Preparation and characterization of PSA/PEDOT conductive composite yarns

Abstract

Polysulfonamide/poly(3,4-ethylenedioxythiophene) (PSA/PEDOT) conductive composite yarns were prepared by the vapor phase polymerization technique. Ferric chloride (FeCl₃) was used as the oxidant initiator with five different concentration settings (20, 40, 60, 80, and 100 g/L). The effects of oxidant concentration on the chemical composition, mechanical properties, and electrical conductivity of PSA/PEDOT composite yarns were analyzed. The surface resistance and mass-specific resistance of conductive yarns were measured to investigate its conductive behavior in terms of oxidant concentration, reaction time, impregnating time, and heating temperature. The effects of the applied voltage and the yarn’s combination structures (knotted, bundled, series, and parallel) on the electrothermal properties were determined using a direct current regulated power. It was concluded that the molecular structure and chemical composition of PSA is not changed significantly with the deposition of PEDOT. The optimized deposition settings for the preparation of the PSA/PEDOT conductive composite yarns were found to be 10 min (reaction time), 60 min (impregnating time), 80℃ (heating temperature), and 80 g/L (FeCl₃ concentration). Correspondingly, the mass-specific resistance of PSA/PEDOT composite yarns could be up to 0.94 Ω g cm⁻². The maximum heating temperature of PSA/PEDOT conductive composite yarns during the electrical heating procedure could be increased rapidly with an increase of applied voltage and then tended to be stable. The electrothermal properties of PSA/PEDOT conductive composite yarns with different combination structures (knotted, bundled, series, and parallel) have been investigated systematically. This study presents a new way to develop conductive polymer based yarns, which can be used as fibrous sensors, connection devices in smart clothing, and for electromagnetic shielding applications.

Keywords

polysulfonamide PEDOT vapor phase polymerization conductive composite yarns heating structure electrothermal properties

Composite yarns with high electrical conductivity have been considered as a key component in the development of smart textiles, which are widely used in fiber optics and piezoelectric sensors.^1,2 Polysulfonamide (PSA) fiber is known to be a functional flame retardant material with excellent heat resistance, flame retardancy, as well as excellent thermal stability. However, the electrical resistance of the raw PSA material is relatively high, which causes some difficulties in its manufacturing procedures and limits its application in the development of functional textile products.³ Among the intrinsically conductive polymers, poly(3,4-ethylenedioxythiophene) (PEDOT) with good stability and monomer control has been considered as a new type of conductive polymer material possessing regular molecular structure, low energy gap, excellent electrical conductivity, etc. Vapor phase polymerization (VPP) has been used for making conducting polymers. This method was originally used to prepare electrically conducting polypyrrole.⁴ The first use of VPP to prepare PEDOT has been described by Kim’s group,⁵ who found that homogeneous thin conductive PEDOT films were uniformly fabricated at nano-level thickness on the substrates. Polymers derived from alkoxythiophenes like 3,4-ethylenedioxythiophene (EDOT) yielding PEDOT on the textile have attracted many researchers’ attention recently.⁶

It was reported that Kim and his team members have developed conductive textiles by polymerization of EDOT to serve as shielding of electromagnetic noise.^7,8 Usually, PEDOT can been deposited by chemical and electrochemical oxidation on a polyester textile, which contributes to a decrease in its conductivity upon stretching, thus enabling the textiles to be used as one kind of strain sensor and the related attempts have been reported.^8,9 Knittel and Schollmeyer reported that the electrical conductivity on textiles made of poly(ethylene terephthalate) and PA could be improved significantly by oxidative polymerization of EDOT with stable and flexible heating or EMI-shielding elements.⁶ Bashir et al. studied an oxidative vapor deposition process which could be used for the coating of flexible textile fibers (viscose) with PEDOT in the presence of FeCl₃ oxidant.¹ Bashir et al. also pointed out that PET with good mechanical properties is one kind of suitable substrate that is better than viscose.¹⁰ Laforgue prepared PEDOT nanofibers by the combination of electro-spinning and VPP;¹¹ it was reported that the conductivity of the prepared nanofiber mats was as high as 60 ± 10 S cm⁻¹. Recent studies have illustrated the prospective applications of conductive textiles.^12–14

In general, PSA staples could be one kind of substrate material and it is propitious to conduct the deposition of PEDOT with the action of FeCl₃. However, very few studies on the deposition of PEDOT on the PSA yarns have been reported. In this study, a self-developed deposition method used for the coating of PEDOT on PSA yarns has been developed and related experiments conducted for the investigation of the newly developed PSA/PEDOT yarns. According to our experimental results, the electrical conductivity and electrothermal properties of PSA/PEDOT conductive composite yarns could be enhanced. There are three sections included in this paper. The following section describes the experimental design and the materials used. Subsequent sections are about the characterization of the prepared PSA/PEDOT yarns and the analysis of the experimental results.

