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Research article Received: 4 June 2012,

Accepted: 6 August 2012,

Published online in Wiley Online Library: 7 September 2012

(wileyonlinelibrary.com) DOI: 10.1002/pat.3073

OCVD polymerization of PEDOT: effect of pre-treatment steps on PEDOT-coated conductive fibers and a morphological study of PEDOT distribution on textile yarns Tariq Bashira,b, Majid Alic, Sung-Woo Choa, Nils-Krister Perssonc and Mikael Skrifvarsa* The functionalization of textile fibers with intrinsically conductive polymers has become a prominent research area throughout the world. A number of coating techniques have already been utilized and optimized to get the uniform layers of conductive polymers on the surface of different substrates. In our previous study, we produced poly(3,4-ethylenedioxythiophene) (PEDOT)-coated conductive fibers by employing oxidative chemical vapor deposition (oCVD) technique. This paper describes the effects of pre-treatment steps, such as surface treatment of textile fibers with organic solvents, drying of oxidant-enriched fibers at variable temperatures and time, and oxidant type on the electrical, mechanical, and thermal properties of PEDOT-coated conductive fibers. Two well-known oxidants, ferric(III)chloride and ferric(III)p-toluenesulfonate (FepTS), were studied, and then their results were compared. In order to verify the PEDOT-coated layer and, to some extent, its impregnation inside the viscose yarns, a morphological study was carried out by using the attenuated total reflectance Fourier transform infrared spectroscopic imaging technique and computed tomography scanning across the obtained conductive fibers. Differential scanning calorimetric and thermogravimetric analysis were utilized to investigate the thermal properties and the contents of PEDOT in PEDOT-coated fibers. The mechanical properties of conductive fibers were evaluated by tensile strength testing of produced fibers. Effects of all of these pre-treatment steps on electrical properties were analyzed with Kiethly picoammeter. This study cannot only be exploited to improve the properties of conductive fibers but also to optimize the oCVD process for the production of conductive textile fibers by coating with different conjugated polymers. Copyright © 2012 John Wiley & Sons, Ltd. Keywords: conductive fibers; oCVD; pre-treatment steps; PEDOT coating; surface morphology

INTRODUCTION

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Conjugated polymers with heterocyclic structures exhibit good electrical conductivity and are being widely used in many sophisticated electronic applications, such as in capacitors, sensors, display windows, and in batteries.[1] In pure form, intrinsically conductive polymers (ICPs) have very rare applications. In order to extend the application areas, these conductive polymers must be applied on the surface of different substrates, such as fabrics, glass, PET, and silicon wafers or must be mixed with other inherently insulating materials, such as polystyrene and polyethylene, etc. The transformation of commercially available textile fibers into conductive fibers by coating with metallic powder, carbon black, and conjugated polymers is a trendy research area throughout the world. These electrically conductive fibers are a key component in future smart and interactive textiles and being widely used as power and signal transmitter in many prospective applications such as strain sensors,[2] ECG measurements,[3] sports and military garments, motions capture devices,[4] and electrotherapy treatment.[5] For all of the above mentioned applications, it is important that the conductive fibers must be strong, flexible, environmentally stable, fast responsive, resistant to chemicals, and have large workable range. A number of techniques have already been investigated to produce the

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conductive fibers, such as melt spinning, wet spinning, and coating of non-conductive fibers with inherently conductive materials, but the conductivity level and mechanical properties of the fibers with melt and wet spinning will be low, and these techniques need some sophisticated arrangements to remove the solvents used.[6,7] On the other hand, textile fibers coated with conjugated polymers are assumed to have good electrical as well as mechanical properties, because textile fibers inherently have good mechanical properties. Poly (3,4-ethylenedioxythiophene) (PEDOT) is one of the most prominent conjugated polymers because of its environmental

* Correspondence to: Professor Mikael Skrifvars, School of Engineering, University of Borås, SE-50190 Borås Sweden. E-mail: mikael.skrifvars@hb.se a T. Bashir, S.-W. Cho, M. Skrifvars School of Engineering, University of Borås, SE-50190, Sweden b T. Bashir Department of Polymer and Process Engineering, University of Engineering and Technology Lahore, Pakistan c M. Ali, N.-K. Persson The Swedish School of Textiles, University of Borås, SE-50190, Sweden

Copyright © 2012 John Wiley & Sons, Ltd.


