Conductivity,superhydrophobicity and mechanical properties of cotton fabric treated with polypyrrole by in-situ polymerization using the binary oxidants ammonium Peroxodisulfate and ferric chloride

Abstract

In this study, polypyrrole deposition and a superhydrophobic coating were applied to cotton fabrics to develop a self-cleaning and conductive fabric with electric heating performance. The binary oxidants ammonium peroxodisulfate and ferric chloride were introduced during the polymerization to adjust the size of the polypyrrole particles for creating diverse nano-scale roughness on the surface of the cotton fabrics and to prevent degradation in the mechanical properties of textiles.

The in-situ polymerization of polypyrrole that introduced the binary oxidants succeeded in depositing polypyrrole particles on the surface of the cotton fabrics. Binary oxidants formed small polypyrrole particles contrary to the single oxidants. In terms of conductivity, the surface resistivity decreased as the FeCl₃ ratio in the oxidants increased. The binary oxidants led to a similar level of conductivity even though the amount of polypyrrole deposition was less than that in the case of the single oxidant. The electrical heating performance improved as the surface resistance was decreased, resulting in an up to 20℃ increase in the surface temperature. On the other hand, the duration of the electro-heating effect was shorter with higher surface temperature. In terms of wettability, a superhydrophobicity with a contact angle of 150° or higher and a shedding angle of less than 10° was achieved under all oxidant conditions because of the nano-scale roughness caused by polypyrrole. Polypyrrole deposition reduced the tensile strength of the cotton fabric and increased its stiffness. The binary oxidants exhibited smaller changes in the mechanical properties of the textile than the single oxidants.

Keywords

polypyrrole deposition binary oxidants conductive fabrics superhydrophobicity electro-heating

Conductive polymer textile composites are subject to functional degradation because of the loss of conductivity when exposed to air and moisture. Washing and management problems caused by contamination during use also remain a challenge that needs to be overcome.^1–4 Therefore, the degradation in conductivity caused by air and water must be prevented to supplement such shortcomings and develop clothing-integrated conductive materials.⁵ Furthermore, convenience in terms of washing and management must be secured while the same characteristics as ordinary textiles are maintained. Superhydrophobicity is defined as a surface with a contact angle of 150° or higher and a shedding angle of less than 10° against water.⁶ When conductive materials have self-cleaning effects against water or soil, their functional degradation and washing problems can be solved. Superhydrophobic surfaces can be realized through dual-scale surface roughness and surface energy adjustment.^7–9

Polypyrrole is a conductive polymer that has the benefits of easy polymerization, high conductivity and excellent atmospheric and thermal stability compared to other conductive polymers.^10–13 With its excellent adhesion force to various fabric substrates, such as polyester, cotton and nylon, polypyrrole has also been actively used for developing smart fabrics.^14–16 The use of in-situ polymerization, in which conductive polymers are diffused into fibers, can form dual-scale roughness for conductivity and superhydrophobicity through a simple process because the nanostructured particles formed by the polymerization of pyrrole monomers into polymer are attached to the fabric surface with micro-scale roughness.^17,18

The shape and conductivity of the conductive polymer are affected by pyrrole concentration, polymerization temperature and time and ratios of the pyrrole and oxidants.¹⁶ The type of oxidant is a factor that most significantly affects the conductive polymer characteristics.^11,19,20

The oxidation-reduction potential of the oxidant influences the rate of polymerization, conductivity and surface morphology of polypyrrole.²¹ The oxidants typically used for polypyrrole polymerization are ammonium persulfate (APS) and ferric chloride (FeCl₃). The oxidation-reduction potential of APS is 1.94 V, while for FeCl₃ it is 0.77 V. When polymerizing polypyrrole, the high oxidation-reduction potential of the oxidant tends to oxidize faster, leading to a faster polymerization rate and resulting in metastable polypyrrole particles in the polymerization process. In order to be stabilized, the polypyrrole particles aggregate each other and form a relatively large-sized particle. Cetiner¹⁸ developed polypyrrole-coated cotton fabrics by oxidation polymerization and observed the surface morphology. As a result, the diameter of the polypyrrole particles was 250 nm when APS was used as an oxidant. On the other hand, the FeCl₃ condition showed a smaller particle diameter than that of the APS condition. This is due to the oxidation-reduction potential difference between the two oxidants. Therefore, it is expected that polypyrrole particles of various sizes can be deposited on the fiber surface by utilizing these characteristics.

FeCl₃ is preferred to APS because a Cl^– ion acts as a dopant, and polypyrrole with better conductivity can be obtained. However, when textile composites are fabricated using polypyrrole, the Fe(OH)₃ formed by FeCl₃ hydrolysis may attack the molecular bonds in the fibers and cause a reduction in tensile strength, resulting in the degradation of the mechanical properties of the textile.¹⁷ Deogaonkar and Patil¹³ suggested that replacing FeCl₃ with APS is desirable only if an appropriate conductivity can be achieved, because FeCl₃ caused corrosion in substrates and stresses the necessity for studies on supplementary measures to maintain high electrical conductivity, while the mechanical properties of textiles are not affected.

As such, binary oxidants are attracting attention in terms of addressing problems caused by single oxidants. Singh et al.²⁰ used FeCl₃ and APS to examine the effects of binary oxidants on the appearance and electrical characteristics of poly(2,5-dimethoxyaniline) (PDMA) films. As a result, they confirmed that the benzenoid structure of PDMA was better aligned as the APS content increased, thereby increasing the crystallinity degree of polymers and improving conductivity. Ma et al.²² fabricated a film by polymerizing polyaniline, a conductive polymer, using the binary oxidants of FeCl₃ and APS. They then examined its conductivity according to the mixing ratio. Consequently, the electrical conductivity of the polyaniline film reached the maximum value when the ratio of FeCl₃/APS was 2:1 because the fast growth of the polyaniline film formed more oligomers, which suppressed the creation of byproducts that interfered with conductivity. Hong and Jang¹² used the binary oxidants of Fe₂(SO₄)₃ and (NH₄)₂S₂O₈·FeCl₃ instead of (NH₄)₂S₂O₈(APS), which is a single oxidant, to synthesize soluble polypyrrole with high electrical conductivity. When the binary oxidants were used instead of APS alone, the electron mobility improved as the anion doping ratio increased, thereby increasing the electrical conductivity. Therefore, in terms of fabricating polypyrrole textile composites, the optimal processing conditions capable of achieving excellent conductivity and maintaining the inherent mechanical properties of the textile can be found using binary oxidants rather than using only FeCl₃, which causes degradation in the mechanical properties of the textile, or APS, which has low conductivity.

These textile composites developed using conductive polymer in particular exhibit conductivity and a heating function caused by resistance heat without any electrical wiring in the textile.^4,23–25 The surface heating elements can reduce the risks of electric shock and electric leakage by operating at a low voltage. Furthermore, they can exhibit excellent thermal efficiency because heat generation is not affected by the pores or partial rupture of the textile.

