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Reactive inkjet printing of PEDOT electroconductive layers on textile surfaces
Synthetic Metals 217 (2016) 276–287
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Reactive inkjet printing of PEDOT electroconductive layers on textile surfaces Z. Stempiena,* , E. Rybickib , T. Rybickic , M. Kozaneckib a b c
Institute of Textile Architecture, Lodz University of Technology, 116 Zeromskiego Street, 90-924 Lodz, Poland Department of Molecular Physics, Lodz University of Technology, 116 Zeromskiego Street, 90-924 Lodz, Poland Institute of Automatic Control, Lodz University of Technology, 18/22 Stefanowskiego Street, 90-924 Lodz, Poland
A R T I C L E I N F O
Article history: Received 22 February 2016 Received in revised form 12 April 2016 Accepted 15 April 2016 Available online xxx This work is in memory of our friend and colleague Professor Edward Rybicki, who passed away in 3rd March 2016 prior to publication, without whom this research would not have been possible. He will be greatly missed. Keywords: PEDOT Textiles Reactive inkjet printing Electroconductive layers Textronics
A B S T R A C T
A simple and original method for the deposition of poly (3,4-ethylenedioxythiophene) (PEDOT) on different synthetic textile fabrics by the reactive inkjet printing technique was proposed. PEDOT coated conducting fabrics were obtained by chemical oxidation of an arbitrarily chosen and constant concentration of 3,4-ethylenedioxythiophene (EDOT) of 0.9 M/dm3 in different alcohols (methyl-, ethyland butyl alcohol) by water solutions of 0.1–0.5 M/dm3 ferric nitrate9H2O on polyacrylonitrile (PAN) and poly(ethylene terephthalate) (PET) fabrics at elevated temperatures. The conducting fabrics were chemically characterised by means of Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy and energy-dispersive spectroscopy (EDS). The morphology of the coatings was observed using optical microscopy and scanning electron microscopy (SEM). The conducting properties (surface resistance) of the fabrics were measured by means of the four-probe method. The optimal conditions for PEDOT deposition on textiles (temperature, oxidant concentration) by reactive inkjet printing were established. The influence of the different alcohols used as an EDOT solvent and the effect of water rinsing on the changes in surface resistance were also discussed. The obtained results show that the proposed method is very simple and gives a good adhesion of the in-situ formed PEDOT to the substrate, at the same time achieving a low surface resistance of ca. 150 V/sq. The variation of the surface resistance vs. concentration of ferric nitrate9H2O for both of the textile fabrics was estimated. A mechanism for the PEDOT deposition and adhesion on the textiles based on electrokinetic phenomena is proposed and verified by changes in the relative resistance of PEDOT/PAN and PEDOT/PET composites during multicyclic bending, washing and dry-cleaning processes. ã 2016 Elsevier B.V. All rights reserved.
1. Introduction Among all conducting polymers, poly(3,4-ethylenedioxythiophene) (PEDOT) is unique because of its high transparency, high conductivity, and good thermal stability [1,2]. These unique properties make PEDOT an excellent material for various applications such as in electrochromics [3], antistatic coatings [4], light-emitting diodes [5,6], sensors [7–9], and textronics [10], among others. Synthesis methods for conducting polymer nanofibres, their potential applications and the challenges associated with their use have also been discussed, for example in chemical, optical and bio-sensors, nano-diodes, field effect transistors, field emission and electrochromic displays, supercapacitors and energy storage, actuators, drug delivery, neural interfaces, and protein purification [11]. Synthesis by chemical
* Corresponding author. E-mail address: [email protected] (Z. Stempien). http://dx.doi.org/10.1016/j.synthmet.2016.04.014 0379-6779/ ã 2016 Elsevier B.V. All rights reserved.
oxidative polymerisation of EDOT monomer in an emulsion technique can lead to a nanosized PEDOT polymer with spherical [12], cylindrical [13] or flake morphology [14–16]. Chemical polymerisation of EDOT with ferric chloride to PEDOT has been commercialised as an antistatic coating in photographic film, printed circuit boards and in the capacitor industry [17,18]. The oxidative chemical polymerisation of PEDOT on fabrics can be achieved using several procedures including liquid phase polymerisation (LPP) [19–25], and vapour phase polymerisation (VPP) [26–29], using such oxidants as FeCl3 or p-toluenesulphonic acid (FepTS) [30]. In the LPP method, textile samples are immersed in a mixture of the monomer (EDOT) and the oxidant agent at a temperature below 10 C to delay the polymerisation process [20]. They are then submitted to a drying step, either at ambient temperature or in a convection oven to complete the polymerisation process. In the VPP method, the textile fabrics are first immersed in an oxidant solution and are then exposed to the EDOT in the vapour phase in a reactor with controlled vacuum and
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temperature. In recent years, growing efforts have been focused on the production of smart fabric devices by printing. Amongst the printing technologies available, screen printing, microcontact printing, gravure, flexographic, offset, inkjet printing and reactive inkjet printing are the most commonly used in the production and fabrication of conductive films and structures with conducting polymer formulations [31–33]. In this paper, an original method for the deposition of poly (3,4ethylenedioxythiophene) (PEDOT) reactive inkjet printing on different synthetic textile fabrics was proposed. PEDOT coated conducting fabrics were obtained by the chemical oxidation of 3,4ethylenedioxythiophene (EDOT) in different alcohols (methyl-, ethyl- and butyl-) by water solutions of ferric nitrate9H2O on polyacrylonitrile (PAN) and poly(ethylene terephthalate) (PET) fabrics at elevated temperatures. The conducting fabrics were characterised chemically by means of Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy and energy-dispersive spectroscopy (EDS). The morphology of the coatings was observed by optical microscopy and scanning electron microscopy (SEM). The conducting properties (surface resistance) of the fabrics were measured by means of the four-probe method. The optimal conditions for PEDOT deposition on the textiles (temperature, oxidant concentration, type of alcohol) by reactive inkjet printing were established. The influence of the different alcohols used as an EDOT solvent and the effect of water rinsing on the changes in surface resistance were also discussed. A mechanism for the PEDOT deposition and adhesion on textiles based on electrokinetic phenomena, and changes in the relative resistance of PEDOT/PAN and PEDOT/PET composites during multi-cyclic bending, washing and dry-cleaning processes is proposed. 2. Materials and methods 2.1. Reagents and materials 3,4-ethylenedioxythiophene (EDOT) 97% was obtained from Aldrich, and used as received. Analytical grade ferric nitrate9H2O obtained from Chempur (Poland) was used without purification. As substrates, two different commercial un-dyed textile fabrics made of synthetic fibres were employed in the experiments. Table 1 shows the fundamental characteristics of the textile substrates used. Before printing, the textile substrates were ultrasonically cleaned with methanol and isopropanol for 20 min in each bath, then rinsed in an ultrasonic bath with deionised water and dried in an oven at the temperature of 40 C. 2.2. Deposition of PEDOT layers on textiles PEDOT electro-conductive layers on flat textile surfaces were printed using the inkjet printing technique, in accordance with the scheme presented in Fig. 1a. For this purpose, a prototype of a digital inkjet printer shown in Fig. 1b was used. It contained (Fig. 1b): (1) nanodispensing hardware, represented by two one-nozzle valve-jet print heads (ReaJet SK 1/080, Germany), (2) drive electronics for generating electrical signals to the print-heads; (3) an ink pump system and
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two ink reservoirs; (4) a two-axis (x-y) motion system for print head positioning with a step resolution of 35 mm, based on stepper motor linear drives; (5) a computer control system with software allowing the selection of a printing pattern and triggering of drops. Before printing, the ink reservoir of print head 1 was filled with an ink composition prepared from a butyl-, ethyl- or methyl alcohol solution of EDOT at a concentration of 0.9 M/dm3, and the ink reservoir of print head 2 was filled with an ink composition prepared from an aqueous solution of ferric nitrate9H2O in the concentration range 0.1–0.5 M/dm3. The printed pattern was designed using a vector graphics editor and converted to a lineart bitmap using raster image processing software. The resolution of the line-art bitmap was 50 dpi and was the same as the resolution of the x-y print heads positioning system (50 steps per inch in the x and y axes). On the basis of the line-art bitmap, the drive electronics system controlled the position of the print heads and the release of drops. The volume of the released drops was ca. 30 nl. The electroconductive layers were deposited line by line onto the surface of the fabrics in such a way that the first nozzle sprayed the selected line of the pattern using a butyl-, ethyl- or methyl alcohol solution of EDOT and then the second nozzle sprayed the same line of the pattern using an aqueous solution of ferric nitrate9H2O, or vice versa. The presented method for the fabrication of PEDOT electro-conductive layers on the surface of fabrics has been patented at the Polish Patent Office [34]. 2.3. Characteristics 2.3.1. Scanning electron microscopy Scanning electron microscopy with EDS was used to determine the morphology of the PEDOT layers and assess the distribution of PEDOT on the textile surface. Energy-dispersive X-ray spectroscopy extends the usefulness of SEM investigations by enabling elemental analysis within regions of a few cubic micrometres. The material was characterised by SEM using a field emission S4700 (Hitachi, Japan) equipped with an energy-dispersive spectrometer (Thermo-Noran, USA). 2.3.2. FTIR spectroscopy FTIR spectra of PAN fabric, a PEDOT reference and PEDOT deposited on PAN were performed on a Nicolet iS50 FT-IR spectrometer (Thermo Fisher Scientific, Inc.) equipped with an ATR accessory with a diamond crystal. The spectral resolution was 2 cm 1 and 256 scans were averaged for all IR spectra. The reference PAN fabric as well as the PEDOT coated PAN were measured without any special treatment. The reference PEDOT sample was used as a powder. The reference PEDOT powder was prepared by dropping 1 ml of EDOT solution in 10 ml of methyl alcohol into 50 ml of an aqueous solution of 0.5 M/dm3 ferric nitrate9H2O, and the mixture was stirred for 36 h. The black sludge formed was then decanted, rinsed several times with bidistilled water and finally rinsed with methyl alcohol. The product was dried at ambient temperature for 24 h. 2.3.3. Raman spectroscopy Raman spectra were collected using a MultiRAM FT Raman spectrometer (Bruker GmbH, Germany) in back-scattering
Table 1 Characteristics of textile substrates used. Fabric material
Fabric weave type
Surface weight, g/m2
Thickness, mm
Warp density, yarns/cm
Weft density, yarns/cm
Polyacrylonitrile (PAN) Polyester (PET)
twill 2/1 twill 3/1
220 150
0.82 0.34
24 24
17 24
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Fig. 1. Reactive inkjet printing of PEDOT layers on textile surfaces: (a) scheme of deposition, (b) view of digital inkjet printer used.
