Effect of Drawing on the Electrical, Thermoelectrical, and Mechanical

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Cite This: ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

Effect of Drawing on the Electrical, Thermoelectrical, and Mechanical Properties of Wet-Spun PEDOT:PSS Fibers Ruben Sarabia-Riquelme,*,† Maryam Shahi,‡ Joseph W. Brill,‡ and Matthew C. Weisenberger† †

Center for Applied Energy Research, University of Kentucky, 2540 Research Park Drive, Lexington, Kentucky 40511, United States Department of Physics and Astronomy, University of Kentucky, Lexington, Kentucky 40506-0055, United States

‡

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S Supporting Information *

ABSTRACT: New electrically conducting and mechanically robust fibers and yarns are needed as building blocks for emerging textile devices. In this work, we describe a continuous wet-spinning process for the fabrication of poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) fibers with high electrical conductivity, excellent mechanical properties, and moderate thermoelectric performance by including a drawing stage in dimethyl sulfoxide. Drawing the fibers induced preferential orientation of the polymer chains in the fiber axis direction. With increased drawing, the room temperature electrical conductivity saturated at approximately 2000 S cm−1. The Seebeck coefficient was found to be rather constant with drawing. Therefore, the thermoelectric power factor saturated with applied draw between 40 and 50 μW m−1 K−2. The thermal conductivities of the drawn fibers were measured between 4 and 5 W m−1 K−1 at liquid nitrogen temperatures. Although the relatively high thermal conductivity negatively affects the ultimate thermoelectric performance, it can be beneficial for other applications such as in electrical interconnections. Additionally, at high draw ratios, the Young’s moduli saturated at near 15.5 GPa with maximum break stresses of 425 MPa. To the best of our knowledge, this Young’s modulus is the highest reported for a PEDOT:PSS material. Further, we investigated the degree of preferred orientation by wide-angle X-ray scattering and found a strong correlation between the orientation of the polymer chains along the fiber axis and the trends observed in the fibers’ properties. In general, the fibers with the highest orientation were also the stiffest and the most conducting fibers. We believe these are important steps toward the continuous fabrication of high performance PEDOT:PSS fibers to be used as building blocks in the emerging field of electronic textiles. KEYWORDS: PEDOT:PSS, fiber, electrical conductivity, thermal conductivity, thermoelectric, conducting polymer deposition of the conducting polymer onto an inert fiber support by using different techniques. Several authors have explored the in situ polymerization of polypyrrole and poly(3,4-ethylenedioxythiophene) (PEDOT) on different substrates, such as silk fiber,6 cotton,7 wool,8 or polyester fabric.8,9 Additionally, aqueous dispersions of PEDOT with the counterion poly(styrenesulfonate) (PSS) or PEDOT:PSS have been used to impregnate nylon fibers,10 commercial polyester fabrics,11 cotton cellulose fibers,12 and even silk yarns.13 Coated fibers have the advantage of being relatively

1. INTRODUCTION Smart electronic textiles cross conventional applications to impart functionalities such as light emission, health monitoring, climate control, sensing, storage and conversion of energy, etc.1−3 However, to realize these smart textile devices, new fibers and yarns that are electrically conductive and mechanically robust are needed as fundamental building blocks.3,4 Conjugated polymers have gained attention in the field of electronic textiles because they are made of earth-abundant elements, have good mechanical properties and flexibility, and can be processed using low-cost large-scale solution processing methods.5 Currently, the main method to fabricate electrically conducting fibers or yarns from conjugated polymers is the © XXXX American Chemical Society

Received: May 6, 2019 Accepted: July 8, 2019 Published: July 8, 2019 A

DOI: 10.1021/acsapm.9b00425 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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ACS Applied Polymer Materials

Figure 1. Scheme of the continuous wet-spinning process for the fabrication of PEDOT:PSS fibers. Insets are (a) image of fiber formation in the coagulation bath (scale bar is 10 mm) and (b) image of dried fiber before entering the DMSO draw bath (scale bar is 2 mm). Diameter of the fiber at this point is around 10 μm. (c) Image of the same fiber as in panel b swollen by DMSO after exiting the DMSO draw bath (scale bar is 2 mm). (d) Image of a fiber on a PTFE spool of 26 mm diameter.