This study aims to develop a conductive fabric with superhydrophobicity through an in-situ polymerization of a conductive polymer onto textile using APS, FeCl₃ and binary oxidants according to the mixing ratios of the two oxidants. For this purpose, polypyrrole, which is a conductive polymer, was directly polymerized onto the cotton fabric surface, while the types and mixing ratios of the oxidants were varied to adjust the surface roughness and to achieve excellent conductivity, maintaining the inherent mechanical properties of the fabric. Silane coating was performed to prevent the conductivity degradation and hydrophobicity. The conductivity, superhydrophobicity and changes in mechanical properties were then compared and observed according to the polymerization conditions of polypyrrole.

Experimental details

Materials

Fabrics of 100% cotton were purchased from Weiqiao Textile Company Ltd (China). The cotton fabrics were in 16’s plain woven with a weight and a thickness of 188 g/m² and 0.42 mm, respectively. The monomer selected for the in-situ polymerization of the conductive polymer was 99% pyrrole (Sigma-Aldrich, USA). Subsequently, 97% anthraquinone-2-sulfonic acid sodium salt monohydrate (AQSA-Na; Sigma-Aldrich, USA) was used as a dopant. Both ammonium peroxodisulfate (Kanto Chemical Co., Inc., Japan) and iron (III) chloride hexahydrate (Kanto Chemical Co., Inc., Japan) were prepared as oxidants. For the hydrophobic surface coating processing, dodecyltrimethoxysilane (DTMS; Tokyo Chemical Industry Co., Ltd, Japan), 99.9% ethyl alcohol anhydrous (Daejung Chemicals and Metals Co., Ltd, South Korea) and 5% acetic acid solution (Daejung Chemicals and Metals Co., Ltd, South Korea) were used. All samples were used without refinement.

Polypyrrole deposition

The study by Huang et al.²³ was consulted and the in-situ polymerization method, in which the pyrrole monomers were polymerized onto the surface of the cotton fabrics through chemical polymerization, was utilized for the fabrication of polypyrrole-deposited conductive textile composites.

A mixed solution was fabricated by dissolving 0.01 mol pyrrole as the monomer and 0.125 mol AQSA-Na as the dopant in 50 ml distilled water. The refined cotton fabrics were cut into 7 cm × 7 cm area, immersed in the mixed solution and maintained at room temperature for 20 min, such that the pyrrole monomers could be sufficiently absorbed by the fabrics.

Meanwhile, APS and iron (III) chloride hexahydrate (FeCl₃·6H₂O), which had different oxidation-reduction potentials, were used as the oxidants for initiating polymerization. The particle size of polypyrrole formed during the polymerization varied according to the redox potential of the oxidants; hence, the binary oxidants were fabricated using APS and FeCl₃. Five mixing ratios according to the mole ratio were then applied, as shown in Table 1.

Table 1.

Mole ratio of oxidants mixture

	A100	AF72	AF55	AF27	F100
APS (%)	100	75	50	25	0
FeCl₃·6H₂O (%)	0	25	50	75	100

APS: ammonium peroxodisulfate.

An oxidant solution was fabricated by dissolving 0.01 mol of the oxidants mixed with APS and FeCl₃ 6H₂O at each ratio in 50 ml distilled water. It was then dropped into the pyrrole monomer-mixed solution, in which cotton fabrics were immersed to start the polymerization. The environment temperature was maintained between 10℃ and 20℃ during polymerization.¹⁶ All the polymerization reactions of polypyrrole deposition occurred under stirring at 25 rpm for 1 h. Upon the completion of the reactions, the samples were removed from the solution, cleaned with distilled water and dried at room temperature.

Hydrophobization

An alkoxysilane compound with low surface energy and environmental hazards was used to hydrophobize the surface of the polypyrrole-deposited cotton fabrics. Based on the method of Wu et al.,²⁶ a 5 vol% mixed solution was made by dissolving DTMS in a 75/25 (v/v) ethanol/water mixed solvent. A sol-gel solution was then fabricated by performing hydrolysis at 35℃ for 48 h. The polypyrrole-deposited cotton fabrics were immersed in the completed solution for 200 s and passed through a roller with 50 psi (4 bar) pressure and 15 rpm speed to remove the excessively absorbed solution. They were then cured in a 100℃ oven for 1 h.

Characterization

Morphologies and chemical compositions

A field-emission scanning electron microscope (FE-SEM; SUPRA 55VP, Carl Zeiss, Germany) was used to observe the surface of the cotton fabrics after the in-situ polymerization of polypyrrole. In this instance, the diameters of the polypyrrole particles that were not aggregated, but independently attached to the surface of the cotton fabrics, were measured to measure the diameters of the polypyrrole particles according to the oxidant mixing ratios by acquiring the images of the cotton fabric surfaces from different positions using ImageJ software.

The add-on ratio was calculated by calculating the weight changes of the cotton fabrics according to polypyrrole deposition and hydrophobic coating (Equation (1)). W_b denotes the weight of the sample before polypyrrole deposition or hydrophobic coating, while W_a is the weight of the sample after polypyrrole deposition or hydrophobic coating

Add-onratio (%) = (\frac{W_{a} - W_{b}}{W_{b}}) \times 100

(1)

An X-ray photoemission spectroscopy (XPS) analysis was conducted to examine the changes in chemical characteristics and bonding states of the samples with polypyrrole deposition and hydrophobic coating. The analysis was conducted in an ultra-high vacuum (UHV) with less than 5 × 10⁻¹⁰ mbar using electron spectroscopy for chemical analysis (ESCA, Sigma Probe, VG, UK). The X-ray source was a micro-focused monochromator source type. The measurement was conducted in the range of 0–1000 eV using A1-Ka.

Conductivity

The surface resistivity for the conductivities of the conductive polymer textile composites was measured using a DC milliohm meter (GOM-804, GW INSTEK, Taiwan) in accordance with AATCC 76-1995. The final surface resistivity was obtained by averaging the five measurements taken at different positions.

The surface temperature was observed according to the applied voltage and time using a thermal imaging camera (C2, FLIR, USA) to investigate the heating characteristics of the conductive polymer caused by the resistance heat. The voltage was limited to 3, 6 and 9 V using alkaline batteries (Duracel®, LR14 and 6LF22). The thermal images of the surface temperature were taken at 1 min intervals for 10 min.

In addition, the voltage were applied to the samples using alkaline batteries (Duracel®, 6LF22) and the heat duration time was measured until the surface temperature of the samples became identical to room temperature in order to examine the heat duration with constant power.