Experimental procedure

Materials and chemicals

PSA staple yarns with a linear density of 115 tex were selected as substrate material, as supplied by Shanghai Tanlon Fiber Co., Ltd, Shanghai. The chemicals used were EDOT of analytical reagent (AR) grade supplied by Herochem, Shanghai., FeCl₃·6H₂O of AR grade supplied by Sinopharm Chemical Reagent Co., Ltd, Shanghai., and acetone and ethanol as analytical reagents supplied by Sinopharm Chemical Reagent Co., Ltd, Shanghai. All the chemicals used were of laboratory grade and they were used as received. The water used was deionized water.

Preparation procedures

The preparation procedure of PSA/PEDOT conductive composite yarns is illustrated in Figure 1. By polymerization of EDOT monomer in the vapor phase, PEDOT partially impregnated into the fiber was deposited on the surface of the PSA staple yarns. The PSA yarns were rinsed in an acetone solution, ultrasonically cleaned for 30 min, and dried in an oven at 80℃. FeCl₃ was applied as an oxidant and its concentrations were selected from 20 g/L to 100 g/L. The pre-sourced PSA yarns were first impregnated with FeCl₃ solution at room temperature for a range of times from 10 min to 60 min, and then dried in air. The oxidant-enriched yarns were then inserted into the self-made reaction chamber for 10–30 min. EDOT monomer was evaporated in a vacuum and its base pressure lowered to about 5 × 10⁻⁵Pa. When oxidant-enriched fibers were exposed to EDOT monomer vapor, a polymerization reaction was started spontaneously and PSA yarns were coated with a darkish blue layer of PEDOT. After polymerization, the PEDOT-coated yarns were washed thoroughly with deionized water and dried in air at room temperature.

Figure 1.

Preparation procedure of PSA/PEDOT conductive composite yarns.

Characterizations

A S-3400N scanning electron microscope (SEM) at a nano-scale resolution was used to characterize the polymerization degree of PEDOT and FeCl₃ on PSA staple yarns. The machine was operated at 5–15 kV.

Cross-section images of PSA/PEDOT conductive composite yarns were taken and digitized by a JNOEC XS-213 optional microscope with CCD digital color camera with the magnifications from 40× to 1000×.

An American Fourier transform infrared (FTIR)/near infrared (NIR) spectroscope was used to characterize the molecular structure and chemical composition of the composite yarns. The spectral data were recorded from 4000 to 650 cm⁻¹ with a 4 cm⁻¹ resolution over 16 scans and with a step size of about 0.125 s.

The thermal stability of PSA/PEDOT conductive composite yarns was characterized by an American STA8000 thermal gravimetric analyzer. The experiment was carried out in a nitrogen atmosphere with a gas flow of 25 mL/min. The samples, with a weight of 5–8 mg, were heated from room temperature to 700℃ at a heating rate of 10℃/min.

One YG065 electronic strength tester was used to characterize the mechanical properties of the samples. The specimen gauge length was 25 cm at an elongation speed of 500 mm/min. Measurements of each specimen are conducted 10 times and the average value was used for result analysis.

A UT70A universal digital multi-meter with a measuring range of 10²–10⁹ Ω was used to measure the resistivity of the PSA/PEDOT conductive composite yarns. The specimens were conditioned at constant temperature in a humid environment for 24 hours before measurement in testing conditions of 20 ± 2℃ temperature and 35 ± 10% relative humidity. For each specimen, the measurements were conducted 10 times and the average value was used for result analysis. The conductivity was characterized using mass-specific resistance as follows

R_{S} = \frac{RNT}{L \times 10^{5}}

(1)

where R is the surface resistance of the samples (Ω), L is the length of sample (cm), N is the number of yarns or fibers, and T is the linear density of samples (tex).

The electrothermal properties were characterized by a JC3003A-3 DC regulated power with the applied voltage of 5–30 V. The experiment time range was from 5 to 120 min, and the specimen gauge length was 3 cm. The infrared (IR) images of PSA/PEDOT conductive composite yarns were taken by an American FLIR T250 IR thermal image instrument. A schematic of the electrothermal experiment for PSA/PEDOT conductive composite yarns with different combination structures, i.e. knotted, parallel, series, and bundled structures, is shown in Figure 2.

Figure 2.

Schematic of electrothermal experiment for PSA/PEDOT conductive composite yarns with different combination structures: (a) knotted, (b) parallel, (c) series, and (d) bundled.

Results and discussion

Cross-section and surface topography and morphology

As illustrated in Figure 3, PEDOT was deposited on the surface of PSA yarns as well as partly into yarns. It was observed that FeCl₃ oxidant was immersed into PSA yarns with the action of solvent osmosis due to the high moisture regain (6.2%) of PSA yarns, which contributes to the polymerization of EDOT inside the yarns.

Figure 3.

Cross-section images of PSA yarns before and after the deposition of PEDOT: (a) untreated PSA yarns and (b) PSA/PEDOT conductive composite yarns.

Figure 4 illustrates the longitudinal microstructures of PSA/PEDOT conductive composite yarns prepared with different FeCl₃ concentrations of 20–100 g/L with an impregnating time of 60 min and reaction time of 10 min.

