt and Daniels, is based in the curing process of n-order reaction that is identified by the exothermic peak at t ≈ 0 for isothermal tests. Otherwise, if the maximum heat evolution is located at 30% and 40% of the curing time, the reaction can be defined as autocatalytic and could not be evaluated by the Borchardt-Daniels method[22]. In order to verify if the presence of acetone affects the reaction order, some isothermal cycles were performed at 90°C during 90 minutes as shown in Figure 4. It can be noticed that the exothermic peak for all the samples is located at the beginning of the isothermal cycle (t ≈ 0), which confirms that the curing process of this epoxy system behaves like a Nth order reaction even with the solvent addition. Since the addition of solvent changes the curing rate of the epoxy system, it was expected that it also affects some important properties of the material – for example, crosslink density. For this reason, some DSC analysis were carried in order to evaluate the changes on the glass transition temperature (Tg) of the resin and indirectly estimate alterations in crosslink degree. Figure 5 presents the Tg of the samples analyzed by the dynamic cycle performed right after the isothermal analysis. The glass transition temperature found for the epoxy system without solvent was 109°C, a value close to the one described in the literature[34]. For samples containing solvent, the values of Tg have decreased from 103°C for the resin with 2 wt.% of acetone to 79°C for the 10 wt.%. This reduction of Tg can indicate that the adopted curing cycle for this resin formulation was not appropriated in order to reach the complete cure of epoxy system. It is supposed that the acetone may works as a barrier for the interaction between the epoxy and hardener during the curing process. This barrier effect, caused by the reduction on the availability of reagents in the reaction medium, has contributed with the reduction in reaction rate. Therefore, the curing reaction takes longer to finish in presence of solvent, which means that, for same conditions of reaction, the crosslink density will be lower. This behavior can be verified indirectly by decrease of glass transition temperatures[6]. Polímeros, 31(1), e2021009, 2021

Another important observation is the occurrence of an exothermic event right after the glass transition temperature for the samples with 0 wt.%, 2 wt.% and 5 wt.% of solvent (highlighted region in Figure 5). These samples were not

Figure 3. The curing degree (α) forecast for Araldite®LY5052/ Aradur®5052 epoxy system as function of time for an isothermal cycle performed at 90 °C.

Figure 4. DSC for the isothermal curing cycle performed at 90 °C during 90 minutes for Araldite®LY5052/Aradur®5052 epoxy system with 0, 2, 5 and 10 wt.% of acetone.

Figure 5. Glass transition temperatures (Tg) for Araldite®LY5052/ Aradur®5052 epoxy system with 0, 2, 5 and 10 wt.% of acetone during the second dynamic run on DSC after the isothermal tests. 5/8


Rodrigues, V. C., Hirayama, D., & Ancelotti Junior, A. C. completely cured on the first isothermal run, despite the prediction of the Figure 3, revealing a difference between the results obtained by the Borchardt-Daniels method and the experimental tests. As discussed during this section, the addition of solvent can decrease the conversion rate during the curing reactions of the epoxy system and reduce the final crosslink density. The sample containing 10 wt.% of solvent was the only one that did not show the exothermic event after the Tg on the dynamic run. According to thermal analysis, the sample was completely cured during the isothermal cycle, however the value of Tg was much lower than expected. The solvent can produce some low-molecular-weight compounds during its evaporation and reduce the portion of the molecules capable to react during the curing process. These results confirm that the curing mechanisms are deeply affected and produce a different material when large amounts of solvent are present in resin during curing process. It is important to notice that the Borchardt-Daniels method is not capable to predict these effects and the reductions of the reactions rate[21], so this must be one of the reasons for the difference between the simulation and the tests performed. However, compared to the other methods, which generally needs a series of dynamic and isothermal tests to model the curing kinetics, the Borchardt-Daniels approach is simpler and can investigate the curing kinetics behavior quickly from just one DSC dynamic test, which makes this method attractive despite its limitations to calculate the kinetic parameters absolute values.

tests were different[37]. Furthermore, the time and temperature adopted for the curing cycle were different for used methods, therefore, the Tg will not have same values. Thus, an important analysis is that, despite the difference between the absolute values of the glass transition temperatures, the samples evaluated by the DMA tests have presented a similar behavior as reported by the DSC analysis. The addition of solvent has decreased the Tg of the epoxy system which was also described by other works[11,25,38].

Figure 6. The curing degree (α) forecast for Araldite®LY5052/ Aradur®5052 epoxy system as function of time for an isothermal cycle performed at 80 °C.

3.2 DMA In this section, thermal and mechanical response of the epoxy samples were measured by DMA tests in order to compare the changes on the glass transition temperatures (Tg) observed on the DSC analysis and also to verify the mechanical changes caused by the addition of solvent. According to DSC isothermal analysis, samples with solvent were partially cured, even after the simulation had predicted a complete curing degree (Figure 3), then the curing cycle used to prepare the samples to the DMA tests were different from analyzed on the DSC in order to ensure a complete curing process to all the samples. In previous studies, it is noted that the epoxy system Araldite® LY 5052 that adopted isothermal curing cycle of 80 °C was totally cured[34], from this information the curing cycle of resin was simulated again by Equation 5 based on isothermal of 80 °C and the results are shown on Figure 6. The simulation demonstrated that all samples reached curing degree of approximately 100% (α≅1) in approximately 140 minutes, for this reason, the curing process for the DMA samples were performed using an isothermal run at 80°C during 180 minutes. The time was extended in order to ensure that all the samples were totally cured. For the analysis of the glass transition temperature changes, the values of the samples Tg were measured by both Storage Modulus (E’) and Tan(δ) method as demonstrated in Figure 7. As the DMA measures the viscoelastic response of the material under oscillation load as function of the temperature and the DSC analyzes the energy involved on the process, absolute value obtained for the Tg from these 6/8

Figure 7. DMA analysis of the glass transition temperature (Tg) for Araldite®LY5052/Aradur®5052 epoxy system with 0, 2, 5 and 10 wt.% of acetone using both methods (a) Storage Modulus (E’) as function of temperature (b) Tan(δ) as function of temperature. Polímeros, 31(1), e2021009, 2021


The effects of residual organic solvent on epoxy: modeling of kinetic parameters by DSC and Borchardt-Daniels method From the Figure 7.a, using the Storage Modulus (E’) method, it can be noticed that the glass transition temperature (Tg) has reduced with the presence of solvent. This behavior is also represented at the Figure 7.b where the Tg was evaluated by the Tan(δ). The glass transition temperature is related with the molecular mobility and depends on the curing degree, so during the curing of thermosetting polymers and after the vitrification process, the crosslink density becomes higher and the mobility is reduced[39]. As mentioned before, the presence of solvent can decrease the crosslink density which increases the mobility of the amorphous regions and reduces the glass transition temperature of the resin. Another important information is that the highest value of storage modulus (E’) of the samples has also decreased with the addition of acetone. Loos (2008) has also reported that the presence of these additives may also decreased Young’s modulus, tensile strength and elongation at break. These changes are due to the lower crosslink degree caused by the presence of solvent which causes a reduction in mechanical properties[29].