as poly(3-alkylthiophenes)18,19 and polyaniline20 fibers. In addition to the mechanical properties and electrical conductivity, other transport properties such as the thermal conductivity and Seebeck coefficient are also of great interest for electronic textiles and can also be affected by the preferential orientation of the polymer chains. For instance, the thermal conductivity at room temperature of an oriented polyacetylene film was measured to be ∼13 W m−1 K−1, while oriented polyethylene fibers may achieve values higher than 30 W m−1 K−1 above 200 K.21,22 On the other hand, the effect of orientation on the Seebeck coefficient is less clear and it has been reported to decrease,23,24 remain the same25 or increase26 with increased orientation in conjugated polymer films. Additionally, PEDOT:PSS has shown potential as an organic thermoelectric material and its thermoelectric properties have been widely studied in the film geometry.27−33 We believe that the study of its thermoelectric properties in the fiber geometry has great scientific value. Motivated by this, we envisioned a continuous wet-spinning process comprised of a coagulation bath, a dimethyl sulfoxide (DMSO) drawing bath and two drying stages. We investigated the effect of varying wet-spinning parameters with special emphasis on the applied draw in the DMSO bath. DMSO swelled the fibers and acted as a plasticizer enabling total draw ratios greater than 2. Drawing the fibers induced preferential orientation of the polymer chains in the direction of the axis of the fiber resulting in outstanding electrical, thermal and mechanical properties. PEDOT:PSS fibers fabricated at high draw ratios achieved room temperature electrical conductivities around 2000 S cm−1 and Young’s moduli around 15.5 GPa. Furthermore, we also investigated the thermoelectric properties of the PEDOT:PSS fibers. The Seebeck coefficient remained practically constant with applied draw and highly drawn fibers achieved thermoelectric power factors between 40 and 50 μW m−1 K−2. However, high thermal conductivities between 4 and 5 W m−1 K−1 were observed at liquid nitrogen temperatures which negatively affects the ultimate thermoelectric performance. By investigating the fiber’s structure using wide-angle X-ray scattering (WAXS), we found a good correlation between preferential orientation of the polymer chains and the enhancements observed in the fiber’s properties. We believe these are important steps toward continuous fabrication of high performance PEDOT:PSS fibers that could

straightforward to fabricate and retain the mechanical properties of the underlying polymer fibers. However, the bulk electrical conductivity of these coated textiles is usually small (often lower than 10 S cm−1),3 which limits their applications. An interesting alternative would be to spin electrically conductive and mechanically robust conjugated polymer fibers, not coated fibers, that could serve as building blocks for electronic textiles. Aqueous dispersions of PEDOT:PSS can be processed into fibers using a traditional wet-spinning process where the polymer solution (dope) is coagulated using a nonsolvent. Okuzaki et al. reported for the first time the coagulation of PEDOT:PSS aqueous dispersion in an acetone bath to form electrically conductive (∼10−1 S cm−1) microfibers.14 Later, they also reported the enhancement of the electrical conductivity of the fibers to 467 S cm−1 by a posttreatment with ethylene glycol.15 Recently, Zhou et al. reported a high electrical conductivity of 2804 S cm−1 by including a drying step at 90 °C right after an isopropyl alcohol (IPA) coagulation bath and washing the fibers 1 h in ethylene glycol to remove excess insulating PSS.16 Recently, Zhang et al.17 published the continuous fabrication of PEDOT:PSS fibers with high conductivities of up to 3828 S cm−1 by switching the coagulation bath from IPA to concentrated sulfuric acid and keeping a residence time of the fiber in the coagulation bath of 10 min. They attributed the enormous increase in electrical conductivity to the very effective removal of excess PSS in the concentrated sulfuric acid bath. All the above reports focused on improving the electrical conductivity by removing the excess of insulating PSS either by posttreatments with ethylene glycol or by coagulating the fiber using sulfuric acid. It must be noted here that Zhou et al. reported using hot drawing as one of the steps for the fabrication of wet-spun PEDOT:PSS fibers.16 However, their reported data stemmed from a single, fixed total draw. A detailed study on the effect of varying the applied draw on the properties of the resultant PEDOT:PSS fibers has hereunto not been reported. Drawing or stretching is a characteristic step of every fiber fabrication process. Drawing induces preferential orientation of the polymer chains in the fiber-axis direction enhancing the mechanical properties of the fiber. Moreover, increased electrical conductivity with increasing draw has been previously reported for other conducting polymer fibers, such B