Figure 4.

SEM micrographs of PSA/PEDOT conductive composite yarns impregnated with different FeCl₃ oxidant concentrations: (a) PSA, (b) 20 g/L, (c) 40 g/L, (d) 60 g/L, (e) 80 g/L, and (f) 100 g/L.

The surface of pure PSA yarns was remarkably smooth, as shown in Figure 4(a). As the oxidant concentrations increased from 20 g/L to 80 g/L, the surface of PSA yarns gradually formed a dense PEDOT layer with a certain thickness and roughness via the continuous polymerization of EDOT. This contributed to the formation of a conductive path, as depicted in Figure 4(b) to (e). The reason behind this phenomenon is that the amount of FeCl₃ absorbed on the surface of PSA yarns increases with increasing oxidant concentration, which is beneficial to the formation of PEDOT. As FeCl₃ oxidant concentration increased from 80 g/L to 100 g/L, the polymerization from EDOT to PEDOT tended to be significant and consequently resulted in the agglomeration of PEDOT particles on the surface of the yarns.

Chemical composition

In the IR spectra of pure PSA yarns shown in Figure 5(a), the stretching vibration absorption of the amide bond (C = O) was at 1656 cm⁻¹, while the stretching vibration absorption of benzene C–C was at 1594 cm⁻¹. The amide absorption band (coupled vibration peak of N–H and C–N) appeared at about 1251 cm⁻¹, which proved the existence of –CONH–. The symmetric and antisymmetric stretching vibration absorptions of sulfone –SO₂ were at 1305 cm⁻¹ and 1147 cm⁻¹. In addition, the opposite-substituted peak of benzene appeared at 834 cm⁻¹, and thus the PSA yarns could be confirmed.¹⁵ The characteristic absorption peak positions of pure PEDOT were at 1483 cm⁻¹, 1356 cm⁻¹, 1207 cm⁻¹, 1187 cm⁻¹, 1054 cm⁻¹, and 977 cm⁻¹, as depicted in Figure 5(c). The vibration absorption peak at 1483 cm⁻¹ and 1356 cm⁻¹ were caused by the stretching vibrations of C = C and C–C in the quinoid structure of EDOT. The vibration absorption peaks of the vinylene group of EDOT were at 1207 cm⁻¹ and 1054 cm⁻¹, and the peak at 977 cm⁻¹ can be attributed to C–S bending.^16,17 Figure 5(b) illustrates the FTIR spectra of PSA/PEDOT conductive composite yarn. Its characteristic absorption peaks include all vibration peaks of PSA and two PEDOT vibration peaks appearing at 1187 cm⁻¹ as bending deformation vibration absorption peak of C–O–C and at 978 cm⁻¹ as vibration absorption peak of C–S.

Figure 5.

FTIR spectra of (a) pure PSA yarn, (b) PSA/PEDOT conductive composite yarn, and (c) pure PEDOT particles.

By analysis and contrast, PSA/PEDOT conductive composite yarns prepared are composed of two components (PSA and PEDOT), and the PSA yarns are coated with PEDOT.

To investigate the effects of oxidant concentration on chemical composition of PSA/PEDOT conductive composite yarn, the oxidant concentration was set, respectively, as (a) 0 g/L, (b) 20 g/L, (c) 40 g/L, (d) 60 g/L, (e) 80 g/L, and (f) 100 g/L. A series of PSA/PEDOT conductive composite yarns were prepared and the IR spectra were analyzed, as shown in Figure 6.

Figure 6.

FTIR spectra of PSA/PEDOT conductive composite yarns prepared with different concentration of FeCl₃: (a) 0 g/L, (b) 20 g/L, (c) 40 g/L, (d) 60 g/L, (e) 80 g/L, and (f) 100 g/L.

The characteristic peak of PSA/PEDOT conductive composite yarns was similar to that of pure PSA, which illustrated that there was no significant change of PSA characteristic absorption peak position and shape before and after the deposition of PEDOT.

The characteristic absorption peak at 3324.9 cm⁻¹ of pure PSA gradually moved to the short-wave direction with an increase of oxidant concentration and there was a blue shift and smooth phenomenon. The reason was that the hydrogen atoms in amide linkage N–H of PSA and oxygen atoms in the vinylene groups of PEDOT tended to form hydrogen bonds in the reaction process of EDOT on the surface of PSA, and so the stretching vibration of N–H in amide linkage moved to the low frequency.¹⁸ With the increase of oxidant concentration, PEDOT deposited on the yarns appeared to undergo agglomeration, which decreased the formation of hydrogen bonds and weakened the blue shift.

The introduction of PEDOT and the increase of oxidant concentration reduced the reflectivity of IR light on PSA/PEDOT conductive composite yarns. In other words, the absorption of IR light on the surface of composite yarns enhanced with the deposition of PEDOT on the PSA yarns.