Superhydrophobicity

The static contact angle was measured using a contact angle measuring device (Theta Lite Optical Tensiometer, KSV Instruments, Finland) to observe the changes in surface wettability of the untreated cotton fabric, polypyrrole-deposited fabrics and polypyrrole-deposited and hydrophobic-coated fabrics. For this purpose, the contact angles within 1 s were measured after the samples were fixed on the slide glass, and water droplets of 3.3 ± 0.3 µl were dropped on the surface from a 1 cm height in the vertical direction from the samples. The final static contact angles were obtained by averaging 10 measurements taken at different positions.

In measuring the shedding angle, the measuring plate on an angle-adjustable cradle was tiled by 0.5°, and water drops of 12.5 ± 0.2 µl were dropped from a 1 cm height from the samples in accordance with the method proposed by Zimmerman et al.²⁷ The angle was set as the shedding angle when the water drops rolled down 2 cm. The final values were obtained by averaging the five measurements taken at different positions.

Mechanical properties

The changes in the mechanical properties of the cotton fabrics according to polypyrrole deposition and hydrophobic coating were assessed by measuring the tensile strength and stiffness. The tensile strength was determine using a universal testing machine (Instron-5543, Instron, USA) in accordance with the ASTM D 5035 strip method, while the stiffness was assessed through the ISO 4606: 2013 measurement method using a fixed angle bending tester. All the final values were obtained by averaging five measurements taken using different parts of the samples.

Results and discussion

Surface morphologies

In the in-situ polymerization in which polypyrrole, a conductive polymer, was diffused into the fibers, the pyrrole monomers were polymerized into a polymer and attached to the fabric surface with micro-roughness as the polymer formed nanostructured particles. Consequently, the textile had a physical roughness for superhydrophobicity because a micro-nano dual-scale roughness was formed on the textile. The binary oxidants of APS and FeCl₃, which had different oxidation-reduction potentials, were used to adjust the size of the polypyrrole particles formed on the surface of the cotton fabrics.

The surface morphologies of the hydrophobic-coated samples were observed using scanning electron microscopy (SEM) after polypyrrole was deposited on the surface of the cotton fabrics using the single oxidants of APS and FeCl₃ or the binary oxidants mixed at the mole ratios of 75:25, 50:50 and 25:75. The surface of the untreated cotton fabric showed the cotton-specific fibril structure (Figure 1(a)), which was relatively smooth. In the case of the polypyrrole-deposited samples, the surface of the fibers was all covered with polypyrrole particles as the pyrrole monomers were sufficiently absorbed into the cotton fabrics by immersing those fabrics in the pyrrole monomer dispersion for 20 min, and the pyrrole diffused into the cotton fabrics formed a polymer when it reacted with the introduced oxidants.

Figure 1.

Scanning electron images (×30,000) of (a) untreated cotton, (b) A100, (c) AF72, (d) AF55, (e) AF27 and (f) F100 polypyrrole-deposited cotton fabrics.

The deposition mechanism of the polypyrrole textile composites through in-situ polymerization is described as follows: firstly, pyrrole is absorbed onto the textile surfaces through a liquid/solid interfacial action, and the textile becomes the center of nucleation. In this instance, when oxidants are added the pyrrole monomer loses electrons, and are oxidized into cation radicals. The two cation radicals then again form a dimer through disproportionation. Linear polypyrrole molecules are formed as reactions continue because of the continuous oxidation process. In the course of these reactions, cation free radicals and other intermediate products are easily absorbed into the surface of solids, such as textiles, and provide places where polypyrrole can grow uniformly by forming the center of nucleation.^16,21 According to Liu et al.,¹⁶ the reactive polypyrrole monomers, dopants and oxidants are present in a dispersed state in a polymerization system; hence, the intermolecular bonding between polypyrrole and cotton fibers is mainly caused by van der Waals attraction and hydrogen bonding.

The diameters of the single particles, which can represent the polypyrrole particles independently distributed on the surface on the cotton fibers, were measured and averaged through ImageJ software. Consequently, the diameter was 254 ± 71 nm for A100, 77 ± 10 nm for AF72, 86 ± 14 nm for AF55, 81 ± 11 nm for AF27 and 167 ± 27 nm for F100, indicating differences according to the oxidants.

As for A100 and F100, which used single oxidants, the different in the size of the formed polypyrrole particles appeared to be attributable to the different oxidation-reduction potentials of the two oxidants. The oxidation-reduction potential of APS (1.94 V) was higher than that of FeCl₃, causing a faster oxidation speed.¹⁹ The fast oxidation speed increased the nucleation and growth rate of polypyrrole, resulting in the formation of metastable particles during the primary nucleation process. The metastable particles aggregated with each other through secondary nucleation to maintain a more stable state, thereby forming larger particles. Meanwhile, the pyrrole monomers absorbed by the textile reacted with the oxidants in a stable manner because the low oxidation-reduction potentials led to slow nucleation, thereby forming a stable polymer with primary nucleation alone.²¹

In contrast, the binary oxidants exhibited a different behavior. Figure 2 shows images taken for 3 min after oxidants were added to the pyrrole monomer dispersion, in which the cotton fabrics were immersed under each oxidant processing condition. The sample fabrics in the A100 and F100 images were visible in the dispersion because of the relative transparency, even though the cotton fabrics turned black after the addition of the oxidants. The solution in AF72, AF55 and AF27 turned green first within 1 min after oxidant addition, then exhibited a completely dark black color after 3 min. This finding confirmed that polypyrrole generation spread from the cotton fabrics to the dispersion in the case of the single oxidants, but it spread in the opposite direction in the case of the binary oxidants.

Figure 2.

Images of the pyrrole polymerization process according to the time after oxidants are inserted.

Hwang et al.²⁸ proposed the nucleation and growth mechanism of polypyrrole in two ways. One is instantaneous nucleation and a two-dimensional growth process, while the other is progressive nucleation and a three-dimensional growth process. In the instantaneous nucleation and two-dimensional growth process, the pyrrole monomers are absorbed into the substrate and form oligomers through instantaneous nucleation. The formed oligomers diffuse in the two-dimensional directions and form a polypyrrole layer. Meanwhile, in the progressive nucleation and three-dimensional growth process, the pyrrole monomers absorbed by the substrate form oligomers in the dispersion state instead of generating nuclei, and the nuclei grow in the three-dimensional growth step, in which nuclei are generated in the vertical direction while polypyrrole layers are superimposed, which is performed after the polypyrrole monolayer is formed.

Therefore, based on the abovementioned theory, the differences in the polypyrrole formation process and particle size according to the oxidants are presumed to be caused by the nucleation and growth mechanisms of the polypyrrole polymerization reaction being different depending on the single and binary oxidants. In the case of the single oxidants shown in Figure 3(a), nuclei were instantaneously generated on the surface of the cotton fabrics, which absorbed the pyrrole monomers through instantaneous nucleation and the two-dimensional growth process. The nuclei growth in the two-dimensional directions completely covered the surface of the cotton fabrics. In contrast, in the case of the binary oxidants, the pyrrole monomers generated nuclei in the dispersion and caused the nuclei growth in the three-dimensional directions while they diffuse into the surface of the cotton fabrics (Figure 3(b)).