OPTIMIZATION OF CVD PROCESS AND SURFACE MORPHOLOGY stability, good electrical properties, transparency in thin oxidized films, and its use in different primitive electronic applications such as, in light emitting diodes,[8] heat generation,[9] EMI shielding,[10] and chemical sensors.[11] Processing of ICPs has always been a challenge because of their stiff aromatic structure,[12] which makes them insoluble and infusible in most of the organic and inorganic solvents.[7] Because of the less solubility of conjugated polymers, it is rather difficult to get highly conductive and uniform layer of conjugated polymers on different substrates. In order to overcome this difficulty, a very interesting approach, called oxidative chemical vapor deposition (oCVD), has been introduced.[1,13,14] Winther-Janssen et al.[15] discussed a base inhibited vapor phase polymerization process to enhance the conductivity of PEDOT layers by reducing the side reactions during the polymerization of EDOT monomer. Xia et al.[16] and Hong et al.[17] reported that different conjugated polymers can be applied on nylon and silk fabrics by using a specific ratio of monomer, oxidant, and doping agent in the polymerization solution. The obtained surface conductivity was, however, relatively low, 0.75 S/cm for PEDOT/nylon-6 and 0.42 S/cm for PEDOT/SF composites. Furthermore, the oCVD technique has widely been used and optimized to produce conductive fabrics by coating a variety of textile substrates with different conjugated polymers.[18–23,13] This paper is the continuation of our previous work in which, we described the production method of conductive textile fibers.[21] We studied the CVD technique with viscose yarn fiber and EDOT monomer in vapor form. The effects of different reaction conditions on electrical as well as mechanical properties were investigated. As the CVD process involves a number of steps, so the overall process time was too long. In this paper, we emphasized on the optimization of oCVD process for the production of conductive fibers by controlling a number of pre-treatment steps, such as solvent treatment of textile fibers, oxidant types, oxidant concentration, and drying of oxidanttreated fibers at variable temperatures and time and their impact on the electrical, mechanical, and properties of PEDOTcoated conductive fibers. In this study, we also tried to investigate the three-dimensional distribution and impregnation of PEDOT polymer coating along the textile fibers by using ATR-IR-imaging spectroscopy and computed tomography (CT) analysis. The produced PEDOT-coated conductive fibers were then characterized by optical microscopy, differential scanning calorimeter (DSC), and thermogravimetric analysis (TGA). Mechanical and electrical properties of obtained fibers were evaluated by tensile testing machine and Kiethly picoammeter.

OCVD polymerization of PEDOT The detailed oCVD polymerization of PEDOT on the surface of viscose fibers has already been described in our previous studies, and the schematic diagram of our used setup is shown in Fig. 1.[21,24] In order to find out the effects of surface treatment agents, the viscose fibers (cut into 5 m length) were first pretreated individually with acetone and ethyl acetate. The solvent-treated viscose fibers were dried at room temperature and then soaked with oxidant (FeCl3) solution. The other reaction conditions, such as, oxidant concentration (15 wt. %), dipping time of viscose fibers in oxidant solution (10 min), drying time of oxidant impregnated viscose fibers at room temperature (30 min), and polymerization time (15 min) were taken from our previous study. At these conditions, we found that better electrical properties of PEDOT-coated fibers can be achieved. All the reaction steps, after the oxidant enrichment, were performed as we did in our previous article.[21] For evaluating the effects of drying time and drying temperature, the viscose fibers were first soaked with oxidant (FeCl3) solution and then dried at different temperature (30, 40, 50, and 60 0C) for variable times (3, 5, 7, and 10 min). The oxidant concentration, dipping time, and polymerization time were kept constant followed by the other polymerization process as discussed before. The effect of oxidant types was evaluated by selecting two oxidants, FeCl3 and FepTS. The viscose fibers were pretreated individually with 15 wt. % solution of FeCl3 and 40 wt. % solution of FepTS for 10 min. The drying time and the polymerization time were kept constant. After all pre-treatment steps, the oxidant-enriched viscose fibers were then introduced into the tubular reactor where the polymerization of PEDOT was carried out on the surface of substrate fibers. Attenuated total reflectance Fourier transform infrared spectroscopic imaging In order to verify the PEDOT-coated layer and, to some extent, impregnated fibrils of the viscose yarns, the attenuated total reflectance (ATR) Fourier transform infrared (FT-IR) imaging was performed using a Perkin-Elmer Spotlight 400 FT-IR coupled with 400 N FT-NIR imaging systems. The samples were mounted on the X-Y stage of the FT-IR microscope and pressed against the ATR-crystal via a force lever. The spectra were taken as an average of two to eight scans between 4000 cm1 and 720 cm1

EXPERIMENTAL Materials

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Figure 1. Schematic diagram of tubular reactor used for oCVD of PEDOT.

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In this work, we used viscose yarn fiber (1220 dtex, 720 No. of filaments, Z100 twist/meter) purchased from CORDENKAW as substrates. For polymerization of PEDOT, we used EDOT (CLEVIOUSW M V2) as monomer, Ferric(III)Chloride (FeCl3) (Sigma-Aldrich, 98%), and Ferric(III)tosylate 40% (FepTS, Clevios C-B 40 V2) as oxidants, and 1-butanol (C4H9OH) (Fisher Scientific) for solution formation. Two organic solvents, ethyl acetate (CH3COOCH2CH3) (Fisher Scientific) and acetone (CH3COCH3) (Fisher Scientific) were selected for surface treatment of viscose fibers. All of these materials were used without of any further modification.