Thermal analysis

Thermal analysis curves of PEDOT, PSA/PEDOT conductive composite yarn, and pure PSA yarn are shown in Figure 7. There was a difference between thermal analysis curves of PEDOT and the two kinds of yarns. There were two stages in the process of thermal decomposition of PEDOT: the water of crystallization was lost on the first stage (room temperature to 100℃) with the weight decreasing 23%. The weight loss also included the volatilization of ethanol and the separation between residual oxidant in the PEDOT main chain and EDOT monomer. The second stage (120–528℃) was mainly caused by the degradation of macromolecules, which underwent severe movement with the rise of temperature and the weight decrease was up to 23%. As the temperature continued to rise, the main chain of the macromolecule was broken and there was a further loss of quality.¹⁷

Figure 7.

Thermogravimetric curves of (a) pure PEDOT, (b) PEDOT-coated conductive composite PSA yarn, and (c) pure PSA yarn.

There were three stages in the process of thermal decomposition of PSA. The first stage was the trace weight loss stage (room temperature to 400℃). There was about 5% of weight loss from room temperature to 100℃, which would be caused by the volatilization of water. From 101℃ to 400℃, the thermogravimetric (TG) curve tended to be a platform with the weight decreasing by 0.712%, which was caused by the volatilization of free water, water of crystallization, and additives. The second stage was the thermal decomposition stage (400–600℃). The decomposition mainly occurred in the C–N of the amide group in nitrogen atmosphere.³ Through the bonding analysis and measurement of PSA structure, the PSA macromolecular chain began to break at about 440℃ with the release of small molecules such as SO₂, NH₃, and CO₂.¹⁹ The weight loss was as high as 42.5% in this stage. The third stage was the carbon stable stage (600–700℃); most of PSA residues were carbides and temperature had less impact on the weight loss of residue.

The TG curve of PSA/PEDOT conductive composite yarn was similar to that of PSA, and also included three stages (trace weight loss stage, thermal deposition stage, and carbon stable stage). As can be seen from Figure 7(b), in the trace weight loss stage (room temperature to 400℃), the weight loss of PSA/PEDOT conductive composite yarns was as high as 20%, which was caused by the decomposition of PEDOT deposited on the PSA yarns. When the temperature exceeded 460℃, the quality retention rate of the composite yarns was not as high as that of pure PSA yarns, as mentioned in the paper by Bashir et al.¹ The reason is that the PSA fiber has excellent thermal stability, which contributes to improving the thermal stability of PSA/PEDOT conductive composite yarns. When the temperature exceeded 540℃, the quality retention of PEDOT was obviously higher than that of PSA, and the quality retention of PSA/PEDOT conductive composite yarns was lower than that of pure PSA yarns, due to small amount of PEDOT deposited on the PSA yarns. To sum up, the thermal stability property of PSA/PEDOT conductive composite yarns is between that of PSA and PEDOT.

Mechanical properties

Weak points of PSA/PEDOT conductive composite yarns increased with cleaning and oxidation treatment, which consequently resulted in loss of strength. As can be seen from SEM micrographs and cross-section images, PEDOT was mainly concentrated on the surface of PSA yarns and rarely into the PSA yarns.

Table 1 shows the physical parameters of PSA/PEDOT conductive composite yarn mechanical properties with different oxidant concentrations. As can be seen from Table 1, the mechanical properties of PSA/PEDOT conductive composite yarns were worse than those of pure PSA yarns. As can be seen from the data, the deposition of PEDOT reduces the strength of PSA yarns, due to the untwisting of the PSA yarns and ectopic phenomenon of fibers in PSA yarns with oxidation dipping, drying, and polymerization treatment. The uniformity of the treated yarns is reduced and the load resistance is also reduced, while the load resistance of the pure PSA yarns with three strands is relatively stronger. The hand-feel of the conductive composite yarns prepared by chemical vapor deposition is hard in compared with pure PSA yarns, and the carrying capacity and breaking elongation tends to be worse. To sum up, the deposition of PEDOT weakens the mechanical properties of PSA yarns. With the increase of oxidant, the mechanical properties of PSA/PEDOT yarns are on the decline, and when it reaches a certain value, the strength of PSA/PEDOT conductive composite yarns tends to be stable.

Table 1.

Physical mechanical properties of PSA/PEDOT conductive composite yarns with various oxidant concentrations

Oxidant concentration (g/L)	Strength at break (cN)	Elongation at break (mm)	Elongation at break (%)	Initial modulus (cN/tex)
0	1914	46.4	18.6	1.37
20	1515	40.7	16.3	1.23
40	1404	39.3	15.7	1.19
60	1359	36.7	14.7	1.08
80	1325	41.3	16.5	1.05
100	1321	39.8	15.9	1.01

Determination of electrical conductivity

The electrical conductivity of PSA/PEDOT conductive composite yarns was determined as a function of FeCl₃ oxidant concentration, reaction time, impregnating time, as well as heating temperature. The parameters, like surface resistance and mass-specific resistance, are particularly important since this study aims to result in development of conductive composite yarns.

