Polímeros: Ciência e Tecnologia (Polimeros), vol.26, n.3, 2016

Polímeros

VOLUME XXVI - N° 3 - JUL/AGO/SET - 2016

adáblios

Para onde você cresça, nós seguiremos.

Esta é a força da parceria global.

A ARLANXEO, joint venture entre LANXESS e Saudi Aramco, é uma empresa líder mundial em borracha sintética.

Como integrante da ARLANXEO a unidade de negócios High Performance Elastomers (HPE) oferece aos seus clientes uma ampla linha de borrachas técnicas para aplicações de alta exigência. Integram este portfólio as borrachas EP(D)M, Nitrílicas, Policloroprenos, Nitrílicas Hidrogenadas e EVM as quais são utilizadas em aplicações de precisão tais como: estanqueidade de automóveis, aditivos de óleo de motor, isolamento elétrico de alta tensão, modificação de plásticos, sistemas de frenagem e arrefecimento, calçados de desempenho e adesivos de multi-finalidades. www.arlanxeo.com.br

http://dx.doi.org/10.1590/0104-1428.2603

Editorial Polímeros, 3th Edition, Aug/2016

Dear readers of Polímeros From August 2016 I had the great pleasure to be appointed as Editor-in-Chief of “Polímeros: Ciência e Tecnologia”.

I know that this will not be an easy task, it will ask for time, patience and effort. I hope to be up to the needs and expectation of our readers. I count upon the help of all our Associate Editors which play a major role judging and speeding the refereeing process:

Adhemar C. Ruvolo Filho (UFSCar/DQ), São Carlos, SP, Brazil.

Alain Dufresne (Grenoble INP/Pagora), Saint Martin d’Heres, RA, France.

Bluma G. Soares (UFRJ/IMA), Rio de Janeiro, SP, Brazil.

César Liberato Petzhold (UFRGS/IQ), Porto Alegre, RS, Brazil.

João B. P. Soares (UAlberta/DCME), Edmonton, AB, Canada.

José António C. Gomes Covas (UMinho/IPC), Guimarães, RN, Portugal.

José Carlos C. S. Pinto (UFRJ/COPPE), Rio de Janeiro, SP, Brazil.

Regina Célia R. Nunes (UFRJ/IMA), Rio de Janeiro, SP, Brazil.

Richard G. Weiss (GU/DeptChemistry), Washington, DC, USA.

Rodrigo Lambert Oréfice (UFMG/DEMET), Belo Horizonte, MG, Brazil.

Among the tasks I set myself, improving the Impact Factor (IF) of Polímeros is imperative and is the one facing the biggest challenge. At the moment (ranking of 2015) the IF stands at 0.498, our goal is to double it in a 4 years term.

I know that this is quite a dream, but if we dream together we can turn it into reality. For that the meticulous work of the anonymous AdHoc’s during reviewing the submitted articles is of utmost importance. A careful analysis can select the good text which upon publication will be read and cited by other authors, truly increasing the IF. The comments also can help the authors to improve the quality of the manuscript and get it accepted. Then comes to the mature work of the associate editors which are responsible to accept or reject the paper. This is the most extenuating task because it balances in the justice’s scale. This “modus operandi” has proven to be the fairest to the authors and most efficient to the improvement of the human scientific knowledge. Polímeros desire to be known and cited by the world researchers, all our efforts will be set for this cause.

I hope be able to contribute to this journal and to the worldwide polymeric society propagating the good practices of Polymer Science and Technology.

Sincerely,

Sebastião V. Canevarolo Jr. Departamento de Engenharia de Materiais – DEMa, Universidade Federal de São Carlos – UFSCar, São Carlos, SP, Brazil caneva@ufscar.br

Polímeros, 26(3), 2016

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P o l í m e r o s - N º 3 - V o l u m e X X V I - J u l / A g o / S e t - 2 0 1 6 - ISSN 0 1 0 4 - 1 4 2 8 - ISSN 1 6 7 8 - 5 1 6 9

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( ve r s ã o

eletrônica)

I n d e x a d a : “ C h e m ic a l A b s t r a c t s ” — “ RA P RA A b s t r a c t s ” — “A l l - R u s s i a n I n s t i t u t e o f S ci e n c e ­T e c h n ic a l I n f o r m a t i o n ” — “ R e d d e R e v i s t a s C i e n t i f ic a s d e A m e r ic a L a t i n a y e l C a r i b e ” — “ L a t i n d e x ” — “ W e b o f S ci e n c e ”

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Polímeros Presidente

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Conselho Editorial

Marco-Aurelio De Paoli (UNICAMP/IQ)

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Membros

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Conselho Editorial

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Adhemar C. Ruvolo Filho (UFSCar/DQ) Ailton S. Gomes (UFRJ/IMA) Alain Dufresne (Grenoble INP/Pagora) Antonio Aprigio S. Curvelo (USP/IQSC) Bluma G. Soares (UFRJ/IMA) César Liberato Petzhold (UFRGS/IQ) Cristina T. Andrade (UFRJ/IMA) Edson R. Simielli (Simielli - Soluções em Polímeros) Elias Hage Jr. (UFSCar/DEMa) Eloisa B. Mano (UFRJ/IMA) João B. P. Soares (UAlberta/DCME) José Alexandrino de Sousa (UFSCar/DEMa) José António C. Gomes Covas (UMinho/IPC) José Carlos C. S. Pinto (UFRJ/COPPE) Júlio Harada (Harada Hajime Machado Consutoria Ltda) Laura H. de Carvalho (UFCG/DEMa) Luiz Antonio Pessan (UFSCar/DEMa) Luiz Henrique C. Mattoso (EMBRAPA) Osvaldo N. Oliveira Jr. (USP/IFSC) Raquel S. Mauler (UFRGS/IQ) Regina Célia R. Nunes (UFRJ/IMA) Richard G. Weiss (GU/DeptChemistry) Rodrigo Lambert Oréfice (UFMG/DEMET) Sebastião V. Canevarolo Jr. (UFSCar/DEMa) Silvio Manrich (UFSCar/DEMa)

Comitê Editorial Sebastião V. Canevarolo Jr. – Editor

Membros

do

Comitê Editorial

Adhemar C. Ruvolo Filho Alain Dufresne Bluma G. Soares César Liberato Petzhold João B. P. Soares José António C. Gomes Covas José Carlos C. S. Pinto Regina Célia R. Nunes Richard G. Weiss Rodrigo Lambert Oréfice

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Produção

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Assessoria Editorial

www.editoracubo.com.br

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“Polímeros” é uma publicação da Associação Brasileira de Polímeros Rua São Paulo, nº 994 13560-340 - São Carlos, SP, Brasil Fone/Fax: (16) 3374-3949

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e-mails: abpol@abpol.org.br / revista@abpol.org.br http://www.abpol.org.br Data de publicação: Setembro de 2016

Apoio:

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Polímeros / Associação Brasileira de Polímeros. vol. 1, nº 1 (1991) -.- São Carlos: ABPol, 1991Versão eletrônica disponível no site: www.scielo.br

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Trimestral v. 26, nº 3 (Jul./Ago./Set. 2016) ISSN 0104-1428 ISSN 1678-5169 (versão eletrônica)

Site da Revista “Polímeros”: www.revistapolimeros.org.br

1. Polímeros. l. Associação Brasileira de Polímeros.

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Polímeros, 26(3), 2016

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Polímeros Seção Editorial

Seção Técnica

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Informes & Notícias ............................................................................................................................................................................E4 Calendário de Eventos ........................................................................................................................................................................E5 Associados...........................................................................................................................................................................................E6

Biodegradable blends of starch/polyvinyl alcohol/glycerol: multivariate analysis of the mechanical properties Juliano Zanela, Maira Casagrande, Marianne Ayumi Shirai, Vanderlei Aparecido de Lima and Fabio Yamashita....................................... 193

Effect of oxidants and anionic surfactants on the morphology and permittivity of polypyrrole and its blends with epoxy resin Preparation of curcumin-loaded nanoparticles and determination of the antioxidant potential of curcumin after encapsulation

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Regiane Aparecida Medeiros Campos, Valdirene Aparecida da Silva, Roselena Faez and Mirabel Cerqueira Rezende............................... 197 Rosana Aparecida da Silva-Buzanello, Mateus Ferreira de Souza, Daniela Alves de Oliveira, Evandro Bona, Fernanda Vitória Leimann, Lúcio Cardozo Filho, Pedro Henrique Hermes de Araújo, Sandra Regina Salvador Ferreira and Odinei Hess Gonçalves.......................... 207

Rheological behavior of acrylic paint blends based on polyaniline

Time domain NMR evaluation of poly(vinyl alcohol) xerogels Elton Jorge da Rocha Rodrigues, Maxwell de Paula Cavalcante and Maria Inês Bruno Tavares................................................................. 221

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Alex da Silva Sirqueira, Dermeval Teodoro Júnior, Marcio da Silva Coutinho, Artur Soares da Silva Neto, Adriana dos Anjos Silva and Bluma Guenther Soares............................................................................................................................................................................ 215

Effects of polypropylene methyl polyhedral oligomeric silsesquioxanes and polypropylene-grafted maleic anhydride compatibilizers on the properties of palm kernel shell reinforced polypropylene biocomposites Muhammad Safwan Mohaiyiddin, Ong Hui Lin, Hazizan Md Akil, Toh Guat Yee, Nik Nur Azza Nik Adik and Al Rey Villagracia............... 228 Emerson Roberto Santos, José Igor Balbino de Moraes, Christine Miwa Takahashi, Victor Sonnenberg, Elvo Calixto Burini, Satoru Yoshida, Herick Garcia Takimoto, Roberto Koji Onmori and Wang Shu Hui................................................................................................................ 236

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Low cost UV-Ozone reactor mounted for treatment of electrode anodes used in P-OLEDs devices

Preparation and characterization of Zn(II) ion-imprinted polymer based on salicylic acrylate for recovery of Zn(II) ions Ebrahim Ahmadi, Javad Gatabi and Zahra Mohamadnia............................................................................................................................... 242

Rheological properties and curing features of natural rubber compositions filled with fluoromica ME 100 Luciana Honorato, Marcos Lopes Dias, Chiaki Azuma and Regina Célia Reis Nunes.................................................................................. 249 María Mejia and Edwin Murillo...................................................................................................................................................................... 254

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Study of the structural, thermal, rheological and film properties of functional copolymers of hydroxyethyl acrylate and methyl methacrylate Avaliação das propriedades elétricas de barras estatóricas fabricadas com resina do tipo éter diglicidílico do bisfenol F (DGEBF) contendo nanopartículas de silica Rafael Novaes da Conceição e João Sinézio de Carvalho Campos................................................................................................................ 262

Microencapsulação do pesticida cipermetrina em blendas de P(3HB/PCL): caracterização e ensaio de liberação in vitro

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Evana Cássia Dall’Agnol, Jaqueline Suave, Marcia Margarete Meier, Valdir Soldi, Denise Abatti Kasper Silva e Ana Paula Testa Pezzin..... 269

Capa: Micrografias de microscopia eletrônica de varredura das micropartículas de P(3HB), PCL; 90/10; 80/20; 70/30; A) sem pesticida e, B) com pesticida.

Elaboração artística Editora Cubo.

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Polímeros, 26(3), 2016

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I N F O R M E S

Global smart polymers market to register Compound Annual Growth Rate (CAGR) of over 19% from 2016 to 2020

E N O T Í C I A S

The global smart polymers market to grow at a CAGR of 19.98% during the period 2016-2020, as per Research and Markets. Growing demand for plastics from healthcare, automotive, food and beverage, oil and gas, construction, and household products has stimulated the need for plastic recycling. The scarcity of petrochemicals, rising costs of raw materials and growing environmental concerns are also fostering the need for recycling.

Evolution of drug delivery systems will be a key trend for market growth as smart polymers are showcasing controlled delivery systems for medications having a short half-life, narrow therapeutic window, liable to gastric and hepatic degradation, and medications that are dynamic at low plasma concentrations. These delivery systems experience numerous difficulties connected with their advancements that are identified with medication stability, drug discharge kinetics, and the conditions under which the system is delivered to the body. Smart polymers sensitive to the proximity of some biomarkers could be valuable in focusing on particular disease conditions. For instance, smart polymers sensitive to folate receptor can be used to convey anti-cancer agents to tumor cells. One of the key drivers for market growth will be application of shape memory polymer in automotive industry. In the automotive industry, shape memory polymers are used in vehicle subsystems. These polymers self‑heal in the case of damage, polymers can also be designed to change appearance or color. In addition, the polymers can be used in sensors in safety systems. Shape memory polymers showcase new platform for variable elements in vehicles. The novel materials include new innovative components that can enhance vehicle performance at lower costs. The Americas is the major revenue contributing region in the smart polymers market and is likely to occupy more than 45% of the overall market revenue by 2020. Much of the region’s growth can be attributed to the growing focus on temperature and pH-sensitive polymers. With the governments trying to improve public healthcare systems, the demand for such smart polymers will increase significantly in the region’s medical industry. The smart polymers market in the region is anticipated to grow at a high CAGR of more than 22% during the predicted period.

Smart polymers have promising applications in the biomedical field as conveyance systems of therapeutic agents, actuators systems, bio-separation devices, cell culture support systems, sensors, or tissue engineering frameworks. Recently, these polymers have come up with new application areas including medicinal diagnostics, pharmaceuticals, implants, and treatments sectors. They have properties like adaptability and compatibility that make them valuable in medical treatments involving the immune system. Manufacturers are formulating new polymeric materials that assist in biosensor designing, drug delivery systems, tissue engineering systems, wound treatment, and other metabolically controllable systems.

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Having enormous potential across different applications, the smart polymers market will witness rapid growth in the coming years. The following companies are the key players in the global smart polymers market: AkzoNobel, Autonomic Materials, Dow Chemical, and Huntsman International.

Issues with use of smart polymers will be a challenge for the market. Smart polymers face challenges with respect to high burst discharge and unpredictable conduct in biphasic discharge profile. For instance, neuronal burst discharges are well-defined as three or more action potentials (or spikes) parted by inter-spike phases of less than or equal to 30 microseconds, or two spikes parted by an intermission of less than or equal to 15 microseconds.

The input of burst discharge to synchronous peak interaction is compared between temporal lobes.

These polymers also face problems regarding general medication, discharge kinetics, conformational integrity during handling, and safeguarding biological actions during discharge. A few patents addressing these issues with smart polymers are being looked into.

Source: Plastemart

ARLANXEO introduces new Therban LT HNBR Elastomers for low temperature applications At K 2016 in Dusseldorf, Germany, ARLANXEO will introduce two new products of Therban hydrogenated acrylonitrile-butadiene rubber (HNBR) for low temperature applications: Therban LT 1707, a fully saturated grade, and Therban LT 1757, a partially saturated grade. Both grades offer the ability to fulfill stringent low temperature requirements up to -40°C, while at the same time maintaining the high dynamically and heat resistant performance of this elastomeric rubber class.

To meet the market need for a further improved balance between low temperature properties and oil resistance, ARLANXEO is now working with high priority on a new generation of Therban elastomers, which will redefine the Therban application window.

These products will push the boundaries for low temperature applications well below -40°C, whilst maintaining excellent oil resistance.

Susanna Lieber, Head of Technical Marketing HNBR at ARLANXEO said: “Together with our customers, we are working on new elastomers to fulfil future market requirements.” Therban LT is a product of the business line NBR/HNBR which is part of the business unit High Performance Elastomers.

High Performance Elastomers (HPE), a business unit of the ARLANXEO group, offers its customers a broad portfolio of technical rubbers. As one of the leading suppliers of synthetic rubbers to the rubber‑processing industry, HPE markets materials which have a wide range of industrial applications. For example, they are used as modifiers for plastic and adhesive raw materials, in gas and oil exploration and production, and in functional components for the automotive and cable industries.

Source: ARLANXEO Polímeros, 26(3), 2016

December

May

Plastics in Africa 2016 Date: 5 December 2016 Location: Dubai - United Arab Emirates Website: www.amiplastics.com/events/event?Code=C795

Polymer Foam – 2017 Date: 2-3 May 2017 Location: Pittsburgh - United States Website: www.amiplastics.com/events/event?Code=C804

Polymers in Flooring – 2016 Date: 6-7 December 2016 Location: Berlin - Germany Website: www.amiplastics.com/events/event?Code=C769

PLASTEC New England Date: 3-4 May 2017 Location: Boston - United States Website: plastec-new-england.plasticstoday.com

Fire Resistance in Plastics 2016 Date: 6-8 December 2016 Location: Cologne - Germany Website: www.amiplastics-na.com/events/Event. aspx?code=C719&sec=7121

Polymer Sourcing & Distribution - 2017 Date: 16-17 May 2017 Location: Hamburg - Germany Website: www.amiplastics.com/events/event?Code=C801

January 1st International Conference on Sustainable Materials Processing and Manufacturing Date: 23–25 January 2017 Location: Skukuza - South Africa Website: www.eiseverywhere.com/ehome/165114 Swiss Plastics 2017 Date: 24–26 January 2017 Location: Lucerne - Switzerland Website: www.swissplastics-expo.ch Polyethylene Films – 2017 Date: 31 January - 2 February 2017 Location: Daytona Beach - United States Website: www.amiplastics.com/events/event?Code=C778

February Gulf Plastics & Polymers Show Date: 1-3 February 2017 Location: Abu Dhabi - United Arab Emirates Website: www.gpps.ae PLASTEC West Date: 7-9 February 2017 Location: Anaheim - United States Website: anaheim.ubmcanon.com/plastics Non-Invasive Delivery of Macromolecules Conference 2017 Date: 12-16 February 2017 Location: Queenstown - New Zealand Website: amn8.co.nz

March Plástico Brasil 2017 Date: 20-24 March 2017 Location: São Paulo - Brazil Website: www.plasticobrasil.com.br

April Feiplastic 2017 Date: 3-7 April 2017 Location: São Paulo - Brazil Website: www.feiplastic.com.br Polymer Testing & Analysis - 2017 Date: 4 - 5 April 2017 Location: Cologne - Germany Website: www.feiplastic.com.br

Plast-Ex Date: 16-18 May 2017 Location: Ontario - Canada Website: plastex.plasticstoday.com Frontiers in Polymer Science Date: 17–19 May 2017 Location: Seville - Spain Website: www.frontiersinpolymerscience.com

June Performance Polyamides - 2017 Date: 6-7 June 2017 Location: Cologne - Germany Website: www.amiplastics.com/events/event?Code=C803 PLASTEC East Date: 13-15 June 2017 Location: New York - United States Website: plastec-east.plasticstoday.com Polymers in Cables – 2017 Date: 20-21 June 2017 Location: Pittsburgh - United States Website: www.amiplastics.com/events/event?Code=C814 Additive Manufacturing and Functional Polymeric Materials Conference Date: 23–26 June 2017 Location: Albufeira – Portugal Website: www.zingconferences.com/conferences/additivemanufacturing-and-functional-polymeric-materialsconference-2017 Europe/Africa Polymer Processing Society Conference Date: 26-29 June 2017 Location: Dresden - Germany Website: http://www.pps2017dresden.de/ Conductive Plastics - 2017 Date: 27-28 June 2017 Location: Cologne - Germany Website: http://www.amiplastics.com/events/event?Code=C792

July 3rd Functional Polymeric Materials Date: 7–10 July 2017 Location: Rome – Italy Website: www.fusion-conferences.com/conference66.php

September Physical Aspects of Polymer Science Date: 13 September 2017 Location: Swansea - United Kingdom Website: paps17.iopconfs.org

Polímeros, 26(3), 2016 E5

Associados da ABPol Patrocinadores

Instituições UFSCar/ Departamento de Engenharia de Materiais, SP SENAI/ Serviço Nacional de Aprendizagem Industrial Mario Amato, SP UFRN/ Universidade Federal do Rio Grande do Norte, RN

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Polímeros, 26(3), 2016

Associados da ABPol As nossas boas vindas....

Ao novo Sócio Patrocinador Retilox Química Especial Ltda.

Agradecemos o valioso apoio!

Luiz Antonio Pessan Presidente

Coletivos A. Schulman Plásticos do Brasil Ltda.

Aditive Plásticos Ltda.

Avamplas – Polímeros da Amazônia Ltda.

CBE – Grupo Unigel Colorfix Itamaster Indústria de Masterbatches Ltda.

Cromex S/A Cytec Comércio de Materiais Compostos e Produtos Químicos do Brasil Ltda.

Formax Quimiplan Componentes para Calçados Ltda.

Imp. e Export. de Medidores Polimate Ltda.

Innova S/A Instituto de Aeronáutica e Espaço/AQI Jaguar Ind. e Com. de Plásticos Ltda Johnson & Johnson do Brasil Ind. Com. Prod. para Saúde Ltda.

Master Polymers Ltda.

Milliken do Brasil Comércio Ltda.

MMS-SP Indústria e Comércio de Plásticos Ltda.

Nexo International Ltda.

Nitriflex S/A Ind. e Com. Politiplastic Politi-ME. Premix Brasil Resinas Ltda.

QP - Químicos e Plásticos Ltda.

Radici Plastics Ltda.

Replas Comércio de Termoplásticos Ltda.

Uniflon - Fluoromasters Polimeros Ind .Com. Imp. Export.Ltda

Polímeros, 26(3), 2016 E7

http://dx.doi.org/10.1590/0104-1428.2420

Biodegradable blends of starch/polyvinyl alcohol/glycerol: multivariate analysis of the mechanical properties Juliano Zanela1,2*, Maira Casagrande2, Marianne Ayumi Shirai3, Vanderlei Aparecido de Lima4 and Fabio Yamashita1 Department of Food Science and Technology, Universidade Estadual de Londrina – UEL, Londrina, PR, Brazil 2 Universidade Tecnológica Federal do Paraná – UTFPR, Dois Vizinhos, PR, Brazil 3 Universidade Tecnológica Federal do Paraná – UTFPR, Londrina, PR, Brazil 4 Universidade Tecnológica Federal do Paraná – UTFPR, Pato Branco, PR, Brazil

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*julianozanela@gmail.com

Abstract The aim of the work was to study the mechanical properties of extruded starch/polyvinyl alcohol (PVA)/glycerol biodegradable blends using multivariate analysis. The blends were produced as cylindrical strands by extrusion using PVAs with different hydrolysis degrees and viscosities, at two extrusion temperature profiles (90/170/170/170/170 °C and 90/170/200/200/200 °C) and three conditioning relative humidities of the samples (33, 53, and 75%). The mechanical properties showed a great variability according to PVA type, as well as the extrusion temperature profile and the conditioning relative humidity; the tensile strength ranged from 0.42 to 5.40 MPa, elongation at break ranged from 10 to 404% and Young’s modulus ranged from 0.93 to 13.81 MPa. The multivariate analysis was a useful methodology to study the mechanical properties behavior of starch/PVA/glycerol blends, and it can be used as an exploratory technique to select of the more suitable PVA type and extrusion temperature to produce biodegradable materials.

Keywords: biodegradable material, extrusion, principal component analysis.

1. Introduction The environmental concerns about the plastic wastes are increasing, promoting the development of suitable alternatives for petroleum based polymers, and starch is a promisor biopolymer from renewable resources to produce biodegradable materials. However, the mechanical properties of these materials are poor, being necessary the development of blends with others biodegradable polymers to improve its mechanical and barrier properties.

Polyvinyl alcohol (PVA) is a biodegradable polymer with vast use in paper and textile industry; it is available industrially with several hydrolysis degree and molecular weight, and these characteristics can affect the properties of the materials produced with PVA blends[1,2], and PVA can be processed by thermoplastic extrusion[3].

Blending starch with polyvinyl alcohol (PVA) can improve the mechanical properties and maintain the biodegradability of starch-based materials how observed in many works[4-7].

To produce biodegradable material by extrusion it is necessary to extrude the blend components producing cylindrical strands, to pelletize the strands, and then to extrude these pellets to produce the material by blown extrusion or flat die extrusion-calendering process. According to Nobrega et al.[8], the mechanical and viscoelastic properties of extruded cylindrical strands from biodegradable polymer blends (starch/poly (butylene adipate co-terephthalate)/ glycerol) were correlated with their capacity to form films by blown extrusion process.

Polímeros, 26(3), 193-196, 2016

The aim of this work was to study the behavior of the mechanical properties of starch/polyvinyl alcohol/glycerol blends using multivariate analysis. The blends were produced as cylindrical strands by extrusion with different polyvinyl alcohol types, and extrusion temperature profiles.

2. Materials and Methods 2.1 Materials The polyvinyl alcohol (PVA) with different degree of hydrolysis (DH) and viscosities (4% aqueous solution) were provided by Sekisui Chemical (Japan): Selvol™ 203 (DH: 88.14%, viscosity: 4.10 cP); Selvol™ 523 (DH: 87.84%, viscosity: 24.50 cP); Selvol™ 540 (DH: 88.04%, viscosity: 49.40 cP); Selvol™ 107 (DH: 98.30%, viscosity: 6.00 cP) and Selvol™ 325 (DH: 98.42%, viscosity: 31.40 cP). The native cassava starch was provided by Indemil (Brazil) and the technical grade glycerol by Dinamica (Brazil).

2.2 Cylindrical strands production The PVA, starch, and glycerol (20:40:40% w/w) were manually homogenized and conditioned in vacuum oven (model Q819V2, Quimis, Brazil) with a vacuum pressure of 0.085 MPa for 90 min at 85 °C to incorporate the glycerol, according the method adapted from Jang and Lee[9]. The blends were extruded in a co-rotating twin‑screw extruder (model D-20, BGM, Brazil) with a six holes (2 mm) die to produce the cylindrical strands, and a screw diameter of 20 mm (L/D = 35).

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Zanela, J., Casagrande, M., Shirai, M. A., Lima, V. A., & Yamashita, F. The screw speed was set at 100 rpm and it was used two extrusion temperature profiles: 90/170/170/170/170 °C and 90/170/200/200/200 °C, totaling 10 different formulations (5 PVA types x 2 extrusion temperature profiles).

2.3 Mechanical characterization The tensile strength, Young’s modulus and elongation at break tests were performed according to ASTM D882-02[10] using a texture analyzer (model TA.XT2i, Stable Micro Systems, England) with an initial distance between grips of 30 mm and a crosshead speed of 0.8 mm/s.

Ten samples from each treatment (50 mm in length) were conditioned at different relative humidity (33±2%, 53±2%, and 75±2% RH) for 72 hours at 23±2ºC before analysis.

2.4 Statistical analyses The multivariate exploratory techniques, Principal Components Analysis (PCA) and Hierarchical Cluster Analysis, were performed using STATISTICA 7.0 software (Statsoft, USA). For PCA, the mechanical parameters (tensile strength, Young’s modulus, and elongation at break) were used as active variables in the derivation of the principal components, and the supplementary variables (temperature,

hydrolysis degree, viscosity, and relative humidity) were projected onto the factor space. The PCA analysis was performed using the covariance matrix. The hierarchical tree was obtained considering the same active variables applied to PCA. The formulations were joined by single linkage as linkage rule, and considering the Euclidean distance as the coefficient of similarity

3. Results and Discussions The results of the cylindrical strands mechanical properties are presented in Table 1. Tensile strength ranged from 0.42 to 5.40 MPa, elongation at break ranged from 10 to 404% and Young’s modulus ranged from 0.93 to 13.81 MPa, showing a great variability according to PVA type, as well as the extrusion temperature profile and the conditioning relative humidity.

The Figure 1a presents the Principal Component Analysis (PCA) plot of active and supplementary variables for mechanical properties of cylindrical strands, and is possible to observe that the two principal components explained 97.28% of total variance.

The tensile strength was positively correlated with the extrusion temperature profile, probably because the extrusion

Table 1. Mechanical properties of PVA/starch/glycerol cylindrical strands.

PVA

Extrusion Temperature Profile (°C) 90/170/170/170/170 °C

Selvol 203 90/170/200/200/200 °C

90/170/170/170/170 °C Selvol 523 90/170/200/200/200 °C

90/170/170/170/170 °C Selvol 540 90/170/200/200/200 °C

90/170/170/170/170 °C Selvol 107 90/170/200/200/200 °C

90/170/170/170/170 °C Selvol 325 90/170/200/200/200 °C

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Relative Humidity Tensile Strength Elongation at Break Young’s Modulus (%) 33% 53% 75% 33% 53% 75% 33% 53% 75% 33% 53% 75% 33% 53% 75% 33% 53% 75% 33% 53% 75% 33% 53% 75% 33% 53% 75% 33% 53% 75%

(MPa) 1.44 ± 0.11 0.83 ± 0.16 0.60 ± 0.25 1.38 ± 0.20 0.72 ± 0.10 0.42 ± 0.06 2.20 ± 0.34 2.39 ± 0.48 1.12 ± 0.43 4.56 ± 0.79 2.77 ± 0.40 1.78 ± 0.19 1.46 ± 0.17 0.95 ± 0.18 1.18 ± 0.23 2.46 ± 0.35 1.83 ± 0.25 1.41 ± 0.37 1.67 ± 0.23 1.28 ± 0.25 0.82 ± 0.08 2.57 ± 0.15 1.52 ± 0.11 1.40 ± 0.11 1.90 ± 0.21 1.49 ± 0.19 1.40 ± 0.14 5.40 ± 0.58 4.06 ± 0.52 3.47 ± 0.47

(%) 61 ± 7 32 ± 5 10 ± 4 92 ± 14 41 ± 12 39 ± 6 166 ± 48 186 ± 47 155 ± 94 391 ± 91 404 ± 95 347 ± 85 119 ± 22 108 ± 32 86 ± 15 149 ± 35 136 ± 27 131 ± 24 62 ± 16 47 ± 6 39 ± 12 95 ± 16 52 ± 8 98 ± 17 83 ± 14 81 ± 16 81 ± 16 262 ± 24 219 ± 47 236 ± 88

(MPa) 5.46 ± 0.37 4.09 ± 0.75 0.93 ± 0.60 4.4 ± 0.72 3.11 ± 0.38 2.50 ± 0.38 5.25 ± 0.91 5.40 ± 1.07 2.48 ± 0.54 7.97 ± 0.71 4.84 ± 0.35 3.38 ± 0.89 2.64 ± 0.61 1.88 ± 0.49 2.60 ± 0.88 5.27 ± 0.70 4.05 ± 0.42 2.84 ± 0.81 7.60 ± 0.52 6.58 ± 0.97 4.33 ± 0.75 13.30 ± 0.93 9.56 ± 0.71 8.35 ± 0.95 5.79 ± 0.74 4.18 ± 0.64 4.18 ± 0.39 13.81 ± 1.57 11.63 ± 1.63 9.03 ± 1.12

Polímeros, 26(3), 193-196, 2016

Biodegradable blends of starch/polyvinyl alcohol/glycerol: multivariate analysis of the mechanical properties process was more efficient at the higher temperature profile, permitting a better interaction among the components (starch/PVA/glycerol).

The PVA hydrolysis degree correlated well with Young’s modulus, probably because the higher the hydrolysis degree the higher the number of hydroxyl groups in substitution of

the acetate groups, enabling more interactions by hydrogen bonds with starch molecules, so increasing their rigidity.

The PVA viscosity correlated well with the elongation at break of the strands, probably because the longer PVA chains.

The conditioning relative humidity had a negative correlation with the mechanical properties, due to the

Figure 1. Classification of the formulations by mechanical parameters. (a) Variable projection by PCA: —— active variable, ------supplementary variable; (b) Scatterplot for the formulations by PCA with grouping suggested by HCA; and (c) Dendogram by HCA analysis with the separation in two distinct clusters (1 and 2).

Polímeros, 26(3), 193-196, 2016

195

Zanela, J., Casagrande, M., Shirai, M. A., Lima, V. A., & Yamashita, F. hydrophilic characteristic of starch/PVA-based materials.

The water acts as plasticizer, consequently materials conditioned at high RH absorb more water, decreasing their mechanical properties[11,12].

In the scatterplot (Figure 1b), the samples could be grouped in two distinct clusters considering the groups suggested by the dendogram of the hierarchical cluster analysis (Figure 1c). The cluster ‘1’ is composed by the strands produced with Selvol 523 and 325 at the higher extrusion temperature profile (90/170/200/200/200 °C) and the three conditioning relative humidities (33, 53 and 75% RH), and with Selvol 107 at the higher temperature profile and 33% RH. The cluster ‘2’ is composed by the remaining strands (below the cutting line of the dendogram) based in their similarities.

According to the dendogram, the higher extrusion temperature profile was more adequate for extrusion process of PVA, and PVA with medium chain size (Selvol 325 Selvol 523) resulted in materials with better mechanical properties.

4. Conclusions The multivariate analysis was a useful methodology to study the mechanical properties behavior of starch/PVA/glycerol blends, and it can be used as an exploratory technique to select of the more suitable PVA type and extrusion temperature to produce biodegradable materials. The PVA with medium chain size, independently of their hydrolysis degree (Selvol 325 and 523), presented the more adequate mechanical properties, and they are promising polymers for future studies.

5. Acknowledgements The authors thank the Federal Technological University of Paraná (Universidade Tecnológica Federal do Paraná – UTFPR), the National Council for Scientific and Technological Development (Conselho Nacional de Desenvolvimento Científico e Tecnológico - CNPq) and the Araucaria Foundation (Fundação Araucaria) for their financial support.

6. References 1. Limpan, N., Prodpran, T., Benjakul, S., & Prasarpran, S. (2012).

Influences of degree of hydrolysis and molecular weight of poly(vinyl alcohol) (PVA) on properties of fish myofibrillar protein/PVA blend films. Food Hydrocolloids, 29(1), 226-233. http://dx.doi.org/10.1016/j.foodhyd.2012.03.007.

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2. Maria, T. M. C., de Carvalho, R., Sobral, P. J. A., Habitante, A. M. B. Q., & Solorza-Feria, J. (2008). The effect of the degree of hydrolysis of the PVA and the plasticizer concentration on the color, opacity, and thermal and mechanical properties of films based on PVA and gelatin blends. Journal of Food Engineering, 87(2), 191-199. http://dx.doi.org/10.1016/j. jfoodeng.2007.11.026.

3. Tang, X., & Alavi, S. (2011). Recent advances in starch, polyvinyl alcohol based polymer blends, nanocomposites and their biodegradability. Carbohydrate Polymers, 85(1), 7-16. http://dx.doi.org/10.1016/j.carbpol.2011.01.030.

4. Zanela, J., Olivato, J. B., Dias, A. P., Grossmann, M. V. E., & Yamashita, F. (2015). Mixture design applied for the development of films based on starch, polyvinyl alcohol, and glycerol. Journal of Applied Polymer Science, 132(43), 42697. http://dx.doi.org/10.1002/app.42697.

5. Sin, L. T., Rahman, W. A. W. A., Rahmat, A. R., & Mokhtar, M. (2011). Determination of thermal stability and activation energy of polyvinyl alcohol-cassava starch blends. Carbohydrate Polymers, 83(1), 303-305. http://dx.doi.org/10.1016/j. carbpol.2010.07.049.

6. Tang, X., & Alavi, S. (2012). Structure and physical properties of starch/poly vinyl alcohol/laponite RD nanocomposite films.

Journal of Agricultural and Food Chemistry, 60(8), 1954-1962. http://dx.doi.org/10.1021/jf2024962. PMid:22217361.

7. Brandelero, R. P. H., Yamashita, F., Zanela, J., Brandelero, E. M., & Caetano, J. G. (2015). Mixture design applied to evaluating the effects of polyvinyl alcohol (PVOH) and alginate on the properties of starch-based films. Stärke, 67(1-2), 191-199. http://dx.doi.org/10.1002/star.201400119.

8. Nobrega, M. M., Olivato, J. B., Bilck, A. P., Grossmann, M. V. E., & Yamashita, F. (2012). Glycerol with different purity grades derived from biodiesel: Effect on the mechanical and viscoelastic properties of biodegradable strands and films.

Materials Science and Engineering C, 32(8), 2220-2222. http:// dx.doi.org/10.1016/j.msec.2012.06.005.