T. BASHIR ET AL. with a resolution of 6 cm1. The FT-IR imaging data were obtained by a linear array detector on 1.56  1.56 mm2 pixel size. Other spectral data and image processing were carried out using the Perkin-Elmer IMAGE software. CT scanning To determine the three-dimensional distribution of PEDOT coating along the viscose fibers, the CT scanning was performed on prepared conductive samples. For this purpose, the highly precised granite-based phoenix nanotomW CT system (GE Sensing & Inspection Technologies, Wunstorf, Germany) equipped with a 180 kV / 15 W high-power nanofocus tube with tungsten target, was used. The tube offers the detectability range from 200 to 300 nm (0.2–0.3 microns) depending on the sample size. During the CT scanning, all the parameters which were used for nanotom CT system are shown in Table 1. For fast and more accurate acquisition of results, the datos|x, phoenix|x-rays proprietary software package, was used.

The thermal properties were investigated by differential scanning calorimetric (DSC) analysis. The PEDOT-coated samples were characterized with a TA instrument Q2000 DSC. The calorimeter cell was flushed by 100 ml min1 of nitrogen. About 3–6 mg of fibers was used in each test using an aluminum crucible. The temperature program was sent in the range from 25 to 400 0C, at a heating rate of 10  C/min. TGA The amount of PEDOT coatings and thermal stabilities of PEDOTcoated conductive fibers were evaluated by TGA. TGA analysis was performed by utilizing TA Instrument Q500 TGA apparatus at a heating rate of 10  C/min from 25 to 600  C under the nitrogen gas. Tensile testing The tensile properties of conductive fibers were investigated by using a Tinius Olsen 10 kN universal testing machine under a crosshead speed of 20 mm/min. We calculated the maximum force at break across the coated fibers. Tensile testing was performed on at least five samples for each type of samples, and then the average values were used. Table 1. Parameters used in nanotom system for CT scanning

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Magnification Voxel size FOD FDD No. of images Image width Image height Scanning time Acceleration voltage e-beam current Tube mode

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For conductivity measurements, the following equation was used: p ¼ R x pr 2 =L Where, r is resistivity (Ω.cm), R (Ω) is surface resistance, r (cm) radius of fiber, and L (cm) is length of fiber. Conductivity is the reciprocal of resistivity. Surface resistance of PEDOT-coated viscose fibers was measured using Keithly 6000 picoammeter. Resistance was measured along (150 mm) long fibers holding between two crocodile clips, at 10 V, 2.5 mA current and at ambient conditions. One fiber was tested several times, and then the mean value was used. Optical microscopy The optical microscopic images of PEDOT-coated viscose fibers were acquired by using Nikon SMZ800 optical microscope.

RESULTS AND DISCUSSION oCVD polymerization of PEDOT on the surface of viscose fiber

Differential scanning calorimetric analysis

Parameters

Conductivity measurements

Sample: PEDOT/viscose 83.4 0.6 2.4 199.9 1440 2294 pixel 1800 pixel 750 ms 70 kV 385 mA 3

In our previous experiments, we investigated that cellulosic fibers (viscose) can successfully be transformed into electro-active fibers by applying PEDOT polymer layers on the fiber surface through oCVD process. The effect of various reaction conditions, such as oxidant (FeCl3) concentration, dipping time of viscose fibers in oxidant solution, drying time of oxidant-treated viscose fibers, and polymerization time, on electro-mechanical properties of PEDOT-coated viscose fibers was carefully examined. We found the best reaction conditions, i.e. oxidant concentration (15 wt. %), soaking time of viscose fibers in oxidant solution (10 min), drying time of oxidant-treated viscose fibers at room temperature (30 min), and polymerization time (15 min),[21] at which we can get maximum electrical conductivity of PEDOTcoated fibers. The oCVD of PEDOT involved three steps: (i) fiber impregnation with oxidant (FeCl3) solution and subsequent drying, (ii) exposition to EDOT monomer vapors, and (iii) doping of PEDOT-coated fibers. When oxidant (FeCl3)-enriched viscose fibers were exposed in EDOT monomer vapor containing environment, polymerization reaction was stared spontaneously, and fibers were coated with darkish blue layer of PEDOT. The mechanism of PEDOT formation on the surface of viscose fibers is shown in (Fig. 2, A ! B).[20] The oxidant (FeCl3) on the surface of viscose fibers oxidized EDOT monomers to form PEDOT polymer chains. In order to increase the conductivity of PEDOT, the PEDOT-coated fibers were then again treated with FeCl3 solution, which was acting as dopant now, shown in (Fig. 2, B ! C). At the end, the doped fibers were washed with methanol in order to remove the un-reacted monomers, oxidant and some other by products produced during the polymerization process. The schematic diagram of PEDOT-coated viscose fibers is shown in (Fig. 2, D). It is assumed that after polymerization, long polymer (PEDOT) chains are formed and are physically entangled and wrapped with each other around the fiber. It might be possible that some other functional groups like, (Fe+, Fe+Cl-4, Na+S-), are also involved in polymer backbone and can take part to establish intermolecular bonding between polymer chains as well. As it has already been studied that a very uniform layer of PEDOT with highest conductivity can be obtained by CVD process, it is very difficult to make this process continuous. Only small piece of substrates can be coated in one batch. Thus, the overall process time for one batch should be minimized, and it