4. Conclusions The effects of acetone addition on the epoxy system Araldite® LY5052/Aradur® 5052 properties and its influence on the curing kinetics were studied. The results showed that the presence of this additives decreases the velocity of the curing reactions and are related with the reduction of the Tg and storage modulus (E’) of the resin. For this reason, the addition of solvent on epoxies formulation should not be disregarded, as it can affect, for example, the curing cycle of composite materials. Furthermore, the epoxy samples containing 10 wt.% of solvent have presented important changes on the curing mechanisms, so that may be a limit for the acetone addition due to the low quality of the final properties of the resin. The Borchardt-Daniels analysis have shown a reduction of some important curing parameters such as activation energy (Ea), reaction order (n) and pre-exponential factor (A) with the addition of solvent on the epoxy formulation. It is important to notice that this method have some limitations and the values obtained for the kinetic parameters are generally overestimated. However, this model provided a great overview of the behavior of epoxy curing kinetics, from only a single dynamic DSC scan, which allowed to relate the variations of the curing process with the changes in the material properties. There are several other methods that presents a better accuracy for the kinetic parameters, but they are more complex and generally take longer to be performed. Therefore, based on the results of this work, the Borchardt-Daniels method presents a great alternative, simpler and faster than the other kinetic methods, to understand the behavior of the curing reactions and the changes caused by the additives, being a very useful model to define the curing cycle of epoxy resins.

5. Acknowledgments The authors would like to thank the financial support from the Brazilian agency CNPq (Conselho Nacional de Desenvolvimento Científico) for the research funding on Polímeros, 31(1), e2021009, 2021

the following projects: 307446/2020-4, 311709/2017-6 and 431219/2018-4.

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28. Huntsman. (2012). Technical Data Sheet - Araldite ® LY 5052 / Aradur ® 5052. Retrieved 2020, June 12, from https://samaro. fr/pdf/FT/Araldite_FT_LY_5052_Aradur_5052_EN.pdf 29. Hong, S. G., & Wu, C. S. (2000). DSC and FTIR analyses of the curing behavior of epoxy/dicy/solvent systems on hermetic specimens. Journal of Thermal Analysis and Calorimetry, 59(3), 711-719. http://dx.doi.org/10.1023/A:1010189301221. 30. Hong, S. G., & Wu, C. S. (1998). DSC and FTIR analysis of the curing behaviors of epoxy/DICY/solvent open systems. Thermochimica Acta, 316(2), 167-175. http://dx.doi.org/10.1016/ S0040-6031(98)00356-6. 31. American Society for Testing and Materials – ASTM (2015). 2041 Estimating Kinetic Parameters by Differential Scanning Calorimeter Using the Borchardt and Daniels Method 1. West Conshohocken, PA: ASTM. https://doi.org/10.1520/E204113E01.2 32. Ma, X., Zhang, F., Han, K., Yang, B., & Song, G. (2015). Evaporation characteristics of acetone-butanol-ethanol and diesel blends droplets at high ambient temperatures. Fuel, 160, 43-49. http://dx.doi.org/10.1016/j.fuel.2015.07.079. 33. Cervi, G., Pezzin, S. H., & Meier, M. M. (2017). Differential scanning calorimetry study on curing kinetics of diglycidyl ether of bisphenol A with amine curing agents for self-healing systems. Revista Materia, 22(2), 3-8. http://dx.doi.org/10.1590/ s1517-707620170002.0183. 34. Raponi, O. A., Raponi, R. A., Barban, G. B., Benedetto, R. M. D., & Ancelotti Junior, A. C. (2017). Development of a simple dielectric analysis module for online cure monitoring of a commercial epoxy resin formulation. Materials Research, 20(Suppl. 2), 291-297. http://dx.doi.org/10.1590/1980-5373mr-2017-0067. 35. Costa, M. L., Rezende, M. C., de Paiva, J. M. F., & Botelho, E. C. (2006). Structural carbon/epoxy prepregs properties comparison by thermal and rheological analyses. PolymerPlastics Technology and Engineering, 45(10), 1143-1153. http://dx.doi.org/10.1080/03602550600887251. 36. de Andrade Raponi, O., Righetti de Souza, B., Miranda Barbosa, L. C., & Ancelotti, A. C. Jr. (2018). Thermal, rheological, and dielectric analyses of the polymerization reaction of a liquid thermoplastic resin for infusion manufacturing of composite materials. Polymer Testing, 71(July), 32-37. http://dx.doi. org/10.1016/j.polymertesting.2018.08.024. 37. Carbas, R. J. C., Marques, E. A. S., Da Silva, L. F. M., & Lopes, A. M. (2014). Effect of cure temperature on the glass transition temperature and mechanical properties of epoxy adhesives. The Journal of Adhesion, 90(1), 104-119. http:// dx.doi.org/10.1080/00218464.2013.779559. 38. Le Craz, S., & Pethrick, R. A. (2011). Solvent effects on cure 1-benzyl alcohol on epoxy cure. International Journal of Polymeric Materials and Polymeric Biomaterials, 60(7), 441-455. http://dx.doi.org/10.1080/00914037.2010.531813. 39. Montserrat, S., & Cima, I. (1999). Isothermal curing of an epoxy resin by alternating differential scanning calorimetry. Thermochimica Acta, 330(1–2), 189-200. http://dx.doi. org/10.1016/S0040-6031(99)00033-7. Received: Nov. 05, 2020 Revised: Mar. 21, 2021 Accepted: Mar. 22, 2021

Polímeros, 31(1), e2021009, 2021


ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.08820

O Induction of defense in apples by sulfated and deacetylated O chichá gum O Carlos Pinheiro Chagas de Lima , Andréia Hansen Oster , Fábio Rossi Cavalcanti , O Regina Célia Monteiro de Paula  and Judith Pessoa Andrade Feitosa *  O Laboratório de Polímeros - LabPol, Departamento de Química Orgânica e Inorgânica - DQOI, Universidade Federal do Ceará - UFC, Fortaleza, CE, Brasil O Empresa Brasileira de Pesquisa Agropecuária - Embrapa Uva e Vinho, Bento Gonçalves, RS, Brasil O Abstract O Elicitors activate the defense mechanism in plants to resist pathogens. Ulvans and glucuronans can act as elicitors, and their activity seems to be related to the sulfate groups, rhamnose and uronic acid monosaccharides. Chichá gum (CHG), O which also contains rhamnose and uronic acid, was sulfated with chlorosulfonic acid/N,N-dimethylformamide and deacetylated with sodium hydroxide solution. The changes were confirmed by infrared spectroscopy. Carbon-13 NMR O revealed that sulfation occurred in galactose and rhamnose units. The apples were sprayed with water (negative control), deacetylated chichá gum (DCHG), and sulfated chichá gum (SCHG). The activity of enzymes guaiacol peroxidase O and polyphenol oxidases and the lignin content were compared with those under the action of a commercial elicitor, benzothiadiazole. DCHG, and especially SCHG, increased the activity of the two enzymes. Only fruits treated with O SCHG showed a significant (p<0.05) increase in lignin content. The plant exudate can be one abundant, renewable and safe source of elicitors. O Keywords: benzothiadiazole (BTH), Sterculia striata, elicitor, polysaccharide, sulfation. O How to cite: Lima, C. P. C., Oster, A. H., Cavalcanti, F. R., Paula, R. C. M., & Feitosa, J. P. A. (2021). Induction of defense in apples by sulfated and deacetylated chichá gum. Polímeros: Ciência e Tecnologia, 31(1), e2021010. https:// doi.org/10.1590/0104-1428.08820 O O 1. Introduction 1