9. Jang, J., & Lee, D. K. (2003). Plasticizer effect on the melting and crystallization behavior of polyvinyl alcohol. Polymer, 44(26), 8139-8146. http://dx.doi.org/10.1016/j.polymer.2003.10.015.

10. American Standard Testing Methods – ASTM. (2002). D-88202: standard test methods for tensile properties of thin plastic sheeting. Philadelphia: ASTM. Annual book.

11. Mao, L., Imam, S., Gordon, S., Cinelli, P., & Chiellini, E. (2002).

Extruded cornstarch - glycerol - polyvinyl alcohol blends: mechanical properties, morphology, and biodegradability.

Journal of Polymers and the Environment, 8(4), 1-7.

12. Chen, L., Imam, S. H., Gordon, S. H., & Greene, R. V. (1997).

Starch- polyvinyl alcohol crosslinked film: performance and biodegradation. Journal of Environmental Polymer Degradation, 5(2), 111-117. http://dx.doi.org/10.1007/BF02763594.

Received: Nov. 09, 2015 Accepted: June 21, 2016

Polímeros, 26(3), 193-196, 2016

http://dx.doi.org/10.1590/0104-1428.2169

Effect of oxidants and anionic surfactants on the morphology and permittivity of polypyrrole and its blends with epoxy resin Regiane Aparecida Medeiros Campos1*, Valdirene Aparecida da Silva1, Roselena Faez2 and Mirabel Cerqueira Rezende1,3 Instituto Tecnológico de Aeronáutica – ITA, São José dos Campos, SP, Brazil Laboratório de Materiais Poliméricos e Biossorventes, Universidade Federal de São Carlos – UFSCar, Araras, SP, Brazil 3 Instituto de Ciência e Tecnologia, Universidade Federal de São Paulo – UNIFESP, São José dos Campos, SP, Brazil 1

2

*remedeiroscampos@gmail.com

Abstract In this work, conductive polymers were prepared based on polypyrrole (PPy) and its blends with epoxy resin. The chemical syntheses of PPy used two oxidants (Fe2(SO4)3 and FeCl3.6H2O) and two surfactants (DBSNa and DBSA). PPy samples and their blends were characterized by scanning electron microscopy, electrical conductivity by four points, and measurements of complex parameters of electric permittivity (ε) and magnetic permeability, in the frequency range of 8.2 to 12.4 GHz. The micrographs of the fractured surfaces show that the PPy synthetized in the presence of surfactants has particles with smaller diameters, and the oxidant sulfate favored the formation of elongated structures, called fillets.

The analysis of the blends found a homogeneous distribution of PPy clusters in epoxy resin matrix, which did not favor the electrical conductivity of these materials. On the other hand, the measurements of the complex parameters of the permittivity show that the blends have increasing values when ​​ the PPy concentration is increased in the epoxy resin.

Keywords: polypyrrole, anionic surfactants, permittivity, blends.

1. Introduction A wide range of polymers is currently available for various purposes. Generally, polymers have been extensively used due to their advantages over other materials such as good mechanical strength, flexibility, environmental stability, low production cost, low density, electrical isolation, and ease processing and shaping. The possibility to associate most of the characteristics of this material class with the property of electrical conductivity has aroused much interest. Hence, the study of conductive polymers has become a widely investigated challenge[1].

The achievement of doped polyacetylene in 1977[2] and accounts of its intrinsic electrical conductivity (in order of magnitude of some metals at room temperature) started a new area of interest in the Intrinsically Conducting Polymers (ICPs). The chemistry of ICPs offers a variety of synthetic methods. The possibility of their incorporation into different types of matrices has been evaluated to meet different requirements of the final application. Thus, the preparation of blends with ICPs has been increasingly studied, so that the interchain and intrachain electron transfer of ICPs is ensured.

Polypyrrole (PPy), along with polyaniline, is one of the most promising conducting polymers due to its excellent chemical stability, ease of synthesis, high electrical conductivity, and electronic properties. The PPy began to receive greater attention after 1979, when Diaz et al.[3] obtained a black film of PPy from the electrolysis of a pyrrole solution in acetonitrile and tetramethylammonium tetrafluoroborate. In addition to

Polímeros, 26(3), 197-206, 2016

obtaining high electrical conductivity (100 S.cm–1), these authors reported that the material can be cycled repeatedly between conductive oxidized and reduced insulator states, showing a redox process between the polymer chain and dopant agent.

Blends of a conventional polymer (insulator) and an ICP can be prepared by various techniques, such as evaporation of the solution containing the mixture of components, polymerization of the conductive polymer in the presence of a conventional polymer matrix, chemical and electrochemical routes[4], or by mechanical blending, for example, in extruders[5]. This material class has been extensively studied and used in different areas, such as electromagnetic interference shields, sensors, static charge dissipation, and electromagnetic radiation absorbers[6-14].

Preparing blends of insulating polymers with ICPs in order to obtain radar absorbing materials (RAM), also called microwave absorbers, has as important data the electric conductivity and the complex parameters of electric permittivity of the material[3,6,13,15,16]. These parameters are affected by the polymer chain size, doping level, dopant type, synthesis method, and procedure used to prepare the blends[16]. Therefore, very strict control of experimental parameters is necessary. Especially in the case of a mixture, it is essential to understand the behavior of the formed phases, because these may modify the final properties of the material.

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

Campos, R. A. M., Silva, V. A., Faez, R., & Rezende, M. C. Hence, the objective of this study is to correlate the morphological aspects of the synthesized polypyrrole in different chemical environments, using two oxidants (ferric sulfate and ferric chloride) and two surfactants (sodium dodecylbenzenesulphonate and dodecylbenzenesulfonic acid) and its blends with epoxy resin, with complex parameters of electric permittivity.

2. Experimental 2.1 PPy synthesis and their blends The chemical synthesis of PPy was performed using two different routes. The first was done with two different oxidants, ferric sulfate (Fe2(SO4)3) and ferric chloride hexahydrate (FeCl3.6H2O), both from Vetec Pa. The second route used the same oxidants but added two different surfactants, sodium dodecylbenzenesulphonate (DBSNa) and dodecylbenzenesulfonic acid, (DBSA), both from Fluka, with 90% purity, as previously described by the authors[10].

The first route used a 0.05 mol solution of pyrrole in 50 mL of distilled water, added drop by drop to the two oxidizing solutions containing 0.1 mol of FeCl3 and 0.05 mol of Fe2(SO4)3, respectively. The reactions were constantly stirred for 4 h at room temperature. The obtained black precipitate of PPy was filtered, washed with distilled water, and dried in a vacuum oven at 50 °C for 16 h.

The second route of PPy synthesis was done in the presence of DBSNa and DBSA surfactants. Solutions containing 0.05 mol of FeCl3 and 0.025 mol of Fe2(SO4)3 were prepared in 50 mL of distilled water. Separately, 50 mL of aqueous solutions with 0.05 mol of DBSNa and DBSA were prepared, respectively. The solutions with the oxidants and surfactants were mixed and constantly stirred for 15 min.

Then, 0.08 mol of pyrrole was transferred into 25 mL of distilled water and added drop by drop to the oxidant/surfactant solution by stirring for 4 h. The PPi obtained was filtered, washed, and dried in a vacuum oven for 16 h.

The PPy/epoxy resin blends were obtained by varying the ratio of PPy in 1, 10, and 20% (w/w) in 4.0 g of an Araldite professional type epoxy resin, commercially available. The homogenized mixtures were manually poured into aluminum molds with a thickness of 4.0 mm. The curing of the epoxy resin occurred at around 60 °C in an oven for 24 h.

2.2 Characterization Samples of PPy and its blends with epoxy resin were characterized by scanning electron microscopy (SEM), in a LEO equipment, model 435 Vip, using cryogenic fracture surfaces, metalized with gold.

Electrical conductivity measurements of the blends were performed using the 4 tip method[17], with 1.27 mm distance between the tips, as presented in a previous study[10]. Measurements were performed in triplicate with an equipment from Cascade Microtech C4s-64 coupled to a Keithley 236 source, a multimeter and an ammeter, using 13 mm diameter and 1 mm thick samples.

The parameters of complex permittivity (ε’- real component, related to the storage; and ε”- imaginary component - related to losses) and magnetic permeability 198

(µ’ and µ”- real and imaginary components, respectively) of samples were evaluated using an vector network analyzer HP 8510C, adapted with a rectangular waveguide in the frequency band from 8.2 to 12.4 GHz. Sample sizes were equal to 22.86 mm × 10.30 mm × 9.0 mm. The Nicolson‑Ross model was used to calculate ε, and μ[18-20].

3. Results and Discussion Scanning electron microscopy was used to evaluate the morphology of the synthesized samples of PPy and its blends with epoxy resin, for the effect of the oxidants and surfactants used. The morphologies of the fracture surfaces of PPy samples without addition of surfactant (PPy-SO4 and PPy-Cl, respectively) are shown in Figure 1. These micrographs illustrate the formation of clusters of globular particles of about 1 µm. The micrographs of samples prepared with the addition of the surfactant show more compact morphology. In the sample of PPy-SO4-DBSNa, for example, there is a deformation and decrease of particles, with the initial formation of thin, elongated structures, called fillets in this study. For the sample of PPy-Cl-DBSNa (with the same surfactant), the behavior is similar, but with a more compact structure without the formation of fillets.

The PPy-SO4-DBSA sample presents a more distinct morphology, with the formation of both grains and larger fillets. The sample of PPy-Cl-DBSA has clusters of particles, but no formation of fillets as in the samples prepared with Fe2(SO4)3.

The elongated structures, called fillets in this study, which are found in the samples synthesized in the presence of sulfate and the two surfactants (PPy-SO4-DBSA and PPy‑SO4 DBSNa), are attributed to the formation of cylindrical micelle, as shown in Figure 2. However, the samples obtained with the oxidant chloride are characterized by the formation of spherical micelles. This behavior is attributed to the larger volume of oxidant sulfate, which when combined with the surfactant induces the formation of thermodynamically more stable structures through the nucleation of cylindrical micelles[21-24]. In the case of DBSNa, the increased volume of this surfactant molecule is favored by the formation of more robust fillets.

Correlation of the observations made by SEM with electrical conductivity data previously published by the authors for these same samples[10] allows to infer that the fillets increase electrical conductivity values (8.9 and 13 S.cm–1 for the PPy-SO4-DBSNa and PPy-SO4-DBSA polymers, respectively). This suggests that this type of structure is formed by more linear polymer chains, which favors the conduction of electrons.

Figure 3 illustrates aspects observed in cluster-rich regions in the blends processed by mixing the six different synthesized PPy polymers (PPy-Cl, PPy-SO4, PPy-Cl-DBSNa, PPy-Cl-DBSA, PPy-SO4-DBSNa, and PPy-SO4-DBSA) in epoxy resin at a percentage of 1% (wt/wt). The analysis of the regions containing the clusters shows that the interaction between the two components of the PPy/epoxy resin blend is good, without the presence of loose particles, and forms smooth resin regions. These aspects are very clear in the micrograph of the PPy-SO4-DBSA sample, which highlights Polímeros, 26(3), 197-206, 2016

Effect of oxidants and anionic surfactants on the morphology and permittivity of polypyrrole and its blends with epoxy resin

Figure 1. SEM of PPy samples with different oxidants and surfactants (5000x).

Figure 2. Schematic representation of spherical and cylindrical micelles[21].

Polímeros, 26(3), 197-206, 2016

the fillets covered by the epoxy resin matrix. The samples obtained with the chlorinated oxidant (PPy-Cl-DBSNa and PPy-Cl-DBSA) show the globular particles of the PPy matrix.

Despite the good interaction of PPy polymers in epoxy resin matrix, the smaller magnifications show that the obtained blends are formed primarily of PPy agglomerates homogeneously distributed in the epoxy resin matrix.

The blends with 1% (wt/wt) of PPy have extensive smooth regions of epoxy resin, which are typical of fractured surfaces of fragile materials found in thermoset matrices.

Figures 4 and 5 show the morphological characteristics of the blends obtained by incorporating 10 and 20% (wt/wt) of PPy into the epoxy resin, respectively. These images shows that the increased concentration of the PPy in the blends creates a more brittle material, as illustrated in Figure 4 for samples PPy-SO4-10%/epoxy resin and PPy SO4-DBSNa-10%/epoxy resin as well as in Figure 5 for the sample PPy-Cl-DBSNa-20%/epoxy resin. The brittle aspect is observed by the presence of regions with loose plates.

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Campos, R. A. M., Silva, V. A., Faez, R., & Rezende, M. C.

Figure 3. SEM of the blends containing 1% (wt/wt) of PPy in epoxy resin (3000x).

However, all the prepared samples present good interaction of the components. Blends with increased concentration of conductive polymer with 10 and 20% (wt/wt) of PPy did not display the fillet structures, suggesting that these were well incorporated into the epoxy resin.

The electrical conductivity measurements of all blends show insulating behavior. This indicates that the percolation threshold was not met because the PPy formed agglomerates into the epoxy resin matrix or that the PPy was undoped during the preparation of the blend with the epoxy resin.

Despite this behavior, the literature[10] shows that some of the same blends studied in this work behave as radar absorbing material and show a good interaction with the electromagnetic wave, attenuating up to 95% of the incident radiation. In this case, one must consider that the mechanisms of loss that occur in the dielectric absorbers, such as those studied in this work, can occur through ohmic losses and/or dielectric losses, Equation 1[25-27]. In such cases, the total electrical conductivity (σΤ) comprises a static component 200

(σ - electrical conductivity) and a dynamic component (ωε”) (Equation 1), where the complex electric permittivity (ε) of a material is defined according to Equation 2[25,26]. σΤ =

σ + ωε " (1)

ε = ε ' − jε " (2)

where: ω is the angular frequency; ε’ - is the real part of electric permittivity, related to the storage component; and ε” - is the imaginary part of electric permittivity, related to the loss component.

From the insulating behavior observed in electrical conductivity (σ) measurements of the studied blends, the variation of complex parameters ε and μ of these materials was investigated. Figure 6 shows the curves of real and imaginary components of the electric permittivity and magnetic permeability of the used epoxy resin. The blends containing 1% (wt/wt) of PPy in epoxy resin had values of Polímeros, 26(3), 197-206, 2016

Effect of oxidants and anionic surfactants on the morphology and permittivity of polypyrrole and its blends with epoxy resin

Figure 4. SEM of the blends with 10% (wt/wt) of PPy in epoxy resin (3000x).

real and imaginary components of electric permittivity very similar for all the six blends prepared. This result shows that this concentration (1% wt/wt) is too low to change the complex parameters of epoxy resin matrix.

The complex parameters show a slight variation with the frequency, with the tendency to decrease with increasing frequency[25,26]. However, in the rated frequency range (8.2 to 12.4 GHz) and the ordered scale used, Figure 6 shows that the complex parameters of epoxy resin varied only a little with increasing frequency. Table 1 shows these values at a frequency of 10 GHz. Figure 6 also shows that the curves of µ’ and µ” are around 1 and 0, respectively.

These values were expected and are typical of dielectric materials[19,26].

Figures 7 and 8 show curves of the real and imaginary components of electric permittivity and magnetic permeability of the blends containing 10 and 20% (wt/wt) of PPy, respectively. Tables 1 and 2 summarize the values of the Polímeros, 26(3), 197-206, 2016

Table 1. Values of complex parameters of electric permittivity of epoxy resin and the blends with 10% (wt/wt) of PPy, at 10 GHz. Epoxy resin

Sample

ε´ 2.7506

ε´´ 0.0941

PPy-Cl-10%/epoxy resin

4.0444

0.8669

PPy-Cl-DBSA10%/epoxy resin

2.7039

0.1585

PPy-Cl-DBSNa10%/epoxy resin

4.1252

0.7789

PPy-SO4-10%/epoxy resin

3.1343

0.2961

PPy-SO4-DBSA10%/epoxy resin

3.2082

0.3974

PPy-SO4-DBSNa10%/epoxy resin

3.3097

0.3935

complex electric permittivity, at 10 GHz, of blends containing 10 and 20% (wt/wt) of PPy, respectively.

Contrary to that observed for the epoxy resin and its blends with 1% (wt/wt) of PPy, Figures 7 and 8 show that the blends with 10 and 20% (wt/wt) of PPy have significant variations of complex parameters of ε. In this case, greater concentration of PPy in epoxy resin increased the values 201

Campos, R. A. M., Silva, V. A., Faez, R., & Rezende, M. C.

Figure 5. SEM images of the blends with 20% (wt/wt) of PPy in epoxy resin (3000x).

Table 2. Values of complex parameters of electric permittivity of epoxy resin and the blends with 20% (wt/wt) of PPy, at 10 GHz. Sample

Figure 6. Complex values of electric permittivity (ε) and magnetic permeability (µ) as function of frequency of epoxy resin.

202

Epoxy resin

ε´ 2.7506

ε´´ 0.0941

PPy-Cl-20%/epoxy resin

6.1410

2.1861

PPy-Cl-DBSA20%/epoxy resin

3.4650

0.5530

PPy-Cl-DBSNa20%/epoxy resin

9.4384

2.6441

PPy-SO4-20%/epoxy resin

3.6339

0.5835

PPy-SO4-DBSA20%/epoxy resin

4.4789

1.0023

PPy-SO4-DBSNa20%/epoxy resin

8.1288

1.9301

of the complex parameter of the blend. Therefore, PPy was not undoped during the preparation of the blends, as these samples show values of real and imaginary components of the electric permittivity higher than those measured for the net epoxy resin (without the addition of PPy).

Polímeros, 26(3), 197-206, 2016

Effect of oxidants and anionic surfactants on the morphology and permittivity of polypyrrole and its blends with epoxy resin

Figure 7. Complex values of electric permittivity (ε) and magnetic permeability (µ) as function of the frequency of the blends containing 10% (wt/wt) of PPy in epoxy resin.

The analysis of results for the blends with 10 and 20% (wt/wt) of PPy-Cl-DBSA in the epoxy resin shows that these samples exhibit anomalous behavior, in both Table 1 and Table 2. This suggests that the synthesis of this PPy sample may have suffered some interference and was disregarded in our analysis.

The correlation of the electrical conductivity values, where the blends behave as insulating materials (σ approximately zero), with the data from complex parameters of electric Polímeros, 26(3), 197-206, 2016

permittivity shown in Tables 1 and 2, it is possible to affirm that the prepared blends have losses mostly by dynamic conductivity (ωε”), as shown in Equation 1. This result is consistent with the morphologies observed in Figures 1, 3, and 4, in which fractured surfaces consist of distributed clusters of PPy particles isolated by amorphous regions of epoxy resin. These characteristics result in behavior of an insulating material, even though electrically conductive particles were added, as shown in literature[10].

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Campos, R. A. M., Silva, V. A., Faez, R., & Rezende, M. C.

Figure 8. Complex values of electric permittivity (ε) and magnetic permeability (µ) as function of the blends containing 20% (wt/wt) of PPy in epoxy resin.

The correlation of morphological aspects observed in Figures 1, 3, and 4 with the permittivity data presented in Table 2 allows also to conclude that the DBSNa surfactant is what favored the production of the most promising PPy samples for RAM processing. In this case, the texture of clusters, which consist of spherical particles of smaller sizes (Figure 1: PPy-Cl-DBSNa and PPy-SO4-DBSNa), favors getting the best values of complex parameters. Despite the presence of the fillets in the net PPy samples, which favor the electrical conductivity, as shown in the literature[10].

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This study shows that this type of structure does not help increase complex parameters of ε in the samples blended with the epoxy resin.

4. Conclusions This study involves polypyrrole polymer blends with epoxy resin, using PPy obtained with two different oxidants (Fe2(SO4)3 and FeCl3.6H2O) and two surfactants (DBSNa and DBSA), respectively. Scanning electron microscopy evidenced that Polímeros, 26(3), 197-206, 2016

Effect of oxidants and anionic surfactants on the morphology and permittivity of polypyrrole and its blends with epoxy resin the synthesis of PPy in the presence of surfactants decreased the diameter of the particles, and the use of oxidizing sulfate favored the formation of elongated structures, called fillets.

The presence of these elongated structures is attributed to steric hindrance of bulky sulfate group in combination with the surfactants, which favored the formation of cylindrical micelles. Although the literature shows that similar samples of PPy are conductive, this study reports that these polymer blends with epoxy resin are insulators. This behavior is attributed to the formation of clusters in the blends, which do not allow the percolation limit to be reached. Measurements of the complex parameters of electric permittivity show that the blends with 10 and 20% (wt/wt) of PPy have increasing values ​​of these parameters with increased concentration of the conductive polymer in the epoxy resin. In this case, the most promising data were obtained for blends containing 20% ​​(wt/wt) of PPy‑Cl‑DBSNa and PPy-SO4-DBSNa (ε’= 9.4384 and 8.1288, and ε”= 2.6441 and 1.9301, respectively), evidencing the positive effect of DBSNa surfactant in the synthesis of PPy. The correlation of the data obtained in this study with results in the literature, for the same samples, shows that the losses of electromagnetic radiation for the studied blends are dominated by the dynamic conductivity (ωε”) and not by ohmic losses (conductivity by four points).

5. Acknowledgements The authors thank CNPq for the financial support (Processes 142314/2010-2 and 303287/2013-6), CAPES/PVNS and the Materials Division of Institute of Aeronautics and Space.

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2. MacDiarmid, A. G., & Maxfield, M. (1987). Organic polymers as electroactive materials. Electrochemical Science and Technology of Polymers, 1, 67-102. http://dx.doi.org/10.1007/978-94-0093413-9_4.

3. Diaz, A. F., Castillo, J. I., Logan, J. A., & Lee, W. Y. (1981).

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Universidade Estadual Paulista “Júlio de Mesquita Filho”, São José do Rio Preto.

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24. Abdiryim, T., Jamal, R., & Nurulla, I. (2007). Doping effect of organic sulphonic acids on the solid-state synthesized 205

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In press.

Received: Mar. 30, 2015 Revised: Dec. 15, 2015 Accepted: May 23, 2016

PolĂ­meros, 26(3), 197-206, 2016

http://dx.doi.org/10.1590/0104-1428.2246

Preparation of curcumin-loaded nanoparticles and determination of the antioxidant potential of curcumin after encapsulation Rosana Aparecida da Silva-Buzanello1, Mateus Ferreira de Souza2, Daniela Alves de Oliveira3, Evandro Bona1, Fernanda Vitória Leimann1, Lúcio Cardozo Filho4, Pedro Henrique Hermes de Araújo3, Sandra Regina Salvador Ferreira3 and Odinei Hess Gonçalves1* Post-graduation Program of Food Technology – PPGTA, Universidade Tecnológica Federal do Paraná – UTFPR, Campo Mourão, PR, Brazil 2 Food Technology and Engineering Department – DALIM, Universidade Tecnológica Federal do Paraná – UTFPR, Campo Mourão, PR, Brazil 3 Department of Chemical and Food Engineering – EQA, Universidade Federal de Santa Catarina – UFSC, Florianópolis, SC, Brazil 4 Department of Chemical Engineering, Universidade Estadual de Maringá – UEM, Maringá, PR, Brazil 1

*odinei@utfpr.edu.br

Abstract Encapsulation of bioactive compounds has been carried out to improve bioavailability and to protect them against harm conditions. However, encapsulation processes are often aggressive and it is important that encapsulated substances keep their biological activity. In this work curcumin was nanoencapsulated using dichloromethane as solvent and ultrasound as dispersion device. Nanoparticles were obtained using different curcumin concentrations and encapsulants (PLLA and Eudragit S100) and the encapsulation efficiency was inferred using spectroscopic and calorimetric techniques as well as optical microscopy. Total phenolic contents and antioxidant activity tests were applied to the curcumin before and after encapsulation and also to blank polymer nanoparticles. Results demonstrated that the encapsulation process had no deleterious influence on its antioxidant activity.

Keywords: curcumin, antioxidant potential, nanoencapsulation, miniemulsification.

1. Introduction Curcumin has attracted much attention due to its significant medicinal potential[1-4]. Industry is also interested in curcumin since its antioxidant properties were demonstrated using a number of different methods[3]. Eybl et al.[5] demonstrated that curcumin protects against lipid peroxidation induced by cadmium, which is a highly carcinogenic compound.

The contribution of each part of the curcumin molecule for the whole antioxidant activity has been subject of some debate[6,7]. Galano et al.[8] shed light on the apparent contradictions among the antioxidant mechanisms proposed by different authors. They claimed that the predominant mechanism depends on the electron withdrawing character of the free radical itself and also on the solvent polarity. Feng and Liu[9] concluded that curcumin acts as an antioxidant mainly through the phenolic hydroxyl group.

Despite its biological properties, large scale use of curcumin by pharmaceutical and food industry has been hindered by its low water solubility and its instability in the presence of light, excessive heat, alkaline conditions, metallic ions and others[10]. Micro and nanoencapsulation have been investigating as a feasible way to protect and to stabilize curcumin until it reaches its destination into the body or to increase its bioavailability[11]. Miniemulsification or emulsification followed by solvent evaporation are suitable

Polímeros, 26(3), 207-214, 2016

techniques to obtain nano or microparticles, respectively[12].

Biocompatible polymers are of interest because they can be readily shaped in submicrometric particles allowing them to carry the encapsulated compound to specific sites inside the body. Recent works demonstrated that curcumin concentration inside nanoparticles can be readily determined by simple methods with the necessary accuracy[13]. However, miniemulsification and emulsification require the use of organic solvents, which is in direct contact with the encapsulated compounds. In the case of curcumin, it is necessary to investigate if decomposition or chemical modifications occurs during the particles production since biological activity is closely related to the molecular structure[8].

In miniemulsification, there is also the need of intense energy to break up the dispersed phase (either organic or aqueous) to achieve submicrometric sizes. The most common devices are high pressure homogenizators and high potency ultrasound devices. Despite the usually low dispersion times required to obtain polymer nanoparticles, there is the need to investigate if the encapsulated compound remains unchanged after the encapsulation process. The objective of this work was to investigate the antioxidant activity of curcumin after its encapsulation on polymer particles by miniemulsification-solvent evaporation technique.

207

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

Silva-Buzanello, R. A., Souza, M. F., Oliveira, D. A., Bona, E., Leimann, F. V., Cardozo, L., Fo., Araújo, P. H. H., Ferreira, S. R. S., & Gonçalves, O. H. The encapsulation process was investigated in respect of different encapsulants in order to determine the encapsulation efficiency and final particles morphology. Finally, total phenolic content (Folin–Ciocalteau colorimetric method) and the antioxidant activity of curcumin before and after the encapsulation were determined by ABTS and β-carotene bleaching method.

2. Materials and Methods 2.1 Materials Folin–Ciocalteau phenol reagent, gallic acid, ABTS [2,2_-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid)], Trolox and curcumin (99.5%) were obtained from Sigma–Aldrich.

Lecithin (Alfa Aesar) was used as surfactant. Methanol (Vetec, 99.8%) and dichloromethane (Vetec, 99.5%) were used in the encapsulation efficiency determination. Distilled water was used as continuous phase in the miniemulsion preparation.

Poly(L-lactic acid) (PLLA, 4,000 g/gmol) was obtained

by L-lactide ring opening polymerization (tinII-octanoate 0.01wt%, 120 °C, 24 hours, Bendix[14]) and used as encapsulant.

Poly(methacylic acid-co-methyl methacrylate) (Eudragit S100, 125,000 g/gmol, Evonik Industries AG) was also used as encapsulant. All chemicals were used as received without further purification.

2.2 Nanoparticle preparation Curcumin loaded nanoparticles were obtained using the miniemulsification-evaporation technique as described by Leimann et al.[15] with some modifications. The procedure varied depending on the encapsulant used as described below.

The formulation used was 22.400 g water, 0.180 g lecithin, 11.500 g dichloromethane, and 0.390 g encapsulant (PLLA and Eudragit S100 or a mixture of them). No co-stabilizer was used since it is known that preformed polymers are effective in preventing emulsion degradation by diffusion[16].

When PLLA was used as encapsulant, PLLA and lecithin were dissolved in dichloromethane for 10 minutes and, after this, curcumin was added and mixed for 5 min (1, 3, 6, 18 wt% of the total encapsulant mass). This solution was added to the distilled water under gentle stirring.

The macroemulsion formed was sonicated (Fisher-Scientific, Ultrasonic Dismembrator 120 W, 1/8” tip) for 180 seconds in a pulse regime (30 s sonication, 10 s pause) and the solvent was evaporated for 18 hours at 40 °C. When PLLA and Eudragit S100 together were used as encapsulant, Eudragit S100 was dissolved in dichloromethane at 60 °C for 20 minutes. After this, the mixture was cooled and the evaporated solvent was added again. From this point, steps were the same as described above.

2.3 Characterization The actual concentration of curcumin ([cur]actual) in the final nanoparticles dispersion (encapsulated and non‑encapsulated) was determined as follows. A nanoparticle dispersion sample (1 mL) was dried at 70 °C for 2 hours in a circulation oven and then diluted in 1 mL of dichloromethane. The polymer was precipitated by adding 1 mL of cold methanol, filtered 208

(Millipore 0.45 µm) and diluted in methanol/dichloromethane 1:1 (vol/vol) to 25 mL. Absorbance was measured at 465 nm (OceanOptics, UV650). The concentration of curcumin effectively encapsulated in the nanoparticles was determined as follows. A nanoparticle dispersion sample (5 mL) was centrifugated at 1,000 rpm for 10 minutes in order to precipitate the non-encapsulated curcumin crystals. 1 mL of the supernatant was dried at 70 °C for 2 hours in a circulation oven and then diluted in 1 mL dicholromethane. The polymer was precipitated by adding 1 mL of cold methanol, filtered (Millipore 0.45 µm) and diluted in methanol/dichloromethane 1:1 (vol/vol) to 25 mL. Absorbances were measured at 465 nm and encapsulation efficiency was calculated using Equation 1. This method was validated elsewhere in respect of linearity, precision and accuracy[13].

EE ( % ) =

[cur ]encapsulated *100 (1) [cur ]actual

Average molar masses of PLLA were determined by Gel Permeation Chromatography (GPC) using polystyrene standards (580 to 3,800,000 g/mol) and tetrahydrofuran as eluent at 1 mL/min. The following apparatus was used: Shimadzu model LC-20A, with a refraction index detector RID-10A, automatic injector SIL-20A, oven CTO-20A and three columns (0.8 x 30 cm, GPC-801, GPC-804 and GPC-807). The samples were dissolved in tetrahydrofuran (THF, Sigma-Aldrich) at approximately 7.0 mg.ml-1, filtered through a 0.45 µm Nylon filter then analyzed at 35 °C. Average particles size (Dz) and polydispersion index (PDI) were determined by Dynamic Light Scattering (DLS, Malvern Zetasizer - Nano Series) using backscattering detection (173°) in samples without previous dilution. PDI was calculated using the standard deviation (σ) of the size distribution by Equation 2.

PDI =

σ2 (2) Dz 2

Particle morphology was investigated by Transmission Electron Microscopy (TEM, Jeol JEM 1011 at 100kV).

Samples were carefully placed on 300 mesh parlodium‑coated cooper TEM grids and stained using osmium tetroxide for 4 hours. The presence of free curcumin crystals in the nanoparticles latex was investigated by optical microscopy (BIOVAL, L2000A). Infrared spectroscopy analysis of PLLA, Eudragit S100, lecithin, curcumin and the curcumin-loaded nanoparticles were carried out using a Frontier PerkinElmer FTIR-UATR with 4 cm–1 resolution and peak normalization.

Differential scanning calorimetry (DSC) of the lyophilized nanoparticles samples were performed using a Simultaneous Thermal Analysis calorimeter (Perkin Elmer STA 6000) from 20 to 390 °C at 10 °C/min and nitrogen at 30 mL/min.

Polymer crystallinity (xc) was calculated using Equation 3. Here, ΔHm is the specific enthalpy of melting and ΔHcm is the specific enthalpy of melting of a 100% crystallinity sample (93.7 J/g for PLLA)[17] and φ is the mass fraction of PLLA in the blend nanoparticles.

1 ∆H χc ( % ) = mc x100 (3) ϕ ∆H m Polímeros, 26(3), 207-214, 2016

Preparation of curcumin-loaded nanoparticles and determination of the antioxidant potential of curcumin after encapsulation 2.4 Total phenolic content The total phenolic content (TPC) was determined according to the Folin-Ciocalteu method[18]. The reaction mixture was composed of 0.1 mL of extracts solutions (concentration of 1,667 mg/L), 7.9 mL of distilled water, 0.5 mL of Folin‑Ciocalteu reagent (a mixture of phosphomolybdate and phosphotungstate) and 1.5 mL of 20% sodium carbonate, placed in amber flasks. The flasks were agitated and allowed to stand for 2 h and the absorbance was measured at 765 nm.

Results (mean value of the triplicate assays) were expressed as milligrams of gallic acid equivalents per gram of the extract (mg GAE/g) using a gallic acid standard curve (concentration range was 0 to 500 mg/mL, R2 = 0.9969).

Table 1. Diameter average particles (Dz), polydispersion index (PDI), curcumin encapsulation efficiency (EE%) for different curcumin concentration.

Added curcumin (wt%) 0 1 3 6 12 p (ANOVA)

DZ (nm)

PDI (-)

EE (%)*

187 ± 6 184 ± 4 171 ± 8 177 ± 6 187 ± 3 0.123

0.223 ± 0.004 0.195 ± 0.005 0.170 ± 0.034 0.160 ± 0.014 0.216 ± 0.009 0.0514

96.6 ± 3.4a 95.8 ± 1.0 a 77.9 ± 5.6 b 30.2 ± 0.5 c 0.0000

*Different letters in the same row indicate statistical differences (p < 0.05).

2.5 Antioxidant potential ABTS•+ radical scavenging assay was carried out based on the procedure described by Re et al.[19] using the synthetic vitamin E dissolved in ethanol, Trolox (6-hydroxy‑2,5,7,8 ‑tetramethylchroman-2-carboxylic acid) (Sigma–Aldrich Co, St. Louis, USA), as an antioxidant standard. ABTS [2,2 -azino-bis-(3-ethylbenzotiazoline-6-sulfonic acid)] was dissolved in water to a concentration of 7.0 mM, and submitted to reaction with 2.45 mM of potassium persulfate for the formation of the radical ABTS•+, that is reduced in the presence of an antioxidant compound hydrogen donor.

The absorbance was measured at 754 nm 6 min after the initial mixing of the samples and standard with the ABTS solution. Results were expressed as Trolox equivalent antioxidant capacity (TEAC) (mM concentration of a Trolox solution whose antioxidant activity is equivalent to the activity of 1.0 mg/mL of sample solution). In order to find TEAC values, a separate concentration response curve for standard Trolox solutions was prepared.

The antioxidant activity from the β-carotene/linoleic acid system (β-carotene bleaching method) was carried out according to the method described by Matthäus[20]. Briefly, an aliquot of 5 mL of a stable emulsion of β-carotene/linoleic acid (40 g of linoleic acid, 400 mg of Tween-20 and 3.34 mg of β-carotene/100 ml distilled water) was added with 0.2 mL of the extract solutions (1667 mg curcumin/mL) and the absorbance was immediately measured at 470 nm against a blank consisting of the emulsion without β-carotene.

The tubes were placed in a water bath at 50 °C and the absorbance was measured every 15 min up to 120 min.

The β-carotene bleaching rate was determined by the difference in absorbance (at 470 nm) values at 0 min and at 120 min (mean of the triplicate experiments) and converted into percentage of antioxidant activity (% AA).

3. Results and Discussion 3.1 Influence of curcumin concentration Table 1 presents the average particles size, polydispersion index and curcumin encapsulation efficiency for different curcumin concentrations (PLLA as encapsulant). Average particles size, polydispersion index and curcumin encapsulation efficiency for different encapsulants (3 wt% added curcumin) are presented in Table 2. Figure 1 presents an image of the latex obtained with 12 wt% curcumin.

Polímeros, 26(3), 207-214, 2016

Table 2. Diameter average particles (Dz), polydispersion index (PDI), curcumin encapsulation efficiency (EE%) for different encapsulants (3 wt% curcumin).