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The Seebeck coefficient was measured using a purpose-built setup. Typically, three 30 mm long segments of fiber were laid between two Peltier devices that allowed for precise control of the temperature and contacts were made using silver paint. Two K-type thermocouples were used to monitor the cold-side and hot-side temperatures. The Seebeck coefficient was extracted as the slope of the ΔV−ΔT plots. Values presented are average values between three specimens and error bars represent standard deviation between specimens within the same sample. Thermal Conductivity Characterization. The thermal conductivities, κ, were measured for specimens from a coagulation bath sample with total draw of 1.58 and DMSO stretched samples with total draws of 1.72, 1.97, and 2.36 using a self-heating technique34,35 (all samples spun into 10 vol % DMSO in IPA); 3−4 specimens from each sample were measured. In short, the resistance of the specimen, mounted in a four-probe configuration, was measured as a function of applied current. If heat exchange by thermal radiation is negligible (see the Supporting Information), the derivative of the resistance with respect to current (for small currents) is given by

serve as the fundamental building blocks for future electronic textiles.

2. EXPERIMENTAL DETAILS Materials. PEDOT:PSS water dispersion was purchased from Heraeus (PH1000; PEDOT:PSS weight ratio of 1:2.5; solid content 1.3 wt %). DMSO and IPA were purchased from VWR. Dope Preparation. The PEDOT:PSS dispersion was placed in a hot plate at 90 °C while magnetically stirring and the mass of evaporated water was monitored until the solid concentration reached 2.5 wt %. Afterward, 5 wt % of DMSO was added and the dope was further stirred for 2 h at room temperature. Then, the dope was bath sonicated for 30 min and finally degassed in a vacuum oven at room temperature for 5 min. Wet-Spinning Process. Figure 1 shows a scheme of the wetspinning setup used in this work. First, the degassed dope was transferred carefully to a 5 cc glass syringe and placed on a syringe pump (KD Scientific) that allowed precise control of the flow rate. A constant flow rate of 0.25 mL/h was used for all the samples collected in this work. The dope passed through a sintered metal disk, with an average pore size of 5 μm, before exiting through a 100 μm diameter capillary spinneret (length to diameter of the capillary, L/D = 5) into the coagulation bath. In this work, two coagulation baths were investigated, pure IPA and 10 vol % DMSO in IPA. After coagulation, the fiber was dried by passing through a heater which kept the air temperature around 120 °C, before reaching the first roller. The ratio between the first roller speed, v1, and the jet velocity, vjet = volumetric flow rate/area of the spinneret orifice, is called the jet draw ratio, DRjet = v1/vjet, and was kept constant at 1.50. Fibers which were not subsequently drawn or “coagulation bath samples” were taken from this first powered roller directly onto a spool. The additional tension needed to take the sample from the first roller to the spool resulted in total draw ratio of 1.58 for coagulation bath samples. For stretched samples, after the first roller, the fiber entered a pure DMSO stretch or draw bath (kept at room temperature) followed by another drying step in a keyhole-cylinder-shaped oven with a maximum air temperature inside the oven of 170 °C. After the oven, drawn and dried fiber could be continuously taken-up on a spool with no interfilament fusion. We defined the ratio between the take-up speed, vtake‑up, and v1, as the DMSO draw ratio, DRDMSO = vtake‑up/v1, and the total draw ratio can then be defined as DRtotal = DRjet·DRDMSO. Typical linear speeds of the fibers at take up were on the order of 1 m/min. Note that the samples measured in this study were collected over the course of several spin runs with dopes made from scratch for each spin run. We report independent characterization data for each of the samples, from varying runs, as a separate data point. That is, two samples may have the same draw ratio, but stemmed from different spin runs. This reveals insight into