2

1

2

1

1

2

*judith@dqoi.ufc.br

Elicitors are molecules capable of induce defense responses. Because these molecules can protect plants against pathogens, they are promising for reducing the use of agrochemicals[1]. Among the various compounds that may act as elicitors, there are the polysaccharides from seaweed[2,3]. The most promising algae-derived polysaccharides for defense induction in plant tissue are ulvans and glucuronans, which are widely available due to the abundance of green algae[3,4]. Ulvans protected tomato plants against Alternaria solani and Xanthomonas vesicatoria[5] and apple fruits against Penicillium expansum and Botrytis cinerea[2]. Ulvans are basically composed of rhamnose (16.8‒45.0%), sulfate groups (16.0‒23.2%), uronic acids (6.5‒19%), xylose (2.1‒12.0%), iduronic acid (1.1‒9.1%), and glucose (0.5‒6.4%)[4]. According El Modafar et al. (2012)[6] the ability of ulvans to induce plant defense may be related to the presence of rhamnose and sulfate groups. Glucuronans, to a lesser extent, and oligoglucuronans, to a greater extent, protected tomato seedlings against Fusarium oxysporum f. sp. Lycopersici[6] and apples against Penicillium expansum and Botrytis cinerea[3]. As glucuronans are composed primarily of residues of glucuronic acids, and these residues may be related to the defense-inducing activity.

Polímeros, 31(1), e2021010, 2021

Three polysaccharide subunits are reportedly related to the elicitation of apples defenses: rhamnose, uronic acid, and sulfate groups. A polysaccharide extracted from the exudate of Sterculia striata, commonly known as chichá gum (CHG), has - in addition to xylose (5.6‒7.7%), galactose (19.3‒23.4%), and acetyl groups (9.6‒10.7%) - rhamnose (23.1‒28.8%), and uronic acids (42.2‒49.2%)[7,8]. Chemical modifications that remove acetyl groups (deacetylation) and add sulfate groups (sulfation) could make the composition of CHG more similar to that of polysaccharides from green algae, making CHG more interesting for apple elicitation tests. In addition, the synthesis and physiological role of sulfated polysaccharides has receiving great attention so far and is another novelty in this work. Apples are one of the most pesticide-treated crops[9]. The ‘Pink Lady’ cultivar was selected as it is susceptible to diseases and thus requires high use of agrochemicals[10]. The objective of this work is to deacetylate and sulfate (for the first time) CHG, characterize the derivatives, and evaluate the ability of these polysaccharides to enhance the protection of “Pink Lady” apples against pathogens. The results are compared with those of a widely used synthetic elicitor, benzothiadiazole (BTH). The carcinogenicity of BTH is

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Lima, C. P. C., Oster, A. H., Cavalcanti, F. R., Paula, R. C. M., & Feitosa, J. P. A. classified as Category 1A[11], which means: “known to have carcinogenic potential for humans[12].

2. Materials and Methods

= DS

2.1 Materials Sterculia striata exudate was obtained from a tree located in Fortaleza-Ceará (Brazil). BTH, sodium hypochlorite, phosphate buffered saline (pH 7.4), citric acid, bovine serum albumin, monolignol (guaiacol) peroxidase, guaiacol, hydrogen peroxide, pyrocathecol, acetone, thioglycolic acid, and 2-hydroxypropyl ether were supplied by Sigma-Aldrich, São Paulo, Brazil. Sodium hydroxide, N,N-dimethylformamide (DMF), and chlorosulfonic acid (CSA) were obtained from Vetec, Brazil. Sodium chloride, hydrochloric acid, and ethanol were purchased from Synth, São Paulo, Brazil.

2.2 Isolation of CHG The gum was isolated by the methodology of Brito et al.[8], with adaptations. An exudate aqueous solution (1% w/v) was filtered through a sintered plate funnel (G1) and the pH was adjusted to 7.0 by adding of 1.0 mol.L‒1 NaOH. Sodium chloride (half of the exudate mass) was added. After 3 h, the gum was precipitated in 2 volumes of ethanol and filtered through a sintered plate funnel (G4). The precipitate was washed three times with ethanol and dried in a desiccator under vacuum. The obtained sample is denoted as CHG.

2.3 Sulfation of chichá gum The sulfation of CHG was based on the method reported by Pires et al. (2013)[13], with several modifications. 1 g sample of the polysaccharide and 50 mL of DMF were placed in a round-bottom flask (250 mL). After 15 h under stirring, another 50 mL aliquot of DMF was added. In an ice bath, 6 mL of CSA was slowly added to the solution, and the reaction was allowed to run for 3 h at room temperature (around 28 °C). Addition of 200 mL of ethanol stopped the reaction. To improve the precipitation of the sulfated derivative, 0.5 g of NaCl was added. The precipitate was retained in a sintered plate funnel (G3), washed twice with ethanol, and dissolved in 200 mL of distilled water. The pH was adjusted to 7.0 with 1.0 mol.L‒1 NaOH, and the solution was dialyzed against distilled water. The sample was recovered by lyophilization, and the sulfated gum (SCHG) was obtained.

2.4 Deacetylation of CHG The procedure reported by Brito et al. (2005)[7] was used for deacetylation of chichá gum. CHG aqueous solution (200 mL, 1% w/v) was mixed with 200 mL of 1.0 mol.L‒1 NaOH solution under stirring. After 20 min, the mixture was neutralized using 6.0 mol.L‒1 HCl, and dialyzed for 4 days, while monitoring the conductivity. The solid obtained by lyophilization is denoted as deacetylayed chichá gum (DCHG).

2.5 Characterization of chichá gum and its derivatives 2.5.1 Determination of the degree of sulfation of SCHG The percentage of carbon (%C) and sulfur (%S) in the sulfated derivative was measured using Perkin Elmer 2/8

2400 Series II CHNS/O microanalyzer (Massachusetts, US). Equation 1 was used to calculate the degree of sulfation (DS):

( % S / Ms ) / ( %C / 6 ×Mc ) (1)

where Ms and Mc are the atomic mass of sulfur and carbon, respectively. 2.5.2 Fourier-transform infrared spectroscopy (FTIR) The absorption spectra of CHG, SCHG, and DCHG in the infrared region were obtained for the samples in KBr pellets by using a Shimadzu IRT Race100 FTIR spectrophotometer, in the range of 400‒4000 cm‒1. 2.5.3 Thermal analysis (TGA, DSC) Samples of the gums (10 mg) were subjected to thermogravimetric analysis on TGAQ50 equipment (TA Instruments) under synthetic air atmosphere at a heating rate of 10 °C min‒1. The dehydration and total decomposition rates were identified to determine the moisture and ash content, respectively. The differential scanning calorimetry curves were obtained on Shimadzu DSC50 equipment under nitrogen atmosphere using a flow of 50 mL min‒1 and 4.0 mg of sample, in the temperature range of 27‒450 °C, at a heating rate of 10 °C min‒1. 2.5.4 Gel permeation chromatography (GPC) Solutions of the gums (0.1% w/v) in 0.1 mol L‒1 sodium nitrate were prepared by magnetic stirring in a water-bath (70 °C) for 12 h, and filtered through a 0.45 μm Millipore membrane. Shimadzu equipment (Kyoto, Japan) consisting of a pump (LC10AD) coupled to a refractive index detector (RID6A) was employed. A Phenomenex precolumn PolySepGFCP (35 mm × 7.80 mm) and PolySepGFCP linear column (300 mm × 7.80 mm) were used for the separation process. A 0.1 mol L‒1 NaNO3 solution was used as the eluent at a flow rate of 1.0 mL min‒1 and sample volume was 50 μL. The standard curve for the molar mass determination was constructed using polystyrene sulfonate standards (logM = 13.92 – 0.99Ve) according to Dupont[14]. 2.5.5 Carbon-13 nuclear magnetic resonance (13C-NMR) The gums were dissolved in D2O with 1% sodium 2,2-dimethylsilapentane-5-sulfonate (DSS) for zero calibration of the chemical shift. CHG was subjected to partial acid hydrolysis to improve the spectral resolution. The spectra were obtained at 70 °C on a Bruker Avance RX500 model spectrometer (Massachusetts, US).