Encapsulants (w:w) E S100: PLLA 0:1 1:1 1:0 p (ANOVA)

DZ (nm)*

PDI (-)

EE (%)*

171 ± 8a 195 ± 9a 232 ± 9b 0.0060

0.170 ± 0.034 0.235 ± 0.023 0.235 ± 0.018 0.1810

95.8 ± 1.0a 43.9 ± 1.4b 34.5 ± 1.0c 0.0004

*Different letters in the same row indicate statistical differences (p > 0.05).

Figure 1. Optical microscope images of the curcumin-loaded PLLA latex (12 wt% curcumin).

Average nanoparticles diameter and polydispersity was not significantly influenced by curcumin (p > 0.05) meaning that the concentrations used were not high enough to affect the equilibrium between breakage and coalescence of the solvent/polymer/curcumin droplets during ultrasound application. This also means that the differences found in the encapsulation efficiency were solely due to the amount of curcumin added initially. Efficiency was high when low concentration of curcumin was used (1 and 3 wt%) but it drastically decreased for higher amounts (6 and 12 wt%).

209

Silva-Buzanello, R. A., Souza, M. F., Oliveira, D. A., Bona, E., Leimann, F. V., Cardozo, L., Fo., Araújo, P. H. H., Ferreira, S. R. S., & Gonçalves, O. H. Free curcumin crystals were easily found in the latex when 12 wt% curcumin was added, meaning that part of the total curcumin was not effectively encapsulated in the nanoparticles. This is in agreement with the encapsulation efficiency values determined by UV-Vis (Table 1). It worth noting that no curcumin macroscopic crystals could be found in the latex when 3 wt% curcumin was added. This behavior was also found by other authors when encapsulating curcumin in yeast cells of Saccharomyces cerevisiae by a diffusion method[21]. The same occurred in the case of spray drying encapsulation using porous starch and gelatin as encapsulants[22] and anti‑solvent precipitation using zein as encapsulant[23]. Results indicate that there is a maximum amount of curcumin that can be encapsulated and the excess is removed from the dichloromethane-PLLA system during the solvent evaporation step forming the micrometric crystals observed in Figure 1.

Although there was difference in the particles diameter for different encapsulant composition, this could not account to the high decrease in the encapsulation efficiency observed when Eudragit S100 was used as encapsulant. This could be attributed to the higher hydrophilicity of the methacrylic acid groups of Eudragit S100 leading to less affinity with curcumin.

3.2 Nanoparticles characterization Figure 2 presents thermograms of pure curcumin, pure PLLA, pure Eudragit S100 as well as curcumin-loaded (3 wt%) PLLA, Eudragit S100 and PLLA/Eudragit S100 blend nanoparticles. In Table 3 the glass transition temperatures and PLLA crystallinity are presented.

Melting temperature (Tm) of curcumin was found to be 171 °C as indicated by the dashed line in Figure 2 which is in accordance with values presented in literature[24-26].

Glass transition temperature (Tg) of PLLA nanoparticles can be observed at 60 °C as a relaxation peak due to its thermal history. Eudragit S100 presented Tg and Tm at 160 °C and 180 °C, respectively, the same values reported elsewhere[27]. In curcumin-loaded nanoparticles the melting peak of curcumin could not be detected. This behavior was also found by other authors[24-26] and can be attributed to the fact that curcumin was successfully encapsulated in the nanoparticles forming a solid solution evidencing the strong interactions between the encapsulants and curcumin.

PLLA powder presented high crystallinity which decreased after the nanoparticles production (from 72% to 48%) meaning that solvent evaporation was slow enough to allow polymer chains conformation in the amorphous phase.

PLLA crystallinity further decreased due to the presence of curcumin, indicating that it might be entrapped inside the nanoparticles hindering the formation of the PLLA crystalline phase.

In the PLLA/Eudragit S100 blend nanoparticles it is possible to observe two separated glass transition: one around 60 °C for PLLA and other around 160 °C for Eudragit S100.

PLLA and Eudragit pure nanoparticles presented the same transition temperatures which is a strong indication that some degree of polymer separation took place[28].

210

Figure 2. Thermograms of blank and curcumin-loaded nanoparticles.

Table 3. Melting temperatures and PLLA crystallinity.

Experimental condition

Tm (°C)

PLLA Blank PLLA nanoparticles

167

ΔHm (J/g) 67.5

161

45.3

48

154

19.1

20

161

26.9

29

160

22.5*

24*

(no curcumin) Curcumin-loaded PLLA nanoparticles (1%wt cur) Curcumin-loaded PLLA nanoparticles (3%wt cur) Curcumin-loaded Eudragit S100/PLLA nanoparticles(3%wt cur)

xc (%) 72

*Considering PLLA:Eudragit S100 mass proportion in the nanoparticles (1:1w/w).

Figure 3 shows the FTIR spectra of blank PLLA nanoparticles (without curcumin), pure curcumin and the curcumin loaded-nanoparticles (3 wt%).

Curcumin is often identified by its phenolic absorption band at 3508 cm–1[10,25]. Other curcumin characteristic bands can also be seen such as the aromatic C-C band (1602 cm–1), olefinic C-H (1428 cm–1), asymmetric C-O-C (1026 cm–1) and phenolic C-O (1276 cm–1). PLLA (C-H at 1450 and 1360 cm–1, asymmetric C-O at 1130cm–1, C=O at 1760 cm–1), lecithin (R–O–P–O–R’ stretching band at 1055 cm–1) and Eudragit Polímeros, 26(3), 207-214, 2016

Preparation of curcumin-loaded nanoparticles and determination of the antioxidant potential of curcumin after encapsulation

Figure 4. Transmission Electron Microscopy of the curcumin‑loaded nanoparticles (3%wt curcumin).

Figure 3. FTIR spectra of pure curcumin, blank PLLA nanoparticles and curcumin-loaded nanoparticles.

S100 absorption bands (carboxyl ester at 1728 cm–1) can also be found[10,29,30] (all absorption bands are indicated in Figure 3). Curcumin phenolic band was attenuated in the nanoparticles when comparing to pure curcumin suggesting that it was located inside the nanoparticles. This can also be attributed to the interaction between curcumin and the encapsulants mostly through hydrogen bonding.

Transmission electron microscopy image of PLLA nanoparticles and PLLA:Eudragit S100 (1:1) nanoparticles are presented in Figure 4 (3 wt% curcumin).

Curcumin-loaded PLLA nanoparticles appeared as homogeneous nanospheres with diameters around 200 nm corroborating the DLS results. On the other hand, PLLA/Eudragit S100 blend nanoparticles presented phase separation morphology in accordance with the DSC results. PLLA can be seen as light gray and Eudragit S100 as black due to differences in contrast under the electron Polímeros, 26(3), 207-214, 2016

beam, indicating that the more hydrophilic PLLA formed an outer shell and Eudragit S100 in the particles core as demonstrated in other works[31]. This core-shell morphology is also supported by the fact that, depending on the process kinetic, the semi-crystalline structure of PLLA could act as a driving force for phase separation[32]. It is worth noting that in both cases no free curcumin crystals could be found corroborating the encapsulation values found (Table 2).

3.3 Antioxidant activity Table 4 presents the antioxidant activity of free curcumin, blank and curcumin-loaded nanoparticles.

Free and encapsulated curcumin presented high phenolic content (in gallic acid equivalents) and antioxidant activity, being effective ABTS scavenging and inhibitor of lipid peroxidation of linoleic acid emulsion. Literature reports that curcumin is an extremely potent lipid-soluble antioxidant proved by different methods including scavenging of a variety of radicals and inhibition of lipid peroxidation[3,33,34].

Phenolic antioxidants usually scavenge free radicals by an electron-transfer mechanism and curcumin acts as an extraordinarily potent H-atom donor in neutral and acidic 211

Silva-Buzanello, R. A., Souza, M. F., Oliveira, D. A., Bona, E., Leimann, F. V., Cardozo, L., Fo., Araújo, P. H. H., Ferreira, S. R. S., & Gonçalves, O. H. Table 4. Antioxidant activity of free curcumin, blank and curcumin-loaded nanoparticles.

Total phenolic

ABTS

β-caroten/ linoleic acid

Sample

content (TPC) (mgGAE/gextract)

% inhibition*

TEAC (μM/gextract)

%AA (after 120 min)ǂ

Free curcumin PLLA NPs

495 ± 8

61 ± 1

824 ± 5

89 ± 1

604 ± 27

68 ± 1

906 ± 6

101 ± 1

167 ± 1

5±1

40 ± 1

16 ± 1

517 ± 9

77 ± 3

1063 ± 9

93 ± 2

55 ± 4

7±1

72 ± 6

6±0

(3 wt% curcumin) Blank PLLA NPs (no curcumin) Eudragit NPs (3 wt% curcumin) Blank Eudragit NPs (no curcumin)

GAE = gallic acid equivalent; TEAC = Trolox equivalent antioxidant activity. *100 μg curcumin/mL solution. ǂ 167 μg curcumin/mL solution.

aqueous solutions[35]. Ak and Gülçin[3] demonstrated the electron donor capacity of curcumin for neutralizing free radicals by forming stable products, which property is associated with antioxidant activity[3]. Antioxidant capacity is widely used as a parameter for medicinal bioactive components since the reduction of inflammatory responses, chronic diseases, DNA damage, mutagenesis, carcinogenesis and inhibition of pathogenic bacterial growth is often associated with the termination of free radical propagation in biological systems[36].

Besides, lipid peroxidation consists of a series of free radical mediated chain reaction processes and is associated with several types of biological damage; therefore, quench those free radicals to terminate the peroxidation chain reactions can improve the quality and stability of food products[3].

Blank nanoparticles (PLLA and Eudragit S100) also presented small antioxidant activity. This may be attributed to lecithin which can act as an oxidative stabilizer. Pan et al.[37] demonstrated that lecithin can decrease the rate of oxidation of encapsulated curcumin in lecithin stabilized emulsions.

In order to normalize the results presented on Table 4 and to allow a fair comparison, the amount of curcumin used in the antioxidant assays (“free curcumin”) was equal to the mass of curcumin actually present in the nanoparticles. This explain the fact that the loaded nanoparticles presented slightly higher values of antioxidant activity than pure curcumin.

It is worth noting that curcumin was extracted from the particles before the antioxidant activity determination simulating the case in which curcumin is released from the nanoparticles. This methodology was also implemented

by Wang et al.[38] to determine the antimicrobial activity of encapsulated curcumin. Results indicated that curcumin kept its antioxidant properties after the encapsulation procedure meaning that the aggressive encapsulation conditions (ultrasound and solvent exposure) did not affect curcumin activity expanding its application also in aqueous systems.

4. Conclusions Curcumin-loaded nanoparticles were obtained with different curcumin concentration and encapsulants.

For PLLA nanoparticles, curcumin was not efficiently encapsulated when more than 3 wt% was added leading to the formation of free microscopic curcumin crystals. For this 212

concentration, differential scanning calorimetry (DSC) and infrared spectroscopy (FTIR) indicated that curcumin was effectively encapsulated forming an amorphous dispersion inside the particle matrix. The same occurred when Eudragit S100 was the encapsulant. When PLLA and Eudragit S100 was used to form blend curcumin-loaded nanoparticles, Transmission Electron Microscopy along with DSC and FTIR strongly suggested that phase separation took place.

The antioxidant activity of free curcumin was compared to the activity of encapsulated curcumin. ABTS, β-carotene and total phenolic content results were very similar before and after encapsulation, showing that the encapsulation conditions (intense ultrasound and solvent exposure) did not compromise the strong antioxidant activity of curcumin.

5. Acknowledgements Authors thank to Conselho Nacional de Pesquisa (CNPq), CAPES and Fundação Araucária for the financial support and scholarships.

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Received: June 03, 2015 Revised: Nov. 01, 2015 Accepted: Feb. 12, 2016

Polímeros, 26(3), 207-214, 2016

http://dx.doi.org/10.1590/0104-1428.2178

Rheological behavior of acrylic paint blends based on polyaniline Alex da Silva Sirqueira1*, Dermeval Teodoro Júnior1, Marcio da Silva Coutinho1, Artur Soares da Silva Neto1, Adriana dos Anjos Silva1 and Bluma Guenther Soares2 Laboratório de Engenharia de Polímeros e de Aplicações Industriais, Centro Universitário Estadual da Zona Oeste – UEZO, Rio de Janeiro, RJ, Brazil 2 Programa de Engenharia Metalúrgica e de Materiais – PEMM, Instituto Alberto Luiz Coimbra de Pós-Graduação e Pesquisa de Engenharia – COPPE, Universidade Federal do Rio de Janeiro – UFRJ, Rio de Janeiro, RJ, Brazil 1

*alexsirqueira@uezo.rj.gov.br

Abstract The rheological properties of acrylic paints and polyaniline (PAni) blends, with different contents of PAni doped by dodecyl benzene sulphonic acid (DBSA) and, dispersed by mechanical stirrer and ultrasonic, were investigated by controlled shear rate testing ramps. The results showed that the commercial acrylic paint had tended to deliver the required stability on the blends, in order to avoid sedimentation process. All samples exhibited non-Newtonian flow behavior (shear thinning), increasing PAni content the flow behavior index (n) decreased (0.41 to 0.11) and power law model were used to fitted the experimental curves. The results showed that the addition of PAni-DBSA affects the viscoelastic behavior of the mixtures due to the interactions between the components in the mixture. The best properties were obtained for samples 90/10 wt % dispersed by ultrasonic, indicating the feasibility of the usage as a conducting paint.

Keywords: polyaniline, acrylic paint, thixotropy.

1. Introduction Inherently Conducting Polymers are considered to be organic materials which exhibit electrical, magnetic and optical properties very similar to metals and semiconductor materials, these kinds of polymers, have conjugated double bonds of polymer chains, which allow an electron flow under specific conditions. The oxidation/reduction processes from polymeric chains, are carried out by charge transfer agents, changing the insulator nature from polymers to a conducting or semi-conducting nature, due to an increase of both electrons mobility and conductivity[1-3].

The chemical doping is a reversible process which occurs by protonation in aqueous acid solution, with no changes on the number of electrons associated with the polymer chains, basically, consists of protons addition to the polymer chains of a doping agent, leading to the shift of electrons from π system[4]. Studies have shown that certain conducting polymers such as Polyaniline and Polypyrrole have been found to offer corrosion protection of iron, steel and aluminum, attributed to its high stability in the air, adhesion and redox properties with the substrate[5-9].

Amongst the class of conducting polymers, Polyaniline (PAni) is unique due to its easy synthesis, low cost, good thermal stability, and reversible doping/dedoping process to control the conductivity[10,11]. When reaction occurs in acidic environment (low pH condition), leads the head‑to-tail coupling of aniline monomers in the para position and green form as a protonated emeraldine, which can be dedoped by oxidizing agent to produce the blue, emeraldine base form of the semi-conducting polymer[12-14].

Polímeros, 26(3), 215-220, 2016

On the other hand, it is well known the insoluble and infusible nature of ICPs has kept them away from the formation of solutions/melt. The mechanical properties of conducting PAni have been investigated in the scientific literature only with respect to its blends, PAni usually acted as particulated conducting filler in a suitable matrix that provided the required mechanical properties. In order to overcome the intractability of ICPs and to bring them into the solution/melt, some techniques such as: introduction of bulky side groups or dopant ions into the polymeric chain or use of polymeric or surfactant stabilizers optimize the formation of ICP solutions[15-17].

Blends of conducting polymers with conventional polymers have been extensively studied[18-21]. Their mixture with conventional polymers consists of a good strategy from the technological point of view[22-24].

As rheology is considered to be an efficient tool for exploring structural properties and molecular interactions of different materials, it will be applied, as a technique for blends macroscopic characterization, on the other hand, it will provide an easily accessible way to correlate the microstructure of a system with its particular rheological responses on PAni-based systems[25-27].

The main of this article will compare the rheological behavior of Acrylic Paints/PAni blends and evaluate the influence of sample preparation methodology on the viscoelastic properties. It known the use of PAni, or another conducting polymer to improve the corrosion protection, but they’re very little paper showing e rheological behavior after the addition of conducting polymer. Rheology will provide

215

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

Sirqueira, A. S., Teodoro, D., Jr., Coutinho, M. S., Silva, A. S., No., Silva, A. A., & Soares, B. G. useful information for prediction of blends behavior during development of conducting paints, due to it determines the performance of the paint during the whole handling cycle, from storage to application and drying[25,28-30].

( dt )

µ =k d γ

n −1

(1)

τ = k γ n (2) ln= µ ln k + ( n − 1) ln γ (3)

2. Materials and Methods 2.1 Materials Commercial acrylic paint was kindly supplied by Akzo Noble (Brazil). The characteristic of the commercial paint is given in Table 1.

The PAni.DBSA was obtained by mixing aniline/DBSA in the molar ratio 1:1 in 75 ml water/methanol mixture (3:1), under vigorous stirring for 7 minutes. Then 20 ml of ammonium persulfate (0.1 mol) as oxidizing agents was slowly added in the solution. The polymerization reaction was carried out at 0 °C under vigorous stirring. After 6 hours, the reaction medium was poured into methanol, filtered, washed several times with methanol, and dried under reduced pressure for 72h[31].

Acrylic paint with doped PAni concentration rises from 5% to 10% w/w were obtained by dispersing using a mechanical stirrer and ultrasonic at room temperature.

Mechanical dispersion was carried out with a constant stirring speed of 300 RPM for 10 minutes. Ultrasonic dispersion was carried out in ultrasonic device, the sonotrode used to have tip diameter 3 mm and made of titanium. The dispersion was carried out in the following conditions: 100 W power, at constant frequency of 30 kHz, and constant amplitude of 50% over five minutes.

2.2 Rheological measurements The rheological characterization was carried out using a shear rate-controlled rotational rheometer (Haake Rotovisco RV20/CV20N) (Couette flow) fitted with a Mooney-Ewart coaxial cylinder sensor (inner cylinder diameter 28.93 mm, outer cylinder diameter 30.0 mm, length 24.0 mm, and sample volume 1.8 cm3). The temperature was controlled with an accuracy of 25±0.1 °C by circulating water in the jacket of the outer cylinder arrangement.

2.2.1 Viscosity measurements The Power-law model was used to study the flow behavior of the formulations:

Where: µ is the viscosity, t is the time, τ is the shear stress; k a constant; γ the shear rate; and n the non-Newtonian power index.

The samples were investigated by controlling shear rate testing ramps from 0 to 500 s–1.

2.2.2 Creep and recovery test In creep and recovery test, generally, elastic deformation may be resembled by a spring model and viscous flow by a dashpot model. The quantity of springs and dashpots and the way in the sample body, which they are connected, can be used to represent different kinds of viscoelastic materials[31].

Creep and recovery tests were carried out under the shear stress of 5 Pa at 25 °C, the variation of the strain in response to the applied stress was measured of a period of 3 minutes, afterwards, the stress was then removed, and changes in strain were registered through a further period of 3 minutes, in order to observe structure recovery, using the evaluation tool from Software Rheowin3, was also possible to provide viscosity zero shear rate and relaxation time determination.

3. Results and Discussions 3.1 Rheological behavior The viscosity is denominating the resistance of a fluid against any force tending to cause the flow. It is one of the most important properties in rheological studies for coating, paint and ink. To obtain a good application characterization, good paint has to be non-Newtonian liquid behavior, which are shear rate dependent. Rheological characterization of paint is to measure the relationship between shear stress and rate of shear strain variation harmonically with time[32].

Figure 1 shows the viscosity variation as a function of shear rate, it is observed that all samples exhibited a typical rheological behavior of non-Newtonian fluid, the viscosity decreased rapidly with the increase of the shear rate, this is called pseudoplastic behavior, and this behavior

Table 1. The characteristic of the commercial paint.

Manufacturer

Commercial paint

Description

Product information Color: white Solid content: 56% Viscosity: 90-100 UK Volatiles: 53-56%

Azko Noble Ltda

Coralar acrilico

Water based paint

Titanium dioxide: 1-5% Ethoxyl alcohol: <0.1% Ammonium: 0.1-1.0% Aluminum silicate: 10-30% Calcium carbonate: 15-40% Application method: air spray, brush or roller

216

Polímeros, 26(3), 215-220, 2016

Rheological behavior of acrylic paint blends based on polyaniline is characteristic of the textural changes in the samples may be induced by the shear rate. For Newtonian fluids, n=1 and ln k=ln µ, but for a pseudoplastic fluid the index n is associated to the viscoelastic parameter and take values smaller 1. The pseudoplastic behavior can be attributed to the polymer chain pulled apart to be arranged in straight chain when the polymer is in shear flows. At low strain region shows non-linear viscosity characterized weak (physical) gel forned by hydrogen bonding between the amine, imine and carboxyl groups. In this work two amounts of conducting polymer were added in the acrylic paint 5% and 10%, by two different dispersion methodologies, mechanical mixer and ultrasonic. Analyzing the addition of PAni 5% w/w, in the range of 50 to 100 s–1, it was not observed any difference in the curve between acrylic paint and the dispersion by mechanical mixer. But, when the ultrasonic methodology was used to disperse PAni, the viscosity curve showed high value. It can be an indication that the sample had better dispersion of PAni in acrylic paint, because the ultrasonic dispersion experiment generates high shear that breaks particle agglomerates in single dispersed particles. It is known, PAni has a high tendency to agglomerate[33], been individual molecules cluster to form primary particles (organic metals), which then form primary aggregates. Primary aggregates cluster to form secondary particles[29]. The weak bonding of dipole force between polymer molecules in a physical gel can be easily broken by the external shear deformation, resulting in the delay of gelation process. Therefore, the increase of viscosity promoted by ultrasonic dispersion is an indication that the sample had better dispersion of PAni in acrylic paint, because the ultrasonic dispersion experiment

generates high shear that breaks particle agglomerates in single dispersed particles.

When increase the amount of 5% to 10% w/w of PAni in acrylic paint, any difference in curve were observed by dispersion methodology. In this condition, 10% w/w of PAni, probably the amount of polymer will determine the curve behavior of viscosity.

When trying to add a filler or an insoluble polymer into the paint formulations some practical aspect appears like precipitate of this additive. An example that rheological characterizations for paint applications is the forecast of settling property. High viscosity in low shear zone is a reflection of good anti-settling property and stable filler suspension in storage condition. The PAni add in Acrylic paint, improve the anti-settling property, but it is difficult specify the mechanism of anti-settling in a commercial acrylic paint due the many variables (morphology, molecular weight, number and type of additives, etc.). However, it is may be reasonably assumed that the anti-settling effect is caused by the formation of hydrogen bonds between PAni and Acrylic resin which keep for a longer time the PAni particles suspended.

The shear stress versus shear rate data for all blends in the range of 0.01 t0 100 s–1, were best fitted to the power‑law model, Equation 2, this model are extensively used to describe the flow properties of non-Newtonian fluids[32,34].

As shown in Figure 1, all blend exhibited pseudoplastic behavior. The values for n, flow indexes, was between 0.41‑ 0.11, as shown in Table 2. Analyzing the addition of PAni in the acrylic paint it is observed a decreased in n values as percentage of PAni increased. This result can indicate the interaction between Acrylic Paint and PAni which will produce a reduction on n values, making the material with lower Newtonian characteristics, which is good for the technological paint applications. In the same way was noticed that the methodologies of dispersing influenced pseudoplastic characteristics.

On the other hand, the apparent viscosity at 100 s–1 (µa) increased as the PAni content increased, except for 5%, as reported in Table 2. This behavior can indicate that when increase the shear rate, starting at 100 s–1, the PAni particles align rapidly in the direction of increasing shear rate and will produce less flow resistance. However, for 10% predominates the high amount of PAni in the paint which will elevate the viscosity values[35].

The consistency index (K) indicates the degree of fluid resistance against the flow. The higher K values the material Table 2. Parameters of power law models for Acrylic paint with PAni. PAni

Figure 1. The dependency of Paint Viscosity in shear rate condition (a) for 5% PAni and (b) for 10% w/w.

Polímeros, 26(3), 215-220, 2016

0 5-M 10-M 5-U 10-U

Apparent viscosity

Ka

at 100 s (Pa.s) 1.21 0.91 2.11 1.12 2.86

(Pa.s) 17.61 53.34 63.84 372.40 1156.0

–1

nb

R2c

0.41 0.11 0.20 0.26 0.23

0.99 0.97 0.98 0.97 0.90

Consistency Index. bFlow Behavior Index. cCorrelation Coefficient.

a

217

Sirqueira, A. S., Teodoro, D., Jr., Coutinho, M. S., Silva, A. S., No., Silva, A. A., & Soares, B. G. is more consistent. The consistency index increase when the amount of PAni increase, 5% to 10% w/w, in the same dispersion methodology. But the high changes in K values were observed when analyzing the dispersion methodology.

Comparing PAni dispersion by mechanical mixer, was obtained 15% increase in 5% to 10% w/w. But for 10% of PAni in acrylic paint by ultrasonic the K value increase around 1000% in relation of commercial acrylic paint.

3.2 Creep and recovery A creep and recovery test is a two-part measurement which provides information about the paint, structural properties at very low stress. Figure 2A shows a typical diagram of the output response from creep and recovery experiment that is the compliance (J) as a function of time. The creep test is characterized by a constant stress is applied, which causes gradual deformation with time, through readjustment of the molecular structure of materials, dominated by the elastic component, followed by the viscoelastic component, where continuous flow occurs. The deformation of the materials is partially restored to its initial state, after unloading the stress on the materials. The more compliant is the molecular structure of sample, then the easier its space network deforms, when under the given shearing stress. The elastic creep compliance (JOC) is determined by linear extrapolation of the viscous regime of the creep curve while the elastic recoverable compliance (JOR) is the difference between the compliance at the end of the creep (t=t2) and recovery experiment (t=t3).

The slope of the line of the viscous regime is also used to determine the zero shear viscosity (µ0). Deformation of the paint was determined from absolute values of compliance and compared by the ratio (J3-J1) /J3[36].

In the recovery experiment, the extent of the recovery gives an indicating of the thixotropic regeneration of the paint. The extent of recovery R (%) has previously been defined as[37].

R ( % ) =  JOR  x100 (4) J3  

A characteristic relaxation time tr may also yield a quantitative assessment of the thixotropic and may be determined as the product:

Figure 2. The Creep test (A) for Acrylic Paint; (B) for Pani 5% and (C) for 10% of PAni. Table 3. Measured creep and recovery parameters for blends of acrylic paint and PAni. Samples 0

5-U

5-M

10-U

10-M

µ0 (Pa.s)

401

8.46×104 1.43×105 1.49×105 7.35×104

R%

1.18

46.09

63.97

46.40

57.91

tr (sec)

664

539

690

393

493

tr = µ0 x JOC (5)

between the paint and polymer chain, because the blends are less readily deformable.

When a polymer is subjected to a constant load, it deforms continuously. The polymer will continue to deform slowly with time indefinitely or until rupture or yielding causes failure. This behavior will describe three regions, if the constant force continuum applied, the first region is the early stage of loading when the creep rate decreases rapidly with time. Then it reaches a steady region which is called the secondary creep stage followed by a rapid increase (tertiary region) and fracture. This phenomenon of deformation under load with time is called creep[34].

The data values of the measured creep and recovery parameters for the acrylic paint are shown in Table 3 and, in Figure 2, the curves. The compliance during the creep test decreased significantly PAni presence. This suggests initially that the PAni really influence in acrylic paint on creep test and in the recovery, producing some interacting

It is interesting to notice the changes in the curve of compliance versus time by the addition of 5 or 10% w/w of PAni. The curve with 5% w/w of Pani in acrylic paint dispersed by mechanical mixer, Figure 2B, showed the lower values of compliance, which can indicate the bad homogenization of PAni in the paint. This suggestion of interpretation can be corroborated by the lower value of viscosity at 100 s-1, it will be easy alignment by the shear rate because the PAni particles are agglomerates. And when use a force to deform paint the agglomerates will act as a barrier. Probably the ultrasonic device deagglomeration not only PAni but others additives in paint that will reduce the resistance. When the amount of PAni increase to 10%, the curve behavior change, the lower values of compliance were obtained to the ultrasonic dispersion, probably due the amount of deagglomeration and dispersion will be less,

218

Polímeros, 26(3), 215-220, 2016

Rheological behavior of acrylic paint blends based on polyaniline It was also observed an increase on thixotropy comparing to acrylic paint, except for composition 95/5 w/w mechanically dispersed, due to the observed decrease on both thixotropy and yield stress value, which indicates a lower interaction between the components from the mixtures, the best properties were obtained with 90/10 w/w dispersed by ultrasonic.

5. Acknowledgements The authors express their sincere thanks to CNPq, CAPES and FAPERJ for financial support.

Figure 3. The Yield stress for conducting acrylic paint.

because there was more PAni in the blend. Generally, the filler increase leads to an increase in the total recovery and characteristic relaxation time of the paint. The recovery values (R) were high to the samples with PAni. It can be attributed to the hydrogen bond that will help in the rebuild the acrylic paint structure. However, relaxation time, for the acrylic paint is similar to acrylic paint and acrylic paint with 5% of PAni. It is interesting noticed that the 10% of PAni reduce the value of relaxation time, probably due the reduction in the agglomeration. But the interpretation of all effect, like percent of deagglomeration, reducing in the PAni and acrylic resin molecular weight, influence of dispersion it is extremely difficult in a commercial paint.

The zero shear viscosity is an indication that the energy to flow to paint and anti-settling. Zero shear viscosity values increased with increasing PAni content and changes were observed by the dispersion method. The behavior of zero shear viscosity was similar to being observed in compliance, high values to dispersion with 5% in mechanical mixer and 10% in the ultrasonic.

3.3 Yield stress Yield stress (τ0) is an important material property for the application of paint[38]. Yield stress measurements have previously been used to indicate the thixotropy of a material as a function of time[39-41]. The Yield stress values of acrylic paint modified with PAni are shown in Figure 3. It is not noticed variation on the yield stress for the addition of 5% of PAni in the acrylic paint. The addition of 10% was observed 50% increase in the yield stress for the mechanical dispersion methodology. But the high value in yield stress was observed when 10% of PAni is dispersed in acrylic paint by ultrasonic. This result can confirm the observation of the high energy dispersion of ultrasonic can reach not only in PAni but in all formulations components.

4. Conclusions Results obtained in this paper showed that Polyaniline addition to acrylic paints, changes the rheological properties of the blends, due to the interactions established between acrylic and Polyaniline.

The creep and recovery tests showed changes in the curves by the addition of PAni and the methodology of dispersion.

The paint blends had better recovery characteristic which is good for the paint application.

Polímeros, 26(3), 215-220, 2016

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Polímeros, 26(3), 215-220, 2016

http://dx.doi.org/10.1590/0104-1428.2093

Time domain NMR evaluation of poly(vinyl alcohol) xerogels Elton Jorge da Rocha Rodrigues1, Maxwell de Paula Cavalcante1 and Maria Inês Bruno Tavares1* Centro de Tecnologia, Instituto de Macromoléculas Professora Eloisa Mano – IMA, Universidade Federal do Rio de Janeiro – UFRJ, Rio de Janeiro, RJ, Brazil

1

*mibt@ima.ufrj.br

Abstract Poly(vinyl alcohol) (PVA)-based chemically cross-linked xerogels, both neat and loaded with nanoparticulate hydrophilic silica (SiO2), were obtained and characterized mainly through time domain NMR experiments (TD-NMR).

Fourier‑transform infrared (FT-IR) and wide angle X-ray diffraction (WAXD) analyses were employed as secondary methods. TD-NMR, through the interpretation of the spin-lattice relaxation constant values and related information, showed both cross-linking and nanoparticle influences on PVA matrix. SiO2 does not interact chemically with the PVA chains, but has effect on its molecular mobility, as investigated via TD-NMR. Apparent energy of activation, spin-lattice time constant and size of spin domains in the sample have almost linear dependence with the degree of cross-linking of the PVA and are affected by the addition of SiO2. These three parameters were derived from a single set of TD-NMR experiments, which demonstrates the versatility of the technique for characterization of inorganic-organic hybrid xerogels, an important class of materials.

Keywords: inorganic-organic hybrid, polymer gels, relaxometry, low-field nuclear magnetic resonance, time domain nuclear magnetic resonance.

1. Introduction Time domain nuclear magnetic resonance (TD-NMR) – also known as relaxometry, wideline or low-field nuclear magnetic resonance (LF-NMR) – is a rapid and inexpensive way to characterize polymeric materials. It is based on the measurement of the time necessary for relaxation to equilibrium of a magnetized atomic nucleus in a nuclear magnetic resonance experiment[1-3]. This process is related to the molecular dynamics of the sample and can reveal many factors acting on the polymer matrix, such as variations in crystallinity, blending of two or more polymers, presence of plasticizing agents or fillers, cross-linking and others[1,4-8].

This versatile tool has been applied in various fields, such as determination of physico-chemical parameters of crude oil samples, polymer-filler interactions in rubber materials, and reaction monitoring and quality assurance in industrial processes, to cite a few examples[9-12].

Polymeric gels are networks containing physically and/or chemically cross-linked segments which can halt the long distance rearrangements between the polymer chains that are responsible for the material’s dissolution and plastic deformation[13]. When polymeric gels are not filled with fluids, they are dubbed xerogels[14]. This class of polymeric material has many applications, such as in tissue engineering, drug delivery, prosthetics and membranes applied in separation processes [14-16].

Various efforts have been made to characterize polymeric gels through TD-NMR[15,17-22], but the majority of the reported methods involve soaking the sample in a solvent[14,19,23,24], with the analysis being conducted under strict temperature conditions[18,20,21], or limiting the chemical nature of the polymeric matrix to elastomers[6,12,18-21].

Polímeros, 26(3), 221-227, 2016

In this work, we obtained xerogels based on chemically cross-linked poly(vinyl alcohol) (PVA) and PVA loaded with nanoparticulate hydrophilic silica (SiO2). These materials were characterized by LF-NMR techniques and supporting spectroscopic characterization methods, such as Fourier-transform infrared (FT-IR) and wide angle X-ray diffraction (WAXD). Our aim was to use the information obtained mainly through TD-NMR to qualify and quantify the changes caused by cross-linking and inorganic filling of the PVA matrix.

2. Experimental 2.1 Materials PVA (99+% hydrolyzed), Mw: 89,000-98,000 g.mol–1 and glutaraldehyde (GA) (25% m/m in H2O) were supplied

by Sigma-Aldrich. Hydrochloric acid (HCl) (37% w/w) was obtained from Vetec, Brazil. SiO2, composed of 12 nm radius particles, was supplied by Degussa. Deionized water was employed during all necessary steps. All reagents were used as received.

2.2 Sample preparation Six 40 mL aliquots of a previously prepared 10% (by weight) aqueous PVA solution were used to obtain the samples, three for neat PVA xerogels and three for PVA/SiO2 xerogels. To each of the PVA solutions, a sufficient volume of a 1 mol.L–1 HCl solution was added to bring the pH of the system to 2±0.1. To each of the PVA/SiO2 solutions, 1% (by weight of polymer) of SiO2 was added to the PVA solution and left under stirring until all the nanoparticles

221

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

Rodrigues, E. J. R., Cavalcante, M. P., & Tavares, M. I. B. were homogeneously suspended. Then, a volume of GA enough to induce 1%, 5% and 10% of nominal cross-linking, based on the molar mass of PVA’s repeating unit, was added to the acidified solutions. Finally, the contents of the six aliquots were poured into polystyrene Petri dishes, covered and left in a fume hood at room temperature for five days to allow completion of the chemical cross-linking reaction.

After this period, the gels were thoroughly washed with deionized water and put in a vacuum oven at 45 °C for 2 days. Additionally, for comparison, a film of neat PVA and one of PVA/SiO2 were produced following the same procedure, except for the acidification and the addition of GA steps. All the materials were stored in a desiccator under vacuum until needed for tests. Table 1 reports the naming scheme of the gels and polymeric films.