Effects of oxidant concentration on electrical conductivity

PSA yarns have good electrical insulation with a surface resistance of 2.6 × 10¹⁶ Ω.¹ Table 2 shows the surface resistance of PSA/PEDOT conductive composite yarns with different oxidant concentration. The surface resistivity of PSA/PEDOT conductive composite yarns significantly decreased owing to the deposition of PEDOT, which made the PSA yarns transform from an insulator (>10¹⁰ Ω) to a semiconductor (10⁻⁴–10¹⁰ Ω).

Table 2.

Surface resistance of PSA/PEDOT conductive composite yarns with different oxidant concentration

Oxidant concentration (g/L)	Surface resistance (Ω)
20	2.73 × 10³
40	1.54 × 10³
60	1.34 × 10³
80	1.04 × 10³
100	1.64 × 10³

Using equation (1), the mass-specific resistance of PSA/PEDOT conductive composite yarns was evaluated from the surface resistance, as shown in Table 3.

Table 3.

Mass-specific resistance of PSA/PEDOT conductive composite yarns with different oxidant concentrations

Oxidant concentration (g/L)	Mass-specific resistance (Ω g cm⁻²)
20	2.46
40	1.38
60	1.21
80	0.94
100	1.48

The mass-specific resistance of PSA/PEDOT conductive composite yarns prepared at different oxidant concentrations was demonstrated. With the increase of oxidant concentration, the change of mass-specific resistance of PSA/PEDOT conductive composite yarns first decreased and then increased. When the oxidant concentration was less than 80 g/L, more EDOT was oxidized to PEDOT with the increasing amount of FeCl₃ absorbed on the surface of PSA yarns, and thus the mass-specific resistance was reduced to 1.05 Ω g cm⁻². When the oxidant concentration was 80–100 g/L, EDOT produced peroxidation with the action of FeCl₃ to generate PEDOT with unconjugated structures,¹ and consequently the electrical conductivity of PSA/PEDOT conductive composite yarns was reduced. In our experiment, the oxidant concentration of 80 g/L was chosen to prepare PSA/PEDOT conductive composite yarns with high electrical conductivity.

Effects of reaction time on electrical conductivity

It is worth noting that reaction time has a great impact on the electrical conductivity of PSA/PEDOT conductive composite yarns, as indicated by the data shown in Table 4. The effect of different reaction time on mass-specific resistance of PSA/PEDOT conductive composite yarns can be seen. The mass-specific resistance of PSA/PEDOT conductive composite yarns gradually increased and the electrical conductivity of became lower with the increase of reaction time within limits. Under the experimental conditions, it took 10 min reaction time to completely oxidize EDOT and then the polymerization of EDOT had almost no influence on the mass-specific resistance of the composite yarns over time. Significant exposure under the condition of 80℃ affected the quality of PEDOT. Therefore in this study, 10 min was selected as the best reaction time.

Table 4.

Surface resistance and mass-specific resistance of PSA/PEDOT conductive composite yarns with different reaction times

Reaction time (min)	Surface resistance (Ω)	Mass-specific resistance (Ω g cm⁻²)
10	1.04 × 10³	0.94
15	1.23 × 10³	1.11
20	1.29 × 10³	1.16
25	1.36 × 10³	1.22
30	1.41 × 10³	1.27

Effects of impregnating time on electrical conductivity

Effects of different impregnating time on the surface resistance and mass-specific resistance values of PSA/PEDOT conductive composite yarns can be seen in Table 5. The mass-specific resistance of PSA/PEDOT conductive composite yarns gradually increased and the electrical conductivity became lower with the increase of impregnating time within the limits.

Table 5.

Surface resistance and mass-specific resistance of PSA/PEDOT conductive composite yarns with different impregnating times

Impregnating time (min)	Surface resistance (Ω)	Mass-specific resistance (Ω g cm⁻²)
10	5.3 × 10²	0.48
20	6.0 × 10²	0.54
30	6.6 × 10²	0.60
40	8.2 × 10²	0.74
50	8.3 × 10²	0.75
60	1.04 × 10³	0.94

With the increase of impregnating time, the change of mass-specific resistance of PSA/PEDOT conductive composite yarns increased gradually, which matched the research result of the Bashir group.¹⁰ They found that the mass-specific resistance of viscose/PEDOT conductive yarns first decreased and then increased as the impregnating time progresses. It took 10 min impregnating time to make EDOT completely polymerize into PEDOT in the presence of FeCl₃ under the conditions of 80 g/L oxidant concentration and 10 min reaction time. EDOT was peroxided with the addition of more FeCl₃ within the yarns to product PEDOT with unconjugated structures over time and there were many smaller segments of PEDOT with low conductivity produced; hence the electrical conductivity of the composite yarns was reduced.

Effects of heating temperature on electrical conductivity

The effect of heating temperature on mass-specific resistance of PSA/PEDOT conductive composite yarns is shown in Table 6. Intermolecular distance of PEDOT on PSA yarns was increased because of heat expansion, which made it difficult for π-electrons to move between PEDOT molecules; thus the electrical conductivity of the composite yarn was reduced. As the temperature increased to 100℃, the decrease of electrical conductivity tended to be stable.