2.6 Biochemical tests 2.6.1 Material preparation and fruit treatments The polysaccharides and BTH were dissolved in distilled water at a concentration of 5 mg mL‒1 for DCHG and SCHG[2,3] and 0.4 mg mL‒1 for BTH[10] (positive control, Ctrl+). ‘Pink Lady’ apples were harvested in Vacaria, Rio Grande do Sul state (Brazil) and the fruits were further sanitized in 2% (v/v) sodium hypochlorite solution. Once dried, the apples were sprayed with distilled water (negative control, Ctrl‒), the gums, and BTH. In a completely randomized experiment, Polímeros, 31(1), e2021010, 2021


Induction of defense in apples by sulfated and deacetylated chichá gum three apple samples (per treatment) were taken at times of 12, 24, 48, and 72 h after spraying (HAS) for pulp sampling. The soluble protein, extracted lignin, and enzyme activity were evaluated. For the chosen time interval, the apples were subjected to B. D. O (Biochemical Oxygen Demand) chamber conditions for 24 h in the dark at 25 ºC. 2.6.2 Enzyme and lignin determination Pulp samples were taken from apple fruits with a spudger and homogenized for 2 min in a mortar in 1:3 (w/v) dilution with 100 mmol L‒1 PBS pH 7.4 buffer containing 25 mmol L‒1 citric acid (cooled). After gauze filtration, the fresh material was centrifuged at 12,400 ×g for 10 min at 4 °C. The crude extract obtained from the supernatant was used for enzymatic determinations[15]. The soluble protein (mgP mL‒1) from the crude extracts was pre-determined using 0.3 mol L‒1 bovine serum albumin (BSA)[16]. To determine the enzyme activity, 100 μL of crude extract (supernatant) was used as the substrate in the enzyme mixtures comprising 2.0 mL of a solution containing 50 mmol L-1 sodium acetate buffer (pH 6.8), 20 mmol L-1 guaiacol and 30 mmol L-1 hydrogen peroxide for guaiacol peroxidase (GPX, EC 1.11.1.7). The procedure was the same for polyphenol oxidases (PPO, EC 1.10.3.1), however without hydrogen peroxide and with 30 mmol L‒1 pyrocathecol instead of guaiacol. The activity of the two enzymes was recorded over the course of 10 min using a spectrophotometer. The GPX activity and PPO were read at 480 and 410 nm, respectively[15]. The relative activity unit (UA) of both oxidases was defined as the change in the enzyme absorbance at the respective wavelengths by one milligram of soluble protein per min (UA mg.P‒1min‒1). For the lignin determination, 0.2 g fresh samples of ‘Pink Lady’ apple pulps were powdered in a mortar with liquid nitrogen for 3 min. The samples were incubated in 85% acetone for 48 h. After centrifugation at 8,000 ×g for 15 min at 7 °C, the dried precipitate was resuspended in 5 mL thioglycolic acid diluted in 2 mol L‒1 hydrochloric acid (1:10, v/v). The suspensions were incubated for 4 h at 25 °C and centrifuged at 8,000 × g. The supernatant was then transferred to a 20 mL tube; 200 µL of 10 mol L‒1 hydrochloric acid was added and the mixture was incubated in an icebath for 4 h. After centrifugation at 8.000 × g, the pellet was homogenized in 5 mL of 0.5 mol L‒1 NaOH and the absorbance was measured at 280 nm. Thioglycolic acid

derivatives (lignin) were measured by comparison with a 10–100 µg mL‒1 standard curve for 2-hydroxypropyl ether[17]. 2.6.4 Statistical analysis Regarding enzyme responses, descriptive statistics and standard deviations between gum-, Bion- treatments and water-pre-treated (Ctrl-) apples were compared with vertical bars beside averages in each hour of time interval. For the lignin determinations, normality (Shapiro-Wilk), homoscedasticity (Bartlett), ANOVA and Tukey tests were run at 5% significance with a specific R script (R version 3.5.0 - The R Foundation, 2018).

3. Results and Discussions 3.1 Characterization of chichá gum and its sulfated and deacetylated derivatives The isolation, sulfation, and deacetylation yields were 52, 105, and 76%, respectively. A yield exceeding 100% is justified because hydrogen atoms (1.0 g mol-1) in the molecular structure are replaced by SO3Na groups (103.1 g mol‒1) during the sulfation process. Other authors have observed sulfation yields above 100%[18,19]. The percentage of sulfur and degree of sulfation achieved were 8.7% and 0.82, respectively. 3.1.1 Infrared absorption spectroscopy (FTIR) The spectra of chichá gum and its derivatives are shown in Figure 1a. The band at 3421 cm‒1 is attributed to the stretching of the O-H bonds[20]. This band was narrower for the SCHG, plausibly because for each sulfate group inserted, one O-H group is lost. For the DCHG, this band was slightly wider. The bandwidth is related to intermolecular interactions. Stronger interaction leads to a wider bandwidth[21]. The bands at 2933 and 1378 cm‒1 were attributed to C-H (CH3) stretching and deformation, respectively[21,22]. These two bands were similar for CHG and SCHG because the sulfation process does not change the number of C-H bonds in the macromolecule. For DCHG, the band at 2933 cm‒1 became narrower and that at 1378 cm‒1 practically disappeared due to loss of the acetyl groups. The bands at 1148, 1066, and 1042 cm‒1 (fingerprint region) correspond to the stretching vibrations of various C-O-C bonds present in carbohydrates in general[22]. The band at 1730 cm‒1, which was absent in the FTIR profile of

Figure 1. (a) FTIR and b) GPC profiles of CHG and its sulfated and deacetylated derivatives. Polímeros, 31(1), e2021010, 2021