Figure 3.

Mechanism of the polypyrrole deposition on the cotton fabrics: (a) single oxidant and (b) binary oxidants.

Although the polypyrrole polymerization methods were different, Hwang et al.²⁸ reported that the mechanism of nucleation varied according to factors such as the oxidizing power of the oxidant, heat treatment and polymerization temperature, and that a higher reactivity led to progressive nucleation and a three-dimensional growth process. The binary oxidants used in this study were APS and FeCl₃, which were mainly used for the polymerization reaction of polypyrrole. In the case of binary oxidants, APS and FeCl₃ reacted with each other to generate highly reactive hydroxyl radicals through the Fenton process as in Equations (2)–(3), where the Fe⁺ ions dissociated from FeCl₃ in the water acted as a catalyst and the APS acted as an oxidant.²⁹ Therefore, under the binary oxidant conditions, the oxidants reacted with the pyrrole monomer dispersed in the solution before they reached the cotton fabric interface as reactivity increased because of the high oxidizing power of the hydroxyl radicals, and the progressive nucleation and growth in the three-dimensional directions were performed. This appeared to have caused the uniform and small-sized polypyrrole particles to be formed on the surface of the cotton fabrics

{Fe}^{3 +} + S_{2} O_{8}^{2 -} \to 2 {SO}_{4}^{- \cdot} + {Fe}^{2 +}

(2)

{SO}_{4}^{- \cdot} + H_{2} O \to {SO}_{4}^{2 -} + OH + H^{+}

(3)

Add-on ratio

Table 2 shows that the add-on proportions caused by polypyrrole deposition with single oxidants were significantly higher than those with binary oxidants. This finding can be attributed to the different nucleation and growth mechanisms according to the oxidants. The amount of polypyrrole deposited on the cotton fabrics was large for the single oxidants because the pyrrole monomers reacted with the oxidants on the surface of the cotton fabrics and diffused into the surface and inside of the fabrics in the two dimensions. However, it was relatively small for the binary oxidants because the pyrrole monomers reacted with the oxidants in the dispersion to generate nuclei, then diffused into the surface of the cotton fabrics in the three-dimensional directions. The add-on difference was insignificant according to the mixing ratio of APS and FeCl₃.

Table 2.

Add-on (%) of the samples after polypyrrole deposition and hydrophobic coating

	A100	AF72	AF55	AF27	F100
Polypyrrole deposition (%)	28.9 ± 5.0	10.9 ± 3.8	11.4 ± 3.9	12.3 ± 2.8	30.0 ± 3.9
Hydrophobic coating (%)	1.6 ± 1.4	1.2 ± 1.1	0.8 ± 1.1	0.8 ± 1.2	0.6 ± 1.7
Total add-on (%)	30.9 ± 2.6	12.3 ± 2.4	12.3 ± 1.6	13.2 ± 1.3	30.8 ± 1.9

In contrast, in the case of the hydrophobic coating, the overall values of the add-on were small at approximately 1%. A hydrophobic layer was formed on the surface of the cotton fabrics, while a siloxane bond (Si-O) was generated by the reaction between the silane group of DTMS and the hydroxyl group of the cotton fabrics.^30,31 The amount of the hydroxyl group that could react with DTMS decreased because the cellulose of the cotton fabrics and polypyrrole reacted with each other due to pyrrole polymerization and was deposited on the fabric surface. However, the amount of hydrophobic coating was reduced because the coating depended on the physical bonding rather than chemical bonding, thereby resulting in relatively insignificant weight changes.

A comparison of the coating amounts according to the oxidants revealed that the amount of hydrophobic coating tended to decrease as the ratio of APS was reduced. Unlike FeCl₃, APS has oxygen in the form of S₂O₈, which participated in the polymerization reaction of polypyrrole and formed a large number of carbonyl groups.³² Therefore, as the carbonyl groups capable of reacting with DTMS increased with the increase of the mixing ratio of APS, the add-on caused by the hydrophobic coating also slightly increased as chemical bonding occurred.

Chemical structure

The chemical composition changes of the polypyrrole-deposited and hydrophobic-coated surface of the cotton fabrics were examined through XPS analysis. Figure 4 shows the high-resolution spectrum analysis results for N1s through which the bonding force and degree of doping in the polypyrrole aromatic ring according to the oxidant treatment conditions can be identified. The main peak at 399.7 eV, which represented the amine (–NH–) group of the pyrrole unit, was found in all the samples deposited with polypyrrole. In contrast, polaron (C–N+) and bipolaron (C=N+), which provided conductivity to polypyrrole, were found near 401 and 402 eV, respectively. No bipolaron structure was detected under the A100 condition. Bipolaron had a form with two polarons as another electron was generated by oxidation in the polypyrrole chain, where polaron was formed, and was generated when the degree of doping was large.^33,34 The FeCl₃ used in this study was both an oxidant and a dopant¹⁶; hence, bipolaron was formed in all AF72, AF55, AF27 and F100, to which FeCl₃ was added by inducing a complete doping effect during the polypyrrole polymerization.

Figure 4.

X-ray photoemission spectroscopy N1s region of the samples polypyrrole-deposited with various ratios of oxidants and hydrophobic coated.

Table 3 shows the quantitative analysis results of the components detected through XPS. Theoretically, the ratios of carbon to nitrogen (C/N) and that of sulfur to nitrogen (S/N) in doped polypyrrole are 4.0 and 2.5, respectively. In this case, one cation exists for every four pyrrole units along the polypyrrole backbone. Therefore, the doping level is 25%.¹⁷ However, in this study, C/N and S/N were higher than the theoretical values because the ratio of C increased, while those of N and S decreased per unit area as the cotton and polypyrrole were bonded to each other because of the OH–NH reaction. S/N increased as the FeCl₃ content in the oxidants increased. S/N is an index for identifying the doping level of polypyrrole, and the rise in the FeCl₃ ratio increased the doping effect in the polypyrrole as Cl⁻ ions discharged from FeCl₃ during the polymerization process acted as a dopant.^16,17 Oxygen acts as an element that interferes with conductivity.³² The ratio of oxygen was 14.0% for A100, 13.0% for AF72, 12.9% for both AF55 and AF27 and 12.6% for F100. The amount of oxygen also increased as the ratio of APS in the oxidants increased. Oxygen already existed in APS in the form of S₂O₈. Carbonyl groups were formed as byproducts when polypyrrole was polymerized; thus, the addition ratio of APS affected the oxygen ratio increase of the final polypyrrole-deposited samples.^17,32,35

Table 3.