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OPTIMIZATION OF CVD PROCESS AND SURFACE MORPHOLOGY

Figure 2. oCVD polymerization mechanism of PEDOT on the surface of viscose fibers (A ! B) Oxidation of EDOT monomer to PEDOT, (B ! C) Doping of PEDOT, (D) Sketch of PEDOT-coated viscose fiber. This figure is available in colour online at wileyonlinelibrary.com/journal/pat

can be done by optimizing the different pre and post treatment steps involve in oCVD polymerization of PEDOT. Also, the commercially available textile fibers usually contain the wax-like materials which act as protective agent against the biological and atmospheric influences. These lubricating substances are necessary to speed up the processing of synthetic fibers on different weaving and knitting machines. These coatings can also affect the polymerization of PEDOT. In this paper, we only focused on pretreatment steps to minimize the overall process time required for the production of one batch of coated fibers and also on the engineering properties of produced conductive fibers. In one batch of oCVD process, now we can coat up to 5 m of viscose fibers, which was only 15 cm in our previous study. The pre-treatment steps including the surface treatment of viscose fibers with different solvents, two different types of oxidants, drying time, and drying temperature of oxidant-enriched fibers were evaluated carefully. Also, the impact of these parameters on electrical, mechanical, and thermal properties was investigated.

show minimum electrical resistance value (6 kΩ) as compared to the PEDOT-coated viscose fibers obtained after surface treatment with acetone (13 kΩ) and ethyl acetate (74 kΩ), ref. Fig. 3. The conductive fibers obtained after surface treatment exhibit lower conductivity values which might be because of the removal of wax-like protective coatings. As it has been explained above that, the removal of protective coatings can change the absorbency characteristics of textile fibers. Hence, more quantity of oxidant (FeCl3) solution must be absorbed by the solvent-treated fibers. It has been reported earlier that the amount or concentration of oxidant must be at optimum value during the polymerization of PEDOT. It should not be lower enough which cannot initiate the polymerization reaction and should not be higher enough which can over oxidize PEDOT polymer chains.[13,25] It can be assumed that in solvent-treated viscose fibers, excess amount of absorbed oxidant caused over oxidation of PEDOT which yields the conjugated structure of PEDOT polymer chains,

Influence of pre-treatment steps on the properties of PEDOTcoated viscose fibers Surface treatment of viscose fibers with different solvents

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Figure 3. Effect of surface treatment on the electrical properties of PEDOT-coated viscose fibers. This figure is available in colour online at wileyonlinelibrary.com/journal/pat

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The surface treatment of viscose fibers was done prior to the oxidant enrichment and polymerization steps, by two well-known solvents, acetone and ethyl acetate, in order to remove the wax-like protective coatings. In our previous study, we used the viscose fibers without of surface modification. It is obvious that after removal of these protective coatings, the frictional as well as the absorbency characteristics of viscose fibers must also be changed. The effect of solvent treatment on the electrical properties of PEDOT-coated viscose fibers is shown in Fig. 3. The PEDOT-coated viscose fibers produced without surface treatment


T. BASHIR ET AL. and hence, the electrical conductivity values of PEDOT-coated viscose fibers were reduced. The solvent treatment of viscose fibers has also a significant effect on the mechanical properties of PEDOT-coated conductive fibers. As it can be seen in Fig. 4, the pure viscose fibers exhibit maximum strength at break i.e. 63 N ( 5.3) which is reduced up to 30 N ( 5) after soaking with oxidant solution and formation of PEDOT coating. The mechanical strength of PEDOT-coated viscose fibers further reduced to 21 N ( 5.1) and 11 N ( 3.1) after treating with ethyl acetate and acetone, respectively. It might again be because of the removal of protective coatings from the surface of viscose fibers, causing excess amount of FeCl3 solution to be absorbed by the viscose fibers. It has been studied that cellulosic fibers can easily be acid hydrolyzed by the acidic nature of FeCl3 solution.[26] Thus, it can be concluded that after the surface treatment of viscose fibers with ethyl acetate and acetone, the excess amount of absorbed FeCl3 solution can speed up the acid hydrolysis process of viscose fibers, and hence, the mechanical properties of PEDOT-coated conductive fibers are reduced. The variations in the thermal properties of PEDOT-coated viscose fibers, before and after surface treatment, are shown in Fig. 5. The previously published TGA results for pure viscose, PEDOT/viscose, and neat PEDOT are included in the figure for comparative reasons. It can be seen that the TGA trends of PEDOT-coated viscose fibers are slightly transformed from pure viscose to pure PEDOT polymer in the temperature range of 25 0C to 360 0C. After surface treatment with ethyl acetate and acetone, the TGA thermograms of PEDOTcoated conductive fibers are more similar to the TGA thermogram of pure PEDOT, and hence, we can conclude that thicker PEDOT polymer coatings were synthesized on the surface of viscose fibers after the removal of wax-like protective coatings, but the conductivity value was lower because of over oxidized structure of PEDOT. From these trends, we can also estimate the amount of PEDOT contents at different temperature ranges. Effect of drying time and drying temperature of oxidant-enriched viscose fibers Zuber et al.[23,27] have explained the effect of humidity on the polymerization of PEDOT; it should be at optimum value to get maximum conductivity of PEDOT. Since, viscose is hygroscopic in nature and can absorb moisture from atmosphere. Hence, drying of oxidant-enriched viscose fibers plays a very important role on getting smooth and highly conductive PEDOT layers on the surface of viscose fibers. In our previous study, we investigated the effect of drying time of oxidant-enriched