Table 6.

Surface resistance and mass-specific resistance of PSA/PEDOT conductive composite yarns at different heating temperatures

Heating temperature (℃)	Surface resistance (Ω)	Mass-specific resistance (Ω g cm⁻²)
Room temperature	1.04 × 10³	0.94
30	2.75 × 10³	1.93
40	3.43 × 10³	2.40
50	4.72 × 10³	3.30
60	6.28 × 10³	4.41
70	7.51 × 10³	5.26
80	8.05 × 10³	5.63
90	1.01 × 10⁴	7.11
100	1.12 × 10⁴	7.84
110	1.11 × 10⁴	7.77

Determination of electrothermal properties

Factors affecting the electrothermal properties of PSA/PEDOT conductive composite yarns are as follows: applied voltage, pressuring time, as well as the yarn’s combination structure, such as knotted, bundled, series, and parallel structures.

Effects of applied voltage on electrothermal properties

IR camera images of PSA/PEDOT conductive composite yarns at different applied voltage for 5 min can be seen in Figure 8. It can be noted that the electrical heating properties of PSA/PEDOT conductive composite yarns were excellent. The temperature of the PSA/PEDOT conductive composite yarns was increased slowly and partially when the voltage was 5 V, whereas it rose faster when the voltage was set to 10–20 V. The heating process was relatively rapid and uniform when the voltage was set to 20–30 V. The electrothermal properties of PSA/PEDOT conductive composite yarns were improved, whereas the heating temperature was affected by the applied voltage (see Figure 14).

Figure 8.

Infrared images of PSA/PEDOT conductive composite yarns at different applied voltages: (a) 0 V, (b) 5 V, (c) 10 V, (d) 15 V, (e) 20 V, (f) 25 V, and (g) 30 V.

The heating temperatures of PSA/PEDOT conductive composite yarns at different direct voltages are listed in Table 7. The maximum and average heating temperature on the surface of the composite yarns were increased with an increase of applied voltage, in agreement with the result of Laforgue.¹¹ It can be seen from equation (2) that the heating temperature of the PSA/PEDOT conductive composite yarns was proportional to the applied voltage U using the same yarns (regarding resistance R as constant) under the same time. It is obvious that the higher voltage corresponds to the higher heating temperature; therefore the electrothermal properties of PSA/PEDOT conductive composite yarns could be improved

\frac{U^{2}}{R} t = mC (t_{w} - t_{0}) + hS (t_{w} - t_{f}) t + C_{1} [(\frac{T_{w}}{100})^{4} - (\frac{T_{2}}{100})^{4}] St

(2)

where m denotes the mass of the conductive yarns tested, t_w and t₀ indicate real temperature and initial temperature of the conductive yarns, S represents surface area of the conductive yarns, h is convective transfer coefficient (a constant), t_f is air temperature, C means heat capacity of the conductive yarns (a constant), C₁ is radiation coefficient of the experimental system (a constant), T_w shows absolute surface temperature of the conductive yarns, and T₂ is absolute surface temperature on another point of the conductive yarns (a constant).

Table 7.

Heating temperature on the surface of PSA/PEDOT conductive composite yarns at different applied voltages

Direct voltage (V)	Maximum temperature (℃)	Average temperature (℃)
0	23.9	23.7
5	26.3	24.1
10	32.9	24.9
15	42.8	25.2
20	65.6	30.6
25	87.8	37.8
30	108	42.1

Effects of pressuring times on electrothermal properties

Figures 9 and 10 shows the heating temperature changes of PSA/PEDOT conductive composite yarns at different pressuring times at 15 V. The experimental data were recorded from the only yarn by IR thermal image instrument at different times, and its mass-specific resistance was 0.94 Ω g cm⁻². According to Figures 9 and 10, the heating process of the PSA/PEDOT conductive composite yarn can be clearly observed when applying a certain voltage across it. As can be seen from Figure 9, it was heated up by leaps from start to 5 min at 15 V. The surface temperature of the PSA/PEDOT conductive composite yarn rose gradually and tended to be stable that the final stable temperature was 34.1℃ over time. Based on the conductive mechanism, the π electrons of PEDOT getting energy made directional moves under strong electrical field and collided with other electrons endowed with accelerating movement and the increase of internal energy, which was possible to jump from π orbital to π* orbital and consequently caused the result that the surface resistance of the PSA/PEDOT conductive composite yarn had decline. According to Joule-Lenz's law²⁰, when the voltage and resistance were given, the internal energy converted by electricity would increase with the increase of time of applied voltage, and so that the heating temperature of the conductive yarns became rising. And yet when the time of applied voltage exceeded a certain value, the collision between π electrons and other electrons reached the upper limit and the heating temperature of the conductive yarns tended to stable. Then the heat generated by electricity would break the conjugate structures of PEDOT over time²¹, and it made the increase of the mass-specific resistance of the PSA/PEDOT conductive composite yarn, the phenomenon of the decrease of heating temperature would appear next.