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Lima, C. P. C., Oster, A. H., Cavalcanti, F. R., Paula, R. C. M., & Feitosa, J. P. A. DCHG, and present in that of CHG and SCHG, is assigned to C=O from the acetyl groups. Intensification of the band at 1250 cm‒1 was observed for SCHG. This absorbance is assigned to the S=O bonds of sulfate groups, and also to the acetyl group[21,23]. For this reason, this band is not present in the spectrum of DCHG. The bands appearing at 818 and 584 cm‒1 are due to the asymmetric and symmetric stretching of the O=S=O bonds of the sulfate group, respectively[24]. 3.1.2 Gel permeation chromatography (GPC) Figure 1b shows that the modified polysaccharides required a larger volume of eluent for detection. This suggests that sulfation and deacetylation degrade the polysaccharide. The average molar masses (MW and MN) of CHG are 15.1 × 106 and 0.42 × 106 g mol‒1, respectively. Average molar masses of the same order of magnitude were reported for a similar polysaccharide (karaya gum), i.e., MW = 16.1 × 106 and MN = 1.1 × 106 g mol‒1 [25]. After deacetylation of CHG, the MW decreased to 4.8 × 106 g mol‒1. A similar value was observed for deacetylated karaya gum, i.e., 1.8 × 106 g mol‒1 [22]. Opposite results, i.e., an increase in the MW after deacetylation of karaya gum (2‒5 × 106 to 12‒16 × 106 g.mol‒1) was reported by Le Cerf, Irinei, & Muller (1990)[26]. However, the authors explained that deacetylation enables water solubilization of high-MW polymeric chains at high pH, while solubilization of the original gum without adjusting the pH was only partial, where only fractions of lower MW were soluble. The molar mass of SCHG declined to 3.8 × 106 g mol‒1 (around four times), but no sulfation studies were found for karaya or CHG to compare. 3.1.3 Thermal analysis (TGA, DSC) SCHG and DCHG were less thermally stable than the CHG (Figure 2a). Degradation began at a lower temperature for SCHG and DCHG (217.5±0.5 °C) than for CHG (236 °C). The moisture content of CHG is close to that of DCHG (18.1±0.1%) and is greater than that of SCHG (15.7%). These values are in the moisture content range of karaya gum: 13%[27] and 20%[22]. The moisture content of DCHG was higher than the values reported for deacetylated karaya gum, i.e., 13‒15%[22]. More residue was generated at 800 °C for SCHG (21.8%) than for CHG (9.2%) and DCHG (4.6%). The increase in the amount of residue after sulfation is due

to the presence of the sodium (Na+) counter ion of the sulfate groups[13]. The lower value for DCHG compared with that of CHG may be due to unintended purification of the gum during deacetylation. Figure 2b shows the DSC curves of CHG and its derivatives. Two events were observed for all polysaccharides. The first (endothermic) is related to water loss, and the second (exothermic) to thermal decomposition. The enthalpy of water loss followed the order: DCHG (677 J g‒1) > CHG (575 J g‒1) > SCHG (345 J g‒1). The order was maintained even when the difference in the moisture content of the samples was taken into account. The maximum degradation occurred at 280±2 °C for the CHG and DCHG, and at 236 °C for the sulfated polysaccharide. The enthalpy of degradation was lowest for SCHG (117 J g‒1), whereas the values for DCHG and CHG were close, i.e., 156 and 163 J g‒1, respectively. Sulfation apparently weakened the chemical bonds of the polysaccharides. 3.1.4 Carbon-13 nuclear magnetic resonance (13C-NMR) Figure 3 shows the 13C-NMR spectrum of SCHG. The signal/noise ratio is somewhat poor; nevertheless, important information can be derived. The spectra of CHG and DCHG presented in Supplementary Material Figure S1 are similar to those reported by Brito et al.[8]. The absence of the band at 23.31 ppm ppm in the DCHG spectrum reaffirms the success of deacetylation and corroborates the FTIR results. The spectra of CHG and DCHG show signals at 60.46, 61.70, 61.21, and 61.63 ppm. These peaks are absent in the profile of SCHG, and can be attributed to carbon 6 of the galactose residues[28,29]. Another possibility is that the peaks may be attributed to the 4-O-methyl glucuronic or 4-O-methyl- galacturonic group, but this polysaccharide does not possess these residues[8]. The two new peaks that emerged in the 13C-NMR spectrum of SCHG at 68.86 and 70.07 ppm, attributed to the sulfated carbon 6 of the galactose residues, shifted by ~6 ppm to lower-field. The peak at 23.31 ppm in the 13C-NMR spectrum of SCHG is due not only to methyl acetyl groups[8,25], but also to the methyl groups of sulfated rhamnose, and was shifted by ~4 ppm. The 13C-NMR spectra of sulfated materials are known to be more complicated because the sulfate groups withdraw electrons from the carbons to which they bond, shifting the signals of these carbons to lower-field, and donate electrons

Figure 2. (a) TGA (in air) and (b) DSC (in N2) curves of CHG its derivatives. 4/8

Polímeros, 31(1), e2021010, 2021


Induction of defense in apples by sulfated and deacetylated chichá gum

Figure 3. 13C-NMR spectrum of SCHG.

Figure 4. GPX activity in Pink Lady apples after spraying with distilled water (control, Ctrl), and DCHG (a), SCHG (b), and BTH (c).

Figure 5. PPO activity in Pink Lady apples after spraying with distilled water (control, Ctrl), and DCHG (a), SCHG (b), and BTH (c).

to neighboring carbons, shifting these carbon signals to higher-field[29,30].

3.2 Enzymes and lignins related to plant defense in ‘Pink Lady’ apples exposed to elicitors

(coniferyl, sinapyl, and p-coumaryl alcohols) and their coupling in the subunits of heterogeneous polyphenols. Lignin is associated with the induction of plant defense as it strengthens the cell walls, making it difficult for pathogens to enter[31,32].

The activity of the guaiacol peroxidase (GPX) and polyphenoloxidase (PPO) enzymes was recorded for gumtreated and water-treated (Ctrl‒) ‘Pink Lady’ apple pulps. The BTH was adopted as a positive control to test the elicitor properties of the gums. The GPX activity increased significantly (p<0.05) with DCHG and SCHG at 12‒48 h (Figure 4a and b). BTH (0.4 mg mL‒1) induced increase in the GPX activity at 12 h, while there was no significant increase (p>0.05) at the other times. GPX plays an essential role in lignin biosynthesis in plant tissues as these enzymes catalyze the cross-linking of phenylpropanoid route monomers

The activity of PPO in apple pulp increased (p<0.05) in the interval 12‒48 h exposure to DCHG (Figure 5b), as reported for GPX. However, in apples exposed to SCHG and BTH, a more transient PPO response was triggered. PPO activity peaked for 24 h for both, but much less intense for apples treated with Bion. At 48 and 72 h, PPO activity was less than to control for fruits treated with SCHG. There are few specific studies to reveal details of the post-harvest enzyme responses in apples. What can be factually inferred is that SCHG gum apparently could promote greater GPX responses at the expense of PPO, and this seems to indicate

Polímeros, 31(1), e2021010, 2021

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Lima, C. P. C., Oster, A. H., Cavalcanti, F. R., Paula, R. C. M., & Feitosa, J. P. A. in “Pink Lady” apples. The amount of lignin extracted from the apples was greater than the control only for fruits sprayed with SCHG. The best defense induction was achieved with the polysaccharide (SCHG) that contained the three subunits associated with plant defense: rhamnose, uronic acid, and sulfate groups. This study demonstrates that plant exudate can be an abundant, renewable and safety source of elicitors.