2.3 FT-IR analysis The analysis was conducted on 1-mm thick films to investigate changes caused by the cross-linking reaction to the polymeric matrix, as well as interactions between PVA and SiO2. The test was run in a Varian Excalibur FT‑IR spectrophotometer, with a zinc selenide crystal for ATR measurements, having the following acquisition parameters: 100 scans, 4 cm–1 spectral resolution, sampling between 600 and 4000 cm–1.

2.4 WAXD analysis Films similar to those employed for FT-IR measurements were subjected to WAXD analysis to verify changes in the molecular order of the PVA, using a Rigaku Miniflex diffractometer, operating with Cu Kα band (1,5418 Å wavelength) radiation. The 2θ angle was varied between 5° and 50°, with sweep speed of 0.05°.s–1.

2.5 TD-NMR analysis

2.5.1 Spin-lattice relaxation constant determination The inversion-recovery pulse sequence was chosen to extract the samples’ spin-lattice relaxation time constant, T1.

The data were processed in commercial software packages, namely WinFit version 2.4.0.0 and Origin version 8.5.

Table 2 shows the analytic parameters used to measure the samples’ relaxation times.

The T1 values displayed are the average between the results from two runs. This mean value was extracted from fitting the experimental points using Equation 1.

M ( τ )= A0 +

 τ −  T A1e 1 

(1)

where M(τ) is the magnetization as a function of time between 180° and 90° pulses; A0 is the DC offset; A1 is a factor proportional to the number of relaxing proton nuclei in each interval; τ is the time interval between 180° and 90° pulses; and T1 is the spin-lattice relaxation time constant. Four different temperatures – 35, 45, 55 and 65 (±0.1) °C – were employed during the measurements, with the sample being left to equilibrate for 5 minutes at each step.

2.5.2 Spin diffusion path T1 relaxation times of solid polymers are not determined solely by dynamic processes. Below the glass transition temperature (Tg), spin diffusion also contributes to the relaxation time value. This diffusion can be modeled as Fickian diffusion and can be useful to understand phase separation and to estimate the size of molecular domains.

The maximum path of diffusion in a 3D matrix, L, can be approximated via Equation 2.

L ~ 6 DT1

(2)

where D is the spin diffusion coefficient (for polymers, its value is approximately 5×10–16 m2.s–1), which depends on the average distance between protons and dipolar interactions[5,25].

Relaxometry was employed to assess the samples’ 2.5.3 Activation energy through spin-lattice relaxation molecular dynamics through spin-lattice relaxation processes.

The activation energy values (Ea) for molecular relaxation All spectra were obtained in a Maran Ultra spectrometer, with processes occurring in the samples were determined through an 18 mm magnet bore, operating at 0.54 T (23.4 MHz for 1H) the method employed by[27]. The spin-lattice relaxation rate and equipped with a heating/cooling module. The samples can be described by Equation 3. were shredded in a mill for better conditioning inside the glass tube used as sample holder. The masses of each sample  1 τ 4τ   

(3) + were measured to be similar to each other, so that the results= C  2 2  2 2  T + ω τ + ω τ 1 1 4      1 could be better compared.

Table 2. Parameters employed in the inversion-recovery pulse sequence.

Table 1. Sample names.

Name PVA PVA/SiO2 P1 P1A P5 P5A P10 P10A

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Nominal cross‑linking 0 0 1% (m/m) 1% (m/m) 5% (m/m) 5% (m/m) 10% (m/m) 10% (m/m)

Percentage of inorganic nanoparticles 0 1% (m/m) 0 1% (m/m) 0 1% (m/m) 0 1% (m/m)

Parameters Values Pulse sequence 180°-τ-90°-Acquisition 90° pulse- μs Automatically set. Typically 7.5 180° pulse - μs Automatically set. Typically 15.

Number of scans 8 Number of τ (logarithmically spaced) 40 Tau sweep - μs 10-2E7 Recycle delay - s 5 Receptor gain - % 3 Probe dead time - μs 8 Receiver dead time – μs 3

Polímeros, 26(3), 221-227, 2016

Time domain NMR evaluation of poly(vinyl alcohol) xerogels where τ is the correlation time; ω is the Larmor frequency; and C is a constant that represents the magnitude of the dipolar interaction between protons.

The correlation time can be understood as being thermally excited, as described in Equation 4. τ (T ) = τ0 e

 Ea     RT 

(4)

where EA is the activation energy; R is the ideal gas constant (here 8,314 J.mol–1.K–1); and τ0 is a pre-exponential factor.

The value of Ea can be obtained by plotting the natural logarithm of the relaxation constant versus the inverse of the absolute temperature[27,28]. The linear regressions were conducted to obtain the coefficients of determination using a routine available in the Origin 8.5 software.

aldehyde C-H and aldehyde carbonyls, which indicates some incomplete cross-linking. Although faint, the increase in the intensity of the absorption bands arising from cross‑linking and the decrease in the intensity of the 1141 cm–1 band (Figure 3b), relative to spatial order in the PVA matrix, can be distinguished in the spectra. Quantification of the influence of these covalent bonds on the polymeric matrix proved to be somewhat difficult and prone to experimental errors. Furthermore, the technique did not show the apparent effect of the inorganic nanoparticles, due to the absence of strong chemical interactions.

3.3 WAXD analysis Diffractograms of all the samples are displayed in Figure 4. Even though PVA does not form crystallites in the strict sense, it does display highly ordered domains that arise

3. Results and Discussion 3.1 Sample analysis Transparent, colorless, free-standing polymeric films were obtained for PVA and PVA/SiO2 samples. Transparent, slightly yellow, free-standing films were obtained for all xerogels. The cross-linking reaction occurred when hydroxyl groups in the PVA and aldehyde groups in the GA underwent an acid-catalyzed acetalization process, producing acetal and hemiacetal bridges between polymer chains. This process is stochastic in nature and has already been shown to depend on factors such as polymer concentration and reactive group tacticity[15,29,30]. A reaction scheme is shown in Figure 1.

Nanoparticulate hydrophilic SiO2 is an amorphous material with many polar oxygenated groups on its surface. These groups promote high affinity for water, polar solvents and media, such as PVA. However, the SiO2 used in this work has a very small surface area (167.6 m2.g–1, determined through BET N2 sorption), so the extension of the interactions between nanoparticles and polymer matrix is expected to be small.

Figure 1. Reaction scheme for the formation of chemical crosslinking between PVA chains and GA.

3.2 FT-IR analysis FT-IR spectra of the samples can be observed in Figure 2a, b. PVA absorptions are reported as: secondary alcohol C-O stretching between 1124-1087 cm–1; aliphatic C-H stretching between 2800-3000 cm–1; and OH broad absorption around 3300 cm–1, which contains contributions from hydrogen bonded moieties. The discrete absorption band at 1141 cm-1 was reported as being from stretching modes of C-C or C-O bonds, when PVA presents intramolecular hydrogen bonds[31]. Therefore, this absorption band is associated with spatial order inside the polymer network.

The absence of absorptions between 1750-1735 cm–1, typical for ester C=O bands, confirms that this PVA is highly hydrolyzed[31,32]. Comparing the spectra of PVA and of PVA/SiO2 there are no apparent differences, which implies weak intermolecular interactions between the polymer matrix and inorganic nanoparticles (Figure 2a).

For better visualization, we selected and expanded regions of the FT-IR spectra from PVA and P10 samples, as shown in Figure 3a, b. All the xerogels display absorption bands at 2850-2750 cm–1 (Figure 3a) and 1720-1750 cm–1 (Figure 3b), corresponding to absorption modes of short-chain aliphatic Polímeros, 26(3), 221-227, 2016

Figure 2. FT-IR spectra for the (a) non-cross-linked materials and (b) cross-linked materials.

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Rodrigues, E. J. R., Cavalcante, M. P., & Tavares, M. I. B.

Figure 3. FT-IR spectra expansion of (a) short-chain aliphatic aldehyde C-H region and (b) aldehyde carbonyls and of C-C or C-O bonds.

Figure 4. Superposed WAXD diffractograms of the samples.

because of the small size of the hydroxyl group, allowing inter/intramolecular hydrogen bonding and leading to more efficient packing of the polymer chains[30,34]. The cross-linking reaction conducted in this work acts by eliminating free hydroxyl groups from the PVA chains, replacing them with acetal and hemiacetal bonds. This effect can be perceived when the xerogelsâ&#x20AC;&#x2122; diffractograms are analyzed. The change is much more pronounced in the highly cross-linked samples.

The presence of SiO2 nanoparticles appears to have no effect on this change. At 10% nominal cross-linking, the intensity of the diffractions coming from the sample is so low that on the scale of the graph it cannot be easily distinguished from the baseline. It is clear that the ordered conformation of the PVA chains was undone by the cross-linking reaction, as this spatial order relies on the presence of hydroxyl groups. Thus, for these systems, WAXD measurements corroborated the results obtained through FT-IR, with respect to the strength of the cross-linking reaction.

3.4 TD-NMR analysis All the acquisition parameters shown in Table 2 underwent various steps of iterative optimization to ensure better results and finer resolution. Once these parameters 224

are set, they can be repeated for each sample in a batch.

When compared with the other characterization techniques, relaxometry offers some advantages. First, by observing only the spin-lattice relaxation processes we could extract up to three different and complementary readings about the samples. Second, TD-NMR probes the whole volume of the sample, providing more accurate and descriptive data about the material being analyzed. Third, if needed, the samples could have been used for other tests, since relaxometry is a nondestructive analytical tool[1,3,4,7].

3.4.1 Spin-lattice relaxation constant determination The spin-lattice relaxation constant, T1, can be interpreted as an estimate of the intermolecular interactions existing in the sample, since this parameter provides information about long chain segments under the frequency used in this experiment. This time constant is very dependent on the correlation times of the proton nuclei in the PVA chains, so that nuclei constrained in rigid domains display long correlation times. This, in turn, translates into greater values of T1. The inverse holds for short T1 values, which arise from mobile, less restricted molecular environments[18,19,33].

PolĂ­meros, 26(3), 221-227, 2016

Time domain NMR evaluation of poly(vinyl alcohol) xerogels The T1 values for PVA and PVA/SiO2 were used as standards against which the values for xerogels were measured. Tables 3 and 4 show the T1 values for the samples. The data are grouped in filled and non-filled samples, for clarity. The magnetization relaxation back to thermal equilibrium via the T1 mechanism is modulated by the molecular mobility (more or less restrained polymer chains) of the sample. The value of T1 decreases with increasing temperature, suggesting a reduction of molecular rigidity of the PVA matrix, which is expected since the system receives thermal energy. This trend occurs despite the degree of cross-linking or the presence of inorganic nanoparticles. The presence of nanoparticles, however, seems to have a dampening effect on the reduction of the samples’ T1 values due to this stimulus, especially at high temperatures. Another trend is the decrease of the T1 values with increased cross-linking between PVA chains, although the intensity of the change in T1 values starts to diminish when the cross-linking degree reaches 5% (m/m), suggesting saturation around this concentration of GA. This behavior is similar to that described by Zhao et al.[18].

The main difference between our results and theirs is that we investigated a vinyl polymer and they examined an elastomer. As previously discussed, the cross-linking process occurs, for the systems studied here, by consumption of hydroxyl moieties present in the polymer. According to the experimental results, we attributed greater lattice rigidity to neat PVA due to the strong hydrogen bonding from hydroxyl moieties in its structure. The conversion of said moieties to (hemi)acetal groups linked by three CH2 bonds, which are somewhat mobile, thus increased the net molecular mobility of the polymer matrix. That being said, the presence acetal bridges diminishes spatial organization and molecular rigidity, as reflected in the T1 values. Furthermore, the nanoparticles’ influence could be perceived when looking at T1 values for a given neat polymer/hybrid pair. For all the formulations, the hybrid displayed a shorter T1 value than its unloaded equivalent, thus showing that T1 is sensitive to the very weak surface interactions between the polymer matrix and the SiO2 nanoparticles. This result is interesting because it allows faster and better interpretation of the extent of the changes brought by the addition of the nanoparticles than the other two common spectroscopic methods employed.

3.4.2 Spin diffusion path analysis Regarding homogeneity at the molecular level, spin-lattice relaxation can be used to describe the extent of similar spin environments at nanometer scales. The parameter L is the theoretical distance where spin diffusion events can take place without causing the averaging of the magnetization. Thus, this value represents the upper limit of similar molecular mobility. In other words, L is an approximation of the size of homogeneous domains inside the material[5,25].

The values for L shown in Figure 5 demonstrate a downward trend with rising quantity of nanoparticles and cross-linking degree. The effect of temperature on this parameter is also demonstrated, with L values for the samples evaluated at 35 and 55 °C. As the results show, the introduction of acetal bridges breaks the spatial organization between PVA chains, and although they are stochastic in nature, the effects of the cross-linking reaction demonstrate an almost linear decay with increasing GA concentration. Comparing PVA with PVA/SiO2, the introduction of SiO2 does little to disturb the homogeneity of PVA chains. Even the smallest amount of GA (P1) had a more pronounced effect on this parameter than that caused by the nanoparticles’ presence. Finally, the technique is sensitive to the effects of temperature on the homogeneity of the polymeric spin domains. With the increase of thermal energy, random movements of long

Figure 5. Spin diffusion path for all samples.

Table 3. T1 values for the non-filled samples at different temperatures.

Sample

T1(ms) at 35 °C

T1(ms) at 45 °C

T1(ms) at 55 °C

T1(ms) at 65 °C

PVA P1 P5 P10

326 267 219 211

286 226 188 188

251 189 159 159

188 142 121 120

Table 4. T1 values for the filled samples at different temperatures.

Sample

T1(ms) at 35 °C

T1(ms) at 45 °C

T1(ms) at 55 °C

T1(ms) at 65 °C

PVA/SiO2

306

243

207

197

246 200 193

205 176 172

172 154 152

127 120 116

P1A P5A P10A

Polímeros, 26(3), 221-227, 2016

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Rodrigues, E. J. R., Cavalcante, M. P., & Tavares, M. I. B. Table 5. Ea values calculated for all the samples.

Sample

Ea (kJ.mol–1)

R2

PVA PVA/SiO2 P1 P1A P5 P5A P10 P10A

15.36 12.93 14.48 15.02 13.35 10.93 11.81 10.05

0.99 0.98 0.99 0.99 0.99 0.99 0.97 0.99

5. Acknowledgements We thank Roberto Pinto Cucinelli Neto for the fruitful discussions about relaxometry and his help during some of the analyses. We also thank Coordenação de Aperfeiçoamento de Pessoal de Ensino Superior (CAPES), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Fundação Carlos Chagas Filho de Amparo a Pesquisa do Estado do Rio de Janeiro (FAPERJ) for funding.

6. References segments of polymer chains contribute to reduction of the spin diffusion path’s length[5,25,26].

3.4.3 Activation energy through spin-lattice relaxation Values for Ea are related to the polymer matrix lateral groups’ rotation movement and to chain end or ramification vibrations [27,28,33]. We treated this quantity as apparent energy of activation. Table 5 lists the values for all samples.

Both cross-linking and the presence of nanoparticles caused a reduction in the value of the systems’ apparent activation energy, albeit through different mechanisms.

As seen before, the SiO2 particles do not interact chemically with the polymeric matrix, but their presence is sufficient to reduce the size of the homogeneous spin domains.

The reduction on the Ea value for PVA/SiO2 must have come from a disorganized, somewhat less restricted environment.

The same rationale explains the effects of cross-linking, as the reaction consumes hydroxyl moieties, reported to be responsible for spatial ordering of PVA chains[30,31]. The results obtained for the apparent Ea values of non-filled samples indicate these values can be used to gauge the extent of cross-linking in this type of xerogel, as they vary linearly with GA concentration.

4. Conclusions We prepared and analyzed SiO2 loaded and neat PVA based xerogels through FT-IR, WAXD and TD-NMR, by means of spin-lattice relaxation mechanisms. The results obtained from the FT-IR and the WAXD analyses demonstrated that the cross-linking reaction reduces the spatial ordering of the PVA polymer chains. There was no chemical interaction between the PVA hydroxyl groups and the SiO2 surface that could be detected via FT-IR, although TD-NMR results did show some level of physical interaction between polymer and nanoparticles. All three TD-NMR experiments performed in this study revealed the effect of the SiO2 presence amid the PVA chains. The data derived from the TD-NMR experiment, such as Ea and T1 constant values, can be readily employed to assess this kind of material’s degree of cross-linking without resorting to swelling-to-equilibrium solvent intake tests, with the added advantage of being faster. The experimental design employed in the work enables clearer and broader characterization of an important class of polymeric materials, the inorganic‑organic hybrid xerogels.

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Received: Mar. 06, 2015 Revised: Oct. 21, 2015 Accepted: Feb. 15, 2016

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http://dx.doi.org/10.1590/0104-1428.2038

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

Effects of polypropylene methyl polyhedral oligomeric silsesquioxanes and polypropylene-grafted maleic anhydride compatibilizers on the properties of palm kernel shell reinforced polypropylene biocomposites Muhammad Safwan Mohaiyiddin1, Ong Hui Lin1*, Hazizan Md Akil2, Toh Guat Yee1, Nik Nur Azza Nik Adik1 and Al Rey Villagracia3 School of Materials Engineering, Universiti Malaysia Perlis – UniMAP, Arau, Perlis, Malaysia School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia – USM, Nibong Tebal, Penang, Malaysia 3 Physics Department, De La Salle University – DLSU, Manila, Philippines

1

2

*hlong@unimap.edu.my

Abstract The effects of the polypropylene-grafted maleic anhydride (PP-g-MAH) and polypropylene methyl polyhedral oligomeric silsesquioxane (PP-POSS) compatibilizers on the mechanical, thermal and physical properties of palm kernel shell (PKS) reinforced polypropylene (PP) were investigated. The production of PP/PKS biocomposites was performed by melt mixing using Brabender Internal Mixer. Mechanical test results showed that the biocomposites with PP-g-MAH have better tensile strength compared to biocomposites with or without PP-POSS. The results also showed an increase in the tensile strength and elongation at break when compatibilizers were added. Polypropylene-grafted maleic anhydride improved the Young’s modulus of the biocomposites, but PP-POSS reduced it. Moreoever, adding compatibilizers in biocomposites reduced the water absorption of the biocomposites. The compatibilizers improved the nucleating ability of filler in the composites. The PP-g-MAH compatibilizer provided better performance in improving nucleating ability to biocomposites compared to PP-POSS. Keywords: biocomposites, palm kernel shell, polypropylene-methyl-polyhedral oligomeric silsesquioxane, mechanical properties, thermal behavior.

1. Introduction Producing environmental-friendly materials due to ecological issues has increasingly captured the attention of the world in government sectors and in the scientific community. The awareness of ecological issues led many government leaders of the countries in the world to introduce green technology to people. Converting wastes to functioning materials through green technology is an approach to solve one of the major ecological issues on wastes. Malaysia, who has become one of the biggest palm oil producer in the world, produces tons of wastes which include empty fruit bunches (EFB), palm kernel shells (PKS), pericap and palm oil effluent. These wastes worsen the disposal problem[1].

Transforming wastes, such as palm kernel shells, into a useful and low-cost biocomposite material would reduce the problem[2].

Moreoever, scientists have shown their enthusiasm in doing research related to natural fillers reinforced biocomposites to produce environmental-friendly materials. In order to reduce the production of petroleum-based thermoplastic, natural fillers from waste have been broadly used. However, thermoplastics are required to undergo surface modification to remove the hydroxyl groups on the surfaces of natural fillers. These hydroxyl groups can be attracted to water

228

molecules that will reduce the mechanical properties of the thermoplastic composites. Therefore, elimination of hydroxyl group should be done in order to make hydrophilic fillers compatible with hydrophobic thermoplastic[3].

Many polymer modifications have been implemented by researchers to overcome this compatibility problem such as the use of Polypropylene-grafted-Maleic Anhyride (PP‑g‑MAH)[4-6], Polypropylene-grafted-Acrylic Acid (PP‑g-AA)[7], Polypropylene methyl Polyhedral Oligomeric Silsesquioxanes (PP-POSS)[8,9], isocyanate group[10,11], Poly(ethylene-co-glycidyl methacrylate) (PEGMA) [12] as compatibilizers. Among the compatibilizers, Polypropylene‑grafted-Maleic Anhydride (PP-g-MAH) compatibilizer is the most effective compatibilizer for lignocellulosic filler and matrix at the interface[13]. Futhermore, the use of PP-g-MAH in lignocellulosics filler filled PP composites has been studied by[14] which accentuated the interest of using rice husk flour and wood flour in PP composites. They found that PP-g-MAH improved the dynamic mechanical thermal properties of the composites.

Thus, Polypropylene-grafted-Maleic Anhydride (PP-g-MAH) compatibilizer was used in this study to improve the properties of polymer composites.

Polímeros, 26(3), 228-235, 2016

Effects of polypropylene methyl polyhedral oligomeric silsesquioxanes and polypropylene-grafted maleic anhydride compatibilizers on the properties of palm kernel shell reinforced polypropylene biocomposites In line with the objective to reduce the PKS wastes and petroleum-based thermoplastics, neat PP, uncompatibilized and compatibilized PP/PKS composites were produced on lignocellulosic filler in this study. The compatibilizers, PP-g-MAH and Polypropylene methyl Polyhedral Oligomeric Silsesquioxanes (PP-POSS), were used to investigate the effects on the mechanical, thermal and water absorption properties of these biocomposites.

2. Experimental Procedures 2.1 Materials In this study, Polypropylene (PP) homopolymer resin grade PX617 with density of 0.9 g/cm3 and Melt Flow Index (MFI) of 1.7 g/ 10 min at 230 °C was purchased from Titan PP Polymers (PP) Sdn. Bhd. The palm kernel shells, that have an average density of 1.4485 g/cm3, were provided by Batu Lintang Oil Palm Mill Sdn. Bhd, Kedah. The compatibilizers, namely, PP-g-MA (polybond 3200) with 5 wt% of maleic anhydride (MA) content, and PP-POSS with 10 wt% of methyl POSS, were supplied

by Uniroyal Polybond Sdn. Bhd. and Sigma-Aldrich (M) Sdn. Bhd, respectively.

2.2 Preparation of palm kernel shell The palm kernel shells (PKS) were first removed from coir and then grounded using a grinder followed by sieving (63 μm) to obtain fine and uniform particle size. After that, the PKS were dried in the oven for 24 hours at 80 °C.

2.3 Preparation of palm kernel shell reinforced polypropylene composites

2.4.2 Un-notched impact test The impact tests were performed according to ASTM D256 using an impact pendulum tester with 7.5 J of impact energy. All the samples were cut into rectangular shape.

At least five samples of each material were tested.

2.5 Morphological study The fractured surface of the tensile area was analyzed using a model JSM-6460 LA JEOL scanning electron microscope (SEM). The fracture ends of samples were sputtered with a thin layer of palladium to avoid electrical charge during the analysis.

2.6 Thermogravimetric analysis (TGA) Thermogravimetric analysis from -50 °C to 200 °C were performed on a Pyris Diamond Thermogravimetric Differential Thermal Analyzer at a heating rate of 10 °C/min in nitrogen atmosphere.

2.7 Differential scanning calorimeter (DSC) analysis The thermal behavior of the polypropylene and biocomposites were examined using a differential scanning calorimeter (DSC) TA instrument analyzer at standard heating/cooling rate of 10 °C/min in nitrogen atmosphere. The temperature range used was -60 °C to 200 °C. First, the samples of about 5 mg were placed in the DSC aluminum pan and heated from -60 °C to 200 °C and hold for 2 minutes. Then, the samples were cooled to -60 °C and hold for 2 minutes. Afterwards, the cooled samples were heated back to 200 °C. Results obtained were analyzed using the Origin Pro software.

2.8 Water absorption test

The PKS and PP matrix were mixed using a Brabender internal mixer fitted with cam blades. The melt mixing was carried out at 180 °C with a rotor speed of 60 rpm.

The polypropylene matrix was preheated in the mixing chamber of the internal mixer for 4 minutes. Then, the PKS powder was carefully inserted into the mixer within 30 seconds. The mixing process of PKS powder and PP was continued for 6 minutes. Afterwards, the compounded specimens were discharged from the mixing instrument.

In preparing the composites with compatibilizer (PP-g-MA or PP-POSS), the compatibilizer was mixed first with the PP matrix before the preheating process. After discharging the specimens from the mixing instrument, the compounded specimens were sheeted with 1 mm thickness using hot press process at temperature of 180 °C for 10 minutes.

The samples were cut into rectangular shape at an approximate dimension of 76.2 × 25.4 × 3.2 mm. The tests were conducted according to ASTM D570-98. The samples were dried at 50 °C for 24 hours and immersed in distilled water at room temperature until a constant weight was reached. The samples were periodically taken out from the water, wiped the surface moisture with a dried white cloth to remove water at the surface of samples, weighed to the nearest 0.001g immediately and replaced in the water.

At least three samples for each composition were used and the results were averaged to obtain a mean value.

2.4 Mechanicaltest

Fillers are known to play an important role in the mechanical properties of thermoplastic composites. Results showed in Figure 1 that the tensile strength of the PKS reinforced PP biocomposite decreased with the increasing of filler content.

2.4.1 Tensile test All the samples were cut into dog-bone shape using Wallace die cutter. The test was carried out using an Instron 5569 tensile testing machine. The crosshead speed of testing is 50 mm/min and the gauge length was set at 50 mm according to ASTM D-638, at room temperature.

At least five samples were tested for each formulation.

Tensile strength, elongation at break and Young’s modulus were recorded and calculated by the instrument software.

Polímeros, 26(3), 228-235, 2016

3. Results and Discussion 3.1 Mechanical test

It can be explained by the different nature of PP matrix with the PKS filler. PP characterized by non-polar nature whereas PKS filler has polar group. This gives poor interfacial interaction between PP and PKS thus, lead to the poor strength of the composite. Poor dispersion of PKS inside PP matrix also contributed to the strength deterioration 229

Mohaiyiddin, M. S., Lin, O. H., Akil, H. M., Yee, T. G., Adik, N. N. A. N., & Villagracia, A. R.

Figure 1. Effect of filler content and compatibilizers on tensile strengths of PKS reinforced PP biocomposites.

of the composites[15]. Nevertheless, PKS reinforced PP biocomposites appeared to have higher tensile strength with the addition of PP-g-MAH and PP-POSS. Encapsulation of PKS particles by the PP-g-MAH compatibilizer occurs in the PKS reinforced PP biocomposites because of the strong polar interaction between PKS particles and PP-g-MAH compatibilizer which lead to the formation of compatibilizer phase between PKS and PP matrix[16]. Well-known cage like structure POSS were proposed to trap PKS particles and lubricate dispersion of PKS in PP matrix[17,18]. PP-g-MAH shows better tensile strength compared to PP-POSS. This is because PP-g-MAH forms chemical bonding with the PKS filler while PP-POSS only has physical bonding with the PKS filler[13,19].

The Young’s modulus of the PKS reinforced PP biocomposites increases with the increasing amount of filler content (as shown in Figure 2). It occurs due to the replacement of polymer matrix by stiffer particulate filler, which improves the overall composites modulus. The increase of the modulus is also due to the fact that the deformation and mobilization of matrix were restricted by the present of particulate filler that introducing a mechanical restraint[20,21].

Incorporating PP-g-MAH compatibilizer into the PKS reinforced PP biocomposites shows not much improvement of Young’s modulus at low filler content. However, the modulus significantly increased at 30 wt% and 40 wt% of filler content by about 300 MPa compared to PKS reinforced PP biocomposites without PP-g-MAH compatibilizer[22].

Explained that, the increment of modulus of composite is due to the PP-g-MAH that solved the incompatibility problem between hydrophilic filler and hydrophobic polymer.

The esterification function of copolymer bonding formed bridge between PKS filler and PP matrix while the hydroxyl group at the PKS formed hydrogen bonds with carboxyl group of PP-g-MAH. The modulus of PP composites decreased slightly when PP-POSS was added. It means that PP-POSS is a plasticizer toward the PP matrix reducing the rigidity of the PP composites[17,23].

Figure 3 shows the effect of compatibilizers on elongation at break of PKS reinforced PP biocomposites. With the increasing of PKS content, elongation at break of PKS reinforced PP biocomposites decreased. The possible reason for this kind of behavior may be attributed to the fact that the restricted deformation of the PKS is generally greater than PP matrix, which restricted the deformation of overall composites. According to Leong et al.[20], the nature of PKS of having a high rigidity may change the mode of failure 230

Figure 2. Effect of filler content and compatibilizers on Young’s modulus of PKS reinforced PP biocomposites.

Figure 3. Effect of filler content and compatibilizers on elongation at break of PKS reinforced PP biocomposites.

of the PP matrix from ductile to almost brittle behavior.

From Figure 3, the addition of PP-g-MAH and PP-POSS improved the elongation at break of PKS reinforced PP biocomposites. This may be due to the homogeneous structure that was developed by good interfacial adhesion between PP matrix and PKS which allowed the composites to have more deformation before break[5]. Polypropylene-methyl-Polyhedral Oligomeric Silsesquioxanes (PP-POSS) shows a significant increase of elongation at break may be due to POSS, which could plasticize the molecular chain of PP matrix thus increasing the flexibility of the PP matrix[8].

Moreover, the impact strength has direct correlation to the adhesion of the filler to the PP matrix. The result shows that PKS particles content and PP-g-MAH compatibilizer improved the impact strength. Figure 4 illustrates the impact strengths of the PKS reinforced PP biocomposites with different filler loading and with and without PP-g-MAH compatibilizer. It is clearly seen that at higher PKS content in PKS reinforced PP biocomposites, there is drastic decreased in impact strength of the composites.

Due to the hydrophilic particulate filler, it tends to agglomerate which initiates the crack propagation, and reduce the ability of the PKS reinforced PP biocomposite to absorb the impact energy through plastic deformation[24]. It can also be explained that irregular shape of PKS (Figure 5) resulted to the inconsistent stress transfer from PP matrix to the PKS filler[25,26]. However, the impregnation of PP-g-MAH into the PKS reinforced PP biocomposites assists the impact strength of the composites. This is because PP-g-MAH improves the wettability between PKS and PP, consequently improve the interfacial bonding in the composites and increase the impact strength[27]. From Figure 4 also, it can be observed Polímeros, 26(3), 228-235, 2016

Effects of polypropylene methyl polyhedral oligomeric silsesquioxanes and polypropylene-grafted maleic anhydride compatibilizers on the properties of palm kernel shell reinforced polypropylene biocomposites

Figure 4. Effect of filler content and compatibilizers on un-notched impact strength of PKS reinforced PP biocomposites.

that PP-g-MAH gives the PKS reinforced PP biocomposites better impact strength compared to PP-POSS[28]. Explained that filler was not well dispersed in PKS reinforced PP biocomposites by using PP-POSS compatibilizer while PP-g-MAH provide better dispersion of PKS in PP matrix.

3.2 Morphological study The SEM micrograph of tensile fracture surfaces of PKS is shown in Figure 5, and the PKS reinforced PP biocomposites with 10 wt% and 40 wt% of filler content are illustrated in Figure 6. In Figure 6a, a few PKS particles are seen compare to tensile fracture surface of Figure 6b. In composite system without compatibilizer, it also can be seen that number of holes of Figure 6b is higher than Figure 6a and large amount of voids due to detachment of PKS particles can be seen in Figure 6b. This is due to the poor bonded interfacial area between matrix and filler causes brittle deformation of the composites[29]. Figures 6c-f show tensile fracture surface of PKS reinforced PP biocomposites with compatibilizers.

Figures 6c-f show less voids compare to Figures 6a and 6b.

PP-g-MAH and PP-POSS compatibilizers help the matrix to encapsulate filler by introducing interfacial bond between the two elements[30]. From Figure 6a, it can be seen that the pull of PP matrix which supported high elongation at break of tensile result[31].

3.3 Water absorption The water absorption of the PKS reinforced PP biocomposites is dependent on the nature of natural filler since the PP exhibit hydrophobic behavior. Figure 7 shows the equilibrium water uptake values, Q∞, of the biocomposites versus different filler content with compatibilizers.

Figure 7 shows that increasing the PKS content in PP matrix will increase the water uptake by the composites.

This is due to the hydrophilic PKS that contains hydroxyl group, which bonds with water molecules through hydrogen bonding[32]. As more filler is added to the PP matrix, more hydrogen bonding formed between hydroxyl group and water molecules, thus increasing the water absorption of the composites[33].

On the contrary, results showed that the water uptake

by PKS reinforced PP biocomposites decreased with the addition of compatibilizers. This can be explained by the polymer modification by PP-g-MAH compatibilizer. Maleic anhydride group of PP-g-MAH forms a hydrogen bond with Polímeros, 26(3), 228-235, 2016

Figure 5. SEM images of Palm kernel shell powder.

some free hydroxyl group of PKS, which reduces the water absorption and it increases the resistance of composites towards water molecules. Polypropylene-grafted-Maleic Anhyride (PP-g-MAH) also enhanced the interaction between the PP and PKS improving the adhesion. However, there are still cracks and voids in the composites causing easy penetration and storage of water through voids[34]. Both compatibilizers led to the reduction of water uptake by the composites. Between the two compatibilizers, the addition of PP-g-MAH shows better water resistant than PP-POSS based on Figure 7. This may be due to less hydrogen bonding because of PP-g-MA layer formed on the filler surface thus, constraint the water from coming into contact with the OH groups in the filler while physical bonding formed

by PP‑POSS and PKS still allowed water penetrate through into the PKS filler[35].

3.4 Differential scanning calorimeter (DSC) Table 1 shows the DSC measurements of neat PP and its biocomposites with compatibilizers containing 10 wt% and 40 wt% of PKS content. The melting point and crystallization temperature were obtained from the main peak of the endothermic and exothermic curves, respectively. On the other hand, glass transition temperature was acquired from the slope of endothermic stepwise change. From Table 1, it can be seen that the melting point temperature decreased with the addition of PKS content[36] reported similar results in his study of flax, hemp and sisal fibers filled polypropylene composites. Table 1 shows that the heat of fusion and crystallinity of PKS reinforced PP biocomposites decreased at a higher filler content. It means that the PKS did not act as a nucleating agent in this system[27]. This may be due to the fact that high PKS content tends to form agglomeration, thus there will be less available nucleation site of PP to crystallize at the interfaces and reduce the crystallinity of the composites. However, impregnation of PP-g-MAH compatibilizer into PKS reinforced PP biocomposites showed higher heat of fusion and crystallinity of the biocomposites.

This is due to the chemical bond formed between the PKS and PP matrix which does not physically constrain the mobilization of the polymer chain. Hence, the addition of MA group from PP-g-MAH compatibilizer improved the 231

Mohaiyiddin, M. S., Lin, O. H., Akil, H. M., Yee, T. G., Adik, N. N. A. N., & Villagracia, A. R.

Figure 6. Tensile fractured surface of uncompatibilized PKS reinforced PP biocomposites and compatibilized PKS reinforced PP biocomposites at magnification X500 (a) 10 wt% PKS (Uncompatibilized); (b) 40 wt% PKS (uncompatibilized); (c) 10 wt% PKS (PP‑g‑MAH); (d) 40 wt% PKS (PP-g-MAH); (e) 10 wt% PKS (PP-POSS); (f) 40 wt% PKS (PP-POSS).

Table 1. DSC parameter analysis of neat PP, uncompatibilized biocomposites (10 wt% and 40 wt% filler) and compatibilized biocomposites (10 wt% filler).

Composites Neat PP Uncompatibilized (10wt%) Uncompatibilized (40wt%) PP-POSS (10wt%) PP-g-MAH (10wt%)

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Tg(°C)

Tc(°C)

Tm (°C)

ΔHf (J/g)

Xc (% crystallinity)

–3.10 –2.50 –2.30 –5.70 –7.20

139.92 138.68 134.88 139.42 138.06

165.31 165.09 163.63 165.08 165.02

85.19 84.75 54.23 81.26 92.10

40.76 40.55 25.95 38.88 44.07

Polímeros, 26(3), 228-235, 2016

Effects of polypropylene methyl polyhedral oligomeric silsesquioxanes and polypropylene-grafted maleic anhydride compatibilizers on the properties of palm kernel shell reinforced polypropylene biocomposites

Figure 7. Effect of filler content and compatibilizers on water absorption of PKS reinforced PP biocomposites.