configuration with a spectral resolution of 2 cm 1. The standard neodymium-doped yttrium aluminium garnet (Nd:YAG) laser (l = 1064 nm) was used as a source of excitation light. The laser power and the number of averaged scans were individually selected for each sample to protect them against overheating and to simultaneously ensure high-quality spectra. Deconvolution of the Raman spectra was performed using PeakFit v.4.12 software after subtraction of linear baseline. 2.3.4. Surface resistance The surface resistance of the printed samples was estimated using the four-point method adapted to textiles. The four-point probe setup used was presented in detail by Stempien et al. [32]. For each fabric, three independent PEDOT prints were tested. The surface resistance was finally calculated as an average value of the measurements and the standard deviation (SD) was determined. 2.3.5. Resistance to bending The bending test equipment is shown in Fig. 2. It consists of a rotating bending system with a stepper motor controlled by a computer system. A textile sample with a PEDOT electroconductive layer is mounted between the fixed and rotating clamps. The mounting edge of the rotating clamp acts as a bending line. It is situated in the rotation axis of the stepper motor, which allows for the repetitive sharp bending of the sample by 30 deg.
The electrical resistance of tested sample was measured in real time using a Picotest M3500A ohmmeter. On the basis of these measurements, the relative resistance R/R0 was determined, where R denotes a resistance measured during sample flexing and R0 the initial resistance of the sample, respectively. 2.3.6. Washing and dry-cleaning processes Changes in the surface resistance of the PEDOT electroconductive layers were estimated after washing and dry-cleaning processes. The details of the washing and dry-cleaning procedures are as follows: a) The washing process was carried out in a Linitest laboratory washing machine (Germany), for 30 min at 40 C. The liquor to sample weight ratio was 100:1. The samples with PEDOT deposited textiles were washed in distilled water containing 0.5% non-ionic surfactant (MARLIPAL O13/70), a product of Sasol (Germany). b) The dry-cleaning process in tetrachloroethylene (PER) was carried out in a Linitest laboratory washing machine (Germany), for 30 min at ambient temperature. The liquor to sample weight ratio was 100:1. After dry-cleaning the samples were left for several hours at room temperature to allow the solvent to evaporate completely.
Fig. 2. Schematic diagram of multi-cyclic bending of textile sample (left) and a textile sample mounted in the clamps during testing (right).
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Fig. 3. Surface resistance of PEDOT prints on PAN fabric vs. hot plate temperature.
3. Results and discussion 3.1. Effect of hot plate temperature on surface resistance Fig. 3 presents the changes in surface resistance of PEDOT prints on PAN textile substrates using EDOT solution in methyl alcohol and 0.3 M/dm3 ferric nitrate9H2O as oxidant with changing temperature, and Fig. 4 shows views taken with an Olympus stereomicroscope SZX-10 under the same temperature conditions. As can be seen in Fig. 3, in the temperature range of 50–70 C almost a linear increase in surface resistance is achieved with a maximum at 70 C. Above this temperature, a sudden decrease in surface resistance (increase in conductivity) is observed. The optimal conditions for the in-situ deposition of PEDOT on textile samples by reactive inkjet-printing seem to be in the range 85–
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90 C, taking into account the boiling points of methyl-, ethyl-, and butyl alcohols as 64.7 C, 78.4 C, and 117.7 C at 760 mm Hg, respectively. It is known from the principles of physical chemistry that a linear decrease in the surface tension of alcohols vs. temperature is observed. At the same time, a steady decrease in the dynamic viscosity of alcohols vs. temperature is also expected. The dissolution of EDOT in all of the alcohols studied can cause an increase both in the surface tension and the dynamic viscosity of the EDOT-alcohol binary mixtures but the changes of these parameters vs. temperature should have similar characteristics. Fig. 4 shows optical microscope images of PEDOT/PAN composites printed at different hot plate temperatures. On the left-hand side in prints up to temperature 70 C clear and fuzzy shadows are observed. An increase in temperature makes the border between the pristine PAN fabric and the PEDOT-deposited PAN textile practically invisible, and sharp edges, especially at temperature 90 C, are observed. Probably, in the temperature range of 50–70 C, a competition between the changes in surface tension and dynamic viscosity caused by temperature (better wetting and spreading of the methanol-EDOT solution on PAN fabric), and evaporation of the solvent lead to rather poor quality inkjet-prints and a worsening of conductivity. In the temperature range of 80– 90 C the solvent evaporation process probably prevails, which contributes to a uniform distribution of the EDOT-alcohol solution on the textile surface and a sudden decrease in the surface resistance is observed. Recently, Wua et al. [35] studied the role of the VPP temperature on the PEDOT polymerisation rate in formation of thin films on glass substrate. The VPP temperature plays key roles on the polymerisation reaction rate and the mobility of the polymer chains as well as the volatilisation of the reactant and product, which result in drastically different properties of the VPP PEDOT films. In the
Fig. 4. Optical microscope images of PEDOT/PAN composites printed at different hot plate temperatures.
Fig. 5. Optical microscope images of PEDOT/PAN composites printed using a butyl alcohol solution of EDOT: (a) PAN substrate; ferric nitrate9H2O concentration (b) 0.1 M/ dm3, (c) 0.25 M/dm3, (d) 0.4 M/dm3.
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Fig. 6. Optical microscope images of PEDOT/PET composites printed by using a butyl alcohol solution of EDOT: (a) PET substrate; ferric nitrate9H2O concentration (b) 0.1 M/ dm3, (c) 0.25 M/dm3, (d) 0.4 M/dm3.
Fig. 7. SEM of PEDOT/PAN composite at magnifications of 4850 and 10,800 after printing.
Fig. 8. SEM of PEDOT/PAN composite at magnifications of 5000x and 10,000 after rinsing in deionised water.
range of 33–57 C, the lattice structure and the morphology of the films can be significantly affected by the VPP temperature. The optimum temperature (46 1 C) resulted in a PEDOT film with the
highest conductivity on the glass substrate (622 S/cm), which is ascribed to its highly ordered crystal structure and smooth morphology. The film conductivity increased with the temperature
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Fig. 9. SEM micrograph (2500) and EDS measurements of the PEDOT/PAN composite after printing.
up to 46 C, and then decreased beyond this temperature. This report may partly correspond to our findings although in our case the situation seems to be more complicated. 3.2. Optical microscope images of PEDOT-deposited textiles Figs. 5–6 and 1S–4S (Supporting information S1) show photographs (taken with an Olympus stereomicroscope SZX-10, magnification 3) of a representative set of samples of PEDOT printed on PAN and PET textile substrates using butyl-, methyl- and ethyl alcohol solutions of EDOT and aqueous solution of ferric nitrate in the concentration range 0.1–0.4 M/dm3. The images presented show that the PEDOT prints on different textile substrates have uniform dark hues, which are characteristic of PEDOT formation. It should be noted that for both textile
substrates, quite good, sharp-edged prints were obtained and a lack of PEDOT spread and coffee-ring effect was observed. This is important from the viewpoint of fabrication of precision electroconductive wires and electrodes on textile surfaces by the inkjet printing technique for textronic applications. It allows for the fabrication of narrow and close placed wires and electrodes with different shapes in different regions of a textronic device. The magnified printed surfaces show a quite good uniform distribution of PEDOT on the textile surfaces. 3.3. Surface morphology of PEDOT layers SEM images of the PEDOT/PAN composite samples in both variants, after printing and rinsing in de-ionised water, were taken at different magnifications from 1000 to 10,000 for the 0.3 M/
Fig. 10. SEM micrograph (2500) and EDS measurements of the PEDOT/PAN composite after rinsing in deionised water.