5. Acknowledgements

Figure 6. Lignin content (TGA derivatives, mg g‒1 FW) extracted from apple (cv. Pink Lady) pulps 72 h after spraying with elicitors. The control (Ctrl‒) apples were sprayed with distilled water. Same letters indicate that the values did not differ significantly according to Tukey’s test (p>0.05).

that GPX may have compensated PPO in the enhanced polymerization of the lignin monomers observed in SCHG treated apples. PPOs are also considered enzymes that are markers of the resistance of plants against pathogens. These enzymes oxidize phenolic compounds to toxic quinones that can act against invading pathogens[3] and are key enzymes for the synthesis of lignin[33]. Because the two enzymes analyzed are involved in the synthesis of lignin, the content of lignin in the fruits was quantified 72 h after treatment (Figure 6). As expected, due to the increase in the activity of GPX and PPO, the fruits treated with DCHG, SCHG, and BTH presented lignin levels numerically higher than that of the control. However, only the fruits treated with SCHG showed a significant (p<0.05) increase in the lignin content. The lignin content of the fruits treated with DCHG and BTH was statistically similar to that of the control (p>0.05). The weak induction of defense compounds (GPX, PPO and lignin) of BTH in “Pink Lady” apples can be explained by the reported low performance of BTH in this cultivar. Marolleau et al.[10] treated Elstar, Fuji, Gala, Golden, and “Pink Lady” apple cultivars with BTH, and reported that the “Pink Lady” cultivar had the lowest level of defense induction. The results suggest ‘de novo’ lignin synthesis triggered by the gums, mainly SCHG, in “Pink Lady” apples. The gum that induced the strongest defense response is the one that simultaneously possesses the three residues suggested by the literature as responsible for the induction of defense compounds: rhamnose, uronic acid, and sulfate groups. This work reaffirms the importance of these residues and suggests that plant exudate polysaccharides should be further explored, as they may be a promising source of defense-inducing compounds in plants.

4. Conclusions The polysaccharide from Sterculia striata (chichá gum,CHG) was deacetylated (DCHG) and, for the first time, sulfated (SCHG). The derivatives were less thermally stable and the average molar mass was lower than that of the original gum. Sulfation occurred at carbon 6 of the galactose and rhamnose residues. Both derivatives of CHG have the ability to induce increased defense-related enzyme activity 6/8

This work was supported by the Coordination Foundation for the Improvement of Higher Education Personnel (CAPES-Brazil), CNPq, the Cearense Foundation for Research Support (FUNCAP-CE-Brazil), INOMAT, and the Brazilian Agricultural Research Corporation (EmbrapaBrazil).

6. References 1. Iriti, M., & Vitalini, S. (2021). Plant immunity and crop yield: a sustainable approach in agri-food systems. Vaccines, 9(2), 1-3. http://dx.doi.org/10.3390/vaccines9020121. PMid:33546315. 2. Abouraïcha, E., El Alaoui-Talibi, Z., El Boutachfaiti, R., Petit, E., Courtois, B., Courtois, J., & El Modafar, C. (2015). Induction of natural defense and protection against Penicillium expansum and Botrytis cinerea in apple fruit in response to bioelicitors isolated from green algae. Scientia Horticulturae, 181, 121-128. http://dx.doi.org/10.1016/j.scienta.2014.11.002. 3. Abouraïcha, E. F., El Alaoui-Talibi, Z., Tadlaoui-Ouafi, A., El Boutachfaiti, R., Petit, E., Douira, A., Courtois, B., Courtois, J., & El Modafar, C. (2017). Glucuronan and oligoglucuronans isolated from green algae activate natural defense responses in apple fruit and reduce postharvest blue and gray mold decay. Journal of Applied Phycology, 29(1), 471-480. http://dx.doi. org/10.1007/s10811-016-0926-0. 4. Stadnik, M. J., & Freitas, M. B. (2014). Algal polysaccharides as source of plant resistance inducers. Tropical Plant Pathology, 39(2), 111-118. http://dx.doi.org/10.1590/S198256762014000200001. 5. Ramkissoon, A., Ramsubhag, A., & Jayaraman, J. (2017). Phytoelicitor activity of three Caribbean seaweed species on suppression of pathogenic infections in tomato plants. Journal of Applied Phycology, 29(6), 3235-3244. http://dx.doi. org/10.1007/s10811-017-1160-0. 6. El Modafar, C., Elgadda, M., El Boutachfaiti, R., Abouraicha, E., Zehhara, N., & Petit, E. (2012). Induction of natural defence accompanied by salicylic acid dependant systemic acquired resistance in tomato seedlings in response to bioelicitors isolated from green algae. Scientia Horticulturae, 138, 55-63. http://dx.doi.org/10.1016/j.scienta.2012.02.011. 7. Brito, A. C. F., Sierakowski, M. R., Reicher, F., Feitosa, J. P. A., & Paula, R. C. M. (2005). Dynamic rheological study of Sterculia striata and karaya polysaccharides in aqueous solution. Food Hydrocolloids, 19(5), 861-867. http://dx.doi. org/10.1016/j.foodhyd.2004.10.035. 8. Brito, A. C. F., Silva, D. A., Paula, R. C. M., & Feitosa, J. P. A. (2004). Sterculia striata exudate polysaccharide: characterization, rheological properties and comparison with Sterculia urens (karaya) polysaccharide. Polymer International, 53(8), 10251032. http://dx.doi.org/10.1002/pi.1468. 9. Tzatzarakis, M., Kokkinakis, M., Renieri, E., Goumenou, M., Kavvalakis, M., Vakonaki, E., Chatzinikolaou, A., Stivaktakis, P., Tsakiris, I., Rizos, A., & Tsatsakis, A. (2020). Multiresidue analysis of insecticides and fungicides in apples from the Greek market. Applying an alternative approach for risk assessment. Polímeros, 31(1), e2021010, 2021


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22. Padil, V. V. T., Senan, C., & Černík, M. (2015). Dodecenylsuccinic anhydride derivatives of gum karaya (Sterculia urens): Preparation, characterization, and their antibacterial properties. Journal of Agricultural and Food Chemistry, 63(14), 3757-3765. http:// dx.doi.org/10.1021/jf505783e. PMid:25797306. 23. Salehi, P., Dashti, Y., Tajabadi, F. M., Safidkon, F., & Rabei, R (2011). Structural and compositional characteristics of a sulfated galactan from the red alga Gracilaria psispérsica. Carbohydrate Polymers, 83(4), 1570-1574. http://dx.doi. org/10.1016/j.carbpol.2010.10.017. 24. Cakić, M., Nikolić, G., Ilić, L., & Stanković, S. (2005). Synthesis and FTIR characterization of same dextran sulphates. Chemical Industry & Chemical Engineering Quarterly, 1(2), 74-78. http://dx.doi.org/10.2298/CICEQ0502074C. 25. Postulkova, H., Chamradova, I., Pavlinak, D., Humpa, O., Jancar, J., & Vojtova, L. (2017). Study of effects and conditions on the solubility of natural polysaccharide gum karaya. Food Hydrocolloids, 67, 148-156. http://dx.doi.org/10.1016/j. foodhyd.2017.01.011. 26. Le Cerf, D. L., Irinei, F., & Muller, G. (1990). Solution properties of gum exudates from Sterculia urens (karaya Gum). Carbohydrate Polymers, 13(4), 375-386. http://dx.doi. org/10.1016/0144-8617(90)90037-S. 27. Singh, B., Sharma, V., & Pal, L. (2011). Formation of Sterculia polysaccharide networks by gamma rays induced graft copolymerization for biomedical applications. Carbohydrate Polymers, 86(3), 1371-1380. http://dx.doi.org/10.1016/j. carbpol.2011.06.041. 28. Singh, B., & Singh, B. (2017). Influence of graphene-oxide nanosheets impregnation on properties of Sterculia gumpolyacrylamide hydrogel formed by radiation induced polymerization. International Journal of Biological Macromolecules, 99, 699-712. http://dx.doi.org/10.1016/j.ijbiomac.2017.03.037. PMid:28284934. 29. Wang, J., Guo, H., Zhang, J., Wang, X., Zhao, B., Yao, J., & Wang, Y. (2010). Sulfated modification, characterization and structure–antioxidant relationships of Artemisia sphaerocephala polysaccharides. Carbohydrate Polymers, 81(4), 897-905. http://dx.doi.org/10.1016/j.carbpol.2010.04.002. 30. Yang, X. B., Gao, X. D., Han, F., & Tan, R. X. (2005). Sulfation of a polysaccharide produced by a marine filamentous fungus Phoma herbarum YS4108 alters its antioxidant properties in vitro. Biochimica et Biophysica Acta, 1725(1), 120-127. http:// dx.doi.org/10.1016/j.bbagen.2005.06.013. PMid:16054758. 31. Lee, M.-H., Jeon, H. S., Kim, S. H., Chung, J. H., Roppolo, D., Lee, H.-J., Cho, H. J., Tobimatsu, Y., Ralph, J., & Park, O. K. (2019). Lignin-based barrier restricts pathogens to the infection site and confers resistance in plants. The EMBO Journal, 38(23), e101948. http://dx.doi.org/10.15252/embj.2019101948. PMid:31559647. 32. Passardi, F., Cosio, C., Penel, C., & Dunand, C. (2005). Peroxidases have more functions than a Swiss army knife. Plant Cell Reports, 24(5), 255-265. http://dx.doi.org/10.1007/ s00299-005-0972-6. PMid:15856234. 33. Lu, Y.-C., Lu, Y., & Fan, X. (2020). Structure and Characteristics of Lignin. In S. Sharma & A. Kumar. Lignin biosynthesis and transformation for industrial applications (pp. 31-32). Switzerland: Springer. http://dx.doi.org/10.1007/978-3-03040663-9_2 Received: Oct. 01, 2020 Revised: Feb. 28, 2021 Accepted: Mar. 24, 2021