Figure 8. TG curves of PKS reinforced PP biocomposites at different filler loading and with compatibilizers.

chemical bond between filler and matrix by enhanced the interfacial adhesion PKS to the PP matrix[14].

Table 1 also shows that the crystallization temperature, Tc, decreased from 139.92 °C for neat PP to 134.88 °C for biocomposites with 40 wt% of PKS. This supports the crystallinity and heat of fusion result of the composites and neat PP whereby PKS reduced the formation of spherulite in the composites. It is suggested that there is a poor interaction between PKS filler with Polypropylene matrix[37]. Nonetheless, PP-g-MAH and PP-POSS compatibilizer assist the PKS to act as nucleation sites for spherulites formation, thus maintaining the value of Tc. The addition of PKS content into PP matrix shows a slight increase in glass transition temperature, Tg. This proves that chemical bonding do not occurs at the interface between PKS filler and PP matrix. Only physical bonding occurs where PKS filler was encapsulated by PP matrix[14]. By further examining the data listed in Table 1, it can be found that the compatibilized PP composites with either PP-g-MAH and PP-POSS have lower Tg. This could be due to the chemical bonding between the filler and matrix where compatibilizers aid in the adhesion of filler toward the matrix thus, acting as a plasticizing agent in composites system[38].

4. Conclusions

3.5 Thermogravimetric analysis (TGA) Thermal stability of the PKS plays an important role to foresee its implementation into PP matrix. Figure 8 shows the thermal behavior of the biocomposites using TGA. Figure 8 illustrates that the 10 wt% of PKS in PP matrix has higher thermal stability compared to 40 w% of PKS. It validates that there is a poor adhesion between the palm kernel shell and polypropylene at higher filler content, which means that it becomes less a stable structure towards heat[39]. On the hand, biocomposites with the presence of PP-g-MAH show remarkable thermal stability than the other composites tested in this study. This indicates strong adhesion of the hydrogen bonds and covalent linkages between the hydroxyl group of PKS and the maleated anhydride group of PP-g-MAH[27]. The biocomposites with the presence of PP-POSS compatibilizer in this study has lower thermal stability compared to PP-g-MAH. This suggested that the lengthy alkyl chains and silicon cage of PP-POSS could create a softer shell. This softer shell may limit the stress transfer from PP matrix to PKS[40].

Polímeros, 26(3), 228-235, 2016

The Polypropylene (PP) was reinforced using palm kernel shells (PKS). Increasing the amount of PKS on the filler significantly increases the Young’s modulus; increases the water uptake; increases the tensile strength at 10 wt% PKS but deteriorates above 10 wt% of PKS; slightly increases the glass transition point temperature; decreases the elongation at break, impact strength, crystallinity, heat of fusion, crystallization temperature, melting point, thermal stability; and producing more holes or voids on the surfaces of the biocomposites. All these effects can be attributed to the presence of the hydroxyl groups in the surface of PKS resulting to poor adhesion of PKS to the PP matrix and attraction to water molecules.

Adding compatibilizers such as PP-g-MAH and PP‑POSS generally increased the adhesion between the PKS and PP matrix by removing the hydroxyl groups on the PKS surface producing less voids and cracks compared to the uncompatibilized biocomposite. Some of their effects differ due to their molecular composition. Using PP-g-MAH as the compatibilizer produced the following results: highest increase of tensile strength, impact strength, Young’s modulus, heat of fusion and crystallinity; slight increase in elongation at break; highest decrease of water uptake improving water resistance; highest thermal stability among the biocomposites in this study; highest decrease in glass transition temperature, crystallization temperature; and slight decrease in melting point.

On the other hand, using PP-POSS as the compatibilizer produced the following results: highest and significant increase in elongation at break; an increase in tensile strength, impact strength and crystallization temperature; significant decrease in Young’s modulus; decrease in water uptake, crystallinity, heat of fusion and glass transition temperature; slight decrease in melting point; and lower thermal stability than using PP-g-MAH as the compatibilizer.

5. Acknowledgements The authors are grateful to the Kuala Lumpur Kepong Berhad, Batu Lintang, Kedah, for providing the palm kernel shells. This study is being supported under grant No: 9002‑00019 by Malaysia Toray Science Foundation.

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Received: Jan. 09, 2015 Revised: Apr. 02, 2015 Accepted: Feb. 15, 2016

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http://dx.doi.org/10.1590/0104-1428.2257

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

Low cost UV-Ozone reactor mounted for treatment of electrode anodes used in P-OLEDs devices Emerson Roberto Santos1*, José Igor Balbino de Moraes2, Christine Miwa Takahashi2, Victor Sonnenberg2, Elvo Calixto Burini3, Satoru Yoshida1, Herick Garcia Takimoto1, Roberto Koji Onmori4 and Wang Shu Hui1 Laboratório de Engenharia de Macromoléculas, Escola Politécnica de Engenharia Metalúrgica e de Materiais, Universidade de São Paulo – USP, São Paulo, SP, Brazil 2 Materiais, Processos e Componentes Eletrônicos, Faculdade de Tecnologia de São Paulo – FATEC, São Paulo, SP, Brazil 3 Instituto de Energia e Ambiente, Universidade de São Paulo – USP, São Paulo, SP, Brazil 4 Laboratório de Microeletrônica, Escola Politécnica de Engenharia Elétrica, Universidade de São Paulo – USP, São Paulo, SP, Brazil

1

*emmowalker@yahoo.com.br

Abstract Low cost UV-Ozone reactor using a high pressure mercury vapor lamp of 80 watts without outer bulb showed good results for treatment of ITO films used as anode electrode in the assembly of P-OLED (polymer-organic light emitting diode) devices. This study revealed 20 minutes as effective treatment time and it was verified also that the effect of UV-Ozone treatment loses its efficiency as the elapsed time increases. It was analyzed with measurements of contact angle using a droplet of PEDOT:PSS polymer. P-OLEDs devices were mounted with architecture: ITO/PEDOT:PSS/PVK/Alq3/Al. The PVK polymer was diluted in organic solvent of 1,2,4-trichlorobenzene with concentrations of: 5, 10, 20 and 30 mg/mL. Results revealed better performance of P-OLED devices for concentration of 5 mg/mL resulting in lower threshold voltage, elevation of electrical current and similar diode curve.

Keywords: ITO film, PVK, P-OLED, HPMVL, UV-Ozone.

1. Introduction Layer-by-layer assembly of monochromatic P-OLED (polymer-organic light emitting diode) devices starts with a TCO (transparent conductive oxide) films chemical pre‑cleaned, they are used as electrode anodes deposited on transparent substrates. This step is complemented by oxidative treatment using UV-Ozone technique[1,2]. This treatment on the surfaces of the TCO films changes their chemical structures removing carbon and hydrocarbon groups contributing to improve the performance of devices[3]. The energy surface is modified by removal of these contaminants increasing the physical contact in the interface between the surface of TCO film and polymeric layer (deposited in posterior step)[4]. The treatments provide also the increase of the TCO workfunction decreasing the interface barrier to injection of holes between the TCO film and adjacent polymer layer promoting better charge carriers then there is a decrease of the threshold voltage of devices[5,6].

After UV-Ozone treatment a polymer known as HTL (hole transport layer) is deposited on TCO[7]. It will inject holes inside the subsequent deposited layer, an emissive polymeric material diluted in any organic solvent[8].

On the emissive material is deposited the ETL (electron transport layer) formed by organic material that will inject electrons coming from electrode cathode film formed by metal deposited on top[9]. The last step is the encapsulation to avoid chemical attack by oxygen and moisture[10].

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In this work, studies of two processes required in the P-OLED assembly to improve the performance of devices, good stability of material and low voltage operation are reported[11]. First, a low cost UV-Ozone reactor assembly due to the lack of manufacturers in Brazil, and second, the use of an organic solvent in different concentrations for the polymer dilution[12].

2. Materials and Methods 2.1 Assembly and analyses of UV-Ozone reactor A UV-Ozone reactor was built with high pressure mercury vapor lamp of 80 watts and ballast supplied by Osram Company. The outer bulb was removed to obtain the discharge tube that generates the ultraviolet rays for the production of ozone from oxygen in air[13]. A metallic box with dimensions: 18.5 × 20.0 × 20.0 cm and two fans held at the lid were used. These fans contribute in the ozone homogenization and help cool the lamp temperature avoiding a possible change of the ozone concentration produced.

The Figure 1a shows the image of reactor and Figure 1b shows the scheme of the complete UV-Ozone apparatus.

A monitor manufactured by IndevR 2B Technologies, 205 model was used for the analyses of ozone produced from 0 to 30 minutes[14]. This procedure was repeated for five times and for each analysis the lamp was cooled to room temperature. A plastic tube with length of ≈1.5 m

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Low cost UV-Ozone reactor mounted for treatment of electrode anodes used in P-OLEDs devices

Figure 1. (a) UV-Ozone reactor; and (b) all components used in the UV-Ozone reactor apparatus.

Figure 2. (a) Contact angle apparatus; (b) method used to obtain the contact angle.

and diameter of ≈7 mm was connected at the back of the monitor and the tube tip (inserted underneath the box) was placed ≈2 cm from the lamp to collect the ozone produced.

A spectroradiometer manufactured by LuzChem, SPR‑03 model connected by optical fiber to the computer, was used to collect the wavelengths produced by the lamp[15].

droplets on the film were immediately measured. Another study was carried out to obtain the “durability” of cleaning (or treatment time), where the TCOs were treated for a specific time and left in the Petri dishes without lid at room temperature.

The contact angles of the samples were measured immediately after exposition to 10 minutes elapsed time.

The irregularities of the analyses were calculated using the equation:

2.2 Analyses of TCOs TCO thin films (ITO - indium tin oxide of 15 Ω/□ deposited on glass) were irradiated at different times in the UV-Ozone reactor and eight droplets of HTL (PEDOT:PSS polymer) were placed on the surface with a micro syringe and contact angle analyses were conducted[16]. The literature reports that a hydrophilic characteristic of ITO films decreases the droplet spreading without treatment[17]. After the UV-Ozone treatment, TCO films surface starts adsorbing chemical elements from atmosphere as carbon and/or hydrocarbons decreasing P-OLEDs performance[18]. UV-Ozone irradiation helps oxygen atoms complete the chemical bonds removed by contaminants from surface resulting in better adherence of the polymer[19].

A webcam manufactured by Philips Company, SPC 530NC model coupled to lens with 30x magnification was used for contact angle measurement and the images, without distortion, produced by software were printed on paper[12].

Contact angles were obtained from left and right side measurements of PEDOT:PSS droplet semi-circles using ruler and protractor to obtain tangent lines. The Figure 2a shows the contact angle apparatus and 2(b) the methodology used.

Before contact angles measurement, the surfaces were exposed to UV-Ozone for: 5, 10 and 20 minutes including an untreated sample used as reference. The angle of PEDOT:PSS Polímeros, 26(3), 236-241, 2016

Irr = ( 3×sd×100%) ÷ artm (1)

where: Irr is the irregularity, sd is the standard deviation and artm is the arithmetic mean.

2.3 Assembly and analyses of P-OLEDs devices P-OLED architecture was mounted using: (a) ITO films cleaned with water and common detergent, then the samples were immersed in isopropyl alcohol and acetone by 30 minutes each using ultrasonic bath; (b) UV-Ozone treatment for a specific time; (c) spin-coating deposition of PEDOT:PSS (HTL supplied by Sigma-Aldrich) at 1,700 rpm and dried at 80 °C for 20 minutes; (d) spin-coating deposition of PVK polymer (supplied by Sigma-Aldrich) diluted in 1,2,4-trichlorobenzene (supplied by Tedia) at 1,700 rpm with: 5, 10, 20 and 30 mg/mL concentration and dried at 50 °C for 60 minutes; (e) evaporation of Alq3 (ETL) synthesized at laboratory; (f) evaporation of electrode cathode formed with aluminum (supplied by Balzers) and (g) encapsulation inside the glove box system at room temperature and humidity below 20% in nitrogen atmosphere. Devices were sealed using glass blades (dimension of 1.7 × 1.7 cm) with calcium oxide (CaO) layer and double-sided rubber 237

Santos, E. R., Moraes, J. I. B., Takahashi, C. M., Sonnenberg, V., Burini, E. C., Yoshida, S., Takimoto, H. G., Onmori, R. K., & Hui, W. S. tape, VHB model (supplied by 3M Company) placed at the edge of the samples[20-22]. Four devices of each sample were built at the same time. Each device presented active area of 3.0 × 3.0 mm. A power source was adjusted for the polarization of the P-OLEDs from 0 to 20 V and the respective electrical current was obtained[23]. The Figure 3a shows the complete architecture of P-OLED device mounted and the Figure 3b shows the sample with four devices.

3. Results and Discussions Results in the Figure 4a revealed a fast increase of ozone concentration in the first minutes, during this period the lamp is still heating and after ≈12 minutes the ozone concentration is more stable ≈1.3 ppm (the literature does not report the minimum ozone concentration requirement for the best condition of UV-Ozone used in TCO treatments).

The reactor geometry and the lamp operation temperature are very important factors for the ozone production, because these parameters contribute to find the specific time of stabilization transforming the oxygen confined in ozone[2]. The reactor revealed efficient gas confinement without escaping to ambient during leakage tests. Ozone concentration near the reactor was monitored and it showed similar results to those found in the laboratory ambient with 0.006 ± 0.002 ppm. For example, the literature reports that for papers storage environment the maximum limit of ozone concentration (defined ozone as pollutant gas) is up to 0.010 ppm and the World Health Organization (WHO) indicates that the ozone concentration above of 0.035 ppm causes health problems in human[17,24].

The literature relates also that the radiation below of 243 nm forms ozone (O3) and radiation between 240 and 320 nm breaks the ozone molecule and in this case, the lamp spectrum in the Figure 4b revealed ultraviolet emission (UV-A from 315 to 400 nm, UV-B from 280 to 315 nm and UV-C from 100 to 280 nm)[25,26].

Figure 3. (a) Complete architecture of P-OLED; and (b) sample mounted with four devices.

For the untreated sample, the surface of TCO film presented a hydrophobic characteristic (as expected), for samples treated mainly for 20 minutes, a better droplet spreading on the TCO film was observed with relative contact angle decrease and increase of the contact area between the polymeric layer and the TCO surface improving the charge carriers transport promoted by improved chemical bonds.

Figure 5 shows the images of PEDOT:PSS droplets on the untreated ITO film and ITO treated for: 5, 10 and 20 minutes.

Arithmetic mean, standard deviation and irregularity are reported in Table 1.

Figure 4. (a) Ozone concentration vs. elapsed time for reactor apparatus; and (b) spectrum of the high pressure mercury vapor lamp of 80 W without outer bulb.

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Low cost UV-Ozone reactor mounted for treatment of electrode anodes used in P-OLEDs devices The study carried out by Satoru Yoshida found up to 35 degrees difference between the untreated and treated ITO films deposited on PET (polyethylene terephthalate) using the same measurement method[27].

To analyze the “durability” of the UV-Ozone treatment, samples were irradiated by 20 minutes and placed in Petri dishes then the contact angles measured from immediately after exposition to 10 minutes elapsed time. The results revealed a significant influence of possible atmospheric elements on the TCO immediately after treatment. Figure 6 shows the contact angles of PEDOT:PSS droplets on the ITO films Table 1. Results of contact angle measurements for untreated and treated for: 5, 10 and 20 minutes with arithmetic mean, standard deviation and irregularity. untreated Arithmetic Mean ± Standard Deviation (degree) Irregularity (%)

55 ± 3

5 10 minutes minutes 54 ± 3 38 ± 3

15

19

23

20 minutes 37 ± 2

13

treated for 20 minutes and analyzed from immediately after exposition to 10 minutes, including the untreated sample.

The Figures 7a to 7d show the I-V (current-voltage) curves of P-OLEDs with PVK diluted in 1,2,4-trichlorobenzene at concentrations of: 5, 10, 20 and 30 mg/mL, respectively.

The I-V curves of P-OLEDs in the Figure 7a mounted with 5 mg/mL concentration solution presented threshold voltages between 10 and 11 V (obtained by curve imaginary tangent line) and electrical current up to 100 mA (the elevation of electrical current is necessary to the light emission increase). A hypothesis for the poor performance in devices mounted with: 10, 20 and 30 mg/mL solution in the Figure 7b to 7d respectively, showed irregularities in curves of diode or increase of voltages with an electrical current up to 50 mA. This aspect can be explained by discontinuity, where incomplete chemical bonds in the polymer causes difficulty in the charge carriers transport increasing the PVK electrical resistance and causing, consequently, increases of the threshold voltage. Another hypothesis is that the all devices emitted light green color and this behavior can be attributed to Alq3 emission instead of PVK, as this polymer is considered a conductor in the

Figure 5. Images of PEDOT:PSS polymer droplets on ITO films: (a) untreated and treated by: (b) 5; (c) 10; and (d) 20 minutes.

Figure 6. Contact angles of PEDOT:PSS droplets on TCO films treated for 20 minutes and analyzed from immediately after the exposition to 10 minutes, including the untreated sample.

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Santos, E. R., Moraes, J. I. B., Takahashi, C. M., Sonnenberg, V., Burini, E. C., Yoshida, S., Takimoto, H. G., Onmori, R. K., & Hui, W. S.

Figure 7. I-V curves of P-OLED devices with PVK diluted in 1,2,4-trichlorobenzene at concentrations of (a) 5; (b) 10; (c) 20; and (d) 30 mg/mL.

literature[28]. The study carried out by Erick Vendruscolo Guerra using this same architecture, but without Alq3 layer revealed no luminance in devices. In other devices he used also a different material called Butyl-PBD as ETL and light blue emission was observed[29,30].

4. Conclusions The UV-Ozone reactor mounted with modified high pressure mercury vapor lamp of 80 W produced stable ozone by elapsed time from ≈12 minutes. In this case, the confined oxygen of the air into reactor (without escape to ambient) and some peak of wavelengths in the range of UV produced by lamp contribute to ozone formation has been mentioned by literature. The analyses of contact angle revealed that the ITO films presented better results for treatment time of 20 minutes improving the spreading of the PEDOT:PSS droplets on the TCO surfaces compared with untreated sample. The analyses showed also that the treatment efficiency decreases after elapsed time of 1 minute. P-OLEDs devices were mounted using ITO films treated with UV-Ozone by 20 minutes and architecture: glass/ITO/PEDOT:PSS/PVK/Alq3/Al. The devices showed better performance of PVK polymer diluted in organic solvent of 1,2,4-trichlorobenzene using 5 mg/mL compared with other concentrations analyzed. These devices presented 240

lower threshold voltage, considerable increase of electrical current and similar curve of diode.

5. Acknowledgements Thanks to CAPES for the financial support and USP for the use of equipments and installations.

6. References 1. Santos, E. R., Correia, F. C., Burini, E. C., Jr., Onmori, R. K., Fonseca, F. J., Andrade, A. M., & Wang, S. H. (2012).

Influence of the transparent conductive oxides on the P-OLEDs behavior. ECS Transactions, 49(1), 347-354. http://dx.doi. org/10.1149/04901.0347ecst.

2. Santos, E. R., Burini, E. C., & Wang, S. H. (2012). UV-ozone generation from modified high intensity discharge mercury vapor lamps for treatment of indium tin oxide films. Ozone Science and Engineering, 34(2), 129-135. http://dx.doi.org/1 0.1080/01919512.2011.649132.

3. He, P., Wang, S. D., Wong, W. K., Cheng, L. F., Lee, C. S., Lee, S. T., & Liu, S. Y. (2003). Vibrational analysis of oxygen-plasma treated indium tin oxide. Chemical Physics Letters, 370(5-6), 795-798. http://dx.doi.org/10.1016/S0009-2614(03)00177-5.

4. Damlin, P., Östergård, T., Ivaska, A., & Stubb, H. (1999).

Light-emitting diodes of poly(p-phenylene vinylene) films electrochemically polymerized by cyclic voltammetry on ITO. Synthetic Metals, 102(1-3), 947-948. http://dx.doi.org/10.1016/ S0379-6779(98)00971-0.

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Low cost UV-Ozone reactor mounted for treatment of electrode anodes used in P-OLEDs devices 5. Sugiyama, K., Ishii, H., Ouchi, Y., & Seki, K. (2000). Dependence of indium–tin–oxide work function on surface cleaning method as studied by ultraviolet and x-ray photoemission spectroscopies. Journal of Applied Physics, 87(1), 295-298. http://dx.doi.org/10.1063/1.371859.

6. Iwama, Y., Cho, D. C., Mori, T., & Mizutani, T. (2003).

Electroluminescence properties of organic light-emitting diodes using ITO with different surface treatments. In Proceedings of the 7th International Conference on Properties and Applications of Dielectric Materials (pp. 718-721). Nagoya: IEEE. 7. Emerson, R. S., Tunísia, E. S., Wang, S. H., Elvo, C. B. J., Marcia, A. Y., Maria, D. P. H. F., Fernando, J. F., & Adnei, M. A. (2008). UV-Ozone treatment on ITO using modified high-pressure mercury vapor lamp for assembly of polymeric devices with RBPV-DODMPPV. In Proceedings of the 6th Ibero-American Congress on Sensors (Ibersensor 2008) (pp.

IB08-112). São Paulo: USP. 8. Santos, E. R., Correia, F. C., Wang, S. H., Hidalgo, P., Fonseca, F. J., Burini, E. C., Fo., & Andrade, A. M. (2010).

Reator de UV-Ozônio com lâmpada a vapor de mercúrio a alta pressão modificada para tratamento superficial de óxidos transparentes condutivos utilizados em dispositivos poliméricos eletroluminescentes. Química Nova, 33(8), 1779-1783. http:// dx.doi.org/10.1590/S0100-40422010000800027.

9. Santos, E. R., Wang, S. H., Correia, F. C., Costa, I. R., Sonnenberg, V., Burini, E. C., Jr., & Onmori, R. K. (2014).

Influência de diferentes solventes utilizados na deposição de filme de poli(9-vinilcarbazol) em dispositivos OLEDs. Química Nova, 37(1), 1-5. http://dx.doi.org/10.1590/S010040422014000100001.

10. Emerson, R. S., Satoru, Y., Elvo, C. B. J., Roberto, K. O., & Wang, S. H. (2013). Comparação de diferentes eletrodos anodos utilizados em dispositivos OLEDs flexíveis. Lumière, 186(15), 54-72.

11. Emerson, R. S., Elvo, C. B. J., & Fernando, J. F. (2009).

Aquaregia and oxygen plasma treatments on fluorinated tin oxide for assembly of pleds devices using OC1C10-PPV as emissive polymer. Sensors & Transducers Journal, 101(2), 22-30.

12. Emerson, R. S., Fábio, C. C., Elvo, C. B. J., Shu, H. W., Marcia, A. Y., Pilar, H., Fernando, J. F., & Adnei, M. A. (2009). New copolymers containing charge carriers for organic devices with ITO films treated by uv-ozone using high intensity discharge lamp. Sensors & Transducers Journal, 101(2), 12-21.

13. Emerson, R. S. (2009). Estudo de tratamentos superficiais em substratos de óxidos transparentes condutivos para a aplicação de dispositivos poliméricos eletroluminescentes (Doctoral thesis). Escola Politécnica de Engenharia Elétrica, Universidade de São Paulo, São Paulo.

14. Kitao, M., Komatsu, M., Hoshika, Y., Yazaki, K., Yoshimura, K., Fujii, S., Miyama, T., & Kominami, Y. (2014). Seasonal ozone uptake by a warm-temperate mixed deciduous and evergreen broadleaf forest in western Japan estimated by the Penmane-Monteith approach combined with a photosynthesis dependent stomatal model. Environmental Pollution, 184, 457-463. http://dx.doi.org/10.1016/j.envpol.2013.09.023.

PMid:24121421.

15. Luzchem Research Inc. (2015). Luzchem Spectroradiometer SPR-03 Specs. Ottawa. Retrieved in 16 May 2015, from http:// www.luzchem.com/products/spectroradiometer.php 16. Vendrami, J. A., Elvo, C. B. J., Wang, S. H., & Emerson, R. S. (2013). Medição de ângulo de contato com webcam. In Anais do 15° Simpósio de Iniciação Científica (pp. 48). São Paulo: Faculdade de Tecnologia de São Paulo.

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17. You, Z. Z., & Dong, J. Y. (2006). Effect of oxygen plasma treatment on the surface properties of tin-doped indium oxide substrates for polymer LEDs. Journal of Colloid and Interface Science, 300(2), 697-703. PMid:16643943.

18. Davenas, J., Besbes, S., Abderrahmen, A., Jaffrezic, N., & Ben Ouada, H. (2008). Surface characterisation and functionalisation of indium tin oxide anodes for improvement of charge injection in organic light emitting diodes. Thin Solid Films, 516(7), 1341-1344. http://dx.doi.org/10.1016/j.tsf.2007.03.163.

19. Sugiyama, K., Ishii, H., Ouchi, Y., & Seki, K. (2000). Dependence of indium-tin-oxide work function on surface cleaningmethod as studied by ultraviolet and x-ray photoemission spectroscopies.

Journal of Applied Physics, 87(1), 295-298. http://dx.doi. org/10.1063/1.371859.

20. Takimoto, H. G. (2013). Estudo de polifluorenos como camada emissora de dispositivos eletroluminescentes eficientes (Master’s thesis). Escola Politécnica de Engenharia Metalúrgica e de Materiais, Universidade de São Paulo, São Paulo.

21. Moraes, J. I. B. (2013). Estudo da camada de TCO e de PVK em dispositivos OLEDs e elaboração de um reator de UVozônio. São Paulo: Faculdade de Tecnologia de São Paulo.

Final Paper.

22. Erik, Y. Y. (2011). Estudo de encapsulamento de dispositivos poliméricos-orgânicos eletroluminescentes. São Paulo: Universidade de São Paulo. Final Paper.

23. Fábio, C. C. (2013). Síntese e caracterização de polímeros contendo 9-9 dioctilfluoreno e 8-oxioctilquinolina para utilização como camada emissora de PLEDs (Doctoral thesis).

Escola Politécnica de Engenharia Metalúrgica e de Materiais, Universidade de São Paulo, São Paulo.

24. National Information Standards Organization – NISO. (2015).

How the information world connects. Bethesda. Retrieved in 17 May 2015, from http://www.niso.org/publications/tr/tr01. pdf 25. Sydney, C. (1930). A theory of upper-atmospheric ozone.

Memoirs of the Royal Meteorological Society, 3(26), 103-125.

Retrieved in 17 May 2015, from http://www.rmets.org/sites/ default/files/chapman-memoirs.pdf 26. UV Disinfection, Aplication Information. (2004). Perfection preserved by the purest of light. Netherlands: Philips.

27. Satoru, Y. (2012). Estudo prospectivo de dispositivos P-OLEDs flexíveis. São Paulo: Faculdade de Tecnologia de São Paulo.

Final Paper.

28. Nguyen, N. D., Le, H. C., Nguyen, T. L., Tran, T. C. T., Tran, Q. T., & Hyung-Kook, K. (2009). Preparation and characterization of nanostructured composite films for organic light emitting diodes. Journal of Physics: Conference Series, 187(1), 1-8. http://dx.doi.org/10.1088/1742-6596/187/1/012029.

29. Erick, V. G. (2011). Estudo do desempenho de dispositivos diodos poliméricos-orgânicos emissores de luz utilizando-se camada PEDOT:PSS. São Paulo: Faculdade de Tecnologia de São Paulo. Final Paper.

30. Erick, V. G., Victor, S., Elvo, C. B. J., Wang, S. H., & Emerson, R. S. (2011). Dispositivos P/OLEDs com PVK e diferentes ETLs. In Anais do 13° Simpósio de Iniciação Científica (pp.

64). São Paulo: Faculdade de Tecnologia de São Paulo.

Received: July 07, 2015 Revised: Nov. 06, 2015 Accepted: Feb. 15, 2016

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http://dx.doi.org/10.1590/0104-1428.2322

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

Preparation and characterization of Zn(II) ion-imprinted polymer based on salicylic acrylate for recovery of Zn(II) ions Ebrahim Ahmadi1*, Javad Gatabi1 and Zahra Mohamadnia2 Department of Chemistry, University of Zanjan, Zanjan, Iran Department of Chemistry, Institute for Advanced Studies in Basic Sciences – IASBS, Gava Zang, Zanjan, Iran 1

2

*ahmadi@znu.ac.ir

Abstract This work describes the synthesis of new ion-imprinted polymers (IIPs) for selective solid phase extraction of Zn(II) ions from aqueous samples. IIPs were synthesized by copolymerization of salicylic acrylate (SA) as a functional monomer and ethylene glycol dimethacrylate (EGDMA) as a crosslinker in the presence of 2,2’-azobisisobutyronitrile (AIBN) as an initiator. The template ions were removed from IIPs particles by leaching with 0.1 M Ethylenediaminetetraacetic acid (EDTA) which leaves cavities in the particles with the capability of selective extraction of the Zn(II) ions. The monomer and the polymer after synthesis have been characterized by 1H NMR, 13C NMR and FT-IR studies. The effect of the pH on the extraction efficiency of Zn(II) ions was studied and optimized in pH around 6. The selectivity of the synthesized IIPs was studied in the presence of Co(II), Cd(II) and Ni(II) ions, and the IIPs showed higher affinity for Zn(II) in the presence of other interfering ions.

Keywords: ion-imprinted polymers, salicylic acrylate, solid phase extraction, Zinc (II) ions.

1. Introduction Zinc is one of the most abundant elements in the Earth’s crust. Zinc has many industrial applications includes; galvanizing iron, production of brass, zinc carbonate, zinc gluconate, zinc chloride, zinc pyrithionse and zinc sulfide.

Also it is used as a negative plate for some batteries, roofing and gutters in building construction, as a coin material, die casting in the automobile industry. Also zinc oxide used in cosmetics and the production of white pigment[1].

Zinc is an essential mineral in human nutrition. Zinc involves in many of human metabolism pathways and it plays an important role in production of the genetic materials and cell division. Zinc exists in many foodstuffs with animal or plant sources. Deficiency of the zinc affects about two billion people in the world and cause many diseases. Also excess of zinc in the human body will lead to ataxia, lethargy and copper deficiency[2-5]. Furthermore, drinking water contains trace amounts of Zn(II) ions especially when stored in the metal containers. Industrial activities (such as mining, coal, waste combustion and steel processing) and toxic waste sites can increase the zinc pollution of the local water recourses to levels that may cause health problems. Thus extraction of the Zn(II) ions from complex matrix is very important for environmental and biological science[3-6].

Analysis and extraction of samples with complex matrix needs special attention in order to avoid matrix interferences beside preconcentration of the analytes and converting them into a more suitable form for the instrumental detection.

The best solution for the complex sample analysis is the selective extraction of the interested analytes using sorbents with tailor-made recognition sites. Molecularly imprinted

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polymers (MIPs) provide this opportunity to synthesize very selective sorbents. MIPs are prepared in the presence of template molecules in order to mold template complementary binding sites[7-9]. MIPs can be synthesized by copolymerzation of a complex of a template and a functional monomer with a cross-linking agent in the environment of a porogenic solvent. After leaching the template molecules, a rigid three dimensional network remains with cavity complemented to the target compounds. MIPs showed many advantages such as high selectivity, easy preparation and low cost that makes them suitable candidate as a sorbent for the sample preparation of very complex samples. If the metal ions are used as templates for imprinting process, the resulting imprinted products are known as ion imprinted polymers (IIPs) that they can recognize metal ions with properties similar to the MIPs[10-14]. IIPs are selective and simple to synthesize. IIPs have been used for other application than solid phase extraction such as catalytic applications[15], chromatographic stationary phases[16], membrane separations[17] and sensors[18-20]. IIPs mainly used for the selective SPE of the trace metal pollutants from complex matrices[21-33].

This selective extraction and clean up cannot possible with the conventional sorbent.

Previously, for ion imprinting of zinc ions, following monomers have been used: styrene[26], methacrylic acid[23], 8-acryloyloxyquinoline[28], 4-vinylpyridine[24], 2-(diethylamino) ethyl methacrylate[22], 4-vinylpyridine[29]. While the following chelating agents have been used for the selective interaction with zinc ions: oxine[26], 3,5,7,20,40-pentahydroxyflavone[23], 2,2’-bipyridyl[24], 8-hydroxyquinoline[22], 2,2’-bipyridyl[29].

Salicylic acid has been extensively used for the extraction

Polímeros, 26(3), 242-248, 2016

Preparation and characterization of Zn(II) ion-imprinted polymer based on salicylic acrylate for recovery of Zn(II) ions of metal ions as a difunctional chelating reagent[34-37].

The chelating property of salicylate structure is due to the existence of an OH group in ortho position relative to the carboxyl or carbonyl group that can react with both hard and intermediate cations. Also salicylate moiety has been used as a chelating reagent in the structure of polymeric resins and has been used for metal ion extraction[38-43].

Furthermore, salicylic acid was used for functionalization of nanoparticles as a novel adsorbent and used for SPE of some heavy metal ions such as Cu(II), Cd(II), Ni(II) and Cr(III) ions[44]. Ion imprinted polymers based on the salicylate structure as chelating group has not been reported before.

Salicylic acid was used in the preparation of a Cu-IIPs but in this work, only a carboxyl group of salicylic acid was available for complexation, while the main chelating agent was 4-(2-Pyridylazo) resorcinol[45]. Another similar work was a preparation of copper salicylate based MIPs for drug delivery purposes, that “copper salicylate” was the template molecule[46].

In this work, a new Zn(II)-ion imprinted polymer based on salicylic acrylate was synthesized and applied for the selective extraction and preconcentration of Zn(II) ions from aqueous solution. Salicylic acrylate as a chelating ligand was synthesized, characterized and its complexation with Zn(II) was studied. Zn(II)-salicylic acrylate complex monomer was synthesized and then polymerized with ethylene glycol dimethacrylate and AIBN as a crosslinking agent and initiator, respectively. The resulted product, (EGDMA‑salicyl acrylate/Zn(II)) was characterized successfully. After removing the template Zn(II) ions from polymeric networks, the Zn(II)‑IIPs have been achieved.

2. Materials and Methods 2.1 Materials All of the reagents were of the highest purity and they used as received. Salicylaldehyde, ethylenediaminetetraacetic acid (EDTA) and ethylene glycol dimethacrylate (EGDMA) were purchased from Merck (Germany). Azobisisobutironitrile (AIBN) was achieved from Aldrich (St. Louis, MO, USA).

Analytical grade zinc chloride dihydrate, cobalt acetate tetra hydrate, nickel sulfate hepta hydrate and cadmium nitrate tetra hydrate were obtained from Fluka (Buchs, Switzerland). Standard solutions of metal ions were prepared in the deionized water. All the other reagents and solvents were achieved from Merck and used as received.

2.2 Apparatus All atomic measurements were carried out using a model Varian-200 flame atomic absorption spectrophotometer (Palo Alto, CA, USA) that equipped with a deuterium background correction system. Zinc absorbance was measured at 213.9 nm using spectral bandwidth (SBW) of 0.7 nm. For other element following condition was used, cobalt (λ: 240.0 nm, SBW: 0.2 nm), nickel (λ: 232.0, SBW: 0.2 nm) and cadmium (λ: 228.8 nm, SWB: 0.7 nm).

Infrared spectra were recorded on a Perkin-Elmer-58B (Billerica, Massachusetts, USA) FT-IR spectrometer using KBr pellets in the range of 4000-200 cm–1. The 1H NMR and 13C NMR spectra were recorded with a Bruker Advance Polímeros, 26(3), 242-248, 2016

DPX-250 MHZ. A model 713 Metrohm digital pH meter was used for pH adjustment.

2.3 Synthesis of salicylic acrylate A 100 mL two necked round bottom flask was charged with salicylaldehyde (8 mmol), THF (12 mL), triethylamine (9.6 mmol). The flask was purged continuously with nitrogen gas and the reaction mixture was cooled in an ice-water bath (–5 °C). Then an amount of 9.6 mmol of acryloyl chloride (diluted in 1.5 mL THF) was added dropwise for 20 minutes with constant stirring in the ice-water bath.