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Figure 4. Effect of surface treatment on the mechanical properties of PEDOT-coated viscose fibers. This figure is available in colour online at wileyonlinelibrary.com/journal/pat

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Figure 5. TGA analysis of PEDOT-coated viscose fibers prepared before and after surface treatment with organic solvents. This figure is available in colour online at wileyonlinelibrary.com/journal/pat

viscose fibers at ambient conditions and found that 30 min drying gives best properties. In this article, we investigated the effect of drying time of oxidant impregnated viscose fibers at variable temperatures instead of room temperature on the electro-mechanical properties of PEDOT-coated conductive fibers. The oxidant-enriched fibers were dried at 30o C, 40o C, 50o C, and 60o C for 3, 5, 7, and 10 min. The electrical properties of PEDOT-coated conductive fibers prepared at variable times and temperatures are shown in Fig. 6. It is clear from the figure that at a specific drying time (3, 5, 7 or 10 min), the resistance values are reduced significantly by increasing the drying temperature, and at a specific temperature (30, 40, 50, or 60o C) again, the resistance values are reduced with increasing the drying time. The minimum electrical resistance value of PEDOT-coated conductive fibers were obtained at 60o C temperature and 10 min drying time i.e. (4 kΩ), which is almost same as that of our previous results, i.e. (3.5 kΩ obtained at 30 min drying time). The electrical properties of PEDOT-coated conductive fibers were improved because it might be possible that with increasing the drying temperature and drying time, the excess amount of absorbed oxidant, which could be used for over oxidation of PEDOT,[25] was deactivated and hence, the better conductivity of PEDOT-coated fibers was achieved. It is also worth noting that almost the same electrical properties of conductive fibers can be obtained at shorter period of time, i.e. with 10 min drying time

Figure 6. Electrical properties of PEDOT-coated viscose fibers produced at variable drying time and temperature. This figure is available in colour online at wileyonlinelibrary.com/journal/pat

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OPTIMIZATION OF CVD PROCESS AND SURFACE MORPHOLOGY which was 30 min in our previous study. Therefore, by controlling the drying time and drying temperature, the overall process time can be reduced. The impact of drying time and drying temperature on the mechanical properties of PEDOT-coated viscose fibers was evaluated by tensile testing of coated fibers. Figure 7 illustrates the mechanical properties of PEDOT-coated conductive fibers prepared at different drying times and temperatures. It is shown in figure that at a specific drying time (3, 5, or 7 min), the mechanical properties of conductive fibers are improved with increasing the temperature, but further increase in drying time up to 10 min has reduced the mechanical properties again. The improvement of mechanical properties with increasing drying temperature might be because of the evaporation of excess amount of absorbed oxidant which causes the hydrolysis of viscose fibers. On the other hand, reduction of mechanical strength with increasing drying time up to 10 min should be because of the shrinkage of the cellulosic fibers which were placed at higher temperatures for longer period of time.[25] The prolonged heating of viscose fibers affects the internal structure of cellulose which initiates the fiber decomposition, and hence, mechanical properties of overall fibers are reduced. Effect of oxidant types and oxidant concentration Oxidants are the initiating agents for the polymerization of PEDOT. The rate of polymerization and the quality of PEDOT polymer strongly depends on the oxidation potential of used oxidants. It should be at optimum value to start the chemical reaction and to avoid the unwanted side reactions. In our previous study, we found that the mechanical properties of viscose fibers were significantly reduced, whereas the electrical