Elemental analysis of the polypyrrole samples deposited for various ratios of oxidants and hydrophobic-coated by the X-ray photoemission spectroscopy technique

	Element (at.%)					S/N	C/N	Proportion of N			S/N-N+
Smaple	C	N	O	S	Si	S/N	C/N	-NH-	-N+	=N+	S/N-N+
A100	74.8	7.4	14.0	2.2	1.6	0.30	10.2	0.71	0.29	0.00	0.0
AF72	75.6	7.9	13.0	2.9	0.6	0.36	9.5	0.59	0.32	0.09	0.0
AF55	75.7	7.6	12.9	2.9	0.9	0.38	10.0	0.55	0.34	0.11	0.0
AF27	75.7	7.2	12.9	3.0	1.1	0.42	10.5	0.54	0.33	0.13	0.0
F100	77.1	5.9	12.6	2.7	1.8	0.46	13.1	0.56	0.34	0.10	0.1

The XPS analysis results showed that the amount of oxygen in the samples generally decreased, and the doping level increased as the FeCl₃ content increased, which seems to make a significant influence on the conductivity of the final polypyrrole-deposited cotton fabrics.

Surface resistivity

Cotton fabrics are insulators that have infinite resistance. However, when their surfaces were deposited with polypyrrole, a conductive polymer, through in-situ polymerization, a current flowed because of the π-conjugation system of polypyrrole and the lighted light-emitted diode (LED) lamps (Figure 5). As a result of the surface resistivity measurement of the polypyrrole-deposited and hydrophobic-coated samples, the resistance was 248 ± 44 Ω/□ for A100, 281 ± 31 Ω/□ for AF72, 228 ± 23 Ω/□ for AF55, 133 ± 12 Ω/□ for AF27 and 42 ± 7 Ω/□ for F100 (Figure 6(a)). When A100 and F100, which used single oxidants, were compared, A100 exhibited a significantly lower conductivity. The oxidants with a high oxidation-reduction potential rapidly oxidized pyrrole, quickly decomposed the newly formed polypyrrole chains, and induced the three-dimensional growth of the polymers by enhancing the cross-linking between the polypyrrole chains. These non-uniform structures reduced the mobility of charges, thereby resulting in a lowered conductivity.²⁸ When APS was used as an oxidant, the conductivity was lower compared to that in FeCl₃ because of the over-oxidation of the polymer conjugation main chain.³⁶ The XPS analysis results also showed that the bipolaron structure did not appear in the polypyrrole structure of A100, which lowered the doping effect and caused a high surface resistivity.

Figure 5.

Photographs of light-emitting diode lamps prepared from polypyrrole-deposited and hydrophobic-coated cotton fabrics. (a) A100, (b) AF72, (c) AF55, (d) AF27, and (e) F100.

Figure 6.

Surface resistivity ((a), red dots) and normalized surface resistivity ((b), blue squares) of the polypyrrole samples deposited for various ratios of oxidants and hydrophobic coated. APS: ammonium peroxodisulfate. (Color online only.)

In contrast, a better conductivity was observed when the binary oxidants were used as the mixing ratio of FeCl₃ increased. FeCl₃ acted as an oxidant and a dopant, and can induce a complete doping effect during the polymerization process. FeCl₃ performed its function as an electron acceptor because it was capable of removing electrons from the pyrrole molecular structure through ion exchange with pyrrole. Therefore, the carrier migration resistance of electrons diminished, while the electron carriers were made, and the band gap was narrowed because of the energy level changes, thereby improving the conductivity of the conductive polypyrrole textile composites.¹⁶

The conductivity improved as the amount of polypyrrole deposited on the substrate increased.¹⁶ However, similar conductivities were observed under both single and binary oxidant conditions even though the polypyrrole add-on in the single oxidant condition was much higher than that in the binary oxidant condition. As for A100, the add-on (28.9 ± 5.0%) was similar to that of F100, but the conductivity was similar to that of AF72, which had the lowest add-on (10.9 ± 3.8%). Meanwhile, AF27 exhibited an excellent conductivity (133 ± 12 Ω/□) even though the add-on of polypyrrole was only 12.3 ± 2.8%.

According to the previous studies, the resistance linearly decreased to a certain level even under identical oxidant conditions, because the polypyrrole-induced weight increment of textiles was higher.^18,37,38 Therefore, the surface resistivity according to the oxidants was normalized under the assumption that the add-on of polypyrrole was the same at 30% to objectively compare the conductivity according to the mixing ratio of the oxidants. Consequently, the surface resistivity proportionally declined as the mixing ratio of APS decreased and that of FeCl₃ increased (Figure 6(b)). In particular, AF72, in which 25% FeCl₃ was added to the oxidants, showed a significantly lower surface resistivity than A100, confirming that even the addition of a small amount of FeCl₃ was effective in improving the polypyrrole conductivity. In other words, the use of the binary oxidants can improve the conductivity because of the synergistic effect of APS and FeCl₃.

The conductivity of the polypyrrole-deposited textile composites was affected by the polypyrrole deposited on the fabric surface and the polypyrrole impregnated into the fabric, indicating that the continuity and connectivity of the conductive particles deposited on the surface and inside of the fabric are important elements in improving the conductivity of polypyrrole textile composites.¹⁴ The high conductivity under the binary oxidant condition is related to the smaller polypyrrole particle sizes compared to those under the single oxidant condition. The abovementioned surface shape observation through SEM showed that the diameters of the polypyrrole particles formed on the surface of the cotton fabrics were 254 ± 71 and 167 ± 27 nm for A100 and F100, respectively (both of which used single oxidants), and 77 ± 10, 86 ± 14 and 81 ± 11 nm for AF72, AF55 and AF27, respectively, which used binary oxidants. Smaller particle sizes can form a continuous pyrrole layer because polypyrrole can be evenly distributed on the surface and inside the fabrics by infiltrating into the spaces among the fibers. As such, excellent conductivity can be achieved if the interconnectivity between the particles of the conductive polymer is improved. Therefore, while the binary oxidant condition had less pyrrole amount than the single oxidant condition, the presence of FeCl₃ increased the doping level of polypyrrole, and small polypyrrole particles less than 100 nm were evenly distributed inside the cotton fabrics, thereby securing continuous paths for electron movement and improving the conductivity.