Then, the reaction mixture allowed achieving to the room temperature. After one hour the reaction was completed and the byproduct, quaternary ammonium salt, was filtered off.

For purification purpose, the obtained product was dissolved in 5 mL dichloromethane and 5 mL of diluted acetic acid was added to the mixture. The organic phase was separated and the process repeated for three times. The solvent was evaporated in vacuum and the residue was dissolve in diethyl ether and petroleum ether. The mixture was heated at 60 °C to separate impurities, and after vacuum evaporation, a yellow oily purified product was achieved.

2.4 Synthesis of Zn(II) salicylic acrylate The binary complex of Zn(II) salicylic acrylate was prepared by addition of an equal amount of salicylic acrylate and ZnCl2.2H2O (0.26 mmol) to the 5 mL of chloroform–methanol mixture (3:2, v/v) under reflux condition with continuous stirring at 60 °C for 4 h. Solvent was evaporated and the residue was dissolved in diethyl ether and n-hexane, then the temperature was reduced to –5 °C. The complex was sedimented on the bottom of the flask.

2.5 Synthesis of Zn(II)-imprinted polymers The Zn(II)-IIP was prepared by bulk polymerization.

Salicylic acrylate (0.26 mmol) was added gently into the 5 mL of chloroform–methanol mixture (3:2, v/v) and then treated with ZnCl2.2H2O (0.26 mmol) under reflex condition with continuous stirring at 60 °C for 4 hours. Afterward, EGDMA (2.6 mmol) and AIBN (6.5 mg) were added slowly to the mixture at room temperature. After 15 min stirring, the polymerization mixture was cooled to 0 °C while purging with N2 and then heated at 60 °C for 18 hours. The resulting polymer was thoroughly washed with an appropriate amount of ethanol and dried at room temperature. The resultant polymer was dried at 70 °C for 24 h.

2.6 Leaching of Zn(II) ions from the Zn(II)-IIPs The synthesized IIPs particles (0.2 g) were treated with 20 mL of 0.1 mol L-1 EDTA for 7 h at 60 °C. Then the solution was filtered and washed with 50 mL water. The washed solution analyzed by FAAS. The removal process was continued until no Zn(II) ion was detected.

2.7 Solid phase extraction of the metal ions The ion imprinted polymer (40 mg) was equilibrated with 25 mL of metal ion solution (80 mg L–1). The pH of the solution was adjusted between 2-7 using either ammonia or 1% acetic acid solution. Extraction was performed by 243

Ahmadi, E., Gatabi, J., & Mohamadnia, Z. stirring the mixture for 1 h at room temperature. Finally, the solution was filtered and washed with 75 mL water and the concentration of Zn(II) ions in the resulting solution was measured by FAAS.

other bands at 1603 cm–1 and 2815 cm–1 are corresponded to the C=C group and stretching vibrations of C-H bond of the aldehyde. Bands around 1410 cm–1 and 1455 cm–1 are attributed to aromatic C-C stretching vibrations and 1135 cm–1 and 1155 cm–1 are related to C-O strong stretching.

3. Results and Discussions

3.2 1H NMR study of salicylic acrylate

Salicylic acrylate ligand has appropriate functional groups (C=O and O-C=O) for complexation and polymerization.

This oily mild yellowish ligand was synthesized successfully and purified by liquid-liquid extraction using diethyl ether and n-hexane as solvent with recovery percentages higher than 89%. The product structure was characterized and confirmed

by FT-IR, 1H NMR and 13C NMR spectra. A scheme for the synthesis procedure is shown in Figure 1.

The 1H NMR spectrum of salicylic acrylate (Figure 3) indicates eight signals. There are two doublets at chemical shift of 6, 6.6 and a quartet at 6.4 ppm that pertain to the methylene and methyne group, respectively. Signals in the chemical shift of 7.1-7.9 ppm belong to the hydrogen’s of the aromatic rings. A singlet signal in the chemical shift of 10.1 ppm refers to the aldehydic hydrogen. Lack of alcoholic signal at 5.2 ppm demonstrated the evolution of the reaction.

3.1 FT-IR study of salicylic acrylate

3.3 13C NMR study of salicylic acrylate

The successful synthesis of salicylic acrylate is demonstrated by the FT-IR spectrum in Figure 2. Two characteristic absorption bands in 1740 cm–1 and 1695 cm–1, are related to the C=O group of both ester and aldehyde. Furthermore,

The 13C NMR spectrum of the salicylic acrylate ligand in the CDCl3 is shown in Figure 4. There are ten different chemical shift signals in the range of 123-188 ppm. The significant signals at 127 and 133 ppm belong to the carbons of the

Figure 1. Scheme for preparation of Zn(II) ion-imprinted polymer.

244

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Preparation and characterization of Zn(II) ion-imprinted polymer based on salicylic acrylate for recovery of Zn(II) ions

Figure 2. The FT-IR spectra of salicylic acrylate.

Figure 5. The comparison of the FT-IR spectra of the Zn, ligand and complex.

spectra. Peak at 753 cm–1 are attributed to ortho substituted aromatic ring. The carbonyl peaks of aldehyde and ester group for the complex are located at 1690 cm–1 and 1731 cm–1, respectively. These peaks have around 10 cm–1 shift toward lower wavenumber relative to the ligand. That is due to the rigid structure of the complex that reduces vibration of the atoms in the complex structure and leads to lower peak intensities. The weak stretching vibration of the aldehyde at the region of 2726 cm–1 in the complex spectra was vanished due of the contribution of the carbonyl group in the complexation with metal ions.

3.5 UV-Vis spectra of ligand and complex Figure 3. 1H NMR spectrum of salicylic acrylate in CDCl3.

The complexation of the salicylic acrylate with Zn(II) ions can be observed in UV-Vis spectra (Figure 6). Strong peak at 200 cm–1 are related to the aromatic ring π→π∗ translation is common for both ligand and complex. Peak at 250 cm–1 are carbonyl π→π∗ translation with 15 cm–1 relocation. Peak at 280 cm–1 is a forbidden n→π∗ translation of carbonyl group that disappeared in the complex spectrum due to involving nonbonding electron pair in complexion with a Zn(II) ion.

The electronic absorption spectra of the complex show a weak shoulder peak in the region of 415-445 nm which are assigned to the spin allowed metal-to-ligand charge transfer (MLCT) d→π∗ transition.

3.6 Preparation and 1H NMR study of polymer

Figure 4. 13C NMR spectrum of salicylic acrylate in CDCl3.

olefin group, indicating alpha and beta carbon near to the carbonyl group, respectively. The signals in the region of 128 and 151 ppm are related to the ipso carbon of the ring connected to the aldehyde and ester groups, respectively.

Other carbons of the aromatic ring are located in the chemical shifts of 123, 126, 130 and 135 ppm. The chemical shifts at 164 and 188 ppm indicate the carbons of the ester and aldehyde groups.

3.4 FT-IR study of Zn(II) salicylic acrylate complex The FT-IR spectra of the ligand, Zn(II) ions and complex were superimposed in Figure 5 for comparison. There are some partial difference between the ligand and the complex Polímeros, 26(3), 242-248, 2016

Copolymer of imprinted poly (EGDMA/salicylic acrylate/ Zn(II)) was performed by in situ polymerization in aqueous solution. The prepared polymer has an Indian red color and characterized by FT-IR and 1H NMR. The polymerization occurs by free radical polymerization mechanism, in which initiator molecule (AIBN) produces radical upon heating at 60 °C. The reaction continues to produce stable type III radical in EGDMA that processes to polymer synthesis. The 1H NMR spectrum of poly (EGDMA/salicylic acrylate/Zn(II)) particles is shown in Figure 7. According to this spectrum the peaks corresponding to the ligand and also EGDMA are completely detectable. The signal in the chemical shift of 10.1 ppm is attributed to the aldehydic hydrogen of salicylic acrylate. The hydrogens of aromatic ring are indicated in chemical shift of 7-7.8 ppm. In a broad area of 1.2-2.2 ppm the aliphatic hydrogens of ligand and crosslinker in polymer network are observable. The signals in the regions of 4.3, 5.6 and 6.2 ppm are related to EGDMA. 245

Ahmadi, E., Gatabi, J., & Mohamadnia, Z.

Figure 6. The comparison of the UV-Vis spectra of the Zn, ligand and complex.

Figure 8. Effect of pH on the extraction of Zn(II) ions using the Zn(II)-IIPs. Table 1. Selectivity experiments in the presence of disturbing ions. The concentration of all metal ions was equal to 80 mg L–1.

Cation/ pH Zn(II) Co(II) Ni(II) Cd(II)

2 60.1 5.7 3.0 4.6

Extraction efficiency (%) 3 4 5 6 66.5 81.7 90.6 94.9 10.2 17.7 28.2 35.5 6.2 7.5 17.1 10.5 6.4 2.0 12.2 19.9

7 90.8 29.3 6.8 27.6

Figure 7. 1H NMR spectrum of poly (EGDMA/salicylic acrylate/ Zn(II)) in DMSO.

following order of selectivity: Fe(III) > Cu(II) > Ni(II) > Co(II) > Zn(II) > Cd(II) > Pb(II), while the same functional group in the current work showed very high selectivity for Zn(II) among Zn(II), Co(II) and Ni(II). This superiority is attributed to Zn(II) ion recognition sites imprinted in the polymer.

3.7 Solid phase extraction and pH optimization

4. Conclusions

The extraction efficiency of the IIP particles was investigated by batch extraction of the Zn(II) ions from aqueous solution. Before the extraction process, the polymer particles completely leached using 0.1 M EDTA solution and then applied for batch solid phase extractions. The most important parameter effective in the extraction efficiency of the IIPs was studied at different pH value in the range of 2-7. The percent of the extraction is shown in the Figure 8.

By increasing the pH, the amount of the extraction increased until pH 5.75 and reached to its maximum amount around 97.88% and then reduced slightly due to increased OH- and consequent Zn(OH)2 precipitation.

In this study an ion imprinted polymer selective for Zn(II) ions was synthesized by copolymerization of salicylic acrylate and cross linking agent EGDMA in the presence of AIBN as initiator and its application for solid phase extraction of the metal ions was investigated. The maximum amount of adsorption of Zn(II) ion was obtained at pH 5.75 equal to 98 percent. The competitive adsorption experiments showed superiority of prepared IIPs toward Zn(II) ion even in the presence of Cd(II), Ni(II) and Co(II) ions which have the similar ionic radius.

3.8 Selectivity of the Zn(II)-IIPs

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Competitive adsorption of zinc ions in the presence of disturbing ions such as Cd(II), Ni(II) and Co(II) was also investigated and the results are shown in Table 1. Although these ions possess a similar ionic radii (Zn(II) = 74 pm, Cd(II) = 71 pm, Ni(II) = 69 pm and Co(II) = 72 pm)[47], the competitive adsorption of Zn(II) ions on the prepared Zn(II)-IIPs in the presence of other metal ions, showed a higher amount of absorption selectivity. It is interesting to compare the selectivity of the prepared Zn(II)-IIPs with a resin prepared by salicylic acid, hexamethylene diamine and formaldehyde in which the salicylate moiety act as a chelating group[48]. This resin for a series of ions showed 246

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Received: Aug. 21, 2015 Revised: Dec. 15, 2015 Accepted: Feb. 15, 2016

PolĂ­meros, 26(3), 242-248, 2016

http://dx.doi.org/10.1590/0104-1428.2352

Rheological properties and curing features of natural rubber compositions filled with fluoromica ME 100 Luciana Honorato1, Marcos Lopes Dias1, Chiaki Azuma2 and Regina Célia Reis Nunes1* 1

Instituto de Macromoléculas Professora Eloísa Mano – IMA, Universidade Federal do Rio de Janeiro – UFRJ, Rio de Janeiro, RJ, Brazil 2 The Open University of Japan, Chiba, Japão *rcnunes@ima.ufrj.br

Abstract This work aims at studying not only the rheological behavior of natural rubber-based compositions by making use of different contents of fluoromica or ME 100 synthetic mica in a natural rubber (NR) matrix, but also the different filler‑filler and matrix-filler interactions before and after curing. The ME 100 content in NR varied from 0 to 10 phr (parts per hundred parts of resin) and the results enabled to conclude on the influence of the mineral filler on the curing parameters, as well as on the limit amount of ME 100 for the best performance resulting from the best filler distribution/interaction in the polymer matrix. All data were compared with those of the unfilled composition. Based on complex viscosity, curing parameters, dynamic modulus and Payne effect tests it was concluded that the mica content limit for the best performance was 7 phr.

Keywords: rheology, natural rubber, synthetic mica, fluoromica.

1. Introduction Natural rubber (NR) is one of the worldwide most widely consumed polymers in view of the combination of its unique properties, such as high tensile and tear strength besides excellent dynamic properties, which renders it a strategic and irreplaceable material for the manufacture of large-sized tires[1,2].

In order that the elastomeric compositions find technological applications there is the need of a formulation including filler system, curing system, processing agents and protection agents such as antioxidants and antiozonants.

Such constituents modify the composite viscosity with direct reflections on processing as well as on the articles’ final properties. Being polymers, the behavior of elastomeric compositions is intermediary between viscous and elastic states, the knowledge of the viscoelastic profile, which can be studied by rheology, being extremely important for the processing prediction as well as for the final properties of the vulcanized elastomer. In the elastomer field, the rheology is mainly studied with the aid of the following equipment: Oscillating Disc Rheometer, Mooney Viscometer and Rubber Processing Analyzer[1-4].

The relevant features exhibited by NR makes it the object of countless research, where the search for new methodologies of chemical modifications and the use of different kinds of fillers has aroused the interest in present research so as to widen its area of application[5-10].

Besides carbon black and silica, which are long-established materials as reinforcement nanoparticles for a wide variety of rubbers, other functionalized or not nanofillers have been studied[6-9]. Among these fillers, synthetic micas which are light and have a similar structure to that of montmorillonite clay are being used as fillers for polymer compositions.

Polímeros, 26(3), 249-253, 2016

In a general way, synthetic micas are prepared from talc by introducing an alkaline metal into the interlamellar galleries. The advantage of using synthetic phyllosilicates as compared with natural mica is due to variables such as purity and composition, it admitting to be modified to suit the matrix to which it is to be incorporated[11-14].

In this work synthetic ME 100 mica free of any surface treatment was used, at contents of from 0 to 10 phr (parts per hundred parts of resin) in composition with natural rubber, an efficient curing system being added to the formulation[2,15,16].

The rheology of all the compositions with NR was assessed based on the results provided by the RPA 2000 rubber processing analyzer, which enables to assess not only the curing parameters, but also the filler-filler and filler-polymer interactions by the Payne Effect before and after crosslink formation[8,9,17,18].

2. Materials and Methods 2.1 Preparation of the composites In this work natural rubber (NR) of Mooney viscosity 82 ML (1+4) at 100 °C was used, and as filler, synthetic mica or Somasif ME 100 fluoromica, supplied by CO-OP Chemical Co., Ltd, Japan, with cationic exchange capacity (CEC) of 120 meq/100g.

The curing system used in this study was the efficient[2,15,16] one, which has the following formulation in phr: NR 100; ZnO 3.5; stearic acid 2.5; Irganox 1010 [Pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate)] 2.0; and TMTD (Tetramethylthiuram disulfide) 3.0.

An unfilled composition (NR/ME 100 0phr) was also obtained aiming at the comparison of results for formulations

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Honorato, L., Dias, M. L., Azuma, C., & Nunes, R. C. R. with 1, 2, 3, 5, 7 and 10 phr of ME 100 mica. All chemicals were used as received.

The blends were performed in accordance with the ASTM D 3184 Method, in a Lab Tech Engineering Company LTDA LRMR-SC-150/0 roll mixer at 30 °C and roll speed of 24:40.

2.2 Rheological properties The properties of the NR compositions related to Complex Viscosity, Curing Rheometric Parameters, Payne Effect and further rheological characterizations were assessed with the aid of the RPA 2000 rubber processing analyzer[17,18].

This equipment is specifically designed for the measurement of properties in cured and uncured rubber compositions so as to comply with international standards such as the ASTM D 5289 and ASTM D 2084 Methods, which were used in this work. As for torque, the measurement tolerance is 0.5% of the working range and for the temperature accuracy is ± 0.3 °C of the test temperature.

2.2.1 Curing rheometric parameters Curing rheometric parameters were assessed with the aid of the RPA instrument at a constant temperature of 150 °C, with oscillating arch of 1°, frequency of 1 Hz, for one hour, on compositions obtained in the rollmill using formulations with the efficient cure system specified in item 2.1. Maximum torque (MH), Minimum Torque (ML), Curing Time (T90), Pre-Curing Time (Ts1) and Curing Rate Index (CRI) were taken and/or calculated from the elastic torque plot (S’) versus time supplied by the RPA equipment[19-22].

2.2.2 Complex viscosity Aiming at studying the effect of ME 100 on NR without the interference of the other additives of the formulation, the assessment of the complex viscosity was the test of choice.

The filler-rubber blends were performed in a rolling mill, under the experimental conditions described in item 2.1, under a homogenization period of 5 minutes for each composition. Unfilled NR (PG) was submitted to the same experimental conditions.

By using the RPA equipment at 100 °C and frequency of 0.5 Hz a plot of complex viscosity as a function of shear rate was obtained for deformations between 0.5° and 80°.

From this plot were extracted figures of complex viscosity for each composition at 0.5° deformation.

3. Results and Discussion 3.1 Complex viscosity The polymer-filler and/or filler-filler interactions influence the viscosity of mixtures and were analyzed before cure by comparing two parameters: one related to the complex viscosity (η*) for compositions with only NR/mica at 0.5º deformation, and the other parameter related to the minimum torque, which is the viscosity of the compositions with the complete additives. The results are displayed on Figure 1 and are compared with those of the unfilled composition.

Data of Figure 1 show that above 5 phr mica has a significant influence on the complex viscosity of the compositions, corroborating the results for minimum torque (ML)[18-22].

According to Chen et al.[20], surface adhesion between the filler and the matrix hinders the flow of elastomeric compositions as a result of the partial formation of phase interactions[19,20]. In this way, data of Figure 1 allow to estimate the NR- ME 100 mica interactions. It can be observed that for both properties studied, complex viscosity and minimum torque, there is increasing growth as a function of the filler content, potentialized for the 7 phr composition, meaning higher rubber-filler interaction. Beyond this formulation, that is, for 10 phr mica the viscosity increase is lower, 7 phr being possibly an indication of the filler limit content in the composition meaning better dispersion and distribution within the elastomeric matrix[23-27].

3.2 Cure rheometric parameters The results of the cure rheometric parameters for the NR/ME 100 and pure gum are listed in Table 1. It can be observed that the presence of the filler does not interfere in the pre-curing time (Ts1) (or safety period of the curing process), but causes a slight increase in the curing period (T90) and as a consequence, a reduction in the curing rate index (CRI) relative to the pure gum, meaning an influence on the process of crosslink formation. The less favorable result is for the composition of the highest fluoromica content, 10 phr.

Assuming that the difference between torques (ΔM = MH - ML) is related to crosslink density[22], the fluoromica incorporation exerts positive influence on the

2.3 Payne effect The polymer-filler and filler-filler interactions were estimated based on the Payne Effect[23-29].

The test was carried out in the RPA instrument by the analysis of the strain scanning applied to the compositions in the range between 0 and 100% and frequency of 1 Hz before and after cure at 60 °C. For the assessment of the post–cure Payne Effect, the compositions were previously submitted to 150 °C in the equipment itself for the curing times (T90) specified for each composition, as reported in the item Curing Rheometric Parameters.

By using RPA it was also possible to study the elastic modulus (G’) as a function of frequency (from 0.1 to 10 Hz) at 150 °C at the deformation of 0.5°.

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Figure 1. Minimum Torque and Complex Viscosity as a function of the ME 100 mica content in uncured NR compositions.

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Rheological properties and curing features of natural rubber compositions filled with fluoromica ME 100 Table 1. Cure rheometric parameters of the NR/ME 100compositions.

(a)

NR/ME 100

ML(a)

MH(a)

MH-ML(a)

Ts1

T90

CRI

(phr) 100/00 100/01 100/02 100/03 100/05 100/07 100/10

(dN.m) 0.09 0.15 0.17 0.17 0.20 0.23 0.24

(dN.m) 7.34 8.11 8.15 8.00 8.30 8.57 7.39

(dN.m) 7.25 7.96 7.98 7.83 8.10 8.34 7.15

(min.)

2.48 2.23 2.36 2.34 2.53 2.57 2.50

(min.)

9.55 10.03 10.45 10.36 10.40 10.58 12.23

(min.–1) 14.14 12.82 12.36 12.46 12.70 12.48 10.27

The torque (ML or MH) measurement tolerance is of 0.5% of the working range.

7 phr composition, corroborating the results shown in Figure 1. The maximum torque (MH), which is related to molecular rigidity, is the lowest for the 10 phr formulation among the filled compositions, this being an indication of the low rubber-filler interaction.

Based on the obtained results the cure rheometric parameters point out to the 7 phr ME 100 composition as the most favorable, predicting the best possible interaction/distribution for this filler content with the elastomeric matrix[3,4,6-9,22,26,27].

3.3 Payne effect The occurrence of a tridimensional network formed by the filler is important since it modifies the physical properties of the elastomeric compositions, significantly affecting the dynamic viscoelastic properties of the rubber articles.

The filler-filler interaction is a ruling factor for hysteresis rise, and is directly related to the breaking and reconstitution of these structures of secondary aggregates in filled rubber compounds when submitted to strains. Such interactions have been studied by Payne (Payne Effect) through the influence of strain amplitude on elastic modulus. The Payne Effect can be calculated by the difference between the elastic moduli before and after cure, with a specific value of strain (∆G’ = G’0 - G’∞) while temperature and frequency are kept constant[23-29].

In the present study the Payne Effect (filler-filler interaction) was calculated by the difference between the elastic modulus at 14% and at 100% strain [∆G’ = (G’14% - G’100%)] in cured and uncured compositions, at 1 Hz and 60 °C, the results obtained being displayed in Figures 2A and 2B respectively.

For the studied strain range the unfilled composition (NR/ME 100 0phr) as well as the filled ones exhibited non linear behavior before as well as after cure. By increasing filler content the distances among aggregates become shorter and therefore there is increased probability of occurrence of a tridimensional network formed by the augmented filler[23,25-28]. Data of Figures 2A and 2B show reduced elastic modulus value with the increase in strain amplitude for all of the mica-reinforced compounds, this phenomenon being explained by the “Payne Effect”[23,27].

In this way, the compositions with fluoromica ME 100, as compared with the pure gum composition had higher G’ values relative to the applied strain, absorbing more energy as a result of their higher rigidity and thus corroborating the results for elastic torque in the curing test.

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Figure 2. Payne Effect: Elastic Modulus for NR/Pure Gum and NR/ME 100 compositions as a function of strain: (A) before cure (B) after cure.

After cure, the curves are shifted towards higher modulus values as a result of filler increase, exception made to the formulation with 10 phr mica where the modulus value is lower than that for the 7 phr composition. This behavior points out to 7 phr filler as limit value for the developed formulation.

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Honorato, L., Dias, M. L., Azuma, C., & Nunes, R. C. R. The accentuated reduction in the G’ value resulting from the strain rate is due to the breaking of the filler‑filler interaction and consequently the dismantlement of the polymer-filler tridimensional network. The increased G’ value observed in the low strain region is in agreement with the Payne Effect that is, an indication of the rubber‑filler interaction[23,27].

In Figure 2B it can be observed that the highest value for G’ is for the 7 phr filler composition.

Figure 3 shows the data for the Payne Effect for cured and uncured compositions calculated from the difference between moduli at 14% and 100% strain [∆G’ = (G’14% - G’100%)].

There is an increase in ∆G’ (Payne Effect) after cure due to

the probable re-aggregation of the filler by the low viscosity of the polymer matrix during crosslinks formation at high temperatures. The Payne Effect will be higher the greater is the value of ∆G’, meaning a higher break of filler-filler interactions, and consequently, higher amount of filler agglomerates in the elastomer matrix. The highest value was obtained for the 7 phr ME 100 fluoromica composition after cure[23,27].

The elastic modulus for the NR/Pure Gum and NR/Mica compositions was also studied as a function of frequency (Figure 4). As relates pure gum the graphic results show that during all the frequency scanning modulus G’ was higher for all of the mica compositions, this being due to interactions, which are quantified by the Payne Effect.

G’ values at low frequencies are lower than those for high frequency as a consequence of the molecular movement differentiated relaxation times, which is hindered by filler addition. Whenever a polymer is strained at higher frequencies the chains are not allowed sufficient time to relax, and the modulus increases[29]. The 7 phr mica composition has a significant higher modulus throughout the whole range of frequency studied due to the better interaction with the NR matrix, corroborating the remaining analyzed results.

4. Conclusions Non-surface treated fluoromica ME 100 added to natural rubber has modified the curing features of the compositions and the rheological properties for the studied contents.

Figure 3. Payne Effect [∆G’ = (G’14% - G’100%)] for the NR/Pure Gum and NR/ME 100 compositions before and after cure.

The cure results led to the conclusion that fluoromica ME 100 interfered in crosslinks formation by the values of the difference between maximum torque and minimum torque, by the increased curing time and by the reduction in the cure rate coefficient.

Based on the Payne Effect theory it was possible to measure the filler-matrix and filler-filler interactions for the NR composites with fluoromica ME 100 before and after cure.

Results for complex viscosity corroborated the remaining tests and indicated 7 phr as the limit ME100 amount for the best performance in natural rubber under the processing conditions reported in this work, resulting from the better filler distribution/interaction in the polymer matrix.

5. Acknowledgements The authors are indebted to CNPq, CAPES and FAPERJ for financial support and to the Grupo Teadit for the donation of natural rubber, additives and further materials employed during the course of this research.

6. References

Figure 4. Elastic Modulus (G’) as a function of frequency for the NR/Pure Gum and NR/ME 100 compositions.

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23. Payne, A. R. (1962). The dynamic properties of carbon black loaded natural rubber vulcanizates. Part I. Journal of Applied Polymer Science, 6(19), 57-63. http://dx.doi.org/10.1002/ app.1962.070061906.

24. Bezerra, F. O., Nunes, R. C. R., & Ito, E. N. (2013). Efeito Payne em nanocompósitos de NBR com montmorilonita organofílica. Polímeros: Ciência e Tecnologia, 23(2), 223-228.

25. Léopoldès, J., Barrès, C., Leblanc, J. L., & Georget, P. (2004).

Influence of filler-rubber interactions on the viscoelastic properties of carbon-black-filled rubber compounds. Journal of Applied Polymer Science, 91(1), 577-588. http://dx.doi. org/10.1002/app.13155.

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27. Fröhlich, J., Niedermeier, W., & Luginsland, H. D. (2005).

The effect of filler–filler and filler–elastomer interaction on rubber reinforcement. Composites. Part A, Applied Science and Manufacturing, 36(4), 449-460. http://dx.doi.org/10.1016/j. compositesa.2004.10.004.

28. Yurekli, K., Krishnamoorti, R., Tse, M. F., McElrath, K. O., Tsou, A. H., & Wang, H. C. (2001). Strucuture and dynamics of carbon black-filled elastomers. Journal of Polymer Science. Part B, Polymer Physics, 39(2), 256-275. http:// dx.doi.org/10.1002/1099-0488(20010115)39:2<256::AIDPOLB80>3.0.CO;2-Z. 29. Barick, A. K., & Tripathy, D. K. (2011). Effect of organically modified layered silicate nanoclay on the dynamic viscoelastic properties of thermoplastic polyurethane nanocomposites.

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Received: Sept. 03, 2015 Revised: Feb. 04, 2016 Accepted: Feb. 24, 2016

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http://dx.doi.org/10.1590/0104-1428.1896

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

Study of the structural, thermal, rheological and film properties of functional copolymers of hydroxyethyl acrylate and methyl methacrylate María Mejia1 and Edwin Murillo2* Grupo de Investigaciones Pirometalúrgicas y de Materiales – GIPIMME, Departamento de Ingeniería de Materiales, Universidad de Antioquia – UdeA, Medellín, Colombia 2 Grupo de Investigación en Materiales Poliméricos – GIMAPOL, Departamento de Química, Universidad Francisco de Paula Santander – UFPS, San José de Cúcuta, Colombia

1

*edwinalbertomurillo@gmail.com

Abstract New functional polymers will be prepared using alkyd resins having high solid content (environmentally friendly) and comb-type structural morphology. Different copolymers of hydroxyethyl acrylate and methyl methacrylate (HEMMA) were synthesized by solution polymerization using azo-bis-(isobutyronitrile) (AIBN) as initiator and dimethylformamide as a solvent. The proportions utilized of AIBN were 0.5 (HEMMA1), 1.0 (HEMMA2), 1.5 (HEMMA3) and 2.0 wt. % (HEMMA4). The conversion percentage of the reaction was higher than 90%. The formation of the copolymers was evidenced by infrared analysis, hydroxyl value, and nuclear magnetic resonance. The intensity of OH group adsorption increased with the molecular weight and hydroxyl value. The polydispersity index was lower than 1.5. All copolymers exhibited a stable region on viscosity at a shear rate between 0.1 and 10 s–1. The copolymers exhibited good thermal stability, flexibility, and adherence.

Keywords: hydroxyethyl acrylate, methyl methacrylate, synthesis, copolymerization, properties.

1. Introduction Acrylic resins have been widely used in the coating and adhesive industries[1,2]. These materials have good resistance to ultraviolet light, hydrolysis, oxidation, and its physics drying is fast[3]. Copolymers obtained from hydrophobic and hydrophilic monomers have a great importance in various applications, such as biocompatible materials, hydrogels, and coatings[4-6]. The hydroxyethyl acrylate (HEA) is a soft monomer and is a bi-functional molecule since it presents an OH group and a double bond[7,8]. The glass transition temperature of the polyhydroxyethyl acrylate (PHEA) is estimated at –15 °C[7,8]. The function of HEA in the copolymers is providing elasticity and the OH groups[7]. MMA is a monofunctional monomer[9] and has been employed in the synthesis of many copolymers[10-12]. MMA and HEA can be polymerized by free radical polymerization[8,12]. HEA has been employed in some synthesis of polymers[13,14].

Reactive monomers (HEA, hydroxyethyl methacrylate, maleic anhydride, etc.) have attracted considerable interest in recent years since they are important precursors for the synthesis of graft copolymers with special properties[5].

They can be employed in applications where high molecular weight polymers are not required[15].

PHEA was synthesized without a solvent by free-radical frontal polymerization (FP) at ambient pressure[16]. HEA and sodium acrylate (AANa) were grafted on the starch backbone in an aqueous solution to obtain biopolymerbased superabsorbent hydrogel[17]. Copolymerizations of HEA/methacrylic acid and ethyl acrylate/HEA by free radicals has been conducted in m-xylene employing a temperature

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range between 70 and 130 °C, and using tertbutyl peroxy benzoate as the initiator[18]. HEA/methacrylic acid copolymers with high HEA content were highly crosslinked and found to swell in dimethylsulfoxide (DMSO)[18].

The polymerization of HEA onto chitosan using persulfate as initiator was performed in an aqueous solution.

The polymerization rate was much more sensitive to the concentration of the HEA than to the concentration of the initiator[19].

Poly(HEA-co-coumaryl acrylate)s were prepared by free radical polymerization using dimethylformamide as solvent and AIBN as an initiator. The copolymers were self-assembled into microspheres in the aqueous phase due to their amphiphilicity[20].

The free radical polymerization is a versatile technique since it presents tolerance to impurities (stabilizers, water, oxygen, etc.), is inexpensive and a wide amount of monomers can be polymerized by this technique[21].

HEMMA copolymers can be used as an alternative to obtaining environmentally friendly alkyd resins (high solid content), but one requirement is that the copolymer have low molecular weight (low viscosity) and also have enough reactive functional groups (OH groups) which can react with acid groups of fatty acids. In this work it is intended to obtain functional HEMMA copolymers by solution polymerization employing different proportions of initiator, since this is an important variable to the synthesis of copolymers with low viscosity and adequate properties that in the future allow obtaining of alkyd resins with high

Polímeros, 26(3), 254-261, 2016

Study of the structural, thermal, rheological and film properties of functional copolymers of hydroxyethyl acrylate and methyl methacrylate solid content. The structural, thermal, rheological and film properties of all materials also will be studied.

2. Experimental Section 2.1 Materials Sigma-Aldrich supplied HEA, MMA, AIBN, sodium chloride, sodium hydroxide, chlorhidric acid and N, N-dimethylformamide (DMF).

2.2 Synthesis of the HEMMA copolymers The amount of HEA (85 g) MMA (15 g) and DMF (200 g) were taken in a reactor. The mixture was heated at 80 °C and stirred at 100 rpm under a nitrogen atmosphere.

In every case, the respective proportion of AIBN (0.5, 1.0, 1.5 and 2.0 wt % with respect to the proportions of HEA and MMA) was added to the reactor and the mixture reacted for 5 h. The solid content in each case was 50 wt%. The reaction conversion related to the formation of HEMMA or HEA and MMA homopolymers was studied by gravimetric analysis and the analysis was conducted in triplicate. To accomplish, the samples were collected at different time intervals during the reaction (these samples were not previously purified) and weighed. A small amount of methyl ethyl hydroquinone (1 wt %) was added to these samples to prevent the reaction during solvent evaporation process. These samples were kept in vacuum oven at 50 °C for 48 h. Finally, the samples were taken out and weighted. It is worth mentioning that samples employed for the determination of the conversion percentage were not used to others analysis but discarded.

The copolymers were named HEMMA1 (0.5 wt% AIBN), HEMMA2 (1.0 wt% AIBN), HEMMA3 (1.5 wt% AIBN) and HEMMA4 (2.0 wt% AIBN). The schematic representation of this reaction is presented in Figure 1.

2.3 Characterization of the HEA/MMA copolymers The characterization was performed on samples that were obtained after a reaction time of five hours. We are discussing the results below from these samples. As mentioned earlier, the samples were initially purified (only to these analyses) before recording infrared, hydroxyl value, NMR, and DSC analyses, and the determination of the HEA homopolymer and

vHEMMA copolymers content. In order to acquire the data, approximately 10 g of samples were precipitated in hot xylene (MMA homopolymer is soluble, but the HEMMA copolymer is insoluble, furthermore in this solvent will be the residual HEA monomer), filtered and dried in an oven at 50 °C for 12 h under vacuum and finally weighted. Furthermore, the samples were subjected to soxhlet extraction with acetone for 12 h (extraction of HEA homopolymer) followed by drying in an oven at 50 °C for 12 h under vacuum, and weighted.

The HEA homopolymer amount was determined employing initial weight of the sample, weight of the dry sample after of the precipitation process in hot xylene. The initial and final weight of the samples after soxhlet extraction process in acetone was used to determine the amount of HEMMA copolymers in the samples.

The HEMMA copolymers was re-precipitated in xylene and used to obtain MMA homopolymer. The solution of xylene (hypothetically containing the residual MMA monomer, residual HEA monomer and MMA homopolymer) was added to acetone and subjected to filtration process to obtain the MMA homopolymer.

Infrared analysis was carried out using a Perkin Elmer equipment model Spectrum One. The spectra were recorded with 8 scans, using resolution of 4 cm–1. The hydroxyl value analysis was made according to AOCS Cd 13-60.

The dynamic light scattering analysis (DLS) was done using a Horiba equipment and at an angle of 90°, by using solutions of the samples in DMF (1 wt%). The 1H NMR analysis was carried out using a Bruker AC 300 MHz spectrometer using dimethyl sulfoxide as the solvent. For gel permeation chromatography (GPC) analysis, the samples were dissolved in tetrahydrofuran and the analysis was performed in Waters HPLC equipment with millennium 2000 software for data acquisition and using polystyrene standards for the quantification. DSC analysis was carried out employing a TA Instrument model Q100 equipment, equipped with the refrigerated cooling system and using a heating and cooling rate of 30 °C/min under nitrogen atmosphere. The decomposition temperature of the samples was determined by thermogravimetric analysis (TGA) using a TA instrument model Q500 equipment at a heating rate of 10 oC/min. The rheological measurements in solution were performed in a Bohling HRNano rotational rheometer (TA instruments) at a strain of 2% using concentric cylinder geometry. Studies of the HEMMA copolymers film properties (gloss, flexibility, and adherence) were performed according to methodologies reported in previous studies[22-24].