properties were extraordinarily improved with increasing oxidant (FeCl3) concentration. In order to improve the electro-mechanical properties of PEDOT-coated conductive fibers, we tried again FeCl3 oxidant along with another commonly used oxidant ferric(III)p-toluenesulfonate (FepTS), and then their results were compared. The optical microscopic images of pure viscose, PEDOT-coated viscose fibers with FeCl3 and FepTS oxidants are shown in Fig. 8. It can be seen in Fig. 8 (C) that by using FepTS oxidant, a very thick but non-uniform darkish blue layer of PEDOT polymer was formed on the surface viscose fibers. On the other hand with FeCl3, the thickness of PEDOT layer was lower enough as compared to FepTS, but it was distributed evenly throughout the fiber surface, ref. Fig. 8 (B). It was found that PEDOT layer formed with FepTS was very brittle as compared to the PEDOT layer obtained with FeCl3 oxidant. Most of the coating was removed even by simple bending the fibers. However, in the case of FeCl3, the coating was almost permanent, and PEDOTcoated conductive fibers exhibited a reasonable flexibility. The comparison of electrical and mechanical properties of viscose fibers before and after PEDOT coating is given in Table. 2. The electrical resistance of PEDOT-coated conductive fibers prepared with FeCl3 oxidant was extraordinarily lower (6.8 kΩ) than the PEDOT-coated conductive fibers produced with FepTS oxidant (54.5 kΩ). However, in the case of FeCl3, the mechanical properties of PEDOT-coated viscose fibers are lesser than the conductive fibers produced by FepTS. Thus, better electrical properties can be achieved with FeCl3 whereas, good mechanical properties can be acquired with FepTS. The reason for the better electrical properties with FeCl3 might be the frequent availability of the active anions (Cl-) which take part in the oxidation of EDOT monomer. Also, the excess amount of ferric chloride oxidant could be utilized as a dopant (oxidation of neutral PEDOT polymer chains), which increases the conductivity level of PEDOT by introducing negative charge along the polymer backbone structure. On the other hand, FepTS oxidant is more sensitive to the humidity level and

Table 2. Electro-mechanical properties of PEDOT-coated conductive fibers produced with ferric chloride and FepTS oxidants

Figure 7. Mechanical properties of PEDOT-coated viscose fibers produced at variable drying time and temperature. This figure is available in colour online at wileyonlinelibrary.com/journal/pat

Sample type

Resistance (kΩ)

Pure viscose FeCl3 FepTS

. . .. . .. . ... 6.8 (0.7) 54.5 (20)

Force at break (N)

Tenacity (N/Tex)

61.3 (5.3) 0.32 (0.03) 29.7 (5.0) 0.14 (0.05) 40.8 (8.8) 0.16 (0.02)

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Figure 8. Optical microscopic images of, (A) pure viscose, (B) PEDOT/viscose with FeCl3, and (C) PEDOT/viscose with FepTS. This figure is available in colour online at wileyonlinelibrary.com/journal/pat


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immediately forms the stabilized structures after being oxidized. It has been reported that by controlling the humidity at a particular level or by suppressing the crystallite formation of FepTS, highly conductive and smooth PEDOT layers can be formed.[23,27] Along with this reason, the high pH value of FepTS oxidant solution might also start several unwanted side reactions, which can limit the conductivity of PEDOT layer. For mechanical properties, it is obvious that FeCl3 oxidant causes the acid hydrolysis of viscose fibers, and hence, the backbone structure of cellulosic substrates partially destroyed, which reduces the mechanical strength of PEDOT-coated conductive fibers. It might not be the case for FepTS oxidant; although it is also acidic in nature, it does not have more diversified effects as FeCl3 has, on the parent structure of viscose fibers that is why the better mechanical properties with FepTS can be achieved. Effect of oxidant type on the thermal properties of PEDOT-coated viscose fibers was evaluated by TGA. The TGA thermograms of neat viscose, PEDOT-coated viscose fibers prepared by FeCl3 oxidant, PEDOT-coated viscose fibers prepared by FepTS oxidant, and pure PEDOT are shown in Fig. 9 (a), (b), (c), and (d), respectively. The thermal stabilities of pure viscose, PEDOT-coated viscose fibers obtained with FeCl3 and neat PEDOT, have already been compared in our previous study.[21] In this study, we compared the results of PEDOT-coated conductive fibers obtained with FepTS oxidant. The thermal degradation of PEDOT-coated conductive fibers prepared with FepTS, from 25 to 200 0C, was almost similar to the pure viscose and PEDOT-coated viscose fibers acquired with FeCl3. From 200 to 300 0C, weight loss was relatively higher in the case of FepTS as compared to the pure viscose and FeCl3, but after 400–600 0C, the thermal stability of FepTS was again almost similar as that of FeCl3 except the remaining wt. % at 600 0C was 25% which was only 15 % in the case of FeCl3. The improved thermal stability of PEDOT-coated viscose fibers with FepTS oxidant and close resemblance of TGA trends with pure PEDOT polymer show that the higher amount of PEDOT was formed on the surface of viscose fibers in the case of FepTS oxidant. In our previous study, we investigated the effects of oxidant (FeCl3) concentrations on the electrical and mechanical properties of PEDOT-coated conductive fibers. The influence of oxidant (FeCl3) concentration on the thermal properties of PEDOT-coated viscose fibers has been discussed in the present study. For this purpose, we took samples of PEDOT-coated viscose fibers prepared at variable FeCl3 oxidant concentrations (3 wt. % to 15 wt. %) and then