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dm3 ferric nitrate9H2O concentration. Figs. 7 and 3S (Supporting information S2) and Fig. 8 and 4S (Supporting information S2) present the SEM images of PEDOT prints on PAN textile substrates after printing and rinsing in de-ionised water, respectively. The presence of PEDOT can be clearly seen on the inkjet-printed PAN fabric. It is evident from Figs. 7 and 8 that in general the fabric yarns were completely coated, and some areas between yarns were coated by small and large granular PEDOT having a non-uniform and non-smooth surface. Overall observation of the SEM study shows that the PAN fabric was successfully covered with PEDOT and that PEDOT penetration onto and into the structure of the fabric provided conductivity to the fabric. The PEDOT/PAN composite samples in both variants, after printing and rinsing in deionised water, were also subjected to EDS survey analyses to identify the elements present on the fabric surface (Figs. 9 and 10). EDS studies revealed that the PEDOT print on the PAN fabric contains mainly carbon, iron and sulphur. The carbon peak on the EDS spectrum may come from PEDOT as well as from the PAN fabric. The sulphur and iron peaks come from EDOT and the oxidant that were used during the in-situ polymerisation of PEDOT by the reactive inkjet printing. Comparison of the SEM images at 2000x magnification with the EDS investigations of the analysed samples show that, on the surface of the PAN fabric, a layer of PEDOT is clearly visible. Bright spots appearing on the Xray maps (Fig. 9) depict places with higher concentrations of the studied element, and also confirm the presence of globular structures, especially in the case of sulphur. 3.4. FTIR and Raman spectra Fig. 11a presents an FTIR spectrum of PEDOT deposited on PAN textile. The IR spectrum is dominated by PAN peaks and only some weak lines (marked by arrows) confirm the presence of PEDOT. These lines are too weak for more detailed analysis. These results suggest that the thickness of the PEDOT film is relatively low and the IR radiation at the wavelengths used passes freely through the PEDOT film without strong absorption. However, a differential spectrum prepared by subtraction of the raw PAN textile spectrum from the spectrum of coated PAN (Fig. 11b) shows that PEDOT successfully coated the support. The positions of the spectral lines in the differential spectrum correspond well to those found in the spectrum of reference PEDOT. In Raman spectroscopy, the spectrum of the PAN textile coated by PEDOT is practically identical with that of the standard PEDOT sample (Fig. 12). The presented results directly confirm the continuity of PEDOT film, as the Raman lines characteristic for PAN are invisible. The strong Raman signal of PEDOT probably resulted from a resonance effect. The Raman spectrum of PEDOT is sensitive to the wavelength of irradiation used for sample excitation [37,38]. The 1064 nm laser line, used to excite the samples in the current investigations, is in resonance with oxidised forms of PEDOT. Thus, the intensity of bands related to polarons and bipolarons should be especially raised. These effects make direct comparison of Raman peak intensities impossible at various excitation wavelengths. It is noteworthy that the resonance conditions strongly depend on the oxidation state of PEDOT, as its absorption in the Vis-NIR region is strictly related to its electronic structure [37,38]. The most interesting spectral region (1200–1700 cm 1) containing PEDOT modes sensitive to oxidation is presented as an inset in Fig. 12 [37–40]. It is notable that according to Garreau et al. [37] the polarons in PEDOT (first oxidation state) correspond to a quinoid structure, while the bipolarons (second oxidation state) to the benzoid form. A multitude of Raman peaks overlapping each other are observed between 1200 and 1700 cm 1 showing that PEDOT occurs in more than one form. Such features are characteristic for various conducting polymers [41]. Peaks located
Fig. 11. FTIR spectra: (a) PEDOT, PAN and PEDOT deposited on PAN textile, where arrows indicate bands corresponding to PEDOT, asterisks to the nitrate ion being a counter ion for PEDOT (assignment based on [36]), and crosses to vestigial water. (b) differential spectrum prepared by subtraction of the raw PAN textile spectrum from the spectrum of coated PAN, where the red numbers correspond to IR absorption bands characteristic for the reference PEDOT sample, while the black numbers correspond to the maxima of the differential spectrum. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
at 1531, 1561, 1427 and 1261 cm 1 are characteristic for polarons. A peak at 1292 cm 1 proved that the bipolarons are also present in the investigated sample. This is the highest frequency mode of the triplet characteristic for bipolarons – 1239, 1268, 1289 cm 1. Also the peak with a maximum at 1455 cm 1 may be assigned to the highest oxidation state of PEDOT. As mentioned above, the lines of the oxidised PEDOT structures are in resonance with the used excited light, thus, the peaks of neutral part of PEDOT chain are weakly visible. However, the presence of bands with maxima at 1496 and 1228 cm 1 suggests that PEDOT is partially in reduced form. The rest of the modes characteristic for this form (1545, 1411, 1271, 1249 cm 1) are probably masked by Raman peaks of polarons and bipolarons. Assignment of the main vibrational bands is not simple and remains controversial (compare [37,38],[40,42]). Table 1S (Supporting information S3) presents a comparison of the PEDOT IR and Raman peaks found in this work with data from the literature. An absorption band with a maximum at 1650 cm 1 is visible in the spectra of all materials (marked in Fig. 11 by cross) and has been
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Fig. 12. Raman spectra of PEDOT, PAN and PEDOT deposited on PAN textile. Deconvolution of the range of Raman spectrum sensitive to PEDOT oxidation is presented in the inset.
assigned to residual moisture adsorbed on the material surface. Lines at 836, 1361 and 1597 cm 1 in the spectrum of standard PEDOT were attributed to the nitrate counter ion [36]. 3.5. Surface resistance Figs. 13 and 14 present the surface resistance of PEDOT prints on PAN and PET textile substrates vs. ferric nitrate9H2O concentration after printing and rinsing in deionised water. A steady decrease in the surface resistance with increasing ferric nitrate9H2O concentration for both the investigated textile substrates was observed. The variation in the surface resistance vs. ferric nitrate9H2O concentration curves is strongly affected by the type of textile substrate used. The exact data for the surface resistance changes for the both PEDOT/PAN and PEDOT/PET composites vs. ferric nitrate9H2O concentration in the concentration range of 0.1–0.5 M/dm3 are shown in Table 2S (Supporting information). The lowest values of surface resistance achieved for both PEDOT/ fabric composites are in bold type. Of the tested textile substrates,
the PAN fabric seems to be the most suitable for the manufacture of highly conductive PEDOT textiles by the reactive inkjet printing technique. The lowest surface resistance for the PEDOT/PAN and PEDOT/PET composites was found at a ferric nitrate9H2O concentration of ca. 0.45 M/dm3 with EDOT solution in methyl alcohol followed by ethyl alcohol. Surprisingly, using EDOT solution in butyl alcohol the lowest surface resistance was observed at very low ferric nitrate9H2O concentrations of 0.2 M/dm3 and 0.3 M/dm3 for the PEDOT/PAN and PEDOT/PET composites, respectively. After rinsing the PEDOT/fabric composites in de-ionised water slight changes in surface resistance increase were observed. The rinsing water was tinted yellow-green as a result of washing out of ferric ions from the PEDOT inkjetdeposited layers. After rinsing in de-ionised water and drying the fabric samples lost their stiffness and became more flexible, maintaining good conductivity. It should be noted that ferric nitrate9H2O proved to be an efficient oxidant during the in-situ deposition of PEDOT on textiles, acting at very low concentrations, taking into account the high water content in the molecule.
Fig. 13. Surface resistance of PEDOT prints on PAN fabric vs. ferric nitrate concentration: (a) after printing, (b) after rinsing in deionised water.
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Fig. 14. Surface resistance of PEDOT prints on PET fabric vs. ferric nitrate concentration: (a) after printing, (b) after rinsing in deionised water.
The effect of different alcohols (ethyl- and butyl-) used in the insitu polymerisation process of PEDOT on nylon 6 has been investigated [19]. The conductivities of PEDOT/nylon 6 composite fabrics treated in ethanol were generally higher than those of fabrics treated in butanol. Moreover, the conductivities of PEDOT/ nylon 6 composite fabrics treated in ethanol increased with increasing polymerisation cycles and reached a maximum value (1.89 S/cm) at five cycles. In the case of a tosylate-doped PEDOT (PEDOT-OTs)-alcohol series, conductivity increased with decreasing solvent chain length. It was suggested that the alcoholic solvents are hydrogen bonded to the counter ion of PEDOT and a compact packing of PEDOT chains is perturbed by the use of alcoholic solvents with longer chain lengths during the polymerisation of PEDOT-OTs, which was supported by the XRD experiment [43]. In our case, change of the EDOT solvent caused an offset on the left in the optimal concentration of oxidant solution at which the lowest surface resistance was observed (i.e. 0.5 M/dm3 for methyl alcohol, 0.45 M/dm3 for ethyl alcohol, and 0.2 M/dm3 for butyl alcohol, respectively) for PAN fabric samples. Similarly, for PET fabric samples, the optimal values of oxidant concentration during the inkjet printed deposition of PEDOT, were respectively 0.45 M/ dm3 for the methyl alcohol solution of EDOT, 0.40 M/dm3 for the ethyl alcohol solution of EDOT, and 0.30 M/dm3 for the butyl alcohol solution of EDOT (cf. Supporting information 2S).
PEDOT layers were printed using a butyl alcohol solution of EDOT and a ferric nitrate9H2O concentration of 0.3 M/dm3. The initial resistances of PAN and PET PEDOT-printed textiles were 663 V and 1442 V, respectively. After the 50,000 bending cycles, the final resistances of the samples studied were ca. 1503 V and 3240 V, respectively. It was observed that the relative resistance for the PEDOT-finished textiles rapidly increased by 100–130% from an initial value due to the formation of cracks in the PEDOT layer and after about 20–30 bending cycles it gradually increased by ca. 1% per 1000 bending cycles. The first rapid increases in relative resistance can be attributed to the limited mobility and brittleness of PEDOT material, which suffers from deteriorating performance because of cracking resulting from its relatively poor elongation and bending properties [44–46]. The further gradual increases in relative resistance can be attributed to structural changes in the PAN and PET fabrics during mechanical deformation caused by multi-cycle bending process. With changes in the fabric structure along the bending line, caused by cyclic elongation of the yarns, damage to the fabric structure was induced and probably the formation of cracks in PEDOT located inside of yarns between the fibres was responsible for the gradual increase in relative resistance.