3. Results and Discussion

Figure 1. A schematic representation of the HEMMA copolymer synthesis.

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The results from the gravimetric analysis are shown in Figure 2. Figure 2a, displays the conversion percentage of the reactions while Figure 2b, shows HEA homopolymers content in the HEMMA copolymers and finally Figure 2c, exhibit the yield of the HEAMMA copolymers, when only these were taken into account. The reaction conversion (Figure 2a) after 5 h was higher than 94%, which increases with the initiator content in the synthesis. It is because the amount of initiator in the reaction mixture decreases the possibility of residual monomer by promoting them in polymerization process. Since no sample was purified initially before this 255

Mejia, M., & Murillo, E. analysis, it can be inferred that the unreacted amount of MMA and HEA was low and the system exhibited a high conversion percentage. This behavior has been observed in the free radical polymerization of HEA[25].

It was apparent that the HEA homopolymer percentage in the samples (Figure 2b), increased with the amount of initiator. This result meant that the initiator concentration favored the formation of HEA homopolymer, possibly this was due to: a) there is high chemistry affinity between free radicals and HEA, b) high amount of HEA employed with respect that of the MMA, c) there is a high affinity between DMF and HEA, since both are highly hydrophilic (it improve the diffusion of species hydrophilic through solvent) and MMA is hydrophobic.

The gravimetric analysis did not display the presence of MMA homopolymer, suggesting that MMA homopolymer was not formed. Using the difference between conversions

percentage of the reaction and the HEA homopolymer percentage in the HEMMA copolymers, was calculated, the conversion percentage (when only the HEMMA copolymers were taken into account) (Figure 2c). It can be visualized that the conversion percentages exhibited a trend with the initiator amount, however, the relation was not statistically significant (Figure 2c).

Figure 3 shows the IR spectra of the MMA and HEA (Figure 3a) and the purified HEMMA copolymers (Figure 3b).

In the MMA spectrum (Figure 3a) a signal at 1439 cm–1 was observed due to bending of -CH3 groups. In the spectrum of the HEA (Figure 3a), the signal at 3416 cm–1 corresponds to stretching of C-OH bonds. The signal that appears at 1725 cm–1 was due to absorption of the carbonyl groups (-C=O), and it appears in MMA and HEA spectra. The signal at 1638 cm–1 was due to stretching of -CH=CH- bonds and that at 1410 cm–1, corresponds to the bending of -CH2.

Figure 2. (a) Conversion percentage of the reactions and (b) HEA homopolymer percentage in the HEMMA copolymers and (c) yield of copolymers.

Figure 3. IR spectra (a) MMA and HEA and (b) HEMMA copolymers.

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Study of the structural, thermal, rheological and film properties of functional copolymers of hydroxyethyl acrylate and methyl methacrylate Figure 3b shows the IR spectra of the HEMMA copolymers.

All samples showed a signal at 3416 cm–1 (C-OH bond) that decreased with the amount of AIBN and can be related to a reduction of HEA units in the copolymers, which is in good agreement with the results obtained to HEA homopolymer percentage in the copolymers (Figure 2b). This result is an indication that the repetitive units of the HEA and MMA were different in the HEMMA copolymers. None HEMMA copolymer exhibited the signal at 1638 cm–1 (stretching of -CH=CH-), which indicates that all monomers reacted, or the residual monomer was extracted during the purification process of the HEMMA copolymers.

The signals that appear at 1439 and 1410 cm–1 were due to -CH3 (MMA) and -CH2 (HEA) groups respectively. At 3416 cm–1 appears the signal for the OH groups. The presence of these signals is an evidence of the formation of the HEMMA copolymers. Considering the results obtained from IR, the HEA homopolymer amount increased from HEMMA1 to HEMMA4. These outcomes are in good agreement with the results obtained from gravimetric analysis (conversion percentage when only the HEMMA copolymers were taken into account).

The spectra of protonic nuclear magnetic resonance (1H NMR) of the HEMMA4 copolymer is presented in Figure 4. The signal around 1 ppm is attributed to -CH3 protons. The -CH and -CH2 backbone protons resonated at 2.3 and between 1.5-1.9 respectively[19]. The signal at 2.5 ppm is attributed to the DMSO protons. Between 2.6 and 2.9 ppm appear two signals, which correspond to protons of CH2O-, which are originated of the etherification reaction between units of HEA in the HEMM4 copolymer; this is favored by the temperature [18], this signal have been observed by others authors in copolymerization reactions of HEA[18], which argue that for every two losses in (-OH) functional groups, there is a formation of two -OCH2 groups.

The signals at 3.4 ppm and 3.6 ppm corresponded to protons of the -OCH3 groups and methylene protons join to OH groups (HO-CH2-) respectively[26]. At 4.1 ppm appear the signals of the methylene protons of -CH2O- which are different that those that experimented etherification reactions[26]. The same signal has been observed by others authors[19,26]. The spectra did not exhibit signals due to protons -CH=CH- (5.3 ppm), this is evidence that the monomers reacted. The presence of different signals indicated that the HEMMA copolymers were formed.

Table 1, shows the respective values of number average molar mass (Mn), weight average molar mass (Mw) and the polydispersity index (PI) of the HEMMA copolymers.

The behavior showed by the HEMMA copolymers with the increase in the AIBN content was expected since with it, the molar mass would be reduced, due to a high number of termination reactions between macroradicals. The lowest Mn and Mw values were due to high amounts of AIBN and polymerization method employed in the synthesis.

The variation in polydispersity was not significant. The Mn values decreased with the amount of HEA homopolymer formed, but Mw did not follow the same trend, which is possibly due to crosslinking reaction exhibited by this material as it was demonstrated by NMR analysis.

The VOH of the HEMMA copolymers (Table 1), decreased with increase in AIBN content employed in the synthesis. This result is in agreement with that obtained with the HEA homopolymer content, since it was expected that when the content of HEA homopolymer is high the amount of the units of HEA in the copolymers will be low.

The results of VOH are in accordance with the results presented by IR analysis. Castor oil with a VOH of 164 mg KOH/g sample has been employed to obtain acrylic ester by esterification reaction between OH groups (castor oil) and acid groups of acrylic acid[27]. In the future, the VOH obtained to the HEMMA copolymers, will allow that these materials be modified with fatty acids to obtain alkyd resins or others hybrid materials.

Figure 5 presents the volume (Figure 5a) and intensity (Figure 5b) size distributions of the HEMMA copolymers.

The hydrodynamic dimensions (volume) of the copolymers (Figure 5a) were nanometric as follows: HEMMA1: 18, HEMMA2: 23, HEMMA3: 15 and HEMMA4: 13.0 nm.

According to the results of GPC, it was expected that the HEMMA1 copolymer would exhibit the higher hydrodynamic dimensions, but this was not the behavior. This was possibly due to HEMMA copolymers exhibiting different processes such as aggregations through interactions between OH groups (hydrogen bonds) or crosslinking reactions (through HEA). All volume size distributions were monodisperse and nanometric.

Figure 4. 1H NMR of the HEMMA4 copolymer.

Table 1. Mn, Mw, PI and VOH values of the HEMMA copolymers.

HEMMA Copolymers HEMMA1 HEMMA2 HEMMA3 HEMMA4

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Mn (g/mol) 7620 7073 6446 5619

Mw (g/mol) 9486 10457 7579 7744

PI 1.2 1.5 1.2 1.4

VOH (mg KOH/g sample) 130 ± 2.0 111 ± 2.0 100 ± 1.0 82 ± 2.0

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Figure 5. Size distributions of the HEMMA copolymers (a) Volume and (b) Intensity.

The intensity size distributions (Figure 5b) present other distributions with diameters (d) higher than 100 nm for the HEMMA2, HEMMA3 and HEMMA4. This is an indication of the presence of aggregates. The intensity distribution is very sensible for detecting the presence of aggregates because large particles scatter more light than small ones. The nanometric hydrodynamic dimensions obtained in this study were mainly nanometric due to the high amounts of AIBN and polymerization method employed in the synthesis. This result was very important because these materials exhibit high structural packing.

The presence of aggregations in the samples HEMMA2, HEMMA3, and HEMMA4, may be attributed mainly to the interactions between OH groups (hydrogen bonds) originated from HEA homopolymers. These copolymers exhibited higher HEA homopolymer content than that of the HEMMA1 copolymer (not exhibited aggregation).

The interactions through OH groups has been reported by some authors to yield hyperbranched polyester polyols by employing good solvents such as DMF and DMSO[28-31].

Since the aggregations appear in intensity size distribution but it does not appear in volume size distribution, it can be inferred that its number is lowest.

Figure 6 shows the rheological behavior of the HEMMA copolymers in solution (in each case the solid content was 50 wt%). The HEMMA copolymer viscosities at 10 s–1 were as follows: HEMMA1: 7.4 Pa.s, HEMMA2: 6.0 Pa.s, HEMMA3: 5.0 Pa.s and HEMMA4: 1.7 Pa.s. The low viscosity of the HEMMA4 may be due to low crosslinking and entanglement degrees. The viscosity decreased with the increasing of the AIBN content employed in the synthesis, and this result was expected because in this same sense occurs a reduction in molar mass. All viscosity values of the HEMMA were low due to low molar mass. All copolymers presented a stable region in viscosity, but this disappeared at shear rates higher than 50 s–1, the fluids hence became shear thinning and this behavior was attributed to disentanglement of the polymer chains, but also to dissociation of interactions. The same behavior has been observed in hydroxyethyl methacrylatebutyl acrylate copolymers[32].

258

Figure 6. Rheological behavior of the HEMMA copolymers.

Since the rheological behavior of the HEMMA copolymers was consistent with the molar mass, it was difficult to identify the influence of the crosslinking process on the rheological behavior of these materials, because this process is not reversible.

According to results obtained by DLS analysis (intensity size distribution), it apparent that only HEMMA2, HEMMA3 and HEMMA4 copolymers exhibited a pseudoplastic behavior due to dissociation of interactions. Moreover,

by DLS analysis it was evidenced that these copolymers exhibited aggregation. However, HEMMA1 copolymer (Figure 5b), did not exhibit aggregations but showed pseudoplastic behavior, due to disentanglement of the chains.

The HEMMA4 copolymer exhibited lower pseudoplastic behavior (reduction in viscosity with the increasing of shear rate) due to lowest molecular weight and reduction in the possibility of entanglement.

Polímeros, 26(3), 254-261, 2016

Study of the structural, thermal, rheological and film properties of functional copolymers of hydroxyethyl acrylate and methyl methacrylate Figure 7 presents the results of the thermal analysis.

Figure 7a shows the thermograms obtained by differential scanning calorimetry (DSC) and Figure 7b the thermograms obtained by thermogravimetric analysis (TGA). All glass transition temperatures (Tg) of the HEMMA copolymers (Figure 7a) were different to values of PHEA (–15 °C)[8] and MMA homopolymer (105 °C)[11]. This is evidence that the HEMMA copolymers were formed. The Tg values of the HEMMA copolymers were as follows: HEMMA1: 32 °C, HEMMA2: 35 °C, HEMMA3: 26 °C and HEMMA4: 33 °C. The results of DSC analysis do not correspond with the molar mass and VOH and this may be due to crosslinking reactions through HEA in the copolymers (Figure 5), interaction between OH groups and different compositions of the copolymers since the MMA is rigid, and HEA is elastic[8,11].

With the increasing on molar mass, there was a reduction in free volume and chain mobility, therefore it was expected that Tg values increased with the molecular weight, but none HEMMA copolymers did not exhibit this behavior.

Furthermore, it was expected that the Tg values would also increase with the VOH, since these values augmented with the OH group numbers in the chain and the probability of occurrence of interactions through the OH groups was higher than when the OH group numbers were low. It can be concluded that the interactions degree between OH groups and crosslinking degree were different for every HEMMA copolymer.

It is apparent that the amount of HEA homopolymer in the HEMMA4 is highest. Also, the number of MMA units in this copolymer was higher than those of the others copolymers. It is possible that HEMMA4 copolymer contains lowest number of HEA units in its structure long with low Mn value yet displaying Tg similar to HEMMA1 and HEMMA2. This result suggests that the distribution and the number of unit of HEA and MMA in HEMMA copolymers were different.

The crosslinking reaction of HEA has been reported[33].

In a previous study, it was reported that the curing rate for HEA was faster than 2-ethylhexyl acrylate, 2-hydroxyethyl methacrylate, and hydroxypropyl acrylate and hydroxypropyl methacrylate[33].

All copolymers exhibited a weight loss between 200 and 350 °C (Figure 7b), which corresponds to oligomers and copolymers of low molecular weight.

Likewise, at approximately 400 °C a great weight loss for all HEMMA copolymers appears. The temperatures of thermal decomposition of the HEMMA copolymers were as follows: HEMMA1: 414 °C, HEMMA2: 416 °C, HEMMA3: 423 °C and HEMMA4: 418 °C. There is no correlation with the molecular weight, initiator amount, HEA homopolymer amount or VOH. Therefore, this behavior is possibly attributed to a different number of interactions, homopolymerizations, and crosslinking reactions.

Table 2 presents the results of adhesion, flexibility and gloss analyses of the HEMMA copolymers. The adhesion and flexibility of the copolymers were good. It has been demonstrated that acrylic polymers exhibit good adhesion[2,34].

In the case of flexibility, this result was expected because the proportion of HEA employed in the synthesis, was high with respect to MMA. The gloss values were very similar to the HEMMA copolymers. Therefore, the behavior shown by the HEMMA copolymers in gloss was possibly due to small variations in refractive index and roughness on the surfaces.

Table 2. Values of gloss, adhesion, and flexibility of the HEMMA copolymers.

HEMMA Copolymers HEMMA1 HEMMA2 HEMMA3 HEMMA4

Gloss (20°)

Adhesion (%)

Flexibility

132 132 137 126

100 100 100 100

Pass Pass Pass Pass

Figure 7. Thermograms of the HEMMA copolymers (a) DSC and (b) TGA. Polímeros, 26(3), 254-261, 2016

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4. Conclusions This study made an important contribution to the synthesis of functional copolymers since HEMMA copolymers were obtained, and they may be an alternative to the synthesis of new materials. With the AIBN content, decreased the amount of HEA in the copolymers, which is attributed homopolymerization of HEA. The initiator concentration improves the conversion percentage of the reaction (take into account HEA homopolymer and HEMMA copolymer formed) and the formation of HEA homopolymer, but it did not improve the formation of HEMMA copolymers. The molar mass and the viscosity of the copolymers decreased with the amount of AIBN. The VOH of the copolymers was between 82 and 130 mg KOH/g sample and their hydrodynamic dimensions were nanometric. They presented an adequate VOH, which possibly would allow esterification reactions between OH groups of the HEMMA copolymers and COOH groups of fatty acids (preparation of alkyd resins type comb).

The viscosity of the copolymers was lower than 10 Pa.s.

The copolymers obtained were amorphous and this may explain the good flexibility that they exhibited.

5. Acknowledgements We would like to thank The Comité Para el Desarrollo de la Investigación (CODI) de la Universidad de Antioquia (Colombia) for their financial support of this investigation.

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Received: Aug. 28, 2014 Revised: Sept. 14, 2015 Accepted: May 16, 2016

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http://dx.doi.org/10.1590/0104-1428.2110

T T T T T T T T T T T T T T T T T T

Avaliação das propriedades elétricas de barras estatóricas fabricadas com resina do tipo éter diglicidílico do bisfenol F (DGEBF) contendo nanopartículas de silica Evaluation of electrical properties of stator bars manufactured with bisphenol F diglycidyl ether resin containing silica nanoparticles Rafael Novaes da Conceição1 e João Sinézio de Carvalho Campos1* Faculdade de Engenharia Química, Universidade Estadual de Campinas – UNICAMP, Campinas, SP, Brasil

1

*sinezio@feq.unicamp.br

Resumo O presente trabalho visa apresentar resultados da aplicação de uma resina epóxi bisfenólica (DGEBF) à base de nanopartículas de sílica (RN) e comparar suas propriedades com a resina convencional de referência epóxi bisfenólica (DGEBA) (RE), atualmente utilizada. Neste sentido fabricaram-se protótipos de barras estatórica, destinadas a hidrogeradores, através do sistema VPI (Vácuo-Pressão-Impregnação) e avaliaram-se as propriedades elétricas pelas técnicas de fator de dissipação e envelhecimento acelerado. Dentre os resultados para as resinas observou-se que: (i) o fator de dissipação e de envelhecimento são praticamente os mesmos para ambas as resinas; (ii) o valor de tip-up resultaram em 0,014% para RE e 0,020% para a resina RN e (iii) a estimativa do tempo de vida útil esta em cerca de 40 anos, o que é aplicável para a maioria das aplicações industriais. Neste sentido sugere-se que a resina RN pode ser uma alternativa a resina RE, com um desempenho elétrico equivalente.

Palavras-chave: barras estatóricas, fita de mica, hidrogerador, nanodielétricos, nanotecnologia.

Abstract The present work aims to present the results of an application of a bisphenolic epoxy resin (DGEBF) containing silica nanoparticles (RN) and compare its properties with a bisphenolic epoxy resin (DGEBA) (RE), currently used. In this context, prototype stator bars for hydrogenerators were manufactured, according to the VPI (Vacuum-Pressure‑Impregnation) system and their electrical properties with the tests of dissipation factor and voltage endurance. Within the results for the resins it was observed that: (i) dissipation factor and voltage endurance are practically the same for both resins; (ii) the resulting values of tip-up were 0.014% for RE and 0.020% for RN resin and (iii) the estimating of the life-time is about 40 years, what is suitable for most industrial applications. In this sense it is suggested that the RN resin can be an alternative to the RE resin, with an equivalent electrical performance.

Keywords: stator bars, mica tape, hydrogenerator, nanodielectrics, nanotechnology.

1. Introdução A crescente demanda por energia elétrica tem pressionado fabricantes de hidrogeradores a desenvolverem máquinas menores e com maior eficiência de geração elétrica desde o início do século XX. A limitação de geração esbarra, entre outros fatores, nas propriedades e tecnologias de materiais compósitos isolantes disponíveis. A busca por novos materiais motiva pesquisadores em todo mundo a desenvolverem alternativas que possam suportar níveis de tensão elétrica acima de 3kV/mm, que é o padrão atual de rigidez dielétrica dos sistemas isolantes[1]. Estima-se que através de uma redução de 15% na espessura de isolação

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seria possível aumentar em 16% a seção transversal de cobre o que aumentaria a eficiência de um hidrogerador em 4%[2].

A descoberta de materiais nanocompósitos isolantes (ou nanodielétricos) em escala de laboratório, com propriedades dielétricas superiores aos convencionais, despertou o interesse de diversos segmentos da indústria com seu potencial diversificado de aplicação. Resultados promissores publicados na literatura motivaram uma pesquisa neste ramo[3-6].

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Avaliação das propriedades elétricas de barras estatóricas fabricadas com resina do tipo éter diglicidílico do bisfenol F (DGEBF) contendo nanopartículas de sílica Um dos principais componentes de um hidrogerador são as barras estatóricas (ou barras Roebel), responsáveis por conduzir a energia elétrica gerada. Barras estatóricas possuem tipicamente uma seção transversal retangular composta por um feixe de condutores isolados individualmente entre si e do núcleo do estator através da isolação principal (Figura 1).

A isolação principal é formada por um compósito a base de camadas de papel de mica e resina, podendo a última ser do tipo epóxi ou poliéster. A função da isolação é passiva na condução elétrica, porém fundamental para que a energia seja transportada sem perdas por descargas elétricas. Ademais garantindo a rigidez mecânica do feixe de condutores e a dissipação térmica eficiente. Um dos processos mais comumente empregados na indústria para fabricação de barras estatóricas é o processo VPI (Vácuo‑Pressão-Impregnação).

Tal processo consiste basicamente de uma impregnação das camadas de fita de mica sob pressão reduzida com posterior aumento da pressão. O que garante o preenchimento por resina dos vazios da fita de mica.

O presente trabalho visa testar uma resina contendo nanopartículas (RN) previamente fabricada, que será testada como elemento de isolação no lugar da resina convencional de referência (RE), para construção de barras estatóricas – protótipos, e testes de caracterização efetuados segundo normas e/ou procedimentos industriais. A resina RE é utilizada desde a década de 60 até a presente data e é um material reconhecido no meio industrial por seus resultados de tempo de vida útil comprovados pelo tempo em operação[8,9]. A resina RN proposta foi obtida segundo estudos recentes desenvolvidos por Meichsner[3], sem o uso de fita de mica, com resultados promissores de envelhecimento acelerado da ordem de um fator 10. A avaliação da resina RN proposta foi efetuada em comparação com as propriedades de isolação da resina RE, convencionalmente utilizada pela Voith Hydro Ltda. Tal resina tem por finalidade eliminar eventuais falhas na isolação, como por exemplo, vazios na isolação, de modo a melhorar a isolação elétrica e reduzir as perdas por transporte de energia.

2. Parte Experimental 2.1 Preparação das resinas A resina utilizada para elaboração de RN é uma epóxi bisfenólica DGEBF na forma líquida de viscosidade 1200‑1600 mPas à 25 °C, contendo nanopartículas de sílica de tamanhos entre 5 e 15nm, possuindo distribuição bi-modal de tamanhos de 5 a 8 nm e de 10 a 15 nm. Sendo o nome comercial Epilox F16-01, fornecida pela empresa VTA GmbH. Misturando-a com endurecedor do tipo anidrido à base de anidrido metil-hexahidroftálico, MHHPA, fornecido na forma líquida pela empresa Huntsman de nome comercial Aradur HY 1102. Após tal mistura obteve-se uma resina homogeneizada para impregnação contendo 18% em peso de nanopartículas de sílica, esta resina proposta foi denominada RN. A homogeneização foi executada utilizando um misturador tipo haste com hélices ao longo de seu comprimento, sendo que a proporção final em massa de resina e endurecedor foi de 1:1 com base no trabalho de Meichsner[3], pois segundo este trabalho a distribuição bimodal de partículas, com tais dimensões e fração de carregamento, apresentou resultados superiores da ordem de um fator 10 em medições dos tempos de falha sob envelhecimento acelerado.

Efetuou-se a avaliação da resina proposta RN por comparação com os resultados de protótipos de barras estatóricas fabricadas pela empresa Voith Hydro Ltda, a qual utiliza uma resina epóxi bisfenólica. Esta resina é líquida sem modificação à base do éter diglicídilico do bisfenol A, DGEBA, de viscosidade 4000-5500 mPas à 25 °C, fornecida pela empresa Huntsman de nome comercial MY‑790, tendo como endurecedor da resina o anidrido MHHPA (nome comercial HY-1102), ambos fornecidos pela empresa Huntsman. Mantendo-se o mesmo procedimento de preparo e a proporção 1:1 em massa entre mistura e endurecedor, esta resina será identificada neste trabalho pela sigla RE.

2.2 Fita de mica A fita de mica utilizada no enfitamento das barras é composta por papel a base de mica fina (160 g/m2), tipo MPM-NC classe 3 muscovita não calcinada[10], depositada sobre um tecido de fibra de vidro com material aglutinante.

Tal fita contém o sal de naftenato de zinco (0,1 a 0,4% em massa de fita) como elemento acelerador na reticulação e cura da resina.

2.3 Barras estatóricas protótipo Barras protótipo foram fabricadas utilizando condutores de cobre isolados individualmente com resina híbrida de epóxi/poliéster e fibra de vidro. Tais barras apresentaram uma seção transversal de 14,5 × 55,4 mm e comprimento de 2.000 mm. Posteriormente esta barra receberá a isolação elétrica complementar.

2.4 Preparação das barras protótipo

Figura 1. Desenho esquemático com os principais materiais que compõe uma barra estatórica: (1) isolação principal; (2) condutores de cobre; (3, 4) são os sistemas anti-corona; (5) cobertura de acabamento; (6) calço tangencial; (7) cunhas de fixação[7].

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As barras protótipo foram enfitadas com 10 camadas de fita de mica e 50% de sobreposição, obtendo-se uma espessura unilateral de isolação de 3 mm, o que para esta tecnologia corresponde à uma tensão de isolação de operação de 13,8kV. Posteriormente são inseridas em moldes e fechados e dispostas em autoclave para impregnação da 263

Conceição, R. N., & Campos, J. S. C. resina, como ilustrado na Figura 2, através do processo VPI (Vácuo- Pressão-Impregnação). Fabricaram-se 18 barras com a resina contendo nanopartículas (RN) e 14 barras com a resina convencional de referência (RE).

Antes de impregnar a resina, realizou-se o processo VPI sob as seguintes condições: colocam-se as barras estatóricas em autoclave para secagem a 65 °C, sob pressão de 0,2 mbar e durante um tempo de 10 horas, para eliminação da umidade.

Após a fase de secagem transfere-se para a autoclave a resina pré-aquecida (60 °C), submergindo as barras com a resina.

Posteriormente eleva-se a pressão no interior da autoclave para 4 bar com a injeção de N2, otimizando a penetração da resina através das camadas de fitas.

Uma vez concluído o processo VPI removem-se as barras da autoclave e submetem-nas ao processo de cura em estufa por aproximadamente 8 horas em temperatura de 140 °C. Após cura, as barras são removidas dos moldes para aplicação da proteção anti-corona em suas extremidades e então submetidas a uma segunda etapa de cura por aproximadamente 12 horas a 120 °C. Após cura da proteção anti-corona as barras estão prontas e preparadas para os ensaios elétricos.

A Figura 3 resume as etapas do processo de obtenção das barras no estudo proposto com nanopartículas.

3. Procedimentos Experimentais 3.1 Fator de dissipação (tan delta) O ensaio de fator de dissipação (tan delta) é comumente empregado como medida de controle de qualidade de produção na fabricação de barras estatóricas para avaliação da integridade da isolação e eventuais falhas decorrentes dos processos de impregnação e cura.

Figura 2. Banheira da autoclave com quatro barras protótipo dentro dos moldes ao fim do processo de impregnação VPI. (Cortesia Voith Hydro Ltda).

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Realizaram-se ensaios do fator de dissipação em todas as barras produzidas com as resinas RN e RE conforme recomendações de ensaio especificadas pela norma IEEE 286[11].

Efetuou-se a medição de tan delta e da capacitância iniciando‑se com uma tensão de 0,2 × Un (Un=13,8 kV) e frequência de 60 Hz, elevando-se a tensão em incrementos de 0,2 × Un até se atingir 1,2 × Un, registrando-se a dissipação a cada incremento. Desta forma foi possível se obter o valor do tip‑up entre 0,2 × Un e 0,6 × Un para cada medição conforme critério de aceitação em EN 50209[12].

Utilizando-se uma ponte de medição de tan delta da marca Tettex modelo 2805 HR, disponibilizado pela empresa Voith Hydro Ltda.

3.2 Envelhecimento acelerado (voltage endurance test) Para se avaliar a expectativa de vida das barras produzidas, as mesmas foram submetidas aos ensaios de envelhecimento acelerado, que é uma prática comum utilizada pelos fabricantes de hidrogeradores. Neste ensaio é avaliado somente o aspecto elétrico, não se considerando as solicitações térmicas, mecânicas e ambientais do componente em operação.

Foram testados dois conjuntos, sendo o primeiro de 6 barras fabricadas com a resina RE e 6 com a resina RN sob tensão constante de 3 × Un=41,4kV (ou rigidez dielétrica de 13,8 kV/mm) de acordo com norma KEMA S13/14[13].

Cabe ressaltar que esta norma KEMA estabelece um tempo mínimo de 10 horas de sobrevivência dos componentes, ou seja, se houver falhas abaixo deste tempo o componente não é recomendado.

Testou-se um segundo conjunto de barras estatóricas seguindo as orientações da norma IEEE 1553[14] relacionada ao ensaio de envelhecimento acelerado. Esta norma possui duas alternativas (programa A e B) em função do tempo de teste e valores de tensão elétrica. No presente trabalho optou-se pelo programa A por permitir um maior tempo de

Figura 3. Fluxograma do processo utilizado para fabricação dos protótipos das barras estatóricas com resina contendo nanopartículas de sílica (RN) ou resina convencional (RE).

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Avaliação das propriedades elétricas de barras estatóricas fabricadas com resina do tipo éter diglicidílico do bisfenol F (DGEBF) contendo nanopartículas de sílica teste e um menor valor de rigidez dielétrica de 10 kV/mm em relação aos norma KEMA S13/14. Deste modo avaliando o compósito obtido em níveis de stress elétrico diferentes.

O equipamento para estes ensaios utiliza um transformador de potência marca EEAT modelo AT1001000CAAMT, com tensão e corrente ajustáveis, sendo tensão de saída 0-100 kVca, corrente de saída 0-1000 mAca e f=60Hz, equipamento pertencente a empresa Voith Hydro Ltda e a Figura 4 ilustra tal equipamento.

4. Resultados Experimentais 4.1 Fator de dissipação (tan delta) Os resultados obtidos nos ensaios do fator de dissipação (tan delta) em função da tensão elétrica para as barras fabricadas com a resina RE e RN podem ser visualizados na Figura 5. Para elaboração do gráfico utilizou-se a média dos resultados de fator de dissipação em cada degrau de elevação da tensão de 0,2 × Un e respectivos desvios padrão obtidos. Observa-se da Figura 5 que para ambas as resinas (RE e RN) o resultado médio do fator de dissipação é praticamente o mesmo entre 0,5 e 0,6%. Nota-se também que o aumento em cada degrau de 0,2 × Un resulta em um aumento muito pequeno do fator de dissipação da ordem de 0,04%. Estes resultados indicam um baixo nível de perdas no dielétrico e uma boa consolidação dos compósitos nos processos de impregnação e cura[8], ou seja, sem a formação de vazios significativos ou defeitos no compósito através da introdução de nanopartículas. Outro fator que contribuiu para uma redução nos resultados foi o uso da proteção

Figura 4. Equipamento para ensaio de envelhecimento acelerado das barras protótipo.

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anti‑corona interna que reduz a distribuição da tensão entre a barra consolidada e a isolação principal[8].

Calculando-se os valores de tip-up encontra-se 0,014% (σ=0,007) e 0,020% (σ=0,007) respectivamente para as resinas RE e RN; satisfazendo os critérios de aceitação de boa isolação, o qual de acordo com a norma EN 50209[12], o valor de tip-up deve ser menor que 0,25%.

Não se encontrou na literatura resultados de fator de dissipação para compósitos similares aos utilizados na fabricação de barras estatóricas a base de fita de mica e resina contendo nanopartículas como no trabalho aqui proposto.

Deste modo não foi possível contrapor os resultados obtidos com a literatura. Alguns artigos comparam resultados de tan delta de amostras obtidas de resina sem versus com nanopartículas variando-se a frequência de ensaio.

Singha et al.[15] realizaram ensaios de tan delta variando-se a frequência de ensaio de 1 MHz a1 GHz para resinas epóxi DGEBA com nanopartículas de TiO2 e Al2O3 e diferentes frações de nanopartículas. Segundo os autores ao comparar-se os resultados de tan delta de um material sem nanopartículas com outro com, não se observou nenhuma diferença no resultado. Singha et al.[15] propõem que haveriam dois processos interagindo que causariam tal comportamento.

Um deles seria o número reduzido de transportadores de cargas disponíveis para condução elétrica.

A introdução de nanopartículas poderia formar um maior número de armadilhas de modo que os transportadores de carga seriam capturados por tais armadilhas. Um segundo efeito seria a introdução de interfaces e emaranhados de cadeias poliméricas no volume da matriz, causado pelas nanopartículas, que poderiam restringir a mobilidade e o deslocamento de cargas elétricas no sistema. Porém, segundo os autores uma melhor compreensão dos fenômenos interfaciais se faz necessário para uma compreensão plena dos resultados. Segundo Tanaka et al.[5] é questionável que os resultados de tan delta possam ser reduzidos pela diminuição do tamanho das partículas (nanomização). Em alguns casos observa-se uma redução nos valores de tan delta e em outros não, e isso estaria intrinsicamente ligado à interação entre a nanopartícula e a matriz polimérica, bem como as condições de processamento.

Figura 5. Resultados médios de tan delta de todas as barras fabricadas com a resina RN e RE. 265

Conceição, R. N., & Campos, J. S. C. 4.2 Envelhecimento acelerado (voltage endurance test) 4.2.1 Norma KEMA S13/S14[13] A Figura 6 apresenta os resultados dos testes de envelhecimento acelerado para as barras estatóricas para as resinas RN e RE, onde se tem o percentual de falhas acumulado em função do tempo de ensaio. O valor de tensão utilizado no ensaio foi de 41,4 kVca e os ensaios realizados em condições ambientes (28 °C e pressão de 1007 mbar), conforme procedimentos estipulados na norma KEMA S13/S14. Utilizou-se para o tratamento dos resultados de probabilidade de falha o método de distribuição de dois parâmetros de Weibull acumulados[16] que é um dos métodos mais apropriados[17] para caracterização de sistemas isolantes sólidos.

Observa-se no gráfico da Figura 6 que para ambas as resinas (RN e RE) as falhas de isolação acontecem para tempos bem superiores ao de 10h, sendo de 98h para RN e de 150h para RE para início de uma falha elétrica. Levando‑se em consideração o valor de parâmetro de escala à 63%, estabelecido pela norma IEEE 930, para o percentual de probabilidade de falha, observa-se da legenda da Figura 6 que os valores são 226 h e 198 h para a resina RN e RE respectivamente. Apesar do maior tempo observado de ruptura de aproximadamente 10% para resina RN, analisando-se o gráfico, se observa que há uma sobreposição dos intervalos de confiança a 95%, o que caracteriza um resultado equivalente para os dois tipos de resina. A maior dispersão dos resultados obtidos é confirmada pelo parâmetro de forma, que é inversamente proporcional à distribuição de dados, de 2,2 e 7,2 para a resina RN e RE respectivamente.

Deste modo conclui-se que os resultados são satisfatórios frente ao critério da norma, porém não se observa nenhuma alteração com o uso de nanopartículas. Yamazaki et al.[18] obtiveram um aumento de 8% nos resultados de tensão de ruptura para amostras de fita de mica e resina contendo nanopartículas. Os autores atribuem a melhora abaixo do esperado a uma dificuldade de difusão das nanopartículas através das camadas de mica. Esta limitação impediria uma dispersão adequada das nanopartículas na matriz polimérica

Figura 6. Probabilidade acumulada de falha em função do tempo de ensaio ou Gráfico de Weibull de envelhecimento acelerado (KEMA S13/14) para as barras produzidas com as resinas RE e RN. (LI=10 h, tempo limite mínimo de acordo com a norma KEMA S13/S14).

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e por consequência o efeito de barreira à propagação de descargas elétricas esperado.

4.2.2 Norma IEEE 1553[12] Com os resultados obtidos nos ensaios de envelhecimento acelerado conforme IEE 1553A fez-se um gráfico de probabilidades acumuladas de Weibull (Figura 7). De acordo com a Figura 7 observa-se que o comportamento das resinas RN e RE são equivalentes e que o tempo mínimo para início de falhas é de aproximadamente 1113 h, o que são resultados satisfatórios, uma vez que o critério de tempo mínimo é de 400 h segundo a norma IEEE 1553-programa A. Como uma das barras fabricadas com a resina RN excedeu 3.400h de ensaio sem falhas optou-se por encerrar o ensaio e utilizar o método da censura singular de acordo com a norma IEEE 930[14] para o cálculo do parâmetro de escala.

Deste modo obteve-se os valores do parâmetro de escala de 2.213 h e 2.292 h para a resina RN e RE respectivamente.

Por este motivo que o valor de 2.099 h para RE apresentado na legenda da Figura 7 difere do mencionado acima.