Figure 9.

Variation curves of the PSA/PEDOT conductive composite yarn heating temperature at different pressuring times.

Figure 10.

Infrared camera images of the PSA/PEDOT conductive composite yarns at 15 V: (a) 0 min, (b) 5 min, (c) 10 min, (d) 15 min, (e) 20 min, (f) 30 min, (g) 40 min, (h) 50 min, (i) 60 min, (j) 80 min, (k) 100 min, and (l) 120 min.

Effects of the yarn’s combination structures on electrothermal properties

The electrothermal properties of PSA/PEDOT conductive composite yarns are closely linked to its specific resistance.

Different combinations of yarn structures can form different resistance network, so as to obtain different electrothermal properties. This paper mainly focused on the effects of different combination structures on the electrical heating properties of PSA/PEDOT conductive composite yarns. The experimental data were analyzed from the yarns with different combination structures by an IR thermal image instrument at a constant DC current (10 mA), and the mass-specific resistance was 0.94 Ω g cm⁻².

Knotted structures

Figure 11(a) shows the fact that if the current flows from one yarn to another, it has to flow through the intersection point composed of the two yarns. In order to study the electrothermal properties of the intersection, the cross-section structure was considered, as shown in Figure 11(b). As an ideal yarn model was assumed and the resistance of the two yarns with knotted structures could be expressed by the series form between length resistance and intersection resistance of the two yarns, so the current must flow through the length resistance and intersection resistance to form a conductive circuit.

Figure 11.

Schematic pattern of PSA/PEDOT conductive composite yarns with knotted structures: (a) knotted structure pattern and (b) cross-sections of knotted structures.

The IR images of PSA/PEDOT conductive composite yarns with knotted structures were taken before and after they were powered on. As can be seen in Figure 12, when both ends of the conductive composite yarns with linkage structures imposed the constant current (10 mA), the heating temperature of knotted point was significantly higher than other parts of the conductive composite yarn since the contact resistance of the yarn’s knotted point was higher than the length resistance. Contact resistance between interlaced conductive yarns would under certain circumstances constitute a problem for sensor applications and electrical routing in interactive textile structures.²²

Figure 12.

Digital photos and infrared images and of PSA/PEDOT conductive composite yarns with knotted structures (at 10 mA): (a) digital photo, (b) without power, and (c) with power for 10 s.

Bundled structures

Figure 13 illustrates the digital photos and IR camera images of the PSA/PEDOT conductive composite yarn with bundled structures (at 10 mA). As can be seen, the PSA/PEDOT conductive composite yarn with bundled structures had admirable electrothermal properties. The local point of the yarn appeared as bright spots, which can be attributed to uneven distribution of oxidant, the resistance was high at the points deposited with less PEDOT and consequently the heating temperature was high.

Figure 13.

Digital photos and infrared images of PSA/PEDOT conductive composite yarns with bundled structures (at 10 mA): (a) digital photo, (b) without power. and (c) with power for 10 s.

Figure 14.

Digital photos and infrared images of PSA/PEDOT conductive composite yarns with series structures (at 10 mA): (a) digital photo, (b) without power, and (c) with power for 10 s.

Series structures

Figure 14 shows the digital photos and IR images of the PSA/PEDOT conductive composite yarn with series structures having good electrothermal properties. The heating rate was fast when charged with electricity for 10 s and there was a temperature rise of 3℃. The whole yarn appeared bright because PEDOT was uniformly deposited on the PSA yarn.

Parallel structures

The digital photos and IR images of the PSA/PEDOT conductive composite yarn with parallel structures can be seen in Figure 15. Compared with the yarns having other combination structures mentioned above, the electrothermal properties of the PSA/PEDOT conductive composite yarn with parallel structures were less outstanding. It is also shown from theoretical analysis that in the series circuit the resistance of the conductive composite yarns can be expressed as follows

R_{total} = 2 R_{0}

(3)

and the resistance of the conductive composite yarns in the parallel circuit can be expressed as follows:

R'_{total} = \frac{R_{0}}{2}

(4)

where

R_{total}

and

R'_{total}

denote, respectively, the electrical resistance of the conductive composite yarns with series and parallel structures, R₀ is the resistance of a single composite yarn.

R'_{total}

is much smaller than

R_{total}

, and it causes the result that the calorific value of the conductive composite yarns with parallel structures is less than that of the conductive composite yarns with series structures, which matches the analysis from the IR images in Figures 14 and 15.

Figure 15.

Digital photos and infrared images of PSA/PEDOT conductive composite yarns with parallel structures (at 10 mA): (a) digital photo, (b) without power, and (c) with power for 10 s.

Conclusions

PSA/PEDOT conductive composite yarns were prepared by the VPP method with the action of FeCl₃ oxidant. The results showed that a dense PEDOT layer can be formed on the surface of PSA, and with the increase of FeCl₃ from 20 g/L to 80 g/L, the change of mass-specific resistance of PSA/PEDOT conductive composite yarns first decreased and then increased. The minimum resistance reached 0.94 Ω g cm⁻² when the oxidant concentration was 80 g/L, the mass-specific resistance of composite yarns gradually increased, and the electrical conductivity of composite yarns became lower with the increase of impregnating time and reaction time within limits.