3.6. Resistance to bending
Figs. 16 and 17 present the changes in surface resistance of PEDOT prints on PAN and PET textile substrates after multicycle washing and dry-cleaning. In Table 2, the initial surface resistances of PEDOT finished textiles selected for washing and dry-cleaning depending on the oxidant solvent used are presented. Figs. 16a and 17a show the changes in surface resistance of inkjet-printed PEDOT layers on PAN and PET fabric samples after washing distilled water containing 0.5% of MARLIPAL O13/70 vs. the number of washing cycles. It can be seen that in the case of the PAN fabric sample, an increasing number of washing cycles causes a steady increase in the surface resistance with a levelling-off tendency after four washing cycles. A quite different situation can be observed in the case of the PET prints (Fig. 17a). An almost linear increase in the surface resistance is observed in this case. This result shows that the inkjet-printed PEDOT layer on PET fabric significantly deteriorates after multi-cycle washing with a steady worsening in surface resistance. Figs. 16b and 17b show changes in the surface resistance of PEDOT prints on PAN and PET fabrics after multicycle dry-cleaning in tetrachloroethylene. The data obtained show that the inkjet deposited PEDOT on both the textile surfaces is very resistant to the multicycle dry-cleaning. The differing behaviour of the PEDOT inkjet-deposited fabric samples is probably due to varying PEDOT adhesion to the surface of the substrates. The PAN fabric may have sulphate and/or sulphonate
Fig. 15 shows the changes in the relative resistance of the PEDOT prints on PAN and PET fabrics after 50,000 bending cycles. The
Fig. 15. Changes in the relative resistance of PEDOT-finished textiles after multicycle bending.
3.7. Washing and dry-cleaning processes
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Fig. 16. Surface resistance of PEDOT prints on PAN fabric: (a) after washing, (b) after dry-cleaning.
Fig. 17. Surface resistance of PEDOT prints on PET fabric: (a) after washing, (b) after dry-cleaning.
Table 2 Initial surface resistances of PEDOT finished textiles selected to washing and dry-cleaning processes depending on the EDOT alcoholic solvent used. Fabric material
Initial surface resistance of PEDOT finished sample, V/sq. Washing
Polyacrylonitrile (PAN) Polyester (PET)
Dry-cleaning
Butyl alcohol
Methyl alcohol
Ethyl alcohol
Butyl alcohol
Methyl alcohol
Ethyl alcohol
269 921
146 1144
229 991
261 912
151 1110
215 995
groups that give it acidic character [47]. The PET fabric has a limited number of acidic carboxylic end groups. In the case of both hydrophobic fabrics, the negative zeta potential values observed in water increased as the anionic surfactant concentration increased suggesting that the amount of surfactant absorbed on the surfaces of these fibres rapidly increased with increased surfactant concentration [48,49]. Recently, Zhang et al. [50] observed a good dispersibility with a positive zeta potential value of +16 mV of synthesised PEDOT microspheres. Probably, in our case a better adhesion of PEDOT on PAN fabric surface is observed than on PET fabric as a result of stronger electrostatic attraction forces between the negatively charged fabric surface and the positively deposited PEDOT during the inkjet printing process, and the formation of stronger anionic bonds than in the case of the PEDOT-deposited PET fabric substrate. However, it can seen that both textile samples studied retain good electro-conductivity after washing or cleaning, which is important from the point of view of their application in textronic use.
4. Conclusion PEDOT formation as a result of the chemical oxidation of EDOT by ferric nitrate9H2O on PAN and PET textile substrates via reactive inkjet printing at a temperature of 85 C is a simple way to achieve very effective conductive textiles. The PEDOT electroconductive layers were deposited on textile substrates placed on a hot plate by printing the selected line of the pattern using first a butyl-, ethyl- or methyl alcohol solution of EDOT and then an aqueous solution of ferric nitrate9H2O. A wide range of modern research techniques confirmed the PEDOT formation on the surface of the textile fabrics. Optical microscopy, SEM-EDS analysis, FTIR, and Raman spectroscopy revealed that PEDOT can be found on the PAN and PET textiles after the in situ finishing via reactive inkjet printing. Changes in surface resistance, measured by means of the four-probe method on the surface of textile substrates, confirmed the formation of the conducting polymer. Talking into account the tested hot plate temperature range 50– 90 C, the optimal value for the in situ deposition of PEDOT on
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textile substrates by reactive inkjet printing seems to be in the range 85–90 C. The lowest surface resistance for the PEDOT/PAN and PEDOT/PET composites was found at a ferric nitrate9H2O concentration of ca. 0.45 M/dm3 with the EDOT solution in methyl alcohol followed by ethyl alcohol. Surprisingly, with the EDOT solution in butyl alcohol the lowest surface resistance was observed at a very low ferric nitrate9H2O concentration of 0.2 M/dm3 and 0.3 M/dm3 for the PEDOT/PAN and PEDOT/PET composites, respectively. The obtained results show that the method proposed is very simple, and can be carried out on the basis of alcohol- and water- containing inks, giving a good adhesion of the formed conductive polymer to the textile substrates and ensuring a very low surface resistance. The mechanism of the PEDOT deposition as well as its adhesion to the textiles on the basis of electrokinetic phenomena, multi-cyclic bending, resistance to washing and dry-cleaning has been proposed and explained. Acknowledgments This research was supported by the Polish National Centre for Research and Development (NCBiR) grant PBS3/A9/34/2015 in the framework of the Applied Research Programme. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. synthmet.2016.04.014. References [1] W. Baik, W. Luan, R.H. Zhao, S. Koo, K. Kim, Synthesis of highly conductive poly (3,4-ethylenedioxythiophene) fiber by simple chemical polymerization, Synth. Met. 159 (2009) 1244–1246. [2] J.H. Chen, C.-A. Dai, W.-Y. Chiu, Synthesis of highly conductive EDOT copolymer films via oxidative chemical in situ polymerization, J. Polym. Sci. Part A Polym. Chem. 46 (2008) 1662–1673. [3] D.M. Welsh, A. Kumar, E.W. Meijer, J.R. Reynolds, Enhanced contrast ratios and rapid switching in electrochromics based on poly(3,4propylenedioxythiophene) derivatives, Adv. Mater. 11 (1999) 1379–1382. [4] F. Jonas, W. Krafft, B. Muys, Poly(3, 4-ethylenedioxythiophene): conductive coatings, technical applications and properties, Macromol. Symp. 100 (1995) 169–173. [5] F. Huang, A.G. MacDiarmid, B.R. Hsieh, An iodine-doped polymer lightemitting diode, Appl. Phys. Lett. 71 (1997) 2415. [6] W.H. Kim, A.J. Mäkinen, N. Nikolov, R. Shashidhar, H. Kim, Z.H. Kafafi, Molecular organic light-emitting diodes using highly conducting polymers as anodes, Appl. Phys. Lett. 80 (2002) 3844–3846. [7] D. Setiadi, Z. He, J. Hajto, T.D. Binnie, Application of a conductive polymer to self-absorbing ferroelectric polymer pyroelectric sensors, Infrared Phys. Technol. 40 (1999) 267–278. [8] H. Yoon, M. Chang, J. Jang, Formation of 1D poly(3,4-ethylenedioxythiophene) nanomaterials in reverse microemulsions and their application to chemical sensors, Adv. Funct. Mater. 17 (2007) 431–436. [9] S.C.J. Meskers, J.K.J. van Duren, R.A.J. Janssen, F. Louwet, L. Groenendaal, Infrared detectors with poly(3,4-ethylenedioxy thiophene)/poly(styrene sulfonic acid) (PEDOT/PSS) as the active material, Adv. Mater. 15 (2003) 613– 616. [10] P. Calvert, D. Duggal, P. Patra, A. Agrawal, A. Sawhney, Conducting polymer and conducting composite strain sensors on textiles, Mol. Cryst. Liq. Cryst. 484 (2008) 291/[657]–302/[668].. [11] Y.-Z. Long, M.-M. Li, C. Gu, M. Wan, J.-L. Duvail, Z. Liu, et al., Recent advances in synthesis, physical properties and applications of conducting polymer nanotubes and nanofibers, Prog. Polym. Sci. 36 (2011) 1415–1442. [12] Y. Xia, M. Wei, Y. Lu, One-step fabrication of conductive poly(3,4ethylenedioxythiophene) hollow spheres in the presence of poly (vinylpyrrolidone), Synth. Met. 159 (2009) 372–376. [13] X. Zhang, J. Lee, G.S. Lee, D. Cha, M.J. Kim, D.J. Yang, et al., Chemical synthesis of PEDOT nanotubes, Macromolecules 39 (2006) 470–472. [14] Y. Lei, H. Oohata, S.I. Kuroda, S. Sasaki, T. Yamamoto, Highly electrically conductive poly(3,4-ethylenedioxythiophene) prepared via highconcentration emulsion polymerization, Synth. Met. 149 (2005) 211–217. [15] H. Behniafar, D. Yousefzadeh, Chemical synthesis of PEDOT/Ag nanocomposites via emulsion technique in silver colloid, Des. Monomers Polym. 18 (2015) 6–11.