characterized by DSC analysis. In Fig. 10, the DSC thermograms of oxidant (FeCl3), pristine viscose, and pure PEDOT are reported. Pure viscose shows a quite narrow peak of decomposition at 336 0C whereas, pure PEDOT shows broad peak of decomposition at 322 0C. Figure 11 illustrates the DSC analysis of PEDOT-coated fiber samples prepared with different concentrations of oxidant (FeCl3) solution. The thermogram of sample prepared with 3 wt. % oxidant concentration, ref. Fig. 11(a), shows very large decomposition peak with a maximum at 256 0C. Along with this, a noticeable exothermic event occurs at 304 0C, which is probably due to the cellulose interaction with residue from the oxidant.[19] With increasing oxidant concentration until (9 wt. %), a progressive expansion of the cellulose decomposition peak is observed, and it slightly sifts towards lower temperatures, ref. Fig. 11(b, c). When oxidant concentration exceeds (15 wt. %), the cellulose decomposition peak not only slightly shifts to lower temperature but two small and noticeable exothermic peaks at 166 0C and 227 0C are also observed. These exothermic peaks indicate the presence of oxidant (FeCl3) contents in PEDOT-coated viscose fibers, which are almost absent in other three thermograms, ref. Fig. 11(d). The exothermic peak, which appeared in (3 wt. %) oxidant sample, is very small in all other samples. It means, with increasing PEDOT layer thickness, the thermal stability of coated viscose fibers increased at higher temperatures and PEDOT layers act as protective coating.

Figure 9. TGA analysis of, (a) neat viscose, (b) viscose/PEDOT/FeCl3, (c) viscose/PEDOT/FepTS, and (d) pure PEDOT. This figure is available in colour online at wileyonlinelibrary.com/journal/pat

Figure 11. DSC analysis of PEDOT/viscose samples prepared with oxidant (FeCl3) concentration, (a) 3 wt. %, (b) 5 wt. %, (c) 9 wt. %, and (d) 15 wt. %. This figure is available in colour online at wileyonlinelibrary.com/journal/pat

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Figure 10. Comparison between DSC analysis of oxidant (FeCl3), pure viscose, and pure PEDOT. This figure is available in colour online at wileyonlinelibrary.com/journal/pat

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OPTIMIZATION OF CVD PROCESS AND SURFACE MORPHOLOGY

SURFACE MORPHOLOGY OF PEDOT-COATED YARNS The uniform distribution and the thickness of the conjugated polymer layers on different substrates are very important factors, which can have a diversified effect on the properties of the obtained products and hence, on the end user applications. In the case of textile-based conductive fibers, coating should be in the range of micro or nano scales, so that the virgin textile properties should not be disturbed. The determination of coating thickness or the distribution of conductive polymers on the coated yarns is always very tricky because of the involvement of many monofilaments. In our case, the viscose yarn fibers were not only covered by PEDOT polymer but also the PEDOT was impregnated to some extent inside the yarn. In order to find out the PEDOT-coated layer and the impregnation inside the viscose yarns, we utilized the ATR-FT-IR imaging technique. It is a powerful technique to find out the spatial distribution of chemical identities within a sample, because we can get full spectral information at each pixel of the obtained IR images. In other words, we can get spatial and chemical information at the same time.[28–30] We acquired the ATR-FT-IR image across the cross section of pure viscose yarns, shown in Fig. 12 (left), where we can see the IR images of some monofilaments of viscose yarn in particular pixel size. The full IR spectra ranging from 4000 to 720 cm1 exist at all of the pixels in the obtained IR image, but we observed the IR spectra between 1800 and 720 cm1 wavenumber. Figure 12 (right) shows the IR spectra taken at different pixels (position 1, 2, and 3) along the IR images; in other words, we obtained the IR spectra at the center of the monofilament (point 1), at the empty space between filaments (point 2) and at the surface of monofilament (point 3). The distinctive absorption peaks related to cellulosic materials at 1640, 1365, 1149, 1014, and 895 cm1 can be seen on the IR spectrum obtained at point 1 (at the core of the monofilament). The detail about these peaks has already been explained in our previous studies.[21] IR spectrum at point 3 (at the surface of monofilament) also has same but less prominent peaks as point 1. Similarly, the ATR-FT-IR image and the IR spectra at two different points along the IR image for pure PEDOT polymer are shown in Fig. 13. The characteristic absorption peaks for doped PEDOT can be observed at 1208 and 1139 cm1 wavenumber.[31] Figure 14 illustrates the IR images and IR spectra for PEDOT-coated viscose yarns. The IR spectra obtained at four different points along the IR images of PEDOT-coated viscose yarns. In the figure, the points 1, 2, 3, and 4