Electro-heating property

Electrical heating of conductive polymers is based on resistance heating. Resistance heating occurs because of the interaction between the electrons and the atomic nuclei. In a conductor with an electric field, the electrons collide with atomic nuclei as they move. In this case, the kinetic and vibration energies of the atomic nuclei increase, and the energy is discharged as heat.^1,37,38 When polypyrrole is dispersed in an isotropic structure, as in polypyrrole textile composites constituting a conductive network in a polymer matrix, the flow of free electrons generates a chaotic heat motion without directions, resulting in heat on the fabric surface.^39,40

After electric wires were connected on both sides of the samples and voltage was applied, the changes in surface temperature were observed through thermal images for 10 min (Figure 7) to investigate the electrical heating characteristics of the polypyrrole-deposited cotton fabrics. The surface temperature increment according to the applied voltage was different according to the oxidant conditions (Figure 8). The surface temperature increment when 3 V voltage was applied was insignificant. The surface temperature instantaneously increased when the voltage was increased to 6 V. Compared to the 3 V condition, a noticeable heating effect was also observed. F100, which showed the lowest surface resistivity, particularly exhibited the best electrical heating performance because it reached up to 42.8℃ with the surface temperature increment of more than 10℃. The changes in the surface temperature over time were obvious in the samples when 9 V was applied, and the heating effect caused by electricity was also clearly observed through the thermal images. The temperature increment was 5.9℃ for A100, 2.9℃ for AF72 and 6.1℃ for AF72, which were all less than 10℃. However, AF27 and F100, which exhibited a low surface resistivity, showed sharp surface temperature increases of 10.1℃ and 20.1℃, respectively.

Figure 7.

Infrared thermal images of the polypyrrole-deposited and hydrophobic-coated cotton fabrics after voltage was applied for 10 min.

Figure 8.

Surface temperature increment of the polypyrrole-deposited and hydrophobic-coated cotton fabrics at (a) 3 V, (b) 6 V and (c) 9 V.

These results indicated that the conductivity of polypyrrole and the applied voltage are very important elements for the surface temperature. In this study, F100 with the lowest surface resistivity exhibited the best heating performance, followed by AF27, AF55, A100 and AF72, which confirmed that the lower surface resistivity led to the higher temperature increase. In addition, the amount of heat linearly increased in proportion to the applied voltage.⁴¹ Consequently, the surface temperature that used electrical heating can be easily changed to various levels by adjusting the voltage applied to the polypyrrole textile composites.

Duration of the electro-heating effect

The heating temperature and duration of the heating function are important when the function is applied to clothing materials, because the power supply must be carried. As such, the heating duration is measured by measuring the surface temperature changes using a 9 V battery with 580 mAh until the battery is discharged to examine the electrical heating performance and the duration of the polypyrrole textile composites over time. The experiment herein was conducted with AF55, AF27 and F100, which exhibited excellent heating performances. Figure 9 shows the results.

Figure 9.

Infrared thermal images (1 h) and surface temperature change during 24 h of the polypyrrole-deposited cotton fabrics at 9 V.

The surface temperature of all the samples sharply increased as soon as the voltage was applied, reaching maximum values within 1 h of voltage application. The temperature increased by 4.1℃ to 27.8℃ for AF55, by 8.2℃ to 32.0℃ for AF27 and by 22.0℃ to 45.8℃ for F100. The surface temperatures then started to decrease and the amount of heat was reduced. The surface temperatures declined to the same level as the environment temperature in approximately 8 h for F100, 25 h for AF27 and 27 h for AF55, and the heating function was terminated.

The conductive polymer was oxidized along the path of the current flow when voltage was applied to the samples, and part of the π-conjugation structure can be destroyed.^24,42 In addition, batteries, which store electrical energy, have limited capacities.⁴³ Therefore, when batteries are used for a long time, the amount of current diminished and the voltage applied to the samples also decreases, resulting in a decrease in the amount of heat. From this perspective, the duration of F100, which exhibited the highest heating temperature, lasted only 8 h because the low resistivity of F100 allowed more current to flow through the sample, thereby dramatically reducing the capacity of the battery. Furthermore, the heat loss caused by convection and radiation seemed to have reduced the heating duration, as the difference between the surface temperature and the environment temperature (23.7℃) increased. In contrast, although AF27 and AF55 had somewhat low surface temperatures (i.e., 28℃ and 26℃, respectively), they exhibited an excellent heating duration by consistently maintaining such temperature for over 20 h.

These results confirmed that when the amount of current is limited, the heating temperature caused by the conductivity and its duration have opposite tendencies. Therefore, polypyrrole textile composites suitable for the final use environment and purpose must be selected and applied based on the maximum heating temperature and duration.

Wettability

Polypyrrole was deposited onto the cotton fabrics under each oxidant condition to examine the surface wettability of the polypyrrole textile composites according to the oxidants. The static contact angle and shedding angle of the hydrophobic-coated samples were then measured using a DTMS sol-gel solution with a low surface energy.

As shown in Table 4, in the case of the refined and untreated cotton fabric (UC), water drops were instantaneously absorbed within 0.05 s of being dropped, and the contact angle was 0°. Cotton is a representative hydrophilic textile and exhibits superb absorption because it is composed of cellulose fibers and has numerous hydroxyl groups on the surface.⁴⁴ Hydrophobicity appeared with a high contact angle of 146.9 ± 2.8° (Table 4) when the surface energy of the untreated cotton fabric was lowered with hydrophobic coating (HC). Cotton fabrics consisted of numerous micro-level cellulose fibers with the average thickness of 10–15 µm crossing each other. The air pockets formed between the water droplets and the cellulose fibers acted as the major roughness that improved the water repellency.⁴⁴ Therefore, reducing the surface energy alone was sufficient for achieving hydrophobicity caused by the inherent micro-roughness of the cotton fabric.

Table 4.

Water contact angles of the samples after treatment with polypyrrole deposition and hydrophobic coating

	UC	A100	AF72	AF55	AF27	F100
Polypyrrole-deposited cotton fabrics	0°	46 ± 49°	49 ± 6°	105 ± 9°	129 ± 14°	132 ± 12°
Polypyrrole-deposited and hydrophobic-coated cotton fabrics	147 ± 3°	161 ± 3°	162 ± 2°	165 ± 2°	162 ± 2°	162 ± 2°

As a result of measuring the contact angle after polypyrrole was deposited on the surface of the cotton fabrics, different wettabilities were observed according to the oxidant conditions. A100 (46 ± 49°), AF72 (49 ± 6°) and AF55 (105 ± 9°) exhibited hydrophilicity, while AF27 (129 ± 14°) and F100 (132 ± 12°) showed hydrophobicity. Meanwhile, all the samples (i.e., A100 (161 ± 3°), AF72 (162 ± 2°), AF55 (165 ± 2°), AF27 (162 ± 2°) and F100 (162 ± 2°)) exhibited superhydrophobicity with contact angles of more than 150° when the contact angle was measured after the surface energy of the polypyrrole-deposited samples was reduced by hydrophobic coating.