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Reactive inkjet printing of PEDOT electroconductive layers on textile surfaces Z. Stempiena,* , E. Rybickib , T. Rybickic , M. Kozaneckib a b c
Institute of Textile Architecture, Lodz University of Technology, 116 Zeromskiego Street, 90-924 Lodz, Poland Department of Molecular Physics, Lodz University of Technology, 116 Zeromskiego Street, 90-924 Lodz, Poland Institute of Automatic Control, Lodz University of Technology, 18/22 Stefanowskiego Street, 90-924 Lodz, Poland
A R T I C L E I N F O
Article history: Received 22 February 2016 Received in revised form 12 April 2016 Accepted 15 April 2016 Available online xxx This work is in memory of our friend and colleague Professor Edward Rybicki, who passed away in 3rd March 2016 prior to publication, without whom this research would not have been possible. He will be greatly missed. Keywords: PEDOT Textiles Reactive inkjet printing Electroconductive layers Textronics
A B S T R A C T
A simple and original method for the deposition of poly (3,4-ethylenedioxythiophene) (PEDOT) on different synthetic textile fabrics by the reactive inkjet printing technique was proposed. PEDOT coated conducting fabrics were obtained by chemical oxidation of an arbitrarily chosen and constant concentration of 3,4-ethylenedioxythiophene (EDOT) of 0.9 M/dm3 in different alcohols (methyl-, ethyland butyl alcohol) by water solutions of 0.1–0.5 M/dm3 ferric nitrate9H2O on polyacrylonitrile (PAN) and poly(ethylene terephthalate) (PET) fabrics at elevated temperatures. The conducting fabrics were chemically characterised by means of Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy and energy-dispersive spectroscopy (EDS). The morphology of the coatings was observed using optical microscopy and scanning electron microscopy (SEM). The conducting properties (surface resistance) of the fabrics were measured by means of the four-probe method. The optimal conditions for PEDOT deposition on textiles (temperature, oxidant concentration) by reactive inkjet printing were established. The influence of the different alcohols used as an EDOT solvent and the effect of water rinsing on the changes in surface resistance were also discussed. The obtained results show that the proposed method is very simple and gives a good adhesion of the in-situ formed PEDOT to the substrate, at the same time achieving a low surface resistance of ca. 150 V/sq. The variation of the surface resistance vs. concentration of ferric nitrate9H2O for both of the textile fabrics was estimated. A mechanism for the PEDOT deposition and adhesion on the textiles based on electrokinetic phenomena is proposed and verified by changes in the relative resistance of PEDOT/PAN and PEDOT/PET composites during multicyclic bending, washing and dry-cleaning processes. ã 2016 Elsevier B.V. All rights reserved.
1. Introduction Among all conducting polymers, poly(3,4-ethylenedioxythiophene) (PEDOT) is unique because of its high transparency, high conductivity, and good thermal stability [1,2]. These unique properties make PEDOT an excellent material for various applications such as in electrochromics [3], antistatic coatings [4], light-emitting diodes [5,6], sensors [7–9], and textronics [10], among others. Synthesis methods for conducting polymer nanofibres, their potential applications and the challenges associated with their use have also been discussed, for example in chemical, optical and bio-sensors, nano-diodes, field effect transistors, field emission and electrochromic displays, supercapacitors and energy storage, actuators, drug delivery, neural interfaces, and protein purification [11]. Synthesis by chemical
* Corresponding author. E-mail address: [email protected] (Z. Stempien). http://dx.doi.org/10.1016/j.synthmet.2016.04.014 0379-6779/ ã 2016 Elsevier B.V. All rights reserved.
oxidative polymerisation of EDOT monomer in an emulsion technique can lead to a nanosized PEDOT polymer with spherical [12], cylindrical [13] or flake morphology [14–16]. Chemical polymerisation of EDOT with ferric chloride to PEDOT has been commercialised as an antistatic coating in photographic film, printed circuit boards and in the capacitor industry [17,18]. The oxidative chemical polymerisation of PEDOT on fabrics can be achieved using several procedures including liquid phase polymerisation (LPP) [19–25], and vapour phase polymerisation (VPP) [26–29], using such oxidants as FeCl3 or p-toluenesulphonic acid (FepTS) [30]. In the LPP method, textile samples are immersed in a mixture of the monomer (EDOT) and the oxidant agent at a temperature below 10 C to delay the polymerisation process [20]. They are then submitted to a drying step, either at ambient temperature or in a convection oven to complete the polymerisation process. In the VPP method, the textile fabrics are first immersed in an oxidant solution and are then exposed to the EDOT in the vapour phase in a reactor with controlled vacuum and
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temperature. In recent years, growing efforts have been focused on the production of smart fabric devices by printing. Amongst the printing technologies available, screen printing, microcontact printing, gravure, flexographic, offset, inkjet printing and reactive inkjet printing are the most commonly used in the production and fabrication of conductive films and structures with conducting polymer formulations [31–33]. In this paper, an original method for the deposition of poly (3,4ethylenedioxythiophene) (PEDOT) reactive inkjet printing on different synthetic textile fabrics was proposed. PEDOT coated conducting fabrics were obtained by the chemical oxidation of 3,4ethylenedioxythiophene (EDOT) in different alcohols (methyl-, ethyl- and butyl-) by water solutions of ferric nitrate9H2O on polyacrylonitrile (PAN) and poly(ethylene terephthalate) (PET) fabrics at elevated temperatures. The conducting fabrics were characterised chemically by means of Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy and energy-dispersive spectroscopy (EDS). The morphology of the coatings was observed by optical microscopy and scanning electron microscopy (SEM). The conducting properties (surface resistance) of the fabrics were measured by means of the four-probe method. The optimal conditions for PEDOT deposition on the textiles (temperature, oxidant concentration, type of alcohol) by reactive inkjet printing were established. The influence of the different alcohols used as an EDOT solvent and the effect of water rinsing on the changes in surface resistance were also discussed. A mechanism for the PEDOT deposition and adhesion on textiles based on electrokinetic phenomena, and changes in the relative resistance of PEDOT/PAN and PEDOT/PET composites during multi-cyclic bending, washing and dry-cleaning processes is proposed. 2. Materials and methods 2.1. Reagents and materials 3,4-ethylenedioxythiophene (EDOT) 97% was obtained from Aldrich, and used as received. Analytical grade ferric nitrate9H2O obtained from Chempur (Poland) was used without purification. As substrates, two different commercial un-dyed textile fabrics made of synthetic fibres were employed in the experiments. Table 1 shows the fundamental characteristics of the textile substrates used. Before printing, the textile substrates were ultrasonically cleaned with methanol and isopropanol for 20 min in each bath, then rinsed in an ultrasonic bath with deionised water and dried in an oven at the temperature of 40 C. 2.2. Deposition of PEDOT layers on textiles PEDOT electro-conductive layers on flat textile surfaces were printed using the inkjet printing technique, in accordance with the scheme presented in Fig. 1a. For this purpose, a prototype of a digital inkjet printer shown in Fig. 1b was used. It contained (Fig. 1b): (1) nanodispensing hardware, represented by two one-nozzle valve-jet print heads (ReaJet SK 1/080, Germany), (2) drive electronics for generating electrical signals to the print-heads; (3) an ink pump system and
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two ink reservoirs; (4) a two-axis (x-y) motion system for print head positioning with a step resolution of 35 mm, based on stepper motor linear drives; (5) a computer control system with software allowing the selection of a printing pattern and triggering of drops. Before printing, the ink reservoir of print head 1 was filled with an ink composition prepared from a butyl-, ethyl- or methyl alcohol solution of EDOT at a concentration of 0.9 M/dm3, and the ink reservoir of print head 2 was filled with an ink composition prepared from an aqueous solution of ferric nitrate9H2O in the concentration range 0.1–0.5 M/dm3. The printed pattern was designed using a vector graphics editor and converted to a lineart bitmap using raster image processing software. The resolution of the line-art bitmap was 50 dpi and was the same as the resolution of the x-y print heads positioning system (50 steps per inch in the x and y axes). On the basis of the line-art bitmap, the drive electronics system controlled the position of the print heads and the release of drops. The volume of the released drops was ca. 30 nl. The electroconductive layers were deposited line by line onto the surface of the fabrics in such a way that the first nozzle sprayed the selected line of the pattern using a butyl-, ethyl- or methyl alcohol solution of EDOT and then the second nozzle sprayed the same line of the pattern using an aqueous solution of ferric nitrate9H2O, or vice versa. The presented method for the fabrication of PEDOT electro-conductive layers on the surface of fabrics has been patented at the Polish Patent Office [34]. 2.3. Characteristics 2.3.1. Scanning electron microscopy Scanning electron microscopy with EDS was used to determine the morphology of the PEDOT layers and assess the distribution of PEDOT on the textile surface. Energy-dispersive X-ray spectroscopy extends the usefulness of SEM investigations by enabling elemental analysis within regions of a few cubic micrometres. The material was characterised by SEM using a field emission S4700 (Hitachi, Japan) equipped with an energy-dispersive spectrometer (Thermo-Noran, USA). 2.3.2. FTIR spectroscopy FTIR spectra of PAN fabric, a PEDOT reference and PEDOT deposited on PAN were performed on a Nicolet iS50 FT-IR spectrometer (Thermo Fisher Scientific, Inc.) equipped with an ATR accessory with a diamond crystal. The spectral resolution was 2 cm 1 and 256 scans were averaged for all IR spectra. The reference PAN fabric as well as the PEDOT coated PAN were measured without any special treatment. The reference PEDOT sample was used as a powder. The reference PEDOT powder was prepared by dropping 1 ml of EDOT solution in 10 ml of methyl alcohol into 50 ml of an aqueous solution of 0.5 M/dm3 ferric nitrate9H2O, and the mixture was stirred for 36 h. The black sludge formed was then decanted, rinsed several times with bidistilled water and finally rinsed with methyl alcohol. The product was dried at ambient temperature for 24 h. 2.3.3. Raman spectroscopy Raman spectra were collected using a MultiRAM FT Raman spectrometer (Bruker GmbH, Germany) in back-scattering
Table 1 Characteristics of textile substrates used. Fabric material
Fabric weave type
Surface weight, g/m2
Thickness, mm
Warp density, yarns/cm
Weft density, yarns/cm
Polyacrylonitrile (PAN) Polyester (PET)
twill 2/1 twill 3/1
220 150
0.82 0.34
24 24
17 24
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Fig. 1. Reactive inkjet printing of PEDOT layers on textile surfaces: (a) scheme of deposition, (b) view of digital inkjet printer used.