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Supplementary Material Supplementary material accompanies this paper. Table S1: Molar mass of crude (GCH), sulfated (SCHG) and deacetylated (DCHG) chichá gum. Figure S1: Carbon-13 nuclear magnetic resonance spectra of crude (CHG) and deacetylated (DCHG) chichá gum. This material is available as part of the online article from http://www.scielo.br/po

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Polímeros, 31(1), e2021010, 2021


ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.08720

Accurate measurement of pitch-based carbon fiber electrical resistivity Caroline Jovine Bouças Guimarães1* , Alcino Palermo de Aguiar1  and Alexandre Taschetto de Castro2  Departamento de Química, Instituto Militar de Engenharia – IME, Rio de Janeiro, RJ, Brasil Seção de Tecnologia de Materiais de Carbono – STMC, Centro Tecnológico do Exército – CTEx, Rio de Janeiro, RJ, Brasil 1

2

*caroljovine@gmail.com

Abstract This study investigated the appropriate methodology required to measure single carbon fibers electrical resistivity. Twoand four-probe methods were evaluated for this measurement. Comparing results for single filaments of pitch-based and PAN-based fibers shows that the two-probe method gives acceptable results for PAN-based fibers, but much higher deviations from adjusted resistivity for pitch-based fibers (>15%). The four-probe method shows small deviations (<1%) for both precursors and is the most suitable for measurements of pitch-based carbon fibers. The four-probe method gives higher accuracy than the two-probe for all samples tested. Keywords: carbon fiber, four-probe method, mesophase-pitch, electrical resistivity. How to cite: Guimarães, C. J. B., Aguiar, A. P., & Castro, A. T. (2021). Accurate measurement of pitch-based carbon fiber electrical resistivity. Polímeros: Ciência e Tecnologia, 31(1), e2021011. https://doi.org/10.1590/0104-1428.08720

1. Introduction Mesophase pitch-based carbon fibers have higher transport properties than most polymers, because of the mesophase pitch’s ability to form highly ordered graphite domains[1-7]. Therefore, they are used as thermal and electrical management materials in applications such as high thermal conductivity radiators[8,9], electronic packaging[10], electromagnetic interference shielding[11], heat storage[9], and radar absorption[12]. Volume resistivity is an important performance indicator for carbon fibers, applied to the evaluation of process parameters along its production steps: spinning[13-15], stabilization[16,17], carbonization[18,19], and graphitization[19-22]. It can also be applied in the evaluation of pre and post-processing steps such as intercalation[23,24], annealing[25], and coating[26-29]. Hence, many researchers use single fiber methods to find the correlation between electrical resistivity and other physical properties[30-32]. First proposed by Wenner[33] in 1915, and adjusted for small, fragile compounds by Coleman[34] in 1975, the four-probe method is commonly used by carbon fiber researchers[14-20,23-28]. However, the carbon fiber resistivity international standard method, ISO 13913, specifies a twoprobe measurement[35], and many authors use which[36-42]. Despite being a simple alternative[43], the two-probe method may be sensitive to contact and lead resistances[44] (Figure 1). Some authors recommend this method only when resistance values are high[45] or when accuracy is not required, as it has a known systematic bias (20–800 Ω)[38-41]. Thus, this information suggests the reference standard single

Polímeros, 31(1), e2021011, 2021

carbon-fiber resistivity test method has some limitations, and it could be inadequate for carbon fibers’ electrical resistivity measurements with highly ordered graphite domains such as mesophase pitch-based carbon fibers. To investigate whether the two methods used in literature are suitable, applied the two- and four-probe to measuring PAN- and pitch-based carbon fibers’ electrical resistivity and testing the effect of contact resistance through the linear fitting sample resistances for different gauge lengths. The results were compared with the datasheet values and literature reports.

2. Materials and Methods Standard and high-modulus grades of PAN- and pitchbased carbon fibers were selected (Table 1). For each sample, electrical resistivity measurements for ten single filaments, obtained at room temperature by the two- and four-point methods, were averaged and compared to the manufacturer datasheet values. Individual filaments were straightened and glued to the specific mounting tab of each method. The two-probe mounting template is a 0.3 mm thickness cardboard, with a 25 mm hole cut out. The four-probe arrangement is a printed circuit board with four parallel copper conduction paths, with the two inner trails separated by 25 mm and the two outer trails by 35 mm. Each carbon monofilament lay on the standard support following a centerline of the mounting template, fixed with a conductive adhesive (Figure 2).

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S S S S S S S S S S S S S S S S S S S S


Guimarães, C. J. B., Aguiar, A. P., & Castro, A. T. Table 1. Electrical resistivity (ρ0), Young Modulus (E), and diameter (D) of commercial fibers specification[46-49]. Manufacturer

Name

ρ0 (µΩ.m)

E (GPa)

D (µm)

Precursor

Cytec Cytec Torayca Torayca

K-1100 P-25 M46J T300

1.2 13 9 17

965 159 436 230

10.0 11.0 5.0 7.0

Pitch Pitch PAN PAN

a Fluke 87 V digital multimeter the electrical resistance (R) of individual filaments. Meanwhile, in the four-probe method, the external contacts were connected through a Keithley’s 6221 DC source, and the internal contacts connected through a Keithley’s 2182A high impedance nano voltmeter connected the others. Thermal voltage’s effects were eliminated by reversing the polarity and averaging the two values[50]. Electrical resistance was obtained according to Equation 1. ρ =

Figure 1. Two-probe method equivalent circuit representation.