The different wettabilities depending on the oxidant conditions are attributed to the following two reasons: the first is the difference in surface energy caused by the chemical composition changes in polypyrrole, and the second is the surface roughness of the dual structure physically formed because of polypyrrole deposition. Polypyrrole, a conductive polymer, consists of a cation-charged conjugated backbone and negatively charged counter-ion. It also has a wide range of wettabilities from hydrophilicity to hydrophobicity depending on the doping level and characteristics of the counter-ions.^6,45 While undoped neutral polypyrrole polymers generally have a low surface energy of approximately 32.1 mJ/m², doped polymers have a higher surface energy because of the presence of radical cations and anions.^46–48 In addition, the surface energy of the doped polypyrrole polymers is 42 mJ/m² for PPy-Cl, 53 mJ/m² for PPy-dodecyl sulfate, 57.8 mJ/m² for PPy-SO₄ and 61.2 mJ/m² for PPy-NO₃, depending on the affinity for moisture or concentration of the doped molecules and the electrical characteristics caused by the radical cations and anions.⁴⁷

In this study, the counter-ions of polypyrrole varied depending on the oxidant conditions, and the sulfonic acid dopant of AQSA-Na and the Cl⁻ of FeCl₃ acted as the counter-ions. In this case, the condition of Cl⁻ doping resulted in the formation of polypyrrole with a lower surface energy; hence, the surface energy of polypyrrole decreased as the FeCl₃ content increased, resulting in hydrophobicity. Therefore, the contact angles of the polypyrrole-deposited cotton fabrics gradually increased as the FeCl₃ content increased from A100 to F100 (Table 4). Furthermore, XPS analysis results indicated that the amount of oxygen increased because of the formation of the carbonyl group caused by the over-oxidation of the polymer conjugation main chain under the oxidant conditions, including APS; thus, hydrophilicity increased, which lowered the contact angle.^32,38

However, the difference in the chemical properties of polypyrrole could not influence the contact angle when the surface energy was lowered by the hydrophobic coating. As a result, the contact angle changed because of the hydrophobic coating produced by the physically formed rough surface of the cotton fabrics. The cotton fabrics have a unique micro-level surface roughness because they have repetitive points made by crossing warps and wefts, and the threads constituting them are composed of several bundles of staple fibers.⁴⁹ Therefore, as the nano-scale roughness was made by polypyrrole polymerization on top of the micro-roughness naturally formed by the cotton fabrics, the formation of the physical dual structure for the superhydrophobic surface reduced the contact area with water droplets, improving the contact angle.

In the case of the shedding angle, all the samples yielded the superhydrophobic surface while recording values less than 10°. As for the oxidant conditions, the single oxidant conditions exhibited slightly lower shedding angles compared to the binary oxidant conditions.

The difference in shedding angle according to oxidant conditions appeared to be a result of the difference in the nano-scale roughness caused by the polypyrrole formed on the surface of the cotton fabrics. Figure 10 exhibits a graph showing the changes in diameter of the polypyrrole particles and shedding angle according to the oxidant mixing ratios. The size of the polypyrrole particles and the shedding angle are in a negative linear relationship.

Figure 10.

Polypyrrole diameter and shedding angle change for various ratios of oxidants. APS: ammonium peroxodisulfate.

Toster and Lewis⁵⁰ studied the contact angles and sliding angles of films according to nano-scale roughness; the films were fabricated by the spin coating method by dispersing silica particles of size 58–1428 nm to Polydimethylsiloxane (PDMS). As a result, the contact angle was hardly affected by the diameter of the nanoparticles and maintained a constant level. In the case of the sliding angle, the high value of 90° for the 58 nm nanoparticle film decreased to 22° for the 1428 nm film, indicating that the contact with the water droplets changed from the Wenzel state to the Cassie state as the nanoparticle size increased. The study by Larsen et al.⁵¹ on the assessment of the sliding angle of a superhydrophobic surface according to the regularity of surface roughness also showed that the sliding angle was higher as the irregularity of the roughness for the roll-off direction increased rather than the ratio of the contact area between the water droplets and the solid surface increased. They explained that this was the result of the strong attraction among the three phases because the contact areas of the solid–liquid–gas were variously arranged due to the irregular surface structure.

Therefore, the reason that the single oxidant conditions recorded lower shedding angles than the binary oxidant conditions in this study appeared to be the nano-scale roughness caused by the polypyrrole particles, which were formed on the surface of the cotton fabrics, formed the Cassie state and induced appropriate configuration of the three phases of solid–liquid–gas. Approximately 254 nm polypyrrole particles were formed when the A100 oxidant was used. This size constituted the proper dual-structured roughness with the cotton fibers, which were 10–15 µm width on average; hence, the contact area with water droplets was reduced, and the Cassie state was reached, thereby recording the lowest shedding angle.

Although somewhat higher shedding angles were observed for the binary oxidant conditions because of the surface roughness caused by the polypyrrole particles, the conditions succeeded in producing a superhydrophobic surface by recording contact angles of more than 150° and shedding of less than 10° angles under all conditions.

Mechanical properties

The tensile strength for each treatment condition was measured to identify the effects of polypyrrole deposition and hydrophobic coating on the mechanical properties of the cotton fabrics. Figure 11 shows the tensile strength retention compared to the untreated cotton fabric.

Figure 11.

Tensile strength of the cotton fabric treated with polypyrrole deposition and hydrophobic coating.

It was shown that the tensile strength of the hydrophobic-coated fabric slightly decreased by 2% compared to the untreated fabric, as the degrees of freedom of the threads decreased. In the case of the polypyrrole-deposited and hydrophobic-coated samples, the tensile strength tended to decrease under all conditions, indicating a tensile strength at the level of 60–84% compared to the untreated cotton fabric. The single oxidant conditions significantly degraded the mechanical properties of the cotton fabrics to the 60% level, while the binary oxidant conditions exhibited relatively high strength retention. The tensile strength was higher in the order of AF27, AF55 and AF72 as the mixing ratio of APS decreased.

The in-situ polymerization of polypyrrole on the surface of a cotton fabric has the benefit of forming a durable polypyrrole layer by increasing the bonding strength between the cotton fabric and polypyrrole. However, when used as an oxidant, FeCl₃ forms Fe(OH)₃ and HCl because of hydrolysis, which is known to attack the hydrogen bonding in the cotton fibers and degrade their mechanical properties.¹⁷ In the case of F100, which used FeCl₃ alone in this study, the degradation in mechanical properties caused by polypyrrole deposition was also observed, maintaining approximately 64% of the strength. As such, previous studies proposed that APS be used as an oxidant instead of FeCl₃ to prevent the degradation in the mechanical properties of textiles.¹³ However, unlike the previous studies, the use of APS alone herein decreased the tensile strength of the cotton fabrics to 60% of the initial strength. H₂SO₄, which was generated after polypyrrole polymerization, significantly contributed to the strength decrease. The cellulose molecular chains of the cotton fabrics immersed in the reaction bath were subjected to hydrolysis by acid substances generated during the polypyrrole polymerization process.^52,53

In the case of binary oxidants, tensile strength retention was relatively higher compared to that in single oxidants, because the binary oxidants underwent the polypyrrole deposition process in three dimensions, in which pyrrole monomers were diffused onto the surface of the cotton fabrics after generating nuclei in the dispersion. In comparison, the single oxidants went through the instantaneous nucleation and two-dimensional growth process, in which nucleation and growth occurred on the surface of the cotton fabrics.^28,54 Therefore, the binary oxidant conditions seemed to have a small influence on the mechanical properties of the cotton fabrics because relatively less direct chemical reactions occurred on the surface of the cotton fabrics in the polypyrrole polymerization process. In addition, the acidity of the solution caused by the resulting H₂SO₄, Fe(OH)₃ and HCl was weak because the added FeCl₃ or APS content was low compared to the single oxidants. In the case of the binary oxidants, in which hydroxyl radicals were generated by the Fenton reaction, the decomposition rate for organic matter increased when the APS concentration was higher.²⁹ Therefore, tensile strength decreased as the APS ratio increased in the order of AF72, AF55 and AF27.