configuration with a spectral resolution of 2 cm 1. The standard neodymium-doped yttrium aluminium garnet (Nd:YAG) laser (l = 1064 nm) was used as a source of excitation light. The laser power and the number of averaged scans were individually selected for each sample to protect them against overheating and to simultaneously ensure high-quality spectra. Deconvolution of the Raman spectra was performed using PeakFit v.4.12 software after subtraction of linear baseline. 2.3.4. Surface resistance The surface resistance of the printed samples was estimated using the four-point method adapted to textiles. The four-point probe setup used was presented in detail by Stempien et al. [32]. For each fabric, three independent PEDOT prints were tested. The surface resistance was finally calculated as an average value of the measurements and the standard deviation (SD) was determined. 2.3.5. Resistance to bending The bending test equipment is shown in Fig. 2. It consists of a rotating bending system with a stepper motor controlled by a computer system. A textile sample with a PEDOT electroconductive layer is mounted between the fixed and rotating clamps. The mounting edge of the rotating clamp acts as a bending line. It is situated in the rotation axis of the stepper motor, which allows for the repetitive sharp bending of the sample by 30 deg.
The electrical resistance of tested sample was measured in real time using a Picotest M3500A ohmmeter. On the basis of these measurements, the relative resistance R/R0 was determined, where R denotes a resistance measured during sample flexing and R0 the initial resistance of the sample, respectively. 2.3.6. Washing and dry-cleaning processes Changes in the surface resistance of the PEDOT electroconductive layers were estimated after washing and dry-cleaning processes. The details of the washing and dry-cleaning procedures are as follows: a) The washing process was carried out in a Linitest laboratory washing machine (Germany), for 30 min at 40 C. The liquor to sample weight ratio was 100:1. The samples with PEDOT deposited textiles were washed in distilled water containing 0.5% non-ionic surfactant (MARLIPAL O13/70), a product of Sasol (Germany). b) The dry-cleaning process in tetrachloroethylene (PER) was carried out in a Linitest laboratory washing machine (Germany), for 30 min at ambient temperature. The liquor to sample weight ratio was 100:1. After dry-cleaning the samples were left for several hours at room temperature to allow the solvent to evaporate completely.
Fig. 2. Schematic diagram of multi-cyclic bending of textile sample (left) and a textile sample mounted in the clamps during testing (right).
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Fig. 3. Surface resistance of PEDOT prints on PAN fabric vs. hot plate temperature.
3. Results and discussion 3.1. Effect of hot plate temperature on surface resistance Fig. 3 presents the changes in surface resistance of PEDOT prints on PAN textile substrates using EDOT solution in methyl alcohol and 0.3 M/dm3 ferric nitrate9H2O as oxidant with changing temperature, and Fig. 4 shows views taken with an Olympus stereomicroscope SZX-10 under the same temperature conditions. As can be seen in Fig. 3, in the temperature range of 50–70 C almost a linear increase in surface resistance is achieved with a maximum at 70 C. Above this temperature, a sudden decrease in surface resistance (increase in conductivity) is observed. The optimal conditions for the in-situ deposition of PEDOT on textile samples by reactive inkjet-printing seem to be in the range 85–
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90 C, taking into account the boiling points of methyl-, ethyl-, and butyl alcohols as 64.7 C, 78.4 C, and 117.7 C at 760 mm Hg, respectively. It is known from the principles of physical chemistry that a linear decrease in the surface tension of alcohols vs. temperature is observed. At the same time, a steady decrease in the dynamic viscosity of alcohols vs. temperature is also expected. The dissolution of EDOT in all of the alcohols studied can cause an increase both in the surface tension and the dynamic viscosity of the EDOT-alcohol binary mixtures but the changes of these parameters vs. temperature should have similar characteristics. Fig. 4 shows optical microscope images of PEDOT/PAN composites printed at different hot plate temperatures. On the left-hand side in prints up to temperature 70 C clear and fuzzy shadows are observed. An increase in temperature makes the border between the pristine PAN fabric and the PEDOT-deposited PAN textile practically invisible, and sharp edges, especially at temperature 90 C, are observed. Probably, in the temperature range of 50–70 C, a competition between the changes in surface tension and dynamic viscosity caused by temperature (better wetting and spreading of the methanol-EDOT solution on PAN fabric), and evaporation of the solvent lead to rather poor quality inkjet-prints and a worsening of conductivity. In the temperature range of 80– 90 C the solvent evaporation process probably prevails, which contributes to a uniform distribution of the EDOT-alcohol solution on the textile surface and a sudden decrease in the surface resistance is observed. Recently, Wua et al. [35] studied the role of the VPP temperature on the PEDOT polymerisation rate in formation of thin films on glass substrate. The VPP temperature plays key roles on the polymerisation reaction rate and the mobility of the polymer chains as well as the volatilisation of the reactant and product, which result in drastically different properties of the VPP PEDOT films. In the
Fig. 4. Optical microscope images of PEDOT/PAN composites printed at different hot plate temperatures.
Fig. 5. Optical microscope images of PEDOT/PAN composites printed using a butyl alcohol solution of EDOT: (a) PAN substrate; ferric nitrate9H2O concentration (b) 0.1 M/ dm3, (c) 0.25 M/dm3, (d) 0.4 M/dm3.
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Fig. 6. Optical microscope images of PEDOT/PET composites printed by using a butyl alcohol solution of EDOT: (a) PET substrate; ferric nitrate9H2O concentration (b) 0.1 M/ dm3, (c) 0.25 M/dm3, (d) 0.4 M/dm3.
Fig. 7. SEM of PEDOT/PAN composite at magnifications of 4850 and 10,800 after printing.
Fig. 8. SEM of PEDOT/PAN composite at magnifications of 5000x and 10,000 after rinsing in deionised water.
range of 33–57 C, the lattice structure and the morphology of the films can be significantly affected by the VPP temperature. The optimum temperature (46 1 C) resulted in a PEDOT film with the
highest conductivity on the glass substrate (622 S/cm), which is ascribed to its highly ordered crystal structure and smooth morphology. The film conductivity increased with the temperature
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Fig. 9. SEM micrograph (2500) and EDS measurements of the PEDOT/PAN composite after printing.
up to 46 C, and then decreased beyond this temperature. This report may partly correspond to our findings although in our case the situation seems to be more complicated. 3.2. Optical microscope images of PEDOT-deposited textiles Figs. 5–6 and 1S–4S (Supporting information S1) show photographs (taken with an Olympus stereomicroscope SZX-10, magnification 3) of a representative set of samples of PEDOT printed on PAN and PET textile substrates using butyl-, methyl- and ethyl alcohol solutions of EDOT and aqueous solution of ferric nitrate in the concentration range 0.1–0.4 M/dm3. The images presented show that the PEDOT prints on different textile substrates have uniform dark hues, which are characteristic of PEDOT formation. It should be noted that for both textile
substrates, quite good, sharp-edged prints were obtained and a lack of PEDOT spread and coffee-ring effect was observed. This is important from the viewpoint of fabrication of precision electroconductive wires and electrodes on textile surfaces by the inkjet printing technique for textronic applications. It allows for the fabrication of narrow and close placed wires and electrodes with different shapes in different regions of a textronic device. The magnified printed surfaces show a quite good uniform distribution of PEDOT on the textile surfaces. 3.3. Surface morphology of PEDOT layers SEM images of the PEDOT/PAN composite samples in both variants, after printing and rinsing in de-ionised water, were taken at different magnifications from 1000 to 10,000 for the 0.3 M/
Fig. 10. SEM micrograph (2500) and EDS measurements of the PEDOT/PAN composite after rinsing in deionised water.