Os valores dos parâmetros de escala e de forma obtidos para os dois tipos de resinas são iguais equivalentes, o que é comprovado graficamente pela sobreposição dos respectivos intervalos de confiança à 95%. Os resultados obtidos para os ensaios de acordo com IEEE 1553[12] confirmam a proximidade de comportamento entre os dois sistemas de resina observados também no item 4.2.1.

Alguns autores obtiveram resultados superiores de envelhecimento acelerado através da incorporação de nanopartículas. Nelson et al.[19] ao utilizarem nanopartículas de TiO2 em epóxi DGEBA conseguiram uma melhora de aproximadamente 4 ordens de magnitude em comparação com o resultado obtido com micropartículas. Segundo os autores tal incremento seria possível através de um maior controle de distribuição de cargas elétricas internas no volume do compósito e também a uma possível polarização interfacial introduzida pela enorme área interfacial das nanopartículas. Roy et al.[20] obtiveram resultados superiores de aproximadamente 3 ordens de magnitude em polietileno com nanopartículas de SiO2 em comparação ao polietileno sem nanopartículas. Segundo IIzuka et al.[21], os quais investigaram o envelhecimento em amostras de resina

Figura 7. Gráfico obtido no ensaio de envelhecimento acelerado conforme IEEE 1553-A para as barras produzidas com as resinas em estudo. (LI=400h, tempo limite mínimo de acordo com a norma IEEE 1553-A).

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Avaliação das propriedades elétricas de barras estatóricas fabricadas com resina do tipo éter diglicidílico do bisfenol F (DGEBF) contendo nanopartículas de sílica

Figura 8. Rigidez dielétrica em função do tempo para as resinas RN e RE.

epóxi/sílica, explicaram que o aumento do tempo de envelhecimento esta associado a dificuldade de formação de arborecimentos na isolação, somada às fortes ligações covalentes estabelecidas entre carga e matriz, poderiam explicar os resultados superiores de envelhecimento.

Contrapondo-se os resultados obtidos nos itens 4.2.1 e 4.2.2 com os exemplos obtidos da literatura, citados anteriormente, é possível concluir-se que para a aplicação aqui proposta não houve um incremento significativo do tempo de envelhecimento acelerado em diferentes solicitações de rigidez dielétrica através do uso de nanopartículas. É possível que as propriedades dielétricas características da mica de impedimento à propagação de descargas elétricas preponderem em relação às propriedades das nanopartículas.

Segundo Yamazaki et al.[18] poderia ocorrer uma filtragem das nanopartículas pelas camadas de fita de mica de modo que estas tenham um comportamento abaixo do esperado.

4.3 Expectativa de vida útil das barras estatóricas Uma estimativa complementar à distribuição de Weibull, comumente utilizada por fabricantes de hidrogeradores, é analisar o gráfico da rigidez dielétrica (tensão suportável por espessura de dielétrico) em função do tempo de ensaio, sendo estes colocados em escala logarítmica[1,9,22,23]. Realiza-se uma extrapolação considerando o valor de rigidez dielétrica para utilização industrial das barras estatóricas. Na Figura 8 são apresentados os resultados da rigidez dielétrica em função do tempo. Realizando-se uma extrapolação no tempo para um valor de 3 kV/mm para rigidez dielétrica obtém-se um tempo de vida útil aproximado de 40 anos, tempo este considerado muito satisfatório de uso deste dispositivo, o qual esta de acordo com os valores práticos observados por fabricantes[24]. Neste ensaio foi avaliado somente o aspecto elétrico, no entanto sabe-se que nas condições reais de operação o equipamento está sujeito à influência da temperatura (efeito Joule), solicitação mecânica Polímeros, 26(3), 262-268, 2016

(vibração) e condições ambientais (ozônio[25]), que não foram consideradas neste estudo.

Apesar dos resultados satisfatórios obtidos no presente trabalho, para as barras fabricadas com resina RN contendo nanopartículas de sílica, não se observa uma melhora significativa dos resultados para o fator de dissipação e envelhecimento acelerado em comparação com a resina RE. Porém, cabe ressaltar que não há na literatura trabalhos com a resina aqui utilizada e que na maioria das referências encontradas na literatura os objetos de estudo são resina epóxi pura versus resina epóxi com nanopartículas em escalas de laboratório e em nosso caso construiu-se em escala piloto (protótipo industrial).

5. Conclusão Foram avaliadas as propriedades elétricas de barras protótipo fabricadas com resinas com e sem nanopartículas, RN e RE respectivamente. Pode-se concluir que: (i) os resultados de fator de dissipação e de envelhecimento são praticamente os mesmos para ambas as resinas; (ii) o valor de tip-up resultaram em 0,014% para RE e 0,020% para a resina RN, sendo que a norma (EN 50209[12]) exige que seja inferior a 0,25% e (iii) a estimativa do tempo de vida útil esta em cerca de 40 anos, que é um tempo médio real para a maioria dos hidrogeradores em operação.

Assim, conclui-se que a resina epóxi bisfenólica DGEBF contendo nanopartículas de sílica pode ser uma alternativa à resina epóxi bisfenólica DGEBA, atualmente utilizada, com um desempenho elétrico equivalente.

6. Referências 1. Sumereder, C., & Weiers, T. (2008). Significance of defects inside in-service aged winding insulations. IEEE Transactions on Energy Conversion, 23(1), 9-14. http://dx.doi.org/10.1109/ TEC.2006.888037.

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Conceição, R. N., & Campos, J. S. C. 2. Marek, P. (2006). New carrier for high voltage insulation materials (Doctoral thesis). TU Graz, Austria.

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IEEE 1553: trial-use standard for voltage-endurance testing of form-wound coils and bars for hydrogenerators. New York: IEEE. 15. Singha, S., & Thomas, M. J. (2008). Permittivity and tan delta characteristics of epoxy nanocomposites in the frequency range of 1 MHz-1 GHz. IEEE Transactions on Dielectrics and Electrical Insulation, 15(1), 2-11. http://dx.doi.org/10.1109/TDEI.2008.4446731.

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18. Yamazaki, K., Imai, T., Ozaki, T., Cho, H., Sekiya, H., Takeuchi, M., Tanaka, M., Asayama, M., & Osako, T. (2012).

Preparation and characteristic evaluation of hydrophobic epoxy-based nanocomposites. In Annual Report Conference on Electrical Insulation and Dielectric Phenomena (CEIDP) (pp. 283-286). Montreal: IEEE. http://dx.doi.org/10. 1109/ CEIDP.2012.6378776 19. Nelson, J. K., & Hu, Y. (2005). Nanocomposite dielectrics: properties and implications. Journal of Physics. D, Applied Physics, 38(2), 213-222. http://dx.doi.org/10.1088/00223727/38/2/005.

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Estudo do efeito do ozônio gerado durante ensaios elétricos em equipamentos de segurança confeccionados em Borracha Natural. Polímeros: Ciência e Tecnologia, 22(2), 142-148. http://dx.doi.org/10.1590/S0104-14282012005000015.

Enviado: Fev. 23, 2015 Revisado: Set. 20, 2015 Aceito: Nov. 09, 2015

Polímeros, 26(3), 262-268, 2016

http://dx.doi.org/10.1590/0104-1428.0343

Microencapsulação do pesticida cipermetrina em blendas de P(3HB/PCL): caracterização e ensaio de liberação in vitro Microencapsulation of cypermethrin pesticide in P(3HB)/PCL blends: characterization and in vitro controlled release Evana Cássia Dall’Agnol1, Jaqueline Suave1, Marcia Margarete Meier2, Valdir Soldi3, Denise Abatti Kasper Silva1 e Ana Paula Testa Pezzin1* Programa de Pós-graduação em Engenharia de Processos, Universidade da Região de Joinville – UNIVILLE, Joinville, SC, Brasil 2 Departamento de Química – DQM, Centro de Ciências Tecnológicas – CCT, Universidade do Estado de Santa Catarina – UDESC, Joinville, SC, Brasil 3 Departamento de Química – DQM, Centro de Ciências Físicas e Matemáticas – CFM, Universidade Federal de Santa Catarina – UFSC, Florianópolis, SC, Brasil

1

*anapezzin@yahoo.com.br

Resumo A aplicação de polímeros biodegradáveis para encapsular pesticidas é uma estratégia que permite, a partir de diferentes proporções entre os polímeros, modificar o perfil de liberação do agente. Este trabalho avaliou a liberação controlada do pesticida cipermetrina encapsulado em microesferas de blendas de P(3HB)/PCL (100/0, 0/100, 97/03, 95/05, 90/10, 80/20 e 70/30) obtidas pelo método de emulsificação-evaporação do solvente. As imagens de microscopia eletrônica de varredura revelam a forte influência da PCL na porosidade das microesferas. As análises de infravermelho mostraram a presença do pesticida em todas as composições de polímeros avaliadas. O ensaio de liberação de cipermetrina sugere que, no intervalo de 4 horas, o teor de cipermetrina liberada é dependente da composição das blendas utilizadas.

As microesferas de PHB e blendas com menores teores de PCL, 97/03 e 95/05 liberaram 75% a 85% enquanto a composição 70/30 liberou 100% de cipermetrina.

Palavras-chave: polímeros biodegradáveis, blendas, microencapsulação.

Abstract This work evaluated the controlled release of cypermethrin pesticide loaded in microspheres of P(3HB)/PCL blends obtained by emulsion-evaporation method. SEM analysis revealed a strong influence of PCL on the porosity of the microspheres. The infrared spectra showed the presence of pesticide in all polymer compositon evaluated. DSC curves showed that with higher content of PCL, decreased the crystallinity degree of polymers, resulting in a faster release of the pesticide. The release assay of cypermethrin suggests that within an interval of four hours the amount of pesticide released varies depending on the composition of the blends.

Keywords: biodegradable polymers, blends, microencapsulation.

1. Introdução Os processos de microencapsulação com polímeros biodegradáveis são intensamente estudados para aplicação em pesticidas e liberação in vitro de fármacos[1]. No caso dos agrotóxicos, essa estratégia é utilizada com o intuito de reduzir os efeitos nocivos e seus impactos sobre os organismos não-alvo[2]. O processo pode ser realizado por diferentes técnicas para carreamento de agrotóxicos[3-5] ou outros agentes biológicos[6-8]. A microencapsulação de

Polímeros, 26(3), 269-276, 2016

pesticidas em dispositivos poliméricos apresenta vantagens importantes sobre as técnicas convencionais: i) reduz a exposição dos trabalhadores aos materiais tóxicos, ii) intensifica a ação dos agentes menos ativos alcançando melhor eficácia biológica e iii) reduz a fitotoxicidade e as perdas por evaporação[9] ou por lixiviação através do solo, reduzindo consequentemente a contaminação ambiental[10,11].

Embora existam esforços consideráveis na investigação

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T T T T T T T T T T T T T T T T T T

Dall’Agnol, E. C., Suave, J., Meier, M. M., Soldi, V., Silva, D. A. K., & Pezzin, A. P. T. de vários aspectos de formulações para liberação in vitro de pesticidas, aplicações comerciais permanecem escassas, em função do alto custo de produção em comparação ao uso de granulados no solo[5].

Entre os materiais que podem ser empregados na microencapsulação, encontram-se os polímeros obtidos de fontes naturais como os polihidroxialcanoatos e os poliésteres sintéticos como as policaprolactonas[12,13].

Os polihidroxialcanoatos são poliésteres termoplásticos sintetizados por várias bactérias a partir de fontes renováveis[14]. O polímero mais importante deste grupo é o poli(3-hidroxibutirato) - P(3HB), biodegradável[15] com ponto de fusão entre 173-180 °C e temperatura de transição vítrea entre –5 e 5 °C[16,17].

A poli(ε-caprolactona) - PCL, é um poliéster sintético biodegradável pertencente à família das policaprolactonas.

Apresenta-se como um polímero semicristalino com grau de cristalinidade (αc), na ordem de 50% e temperatura de transição vítrea (Tg) em torno de –70 °C. Em geral, a PCL apresenta a capacidade de formar blendas com vários polímeros em um vasto campo de composições, e tem sido usada como plastificante polimérico[18-20]. Num estudo de biodegradação de filmes homopoliméricos de P(3HB), P(3HB-V) e PCL em solo, Rosa et al. [21] verificaram que P(3HB) degradou mais rapidamente que PCL e P(3HB-V) não apresentou degradação no período avaliado de 90 dias.

Uma forma de diversificar as propriedades dos homopolímeros é por meio de mistura de dois polímeros biodegradáveis, aumentando o leque de aplicações do polímero resultante e permitindo controlar sua taxa de biodegradação[22]. Assim, blendas com diferentes composições mostram cinéticas de degradação variadas baseados no grau de cristalinidade do polímero e no volume da área superficial[15,23]. As blendas de P(3HB)/PCL tem sido estudadas e descrita por diversos autores[13,18,24]. Os resultados têm demonstrado que é possível modular a degradação destas blendas variando a composição de polímeros. Duarte et al.

[18] relataram miscibilidade parcial para as blendas de PHB/PCL na composição (70/30). A estratégia de formação de blendas com polímeros biodegradáveis tem sido aplicada nas composições com polímeros usuais como o poli(cloreto de vinila)[25] e o polietileno de baixa densidade[26].

Na classe dos piretróides clorados, que são amplamente utilizados para o controle de agentes patogênicos, encontra-se a cipermetrina, um inseticida sintético, sendo extremamente eficaz contra uma ampla gama de insetos, incluindo muitos lepidópteros responsáveis por pragas do algodão, frutas e hortaliças, e está disponível como concentrado emulsionável ou pó molhável. O inseticida é extremamente tóxico para peixes e organismos aquáticos[27] apresentando toxicidade por via oral em ratos, DL50 = 70 mg kg–1 e possui elevada solubilidade em solventes orgânicos[28] e reduzida solubilidade em água (9 μg/L)[29]. No entanto, a utilização de pesticidas pode ser benéfica, diminuindo a perda de culturas, tanto antes como depois da colheita[29], mas por outro lado há um desejo social crescente para reduzir a utilização de pesticidas na agricultura e horticultura em função de seus efeitos adversos nos seres humanos[30-32]. Recentemente Xia et al.

[33] demonstraram que a encapsulação de cipermetrina em nanocápsulas de poliestireno atingiu 61% de eficiência 270

pelo método de polimerização da emulsão de estireno em presença de diferentes iniciadores. Em 2011, Bang et al.[34] descreveram a encapsulação de cipermetrina em nanolipossomas com paredes de quitosana. Os autores observaram dependência entre o perfil de liberação da cipermetrina e a carga superficial intrínseca da superfície dos lipossomas sintetizados. Zhu et al.[35] descreveram que a encapsulação da cipermetrina em nanocápsulas de gelatina e acácia atingiu 60% de eficiência. Zeng et al.[36] prepararam nanosuspensão de cipermetrina em ciclohexanona e dimetilformamida atingindo 87% de eficiência de encapsulação. A liberação da cipermetrina foi muito mais rápida neste sistema quando comparado a formulação comercial e a sistemas preparados por microemulsões. Em recente levantamento da literatura, não foram encontrados artigos que descrevessem o uso de polímeros biodegradáveis da família dos poliésteres para encapsulação da cipermetrina.

De acordo com o exposto e visando contribuir com o desenvolvimento das técnicas de microencapsulação de pesticidas, este trabalho teve como objetivo estudar a matriz polimérica e o agente microencapsulado e avaliar a influência de diferentes blendas de P(3HB)/PCL sobre o tempo de liberação in vitro do pesticida cipermetrina.

2. Metodologia 2.1 Materiais Poli(3-hidroxibutirato), P(3HB) (Mw = 342 000 g mol–1) em pó, foi doado pela PHB-Industrial S.A. A poli(ε-caprolactona), PCL (Mn = 80 000 g mol–1) em forma de pellets, foi proveniente da Aldrich. A cipermetrina técnica com pureza de 93% foi gentilmente fornecida por Elize Chemical Ltda. Também foram utilizados neste trabalho gelatina em pó aparência de pó bege, tamanho médio de partícula 600 µm, viscosidade 2,5-3,5 cp, pH 5-6, (Vetec), tensoativo polissorbato 80 (Tween 80, Vetec), clorofórmio (Synth) e álcool comercial (96 °GL, Vetec).

2.2 Síntese das microesferas As microesferas de P(3HB)/PCL foram preparadas pelo método de emulsificação-evaporação do solvente, conforme descrito anteriormente[37]. Os experimentos foram feitos com diferentes composições poliméricas P(3HB)/PCL: 100/0, 97/03, 95/05, 90/10, 80/20, 70/30, e 0/100 (m/m).

Preparou-se soluções dissolvendo 0,350 g dos polímeros, nas proporções de P(3HB) e PCL para atingir as composições desejadas, em 5 mL de clorofórmio.

Primeiramente, dissolveu‑se P(3HB) em clorofórmio a 60 °C. Após resfriamento da solução, PCL e cipermetrina foram adicionados à solução anterior de modo que as soluções sempre contivessem 70% e 30% em massa de polímero e cipermetrina, respectivamente. A solução contendo os polímeros e o inseticida foi emulsificada em solução aquosa previamente preparada (200 mL) com gelatina em pó (1% m/v) e polissorbato 80 (0,05% m/v).

A emulsão formada permaneceu sob agitação constante (700 min–1) por 24 h a 25 °C em agitador magnético, permitindo a evaporação do clorofórmio e a solidificação das microesferas poliméricas. A suspensão formada foi Polímeros, 26(3), 269-276, 2016

Microencapsulação do pesticida cipermetrina em blendas de P(3HB/PCL): caracterização e ensaio de liberação in vitro filtrada sob vácuo utilizando-se papel filtro qualitativo.

Em seguida, as microesferas retidas no papel filtro foram lavadas (uma vez) com cerca de 200 mL de água destilada e após secagem à temperatura ambiente, foram armazenadas em dessecador para realização das análises de caracterização.

Tendo em vista que a quantidade de microesferas obtidas pelo método de emulsificação‑evaporação do solvente empregado foi inferior a 0,5 g, foram utilizadas microesferas provenientes de três lotes para a realização das análises.

2.3 Caracterização das microesferas 2.3.1 Microscopia eletrônica de varredura (MEV) A morfologia das microesferas com e sem cipermetrina, foi avaliada utilizando-se um microscópio eletrônico de varredura Zeiss DSM 940A, empregando tensão de 5 kV. As microesferas foram fixadas em suporte metálico e recobertas com uma fina camada de ouro num metalizador de amostras Sputer Coater BALTEC SCD 050.

2.3.2 Espectroscopia de infravermelho com transformada de Fourier (FT-IR) Filmes de P(3HB) e PCL foram preparados por evaporação de solvente, com e sem cipermetrina, usando clorofórmio como solvente. As mesmas composições usadas para as microesferas estudadas foram feitas com os filmes. As amostras foram inseridas no espectrofotômetro e submetidas a 12 varreduras na região de 4000 a 400 cm-1, utilizando uma resolução de 4 cm–1 em um Espectrofotômetro Perkin-Elmer One B. 2.3.3 Calorimetria exploratória diferencial (DSC) As medidas foram conduzidas em um MDSC da TA Instruments para determinação das temperaturas de transição vítrea (Tg), de cristalização (Tc), de fusão (Tm), entalpia de fusão (ΔHm) e grau de cristalinidade.

As amostras foram seladas em panelas de alumínio, aquecidas de 25 até 190 °C a 10 °C min–1 e mantidas nessa temperatura por 2 min. Em seguida, foram resfriadas a 15 °C min–1 de 190 a –100 °C e mantidas nessa temperatura por 2 min. Um segundo aquecimento foi realizado de –100 a 190 °C a 10 °C min–1. O fluxo de argônio foi mantido em 50 mL min–1.

2.3.4 Liberação in vitro da cipermetrina Os ensaios de liberação in vitro da cipermetrina foram realizados em duplicata utilizando-se as microesferas geradas a partir dos três lotes. Precisamente 120 mg de microesferas de cada composição de blenda com cipermetrina foram pesadas e transferidas para erlenmeyers, contendo 100 mL de uma solução previamente preparada, na proporção 60:40 (v/v) água destilada:etanol, devidamente vedados. A mistura permaneceu sob agitação constante a 100 min-1 (rotatória) em Shaker B. Braun Certomat® S, a temperatura ambiente, em torno de 25 (± 2) °C. Em intervalos de tempo previamente determinados (1, 2, 3, 4, 26 e 50,5 h), as microesferas foram separadas por filtração e novamente dispersas em 100 mL da solução água:etanol. Dos meios de liberação filtrados, retirou-se alíquotas de cerca de 10 mL para posterior análise por espectrofotometria no ultravioleta. A concentração de cipermetrina liberada (36 mg/L considerando-se 100%) foi Polímeros, 26(3), 269-276, 2016

determinada analisando-se as alíquotas dos filtrados em um espectrofotômetro UV-Vis Hach DR/4000U. Mediu-se a absorbância da solução a 276 nm (ε = 5,26 L g–1 cm–1).

As condições experimentais adotadas consideraram as condições sink e consideram 100% de encapsulação, visto que a cipermetrina é insolúvel no ambiente aquoso, presente no processo de emulsificação (método de obtenção).

O percentual (em massa) de cipermetrina encapsulada nas microesferas foi calculado considerando que o teor de cipermetrina que foi dissolvida no meio aquoso durante a síntese foi insignificante, devido a sua reduzida solubilidade em água. Portanto, toda a cipermetrina permaneceu na fase oleosa que, após secagem das microcápsulas, é a fase polimérica. No cálculo considerou-se o rendimento da síntese de microcápsulas contendo cipermetrina, conforme descrito na Tabela 1, permitindo calcular o teor de cipermetrina encapsulada. Conhecendo o teor de cipermetrina encapsulada foi possível determinar o percentual de cipermetrina liberada de cada grupo experimental.

3. Resultados e Discussão 3.1 Caracterização das microesferas As micrografias das microesferas sem o pesticida (Figura 1A) revelaram que as microesferas de P(3HB) puro apresentaram ausência de poros na superfície, enquanto as microesferas de PCL puro apresentaram superfície com prolongamentos lisos, descontínuos e presença de poros. As microesferas formadas pela blenda P(3HB)/PCL sem cipermetrina nas composições 90/10, 80/20 e 70/30 apresentaram superfícies com aspecto de filamentos entrelaçados, especialmente visíveis na blenda 80/20.

As blendas P(3HB)/PCL nas composições 97/03 e 95/05 geraram microesferas com morfologias homogêneas, porém rugosas, similares à das microesferas de P(3HB) puro. Para as microesferas contendo cipermetrina microencapsulada (Figura 1B), a morfologia das microesferas de P(3HB) revelou uma superfície homogênea e ausência de poros, enquanto as de PCL apresentaram duas regiões distintas, uma compacta e outra com filamentos entrelaçados formando regiões de grande porosidade. As blendas de P(3HB)/PCL contendo cipermetrina formadas pelas composições 97/03, 95/05, 90/10, 80/20 e 70/30 possuem a característica de aumento da porosidade superficial proporcional ao teor de PCL na blenda. A ocorrência dessa morfologia, entrelaçada, Tabela 1. Rendimento e teor de cipermetrina encapsulada nas microesferas.

Composição Blenda

Rendimento %

P(3HB)/PCL 0/100 100/0 70/30 80/20 90/10/ 95/05 97/03

80,5 83,8 85,1 76,4 78,8 80,9 84,3

Teor de cipermetrina encapsulada (mg)* 37,2 35,8 35,5 39,4 38,0 37,1 37,9

* em 120 mg de microcápsulas.

271

Dall’Agnol, E. C., Suave, J., Meier, M. M., Soldi, V., Silva, D. A. K., & Pezzin, A. P. T. pode ser associada à separação de fases, fato relacionado à imiscibilidade do P(3HB) e da PCL nesta faixa de composição. Estudos relataram a imiscibilidade de blendas de P(3HB) e PCL [13.22].

Com o objetivo de avaliar a presença de cipermetrina e se houve a ocorrência de interação química entre o pesticida e os polímeros P(3HB), PCL foram preparados filmes das blendas na ausência dos demais componentes utilizados no processo de emulsificação, possibilitando maior clareza dos espectros de FTIR e identificação das principais bandas associadas à cipermetrina. A presença do pesticida e os polímeros puros de P(3HB) e PCL pode ser observada nos espectros de infravermelho mostrados nas Figuras 2A, B, respectivamente. Em ambos os espectros, pode-se verificar a presença de bandas características da estrutura do pesticida cipermetrina em 3065 cm–1 (a’ e e’)

representando a deformação axial da ligação C-H em anel aromático, 2975 – 2959 – 2933 cm-1 evidenciando as ligações C – H da região alifática, 1586-1486 cm–1 (b’ e f’) referentes a deformação axial da ligação C = C no anel aromático, 781-753 cm–1 (c’) correspondente a ligação C - Cl, 691 cm–1 (d’e g’) deformação angular fora do plano de C-H do anel, referindo-se aos anéis aromáticos presentes na estrutura da cipermetrina[38]. Estas observações confirmam a presença do pesticida nos filmes.

As curvas de DSC para os polímeros puros e as blendas de P(3HB)/PCL foram obtidas a partir do segundo aquecimento, possibilitando uma análise mais apurada entre as amostras, pois a história térmica delas foi apagada no primeiro aquecimento. As curvas apresentam picos de transição vítrea (Tg), cristalização (Tc) e fusão (Tm) como pode-se observar na Figura 3.

Figura 1. Micrografias de microscopia eletrônica de varredura das micropartículas de P(3HB); PCL; 90/10; 80/20; 70/30; (A) sem pesticida e; (B) com pesticida.

Figura 2. Espectros de FT-IR (A) do filme de PHB (a) sem pesticida; (b) com cipermetrina e; (c) cipermetrina e (B) do filme de PCL (a) sem pesticida; (b) com cipermetrina e; (c) cipermetrina.

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Polímeros, 26(3), 269-276, 2016

Microencapsulação do pesticida cipermetrina em blendas de P(3HB/PCL): caracterização e ensaio de liberação in vitro As Tg’s, Tc’s, Tm’s e entalpias de fusão (ΔHm) das blendas P(3HB):PCL foram determinadas a partir das curvas de DSC e os valores estão sumarizados na Tabela 2. Os valores de ΔHm foram calculados a partir da área do pico de fusão e normalizados com relação à porcentagem de cada polímero na blenda analisada. Observa-se a Tg do P(3HB) puro em –5 °C, a Tc em 42 °C e a fusão dos cristais em 155 °C, concordando com valores encontrados na literatura[17,18,39].

Com a adição do pesticida houve somente um deslocamento da Tc de 42 para 54 °C, enquanto a Tg e Tm exibiram comportamento praticamente constante. Na curva da PCL, a Tg observada foi em torno de –66 °C, a Tm em torno de 55 °C e a Tc não foi observada. Valores encontrados na literatura confirmam estes resultados[22, 40]. Com a adição da cipermetrina, ocorreu um aumento na Tg de –66 para –43 °C, dificultando a movimentação das cadeias de PCL, enquanto a Tm permaneceu sem alteração significativa. Analisando a blenda com composição 97:03, sem pesticida, observou-se a Tg em cerca de –5 °C, Tc em 42 °C e Tm a 158 °C para o P(3HB), enquanto que para a PCL verificou-se Tm próxima de 54 °C, sendo as demais temperaturas não detectadas pela análise, devido a ínfima quantidade deste polímero na composição. Com a incorporação da cipermetrina, os valores observados referentes ao P(3HB), para Tg, Tc e Tm foram –2, 52 e 159 °C respectivamente, e para a PCL não foi possível identificar alterações na Tm devido a ocorrência desta fusão próxima a Tc do P(3HB) presente na blenda.

Nota-se semelhança nos dados da blenda 97/03 pura quando

Figura 3. Curvas de fluxo de calor, obtidas por DSC, para microesferas sem e com cipermetrina, respectivamente: (a e a’) para o P(3HB); (b e b’) para a blenda 97/03; (c e c’) para 90/10; (d e d’) para 70/30; (e e e’) para PCL, todas no segundo aquecimento.

comparados com os valores das microesferas de P(3HB), também puras. Este evento pode ser explicado pela maior percentagem do polímero na blenda. Na composição 90/10, sem cipermetrina, os valores de Tg, Tc e Tm verificados para o P(3HB) da blenda, foram de aproximadamente –4, 42 e 157 °C, respectivamente. Para a PCL da blenda, observou-se a Tm em 55 °C enquanto as demais temperaturas não foram identificadas. Com a adição do pesticida na blenda, a Tg do P(3HB) permaneceu praticamente na mesma temperatura, enquanto houve deslocamento das demais temperaturas referentes ao P(3HB), a Tc aumentou de 42 para 54 °C e a Tm diminuiu de 157 para 143 °C, enquanto que para a PCL a Tg ocorreu em –45 °C e a Tm diminuiu de 55 °C para 40 °C. A blenda 70/30, sem incorporação do pesticida, apresentou Tg em –3 °C, Tc próxima de 43 °C e Tm em 159 °C, referentes ao P(3HB). Para a PCL, identificou-se somente a Tm próxima de 55 °C. Após adição da cipermetrina a blenda analisada demonstrou alteração relacionada ao P(3HB), como a elevação da Tc de 43 para 55 °C, enquanto que para a PCL foi possível observar a Tg em –43 °C, a Tc em –12 °C e decréscimo na Tm de 55 para 43 °C. A partir dos dados de entalpia de fusão (ΔH0m) para o polímero 100% cristalino, ΔH0m = 142 J g–1 para o P(3HB)[41] e ΔH0m = 146 J g–1 para o PCL[14] o grau de cristalinidade (αc) dos componentes na blenda foi calculado (αc %), através da razão de ΔHm pelo ΔH0m. A cristalinidade de um polímero pode ser considerada como um “arranjo ordenado”, uma repetição regular de estruturas atômicas moleculares. Os valores obtidos no cálculo da cristalinidade dos homopolímeros e das blendas estão expostos na Tabela 2. Para os polímeros puros foi obtido grau de cristalinidade de 56% para o P(3HB) e 42% para a PCL, valores de cristalinidade próximos ao encontrado na literatura[22].

Com a incorporação do pesticida observa-se um decréscimo na entalpia de fusão do PHB e da PCL, resultando na redução da cristalinidade destes polímeros para 42 e 37%, respectivamente, o que sugere uma distribuição homogênea nas matrizes. Nas blendas de P(3HB)/PCL, também houve redução da ΔHm dos polímeros após a adição do pesticida.

Pela análise realizada percebe-se que quanto maior o teor de PCL na blenda, menor foi o grau de cristalinidade observado, o que significa uma menor organização dos cristais da blenda, influenciando diretamente na liberação do pesticida.

Tabela 2. Valores de temperatura de transição vítrea (Tg), temperatura de cristalização (Tc), temperatura de fusão (Tm), entalpia de fusão (ΔHm), grau de cristalinidade (αc), obtidos por DSC (2o aquecimento) dos homopolímeros e blendas de P(3HB)/PCL. Composição (PHB/ PCL)

PHB

Tg (°C) PCL

PHB

Tc (°C) PCL

PHB

Tm (°C) PCL

PHB

PCL

PHB

PCL

100/0 100/0 + Cip 0/100 0/100 + Cip 97/03 97/03 + Cip 90/10 90/10 + Cip 70/30 70/30 + Cip

–5 –4 –5 –2 –4 –2 –3 -

–66 –43 –45 –43

42 54 42 52 42 54 43 55

-

155 153 158 159 157 143 159 160

55 53 54 55 40 55 43

79 59 83,42* 54,32* 68,4* 54* 39,2* 26,6*

61 54 0,03* 0,6* 0,1* 5,7* 3*

56 42 59 38 48 38 28 19

42 37 0,02 0,4 0,07 4 2

–12

ΔHm (J/g)

αc (%)

- Valores não encontrados nas curvas de DSC analisadas. * Valores já relacionados com a percentagem do polímero na blenda

Polímeros, 26(3), 269-276, 2016

273

Dall’Agnol, E. C., Suave, J., Meier, M. M., Soldi, V., Silva, D. A. K., & Pezzin, A. P. T. 3.2 Ensaio de liberação in vitro Com a realização do ensaio de liberação in vitro do pesticida cipermetrina, obteve-se o perfil de liberação deste para o meio, apresentado na Figura 4.

Os resultados permitem observar que as microcápsulas compostas por PCL e pela blenda P(3HB)/PCL 70/30 liberaram praticamente toda a cipermetria após 2 horas do início do ensaio de liberação. As cadeias poliméricas destes dois grupos apresentaram menor grau de cristalinidade em relação aos demais grupos, conforme demonstrado nas análises de DSC. As cadeias poliméricas amorfas estão mais desorganizadas em relação às cadeias com maior grau de cristalinidade, permitindo a liberação de teores elevados de cipermetrina em curto espaço de tempo. Por outro lado, polímeros com maior grau de cristalinidade, devido a maior organização das cadeias poliméricas, apresentam teores de liberação inferiores neste mesmo intervalo de tempo ou necessitaram de mais tempo para que atingissem teores de liberação próximos a totalidade. Vale destacar que o processo de liberação envolve inicialmente a etapa de difusão de água entre as cadeias poliméricas para que, na sequência, a cipermetrina possa difundir e ser liberada.

Devido a maior flexibilidade das cadeias amorfas, a etapa de difusão de água é facilitada em relação aos polímeros com maior grau de cristalinidade.

Como o percentual de cipermetrina liberada foi calculado a partir de valores teóricos de cipermetrina encapsulada em cada grupo experimental, observou-se percentuais acima de 100% para o grupo P(3HB)/PCL 70/30, indicando alguns desvios.

As curvas de liberação de cipermetrina seguem comportamento exponencial, conforme Equação 1, cujos valores dos parâmetros Yo, A, t e R2 (coeficiente de correlação) são apresentados na Tabela 3. −x

= y A

w

+ y f (1)

onde y = percentual de cipermetrina liberado no tempo t A = constante -x/w = fator exponencial yf = percentual de cipermetrina liberado no tempo final Analisando os resultados da Tabela 3 não foi possível observar uma correlação entre a composição das blendas e os parâmetros da equação exponencial. Isso se deve ao fato de diferentes fatores estarem associados à liberação de cipermetrina, como: cristalinidade da fase polimérica; separação de fase dos componentes de algumas composições, causando poros na superfície das microesferas e efeito de difusão.

Embora, um estudo detalhado da cinética de liberação não tenha sido realizado, é possível observar que até 5 h a liberação pode ser modulada, sendo que mais cipermetrina foi liberada para teores elevados de PCL presente, concordando com a maior porosidade superficial observada nas microesferas e com o grau de cristalinidade menor, conforme descrito anteriormente.

274

Figura 4. Perfil de liberação in vitro da cipermetrina a partir das microesferas de P(3HB)/PCL nas composições 100/0, 0/100, 97/03, 95/05, 90/10, 80/20 e 70/30.

Tabela 3. Parâmetros associados à equação exponencial que descreve a liberação controlada de cipermetrina nas diferentes composições poliméricas.

Composição Blenda P(3HB)/ PCL 0/100 70/30 80/20 90/10 95/05 97/03 100/0

yo

A

102,95 106,47 95,841 100,16 95,857 98,723 102,10

–78,92 –37,32 –41,76 –65,07 –57,25 –66,99 –67,91

t

0,6756 0,965 2,468 3,428 1,974 3,148 4,555

R2

0,9814 0,9887 0,9869 0,9952 0,9938 0,9971 0,9931

4. Conclusões As análises de MEV revelaram forte influência da PCL sobre a porosidade das blendas. A análise dos espectros de infravermelho revelou a presença da cipermetrina nos filmes, tanto puros quanto nas blendas. Nos DSC observou-se uma interação maior entre o pesticida cipermetrina com o polímero PCL do que com o P(3HB). Quanto maior o percentual de PCL nas blendas, menor o grau de cristalinidade das microesferas. O estudo preliminar da liberação in vitro da cipermetrina sugere que dentro de um intervalo de 4 horas a quantidade de pesticida liberado varia em função da composição das blendas sendo menor na presença de maior conteúdo de P(3HB) na matriz.

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