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refer to the outermost surface of the coated filament, core of the filament, space between different filaments, and at the interface of two coated filaments, respectively. The IR spectrum at point 4 is just for the air present between the empty spaces of monofilaments. It is clear from the figure that the IR spectrum obtained at the core of the monofilament (point 2) is exactly same as it was observed in pure viscose yarns. Also, the IR spectrum corresponding to the outer surface of the coated filament (point 1) is approximately same as the IR spectrum obtained at the outer surface of the pure viscose yarns. In other words, we can say that the monofilament was not coated with PEDOT. However, the IR spectrum acquired at point 4 shows almost similar absorption peaks at 1210 and 1150 cm1 wavenumber, which were observed in the IR spectra of pure PEDOT polymer (Fig. 13). From these spectroscopic results, we can conclude that the viscose yarn fibers were not coated completely with PEDOT polymer and the polymer was impregnated inside the yarns. From ATR-FT-IR spectroscopic imaging technique, we got some information about the PEDOT impregnation, but still the mystery of coating thickness is not resolved. For further analysis, we used the CT scanning technique. It is a very valuable technique which can be used to get information about small

Figure 14. ATR-FT-IR spectroscopic images and IR spectra of PEDOTcoated viscose yarn fibers. This figure is available in colour online at wileyonlinelibrary.com/journal/pat

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Figure 12. ATR-FT-IR images and IR spectra of neat viscose yarn fibers. This figure is available in colour online at wileyonlinelibrary.com/journal/pat

Figure 13. ATR-FT-IR spectroscopic images and IR spectra of pure PEDOT polymer. This figure is available in colour online at wileyonlinelibrary.com/ journal/pat


T. BASHIR ET AL. particle sizes, composition, internal distances, or internal wall thickness of very complex samples, where the commonly used optical scanners can never reach. In order to get better knowledge about material’s distribution, the two-dimensional and three-dimensional images can be obtained by CT scanning technique. The obtained CT images for a complex object must have different resolutions depending on the density variations for different parts of the object. In Fig. 15, the xy sliced view of PEDOT-coated yarns is shown. It can be observed in the figure that almost all the monofilaments have same appearance except some dense particles can

be seen at the outer surface of the yarns. These dense particles must be the thick and uneven PEDOT coating across the conductive yarns. It is very difficult to see any coating or coating thickness at the inner part of the coated samples. It might be because of the similar densities of the PEDOT (1.5 g/cm3) and viscose (1.52 g/cm3).[32] As in our previous studies, we analyzed our coated samples with SEM and in this study with optical microscope, we can say that viscose yarn fibers were very well (not 100%) coated with PEDOT polymer. Furthermore, the xz sliced view and the three-dimensional view of the PEDOT-coated yarns is shown in Fig. 16(left) and Fig. 16(right), respectively. In xz sliced view, we can see some dense particles on the surface of the analyzed sample, similarly in 3-D view at outer most surface of the sample the whitish part can be observed, which is because of the dense particles of the PEDOT polymers. From CT analysis, it can be revealed that only on the surface of the coated samples, very thick PEDOT layers exists, but the impregnation of the PEDOT inside the coated yarns cannot be observed with CT technology, which was confirmed with IR imaging technique.

CONCLUSIONS

Figure 15. Computed tomography scanning of PEDOT-coated viscose fibers, cross-sectional view.

It was concluded that the commercially available textile fibers can successfully be transformed into electroactive fibers by oCVD technique. Results show that the pre-treatment steps, prior to the polymerization of conjugated polymers, have a significant effect on the electrical, mechanical, and thermal properties of produced fibers. It was investigated that after the surface treatment of textile fibers with organic solvents, the electrical and mechanical properties of produced conductive fibers was reduced. TGA analysis of PEDOT-coated conductive fibers obtained after surface treatment revealed that more PEDOT coating was formed on the surface of viscose fibers. By controlling the drying time and temperature of oxidant impregnated viscose fibers, not only the better electro-mechanical properties of PEDOT-coated

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Figure 16. 2-D and 3-D CT images of PEDOT-coated viscose fibers.

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OPTIMIZATION OF CVD PROCESS AND SURFACE MORPHOLOGY conductive fibers was obtained but also the overall process time can be optimized. Two different types of oxidants, FeCl3 and FepTS, were studied, and it was concluded that better electrical properties was obtained with FeCl3 whereas, the better mechanical properties was achieved with FepTS oxidant. The DSC analysis show that with increasing oxidant (FeCl3) concentration, the thicker PEDOT layers were formed on the surface of viscose fibers, which act as protective coatings during thermal decomposition. From ATR-FT-IR spectroscopic imaging technique and CT scanning method, we can get some knowledge about the coating distribution along the sample as well as the impregnation of the PEDOT inside the viscose yarns. The results of this study cannot only be utilized to get the better electro-mechanical properties of conductive textile fibers but also the overall oCVD coating process can be optimized.

Acknowledgements The authors express their gratitude to GE Sensing & Inspection Technologies, Wunstorf, Germany for CT analysis and Stiftelsen Svensk Textilforskning, Sweden for providing funding to this project.

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Ocvd polymerization of pedot