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dm3 ferric nitrate9H2O concentration. Figs. 7 and 3S (Supporting information S2) and Fig. 8 and 4S (Supporting information S2) present the SEM images of PEDOT prints on PAN textile substrates after printing and rinsing in de-ionised water, respectively. The presence of PEDOT can be clearly seen on the inkjet-printed PAN fabric. It is evident from Figs. 7 and 8 that in general the fabric yarns were completely coated, and some areas between yarns were coated by small and large granular PEDOT having a non-uniform and non-smooth surface. Overall observation of the SEM study shows that the PAN fabric was successfully covered with PEDOT and that PEDOT penetration onto and into the structure of the fabric provided conductivity to the fabric. The PEDOT/PAN composite samples in both variants, after printing and rinsing in deionised water, were also subjected to EDS survey analyses to identify the elements present on the fabric surface (Figs. 9 and 10). EDS studies revealed that the PEDOT print on the PAN fabric contains mainly carbon, iron and sulphur. The carbon peak on the EDS spectrum may come from PEDOT as well as from the PAN fabric. The sulphur and iron peaks come from EDOT and the oxidant that were used during the in-situ polymerisation of PEDOT by the reactive inkjet printing. Comparison of the SEM images at 2000x magnification with the EDS investigations of the analysed samples show that, on the surface of the PAN fabric, a layer of PEDOT is clearly visible. Bright spots appearing on the Xray maps (Fig. 9) depict places with higher concentrations of the studied element, and also confirm the presence of globular structures, especially in the case of sulphur. 3.4. FTIR and Raman spectra Fig. 11a presents an FTIR spectrum of PEDOT deposited on PAN textile. The IR spectrum is dominated by PAN peaks and only some weak lines (marked by arrows) confirm the presence of PEDOT. These lines are too weak for more detailed analysis. These results suggest that the thickness of the PEDOT film is relatively low and the IR radiation at the wavelengths used passes freely through the PEDOT film without strong absorption. However, a differential spectrum prepared by subtraction of the raw PAN textile spectrum from the spectrum of coated PAN (Fig. 11b) shows that PEDOT successfully coated the support. The positions of the spectral lines in the differential spectrum correspond well to those found in the spectrum of reference PEDOT. In Raman spectroscopy, the spectrum of the PAN textile coated by PEDOT is practically identical with that of the standard PEDOT sample (Fig. 12). The presented results directly confirm the continuity of PEDOT film, as the Raman lines characteristic for PAN are invisible. The strong Raman signal of PEDOT probably resulted from a resonance effect. The Raman spectrum of PEDOT is sensitive to the wavelength of irradiation used for sample excitation [37,38]. The 1064 nm laser line, used to excite the samples in the current investigations, is in resonance with oxidised forms of PEDOT. Thus, the intensity of bands related to polarons and bipolarons should be especially raised. These effects make direct comparison of Raman peak intensities impossible at various excitation wavelengths. It is noteworthy that the resonance conditions strongly depend on the oxidation state of PEDOT, as its absorption in the Vis-NIR region is strictly related to its electronic structure [37,38]. The most interesting spectral region (1200–1700 cm 1) containing PEDOT modes sensitive to oxidation is presented as an inset in Fig. 12 [37–40]. It is notable that according to Garreau et al. [37] the polarons in PEDOT (first oxidation state) correspond to a quinoid structure, while the bipolarons (second oxidation state) to the benzoid form. A multitude of Raman peaks overlapping each other are observed between 1200 and 1700 cm 1 showing that PEDOT occurs in more than one form. Such features are characteristic for various conducting polymers [41]. Peaks located
Fig. 11. FTIR spectra: (a) PEDOT, PAN and PEDOT deposited on PAN textile, where arrows indicate bands corresponding to PEDOT, asterisks to the nitrate ion being a counter ion for PEDOT (assignment based on [36]), and crosses to vestigial water. (b) differential spectrum prepared by subtraction of the raw PAN textile spectrum from the spectrum of coated PAN, where the red numbers correspond to IR absorption bands characteristic for the reference PEDOT sample, while the black numbers correspond to the maxima of the differential spectrum. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
at 1531, 1561, 1427 and 1261 cm 1 are characteristic for polarons. A peak at 1292 cm 1 proved that the bipolarons are also present in the investigated sample. This is the highest frequency mode of the triplet characteristic for bipolarons – 1239, 1268, 1289 cm 1. Also the peak with a maximum at 1455 cm 1 may be assigned to the highest oxidation state of PEDOT. As mentioned above, the lines of the oxidised PEDOT structures are in resonance with the used excited light, thus, the peaks of neutral part of PEDOT chain are weakly visible. However, the presence of bands with maxima at 1496 and 1228 cm 1 suggests that PEDOT is partially in reduced form. The rest of the modes characteristic for this form (1545, 1411, 1271, 1249 cm 1) are probably masked by Raman peaks of polarons and bipolarons. Assignment of the main vibrational bands is not simple and remains controversial (compare [37,38],[40,42]). Table 1S (Supporting information S3) presents a comparison of the PEDOT IR and Raman peaks found in this work with data from the literature. An absorption band with a maximum at 1650 cm 1 is visible in the spectra of all materials (marked in Fig. 11 by cross) and has been
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Fig. 12. Raman spectra of PEDOT, PAN and PEDOT deposited on PAN textile. Deconvolution of the range of Raman spectrum sensitive to PEDOT oxidation is presented in the inset.
assigned to residual moisture adsorbed on the material surface. Lines at 836, 1361 and 1597 cm 1 in the spectrum of standard PEDOT were attributed to the nitrate counter ion [36]. 3.5. Surface resistance Figs. 13 and 14 present the surface resistance of PEDOT prints on PAN and PET textile substrates vs. ferric nitrate9H2O concentration after printing and rinsing in deionised water. A steady decrease in the surface resistance with increasing ferric nitrate9H2O concentration for both the investigated textile substrates was observed. The variation in the surface resistance vs. ferric nitrate9H2O concentration curves is strongly affected by the type of textile substrate used. The exact data for the surface resistance changes for the both PEDOT/PAN and PEDOT/PET composites vs. ferric nitrate9H2O concentration in the concentration range of 0.1–0.5 M/dm3 are shown in Table 2S (Supporting information). The lowest values of surface resistance achieved for both PEDOT/ fabric composites are in bold type. Of the tested textile substrates,
the PAN fabric seems to be the most suitable for the manufacture of highly conductive PEDOT textiles by the reactive inkjet printing technique. The lowest surface resistance for the PEDOT/PAN and PEDOT/PET composites was found at a ferric nitrate9H2O concentration of ca. 0.45 M/dm3 with EDOT solution in methyl alcohol followed by ethyl alcohol. Surprisingly, using EDOT solution in butyl alcohol the lowest surface resistance was observed at very low ferric nitrate9H2O concentrations of 0.2 M/dm3 and 0.3 M/dm3 for the PEDOT/PAN and PEDOT/PET composites, respectively. After rinsing the PEDOT/fabric composites in de-ionised water slight changes in surface resistance increase were observed. The rinsing water was tinted yellow-green as a result of washing out of ferric ions from the PEDOT inkjetdeposited layers. After rinsing in de-ionised water and drying the fabric samples lost their stiffness and became more flexible, maintaining good conductivity. It should be noted that ferric nitrate9H2O proved to be an efficient oxidant during the in-situ deposition of PEDOT on textiles, acting at very low concentrations, taking into account the high water content in the molecule.
Fig. 13. Surface resistance of PEDOT prints on PAN fabric vs. ferric nitrate concentration: (a) after printing, (b) after rinsing in deionised water.
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Fig. 14. Surface resistance of PEDOT prints on PET fabric vs. ferric nitrate concentration: (a) after printing, (b) after rinsing in deionised water.
The effect of different alcohols (ethyl- and butyl-) used in the insitu polymerisation process of PEDOT on nylon 6 has been investigated [19]. The conductivities of PEDOT/nylon 6 composite fabrics treated in ethanol were generally higher than those of fabrics treated in butanol. Moreover, the conductivities of PEDOT/ nylon 6 composite fabrics treated in ethanol increased with increasing polymerisation cycles and reached a maximum value (1.89 S/cm) at five cycles. In the case of a tosylate-doped PEDOT (PEDOT-OTs)-alcohol series, conductivity increased with decreasing solvent chain length. It was suggested that the alcoholic solvents are hydrogen bonded to the counter ion of PEDOT and a compact packing of PEDOT chains is perturbed by the use of alcoholic solvents with longer chain lengths during the polymerisation of PEDOT-OTs, which was supported by the XRD experiment [43]. In our case, change of the EDOT solvent caused an offset on the left in the optimal concentration of oxidant solution at which the lowest surface resistance was observed (i.e. 0.5 M/dm3 for methyl alcohol, 0.45 M/dm3 for ethyl alcohol, and 0.2 M/dm3 for butyl alcohol, respectively) for PAN fabric samples. Similarly, for PET fabric samples, the optimal values of oxidant concentration during the inkjet printed deposition of PEDOT, were respectively 0.45 M/ dm3 for the methyl alcohol solution of EDOT, 0.40 M/dm3 for the ethyl alcohol solution of EDOT, and 0.30 M/dm3 for the butyl alcohol solution of EDOT (cf. Supporting information 2S).
PEDOT layers were printed using a butyl alcohol solution of EDOT and a ferric nitrate9H2O concentration of 0.3 M/dm3. The initial resistances of PAN and PET PEDOT-printed textiles were 663 V and 1442 V, respectively. After the 50,000 bending cycles, the final resistances of the samples studied were ca. 1503 V and 3240 V, respectively. It was observed that the relative resistance for the PEDOT-finished textiles rapidly increased by 100–130% from an initial value due to the formation of cracks in the PEDOT layer and after about 20–30 bending cycles it gradually increased by ca. 1% per 1000 bending cycles. The first rapid increases in relative resistance can be attributed to the limited mobility and brittleness of PEDOT material, which suffers from deteriorating performance because of cracking resulting from its relatively poor elongation and bending properties [44–46]. The further gradual increases in relative resistance can be attributed to structural changes in the PAN and PET fabrics during mechanical deformation caused by multi-cycle bending process. With changes in the fabric structure along the bending line, caused by cyclic elongation of the yarns, damage to the fabric structure was induced and probably the formation of cracks in PEDOT located inside of yarns between the fibres was responsible for the gradual increase in relative resistance.