In addition, the flexural stiffness G was calculated herein to examine the degree of the drape and the stiffness of the cotton fabrics treated with polypyrrole deposition and hydrophobic coating. Figure 12 shows the results, indicating that the G value of the untreated cotton fabric and the hydrophobic-coated cotton fabrics (HC) was the same at 0.022. This finding implied that the hydrophobic coating performed herein did not significantly affect the stiffness of the fabrics. In contrast, in the case of the samples treated with polypyrrole deposition and hydrophobic coating, the flexural stiffness increased.

Figure 12.

Conventional flexural stiffness of the cotton fabric treated with polypyrrole deposition and hydrophobic coating.

The flexural stiffness of a fabric is affected by various factors, such as initial elasticity, yarn characteristics, texture composition and weight, as well as fabric thickness.⁵⁵ The experimental results indicated that the thicknesses of the fabrics increased after polypyrrole deposition (Table 5), and this may have served as a factor that increased the flexural stiffness. Furthermore, the polypyrrole used in this study is known as a rigid substance with conjugated double bond structures.⁵⁶ Therefore, the polypyrrole particles formed through the pyrrole polymerization can be estimated to disperse even into inside the fabrics, form rigid conductive layers and restrict the movement of the threads, making the fabrics stiff. As a consequence, A100 and F100, whose thicknesses increased by more than 10% because of the large polypyrrole deposition amounts, exhibited stiffer and more rigid appearances because of deposition and coating. F100, with small polypyrrole particles, exhibited the highest flexural stiffness. Meanwhile, in the case of the binary oxidants, the flexural stiffness did not significantly increase compared to the untreated cotton fabric because the polypyrrole deposition amount was small, and the formed polypyrrole particles were as small as less than 100 nm. However, AF72 exhibited a significant increase in the flexural stiffness compared to the other conditions, despite the similar thickness increase and the polypyrrole deposition amount in AF55 and AF27. Such a difference appeared to be caused by the reduced freedom of the threads because the diameters of the polypyrrole particles deposited on AF72 were smaller than those on the other samples, and conductive layers were formed more densely inside the fabric.

Table 5.

Thicknesses of the samples that were polypyrrole-deposited with various ratios of oxidants and hydrophobic-coated

		A100	AF72	AF55	AF27	F100
Untreated	Thickness (mm)	0.42	0.42	0.42	0.42	0.42
Polypyrrole deposition	Thickness (mm)	0.49	0.42	0.43	0.43	0.48
	Change ratio (%)	+16.7	0.0	+2.4	+2.4	+14.3
Hydrophobic coating	Thickness (mm)	0.49	0.42	0.42	0.42	0.48
	Change ratio (%)	0.0	0.0	–2.4	–2.4	0.0

As assessment of flexural stiffness revealed that the binary oxidant conditions were more favorable than the single oxidant conditions in maintaining the flexibility of the fabric during the polypyrrole deposition process. The hydrophobic coating process did not significantly affect the flexural stiffness of the fabric.

Conclusion

The conductivity, wettability and mechanical properties of superhydrophobic polypyrrole textile composites, which can be used as clothing materials, were investigated herein according to the oxidant treatment conditions to select the optimal conditions.

In terms of conductivity, the surface resistivity decreased as the FeCl₃ ratio in the oxidants increased. In the case of using binary oxidants, the surface resistivity was higher than when FeCl₃ alone was used. However, excellent conductivity was achieved despite the small deposition amount because small-sized polypyrrole particles were formed that diffused inside the fabrics, thereby securing continuous paths for electron movement. The lower surface resistivity led to the higher surface temperature by the electrical heating property of polypyrrole textile composites. However, the duration of the electro-heating effect of F100, which exhibited the highest heating temperature, was shorter than those of AF27 and AF55 because the heat loss caused by the difference between the surface temperature and the environment temperature increased. Although the binary oxidant-treated samples had somewhat lower surface temperatures than those of the single oxidant conditions, they exhibited an excellent heating duration by consistently maintaining such temperature for 20 h.

A superhydrophobic surface could be achieved because of the micro-nano dual-scale roughness formed on the cotton fabric surfaces by polypyrrole deposition with the aid of oxidants. The contact angles of the samples treated with binary oxidants were higher than those of the single oxidant conditions with the values up to 165°; however, the shedding angle was slightly higher. However, superhydrophobic surfaces were obtained under all oxidant conditions with contact angles higher than 150° and shedding angles less than 10°.

In terms of the mechanical properties of the fabrics, the Fe(OH)₃ and HCl generated during the polymerization process broke the hydrogen bonds of the cotton fabrics, caused hydrolysis and degraded their mechanical properties, thereby significantly lowering the tensile strength and making the fabric stiff. However, the influence of the binary oxidants on the fabrics was relatively smaller than that of the single oxidants because polypyrrole was not directly generated on the surface of the cotton fabrics due to the difference in the polymerization mechanism and, thus, less damage was caused by the acid substances generated in the polymerization process, and because the changes in the thickness and weight caused by the add-on were insignificant.

In this study, in-situ polymerization of polypyrrole and a superhydrophobic coating were applied to cotton fabrics to develop self-cleaning and conductive textile composites with electric heating performance. In particular, the binary oxidants introduced into polypyrrole polymerization enabled it to form various nano-roughnesses on the surface of the cotton fabric by controlling the size of polypyrrole particles. The binary oxidants condition satisfied excellent conductivity, rather moderate surface temperature, longer heating duration and superhydrophobicity while maintaining flexibility and mechanical properties. This could also solve the problems in single oxidant conditions, such as low conductivity of APS and the degradation in mechanical properties of FeCl₃.

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 the National Research Foundation (NRF) of Korea funded by the Korean government (MSIP) (Grant No. 2015R1A2A2A03002760, NRF-2018R1A2B6003526 and No. NRF-2016M3A7B4910940).

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