Based on the FTIR analysis, the characteristic absorption peak of composite yarns includes the characteristic absorption peak of PSA appearing at 1656 cm⁻¹, 1251 cm⁻¹,1305 cm⁻¹, and 1147 cm⁻¹ and two vibration peaks of PEDOT appearing at 1187 cm⁻¹ and 978 cm⁻¹.

TG analysis indicates that the thermal stability properties of PSA/PEDOT conductive composite yarns are between PSA and PEDOT. The results showed that the mechanical properties of PSA/PEDOT conductive composite yarns were lower than those of pure PSA yarns.

The experimental results of electrothermal measurements illustrate that the heating temperature of PSA/PEDOT conductive composite yarns can be increased with an increase of applied voltage. The heating temperature first increased then tended to be stable with an increase of heating times under certain applied voltages. The maximum heating temperature of PSA/PEDOT conductive composite yarns with knotted structures is higher than that with bundled, parallel, and series structures. The composite yarn with bundled structures had uniform bright spots throughout the whole yarn, while that with series structures appeared uniformly bright. The electrothermal effect of the composite yarns with parallel structures was not so obvious because of the small electrical resistance under this structure.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the SUES Research Innovation Fund (grant number E1-0903-14-01159-14KY0905).

References

Bashir

Skrifvars

Persson

. Production of highly conductive textile viscose yarns by chemical vapor deposition technique: a route to continuous process. Polym Adv Technol 2011; 22: 2214–2221.

Xue

Tao

. Morphological and electromechanical studies of fibers coated with electrically conductive polymer. J Appl Polym Sci 2005; 98: 1844–1854.

Chen ZM. Preparation and characterization of functional polysulfonamide composites. Shanghai University of Engineering Science, China, 2011.

Mohammadi

Hasan

Liedberg

. Chemical vapour deposition (CVD) of conducting polymers: polypyrrole. Synth Met 1986; 14: 189–197.

Kim

Won

. The preparation and characteristics of conductive poly(3,4-ethylenedioxythiophene) thin film by vapor-phase polymerization. Synth Met 2003; 139: 485–489.

Knittel

Schollmeyer

. Electrically high-conductive textiles. Synth Met 2009; 159: 1433–1437.

Kim

Chun

. Characteristics of electrically conducting polymer-coated textiles. Mol Cryst Liq Cryst 2003; 405: 161–169.

Kim

Song

. EMI shielding intrinsically conducting polymer/PET textile composites. Synth Met 2003; 135: 105–106.

Malinauskas

. Chemical deposition of conducting polymers. Polymer 2001; 42: 3957–3972.

10.

Bashir

Skrifvars

Persson

. Synthesis of high performance, conductive PEDOT-coated polyester yarns by OCVD technique. Polym Adv Technol 2012; 23: 611–617.

11.

Laforgue

. All-textile flexible supercapacitors using electrospun poly(3,4-ethylenedioxythiophene) nanofibers. J Power Sources 2011; 196: 559–564.

12.

Odhiambo

Mey

Hertleer

. Discharge characteristics of poly(3,4-ethylene dioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) textile batteries; comparison of silver coated yarn electrode devices and pure stainless steel filament yarn electrode devices. Text Res J 2014; 84: 347–354.

13.

Varesano

Tonin

. Improving electrical performances of wool textiles: synthesis of conducting polypyrrole on the fiber surface. Text Res J 2008; 78: 1110–1115.

14.

Bashir T. Conjugated polymer-based conductive fibers for smart textile applications. Chalmers University of Technology, Sweden, 2013.

15.

Chen WJ. Study on properties and structure of electrospun functional PSA composite nanofibres. Shanghai University of Engineering Science, China, 2012.

16.

Zhang S. Synthesis of conducting polymer PEDOT dispersion and application in antistatic coating. South China University of Technology, China, 2010.

17.

Zhu W. Preparation and characterization of poly(3,4-ethylenedioxythiophene)/polyaniline composite anode materials in Zinc electrodeposition. Kunming University of Science and Technology, China, 2011.

18.

Zhu

. Analysis of polymer structures, Beijing: The Science Publishing Company, 2010.

19.

Zhang

Ren

. Thermal stability analysis of PSA fiber. J Donghua Univ 2005; 2: 13–16.

20.

Tan HL. Prepapation of carbon system electrothermal coating with controllable temperature and study on electrothermal property. Hunan University, China, 2010.

21.

Yang

Shang

. Vapor phase polymerization of 3,4-ethylenedioxythiophene on flexible substrate and its application on heat generation. Polym Adv Technolo 2011; 22: 1049–1055.

22.

Gunnarsson

Karlsteen

Berglin

. A novel technique for direct measurements of contact resistance between interlaced conductive yarns in a plain weave. Text Res J 2015; 85: 499–511.