π D2 4L

⋅R

(1)

The resistance of each sample was also measured by both methods, at different lengths in the 2–15 mm range (Figure 3) to estimate the contact resistance (Rc) and the adjusted resistivity value (ρa). These parameters can be obtained by linearly fitting the resistance (R*) of different gauge lengths (L*), according to Equation 2, assuming that the cross-sectional carbon fiber area (A) and the contact resistance are constant. The adjusted electrical resistivity values were compared to the averages electrical resistivity at the fixed 25 mm distance. R* = ρ a ⋅

L* + Rc A

(2)

3. Results and Discussions

Figure 2. Two and four-probe mounting tab for single filament.

Figure 3. Mounting tab with different gauge length sample for two and four-probe method.

A Mitutoyo CD-6” AX-B digital caliper was used to measure the distances between the two inner points (L) at which the fiber no longer touches the conductive adhesive. An Olympus BX41 confocal microscope was used to measure the diameter (D) at three distinct points along the filament length at 1000x magnification. In the two-probe method, 2/6

Table 2 shows the average electrical resistivity (ρ i) and relative deviation (Di) from the manufacturers’ values (ρ0) of each carbon fiber for both tested methods. For the two-probe method, the electrical resistivity relative deviations of the PAN-based fibers are less than 4%, while the pitch-based deviations exceed 10%. In contrast, all relative deviations for the four-probe method are less than 2%. Besides that, at 95% confidence interval Student’s t-test[51] results in no statistically significant difference between the manufacturers’ values (ρ0) and the four-probe method electrical resistivity (ρ IV ) (Table 2), since t-values (tIV) modulus are less than the critical t-value (tcrit = 2.26[51]). On the other hand, the two-probe showed a significant difference between these values for K-1100 and P-25 fiber since the t-values modulus is higher than the t-critical. These results suggest that pitchbased carbon fiber manufacturers do not follow the single filament method proposed by ISO 13913 international standards. The four-probe standard deviations are smaller than the two-probe for all samples (Table 2). Besides, the former, by statistical F-test[51], provide better precision at 95% confidence level, since all samples F-value (Table 2) are superior to the critical F-value (Fcrit = 3.31[51]). These Polímeros, 31(1), e2021011, 2021


Accurate measurement of pitch-based carbon fiber electrical resistivity Table 2. Average electrical resistivity ( ρ i), manufacturers declared value (ρ0), standard deviation (σi), relative deviation (RDi) from manufacturers provided value, t and F values. Name K-1100 P-25 M46J T300

ρ II (µΩ.m) 1.4 11.7 8.9 16.4

Two-probes method σII (µΩ.m) RDII (%) 0.3 1.2 0.6 1.7

16.7 10.1 1.1 3.5

tII

ρ IV (µΩ.m)

2.30 -3.43 -0.53 -1.12

1.2 12.8 8.9 16.8

Four-probes method σIV (µΩ.m) RDIV (%) 0.2 0.6 0.3 0.8

tIV

0.0 1.9 1.1 1.2

0.00 -1.05 -1.05 -0.79

F 3.36 4.00 4.00 4.52

Figure 4. Commercial carbon fibers fit of (a) two-probe and (b) four-probe methods.

results suggest that the four-probe is more accurate than the two-probe method. Table 3 shows each carbon fiber’s literature data electrical resistivity ( ρ L ). Comparing these to two- and four-probe the electrical resistivity ( ρ II and ρ IV ) by Student’s t-test concludes that is no statistically significant difference in a 95% confidence interval (Table 3) since the t-values (tIV) modulus are less than the critical t-value for all sample. On the other hand, there is a significant difference between literature data and two-probe electrical resistivity values for K-1100 fiber since t-values modulus is higher than the t-critical. This result indicates that the two-probe may not be a suitable method to estimate the pitch-based carbon fibers’ electrical properties. Figure 4 shows the correlations between measured electrical resistance and gauge length, fitted by a straight line, for both methods. Contact resistance is given by the vertical axis intercept, and electrical resistivity by the line slope (Table 4). All correlation coefficients (R2) were higher than 0.999, representing a good fit. Contact resistances varied from about 20 – 220 Ω, with the highest values from PAN-based carbon fibers. For these fibers, there was no significant difference between two and four probes contact resistance. However, for pitch-based carbon fibers, the contact resistance obtained by the twoprobe method is significantly higher than by the four-probe method. These results confirm the higher accuracy of the four-probe method[44]. For all samples, the adjusted electrical resistivity obtained by the two and four-probe methods were identical. Polímeros, 31(1), e2021011, 2021

Table 3. Literature data (ρL), standard deviation (σi), and t values for two- and four- probe results. Name K-1100 P-25 M46J T300

ρL (μΩ٠m)

tII-value

tIV-value

1.17[52] 13.7[53] 9.3[54] 16.8[55]

2.65 -1.84 -1.84 -0.74

0.47 2.11 -1.95 0.00

Comparison of average values for the adjusted electrical resistivity (ρa) and the electrical resistivity ( ρi) obtained by the four-probe method, by Student’s t-test[51] show no statistically significant difference in a 95% confidence interval. For the two-probe method, on the other hand, there is a significant difference between these values for K-1100 and P-25 fibers, which are both pitch-based. Besides having low resistivity, the P-25 and K-1100 fibers’ electrical resistances are the lowest because they have the largest diameter (Table 1), so its values are more affected by contact resistance (Table 5). The lowest electrical resistance fiber, K-1100, presented the highest relative deviation from adjusted resistivity, while the highest electrical resistance fiber, M46J, presented the smallest difference. This effect is more prominent in two-probe measurements, which is the method that has higher contact resistances. The coefficient of variation (Table 5), variability estimator, from pitch-based carbon fibers is higher than PAN-based; this occurs because pitch-based fibers tend to be more heterogeneous than PAN-based[56], which intensifies measurement noise. 3/6


Guimarães, C. J. B., Aguiar, A. P., & Castro, A. T. Table 4. Contact resistance (Rc), adjusted electrical resistivity (ρa), and coefficient of determination (R2) Name K-1100 P-25 M46J T300

RIIc (Ω)

Two-probe method ρIIa (μΩ٠m)

17 25 90 215

1.2 12.9 8.9 16.7

R2

RIVc (Ω)

Four-probe method ρIVa (μΩ٠m)

R2

0.99936 0.99979 0.99992 0.99993

5 8 92 211

1.2 13.0 8.9 16.7

0.99999 0.99966 0.99973 0.99998

Table 5. Average electrical resistance ( R ), relatives deviation (RD) from adjusted resistivity, and relative standard deviation (RSD). Name K-1100 P-25 T300 M46J

R (kΩ) 0.5 2.8 10.0 11.4

RDII (%)

RDIV (%)

RSDII (%)

RSDIV (%)

16.7 9.3 2.0 0.3

0.8 0.5 0.4 0.3

23 10 7 10

15 5 3 5

4. Conclusions The two-probe method specified by ISO resulted in up to 2% relative deviation from adjusted resistivity for PAN-based fibers and over 15% deviation for pitch-based fibers. On the other hand, the four-probe method achieved less than 1% relative deviation from adjusted resistivity for all tested fibers, producing accurate and consistent results, even when measuring low resistances. We conclude that the two-probe method is particularly inadequate for determining pitch-based carbon fiber’s electrical resistivity due to its inability to measure low electrical resistances accurately. For PAN-based fibers, the two-probe method gives acceptable results, but with lower accuracy than the four-probe method unless its values are corrected by linear fitting of the resistance of different gauge lengths.

5. Acknowledgements We thank the Brazilian Army Technological Center (Centro Tecnológico do Exército - CTEx) for supporting this work.

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