3.6. Resistance to bending
Figs. 16 and 17 present the changes in surface resistance of PEDOT prints on PAN and PET textile substrates after multicycle washing and dry-cleaning. In Table 2, the initial surface resistances of PEDOT finished textiles selected for washing and dry-cleaning depending on the oxidant solvent used are presented. Figs. 16a and 17a show the changes in surface resistance of inkjet-printed PEDOT layers on PAN and PET fabric samples after washing distilled water containing 0.5% of MARLIPAL O13/70 vs. the number of washing cycles. It can be seen that in the case of the PAN fabric sample, an increasing number of washing cycles causes a steady increase in the surface resistance with a levelling-off tendency after four washing cycles. A quite different situation can be observed in the case of the PET prints (Fig. 17a). An almost linear increase in the surface resistance is observed in this case. This result shows that the inkjet-printed PEDOT layer on PET fabric significantly deteriorates after multi-cycle washing with a steady worsening in surface resistance. Figs. 16b and 17b show changes in the surface resistance of PEDOT prints on PAN and PET fabrics after multicycle dry-cleaning in tetrachloroethylene. The data obtained show that the inkjet deposited PEDOT on both the textile surfaces is very resistant to the multicycle dry-cleaning. The differing behaviour of the PEDOT inkjet-deposited fabric samples is probably due to varying PEDOT adhesion to the surface of the substrates. The PAN fabric may have sulphate and/or sulphonate
Fig. 15 shows the changes in the relative resistance of the PEDOT prints on PAN and PET fabrics after 50,000 bending cycles. The
Fig. 15. Changes in the relative resistance of PEDOT-finished textiles after multicycle bending.
3.7. Washing and dry-cleaning processes
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Fig. 16. Surface resistance of PEDOT prints on PAN fabric: (a) after washing, (b) after dry-cleaning.
Fig. 17. Surface resistance of PEDOT prints on PET fabric: (a) after washing, (b) after dry-cleaning.
Table 2 Initial surface resistances of PEDOT finished textiles selected to washing and dry-cleaning processes depending on the EDOT alcoholic solvent used. Fabric material
Initial surface resistance of PEDOT finished sample, V/sq. Washing
Polyacrylonitrile (PAN) Polyester (PET)
Dry-cleaning
Butyl alcohol
Methyl alcohol
Ethyl alcohol
Butyl alcohol
Methyl alcohol
Ethyl alcohol
269 921
146 1144
229 991
261 912
151 1110
215 995
groups that give it acidic character [47]. The PET fabric has a limited number of acidic carboxylic end groups. In the case of both hydrophobic fabrics, the negative zeta potential values observed in water increased as the anionic surfactant concentration increased suggesting that the amount of surfactant absorbed on the surfaces of these fibres rapidly increased with increased surfactant concentration [48,49]. Recently, Zhang et al. [50] observed a good dispersibility with a positive zeta potential value of +16 mV of synthesised PEDOT microspheres. Probably, in our case a better adhesion of PEDOT on PAN fabric surface is observed than on PET fabric as a result of stronger electrostatic attraction forces between the negatively charged fabric surface and the positively deposited PEDOT during the inkjet printing process, and the formation of stronger anionic bonds than in the case of the PEDOT-deposited PET fabric substrate. However, it can seen that both textile samples studied retain good electro-conductivity after washing or cleaning, which is important from the point of view of their application in textronic use.
4. Conclusion PEDOT formation as a result of the chemical oxidation of EDOT by ferric nitrate9H2O on PAN and PET textile substrates via reactive inkjet printing at a temperature of 85 C is a simple way to achieve very effective conductive textiles. The PEDOT electroconductive layers were deposited on textile substrates placed on a hot plate by printing the selected line of the pattern using first a butyl-, ethyl- or methyl alcohol solution of EDOT and then an aqueous solution of ferric nitrate9H2O. A wide range of modern research techniques confirmed the PEDOT formation on the surface of the textile fabrics. Optical microscopy, SEM-EDS analysis, FTIR, and Raman spectroscopy revealed that PEDOT can be found on the PAN and PET textiles after the in situ finishing via reactive inkjet printing. Changes in surface resistance, measured by means of the four-probe method on the surface of textile substrates, confirmed the formation of the conducting polymer. Talking into account the tested hot plate temperature range 50– 90 C, the optimal value for the in situ deposition of PEDOT on
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textile substrates by reactive inkjet printing seems to be in the range 85–90 C. The lowest surface resistance for the PEDOT/PAN and PEDOT/PET composites was found at a ferric nitrate9H2O concentration of ca. 0.45 M/dm3 with the EDOT solution in methyl alcohol followed by ethyl alcohol. Surprisingly, with the EDOT solution in butyl alcohol the lowest surface resistance was observed at a very low ferric nitrate9H2O concentration of 0.2 M/dm3 and 0.3 M/dm3 for the PEDOT/PAN and PEDOT/PET composites, respectively. The obtained results show that the method proposed is very simple, and can be carried out on the basis of alcohol- and water- containing inks, giving a good adhesion of the formed conductive polymer to the textile substrates and ensuring a very low surface resistance. The mechanism of the PEDOT deposition as well as its adhesion to the textiles on the basis of electrokinetic phenomena, multi-cyclic bending, resistance to washing and dry-cleaning has been proposed and explained. Acknowledgments This research was supported by the Polish National Centre for Research and Development (NCBiR) grant PBS3/A9/34/2015 in the framework of the Applied Research Programme. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. synthmet.2016.04.014. References [1] W. Baik, W. Luan, R.H. Zhao, S. Koo, K. Kim, Synthesis of highly conductive poly (3,4-ethylenedioxythiophene) fiber by simple chemical polymerization, Synth. Met. 159 (2009) 1244–1246. [2] J.H. Chen, C.-A. Dai, W.-Y. Chiu, Synthesis of highly conductive EDOT copolymer films via oxidative chemical in situ polymerization, J. Polym. Sci. Part A Polym. Chem. 46 (2008) 1662–1673. [3] D.M. Welsh, A. Kumar, E.W. Meijer, J.R. Reynolds, Enhanced contrast ratios and rapid switching in electrochromics based on poly(3,4propylenedioxythiophene) derivatives, Adv. Mater. 11 (1999) 1379–1382. [4] F. Jonas, W. Krafft, B. Muys, Poly(3, 4-ethylenedioxythiophene): conductive coatings, technical applications and properties, Macromol. Symp. 100 (1995) 169–173. [5] F. Huang, A.G. MacDiarmid, B.R. Hsieh, An iodine-doped polymer lightemitting diode, Appl. Phys. Lett. 71 (1997) 2415. [6] W.H. Kim, A.J. Mäkinen, N. Nikolov, R. Shashidhar, H. Kim, Z.H. Kafafi, Molecular organic light-emitting diodes using highly conducting polymers as anodes, Appl. Phys. Lett. 80 (2002) 3844–3846. [7] D. Setiadi, Z. He, J. Hajto, T.D. Binnie, Application of a conductive polymer to self-absorbing ferroelectric polymer pyroelectric sensors, Infrared Phys. Technol. 40 (1999) 267–278. [8] H. Yoon, M. Chang, J. Jang, Formation of 1D poly(3,4-ethylenedioxythiophene) nanomaterials in reverse microemulsions and their application to chemical sensors, Adv. Funct. Mater. 17 (2007) 431–436. [9] S.C.J. Meskers, J.K.J. van Duren, R.A.J. Janssen, F. Louwet, L. Groenendaal, Infrared detectors with poly(3,4-ethylenedioxy thiophene)/poly(styrene sulfonic acid) (PEDOT/PSS) as the active material, Adv. Mater. 15 (2003) 613– 616. [10] P. Calvert, D. Duggal, P. Patra, A. Agrawal, A. Sawhney, Conducting polymer and conducting composite strain sensors on textiles, Mol. Cryst. Liq. Cryst. 484 (2008) 291/[657]–302/[668].. [11] Y.-Z. Long, M.-M. Li, C. Gu, M. Wan, J.-L. Duvail, Z. Liu, et al., Recent advances in synthesis, physical properties and applications of conducting polymer nanotubes and nanofibers, Prog. Polym. Sci. 36 (2011) 1415–1442. [12] Y. Xia, M. Wei, Y. Lu, One-step fabrication of conductive poly(3,4ethylenedioxythiophene) hollow spheres in the presence of poly (vinylpyrrolidone), Synth. Met. 159 (2009) 372–376. [13] X. Zhang, J. Lee, G.S. Lee, D. Cha, M.J. Kim, D.J. Yang, et al., Chemical synthesis of PEDOT nanotubes, Macromolecules 39 (2006) 470–472. [14] Y. Lei, H. Oohata, S.I. Kuroda, S. Sasaki, T. Yamamoto, Highly electrically conductive poly(3,4-ethylenedioxythiophene) prepared via highconcentration emulsion polymerization, Synth. Met. 149 (2005) 211–217. [15] H. Behniafar, D. Yousefzadeh, Chemical synthesis of PEDOT/Ag nanocomposites via emulsion technique in silver colloid, Des. Monomers Polym. 18 (2015) 6–11.
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