the reproducibility of the data. Mechanical Characterization. Tensile tests were performed using an automatic single-fiber test system, FAVIMAT+ from Textechno. The pretension was 0.50 cN/tex and the test speed was 5.0 mm/min over a gauge length of 25.4 mm. Values presented in this work are average values of at least 5 fibers per sample and error bars represent standard deviation between fibers. Electrical and Thermoelectric Characterization. A 30 mm long segment of fiber was laid between two copper tape strips and contacted using silver paint. Then, the resistance was measured by the 2-probe method using a Keithley 2100 microvoltmeter. Initially, we measured resistance using a 4-probe method to eliminate contact resistance. However, the contact resistance was found to be in all cases small (100%), so measurements were instead made at liquid nitrogen temperature, where the radiation uncertainty was ∼±3%. Also, measurements were done for presumably dehydrated samples in vacuum. Details of the analysis and the measurement technique are given in the Supporting Information. Scanning Electron Microscopy (SEM). Imaging was performed on a Hitachi S-4800 field emission SEM at 10 kV accelerating voltage and 10 μA beam current. Gold sputtering of the samples was not needed due to the conductive nature of the fibers. Since the electrical conductivity is dependent on diameter, each specimen tested for electrical resistance was then placed in the SEM to obtain its average diameter. For each specimen 10 to 15 diameter values were measured at different points, which gave a total of 50−75 measured values per sample. The average value was taken as the average diameter of the sample and error bars represent the standard deviation within the same sample. In the case of nonround fibers which were spun into a 100% IPA coagulation bath, the reported diameters were for an equivalent circular area. For imaging the cross sections, a bundle of fibers was immersed in liquid nitrogen and fractured using a razor blade. The bundle was then transferred to the SEM for imaging. Wide-Angle X-ray Scattering (WAXS). Measurements were performed in transmission mode using the Xenocs Xeuss 2.0 SAXS/ WAXS system located at the Electron Microscopy Center of the University of Kentucky. The source was GeniX3DCu ULD 8 keV with wavelength of 1.54189 Å. Typically, after completing electrical, thermoelectric, mechanical and SEM characterizations, the remaining fibers on the spool were cut and aligned into a bundle and placed in an aperture card. The aperture card with the aligned fiber bundle was then transferred to the WAXS sample holder and placed at 101.17 mm from the 2D detector (Dectris Pilatus 200 K). Exposure time was 600 s. Data processing to obtain the integrated diffracted intensity versus 2θ and azimuthal angle, Ψ, was performed using the software Foxtrot provided by Xenocs. To measure the WAXS diffraction pattern of an unoriented PEDOT:PSS sample, a film was prepared by drying some drops of dope on a flat surface at room temperature. The dried film could be peeled off the surface and transferred to the WAXS sample holder for characterization.

3. RESULTS AND DISCUSSION Figure 1 shows a scheme of the continuous wet-spinning setup used in this work. The prepared PEDOT:PSS dope (see experimental details for preparation methods) was pumped out of the syringe passing through a 5 μm syringe filter. Initial tests without the filter were performed but frequent breakage of the C


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Figure 2. (a) Plot of diameter versus total draw ratio of the PEDOT:PSS fibers. As expected, the diameter of the fibers decreased with increased draw ratio. Plotted values are average values of 50−75 diameter measurements performed on 5 different specimens (10−15 per specimen) and error bars represent the standard deviation within specimens of the same sample. (b and c) SEM cross-section image of fibers spun into a coagulation bath of IPA and 10 vol % DMSO in IPA, respectively, and total draw ratio of 1.67. Insets are close-ups to one of the fibers. The crosssectional shape was not circular for fibers spun into pure IPA but became circular when 10 vol % DMSO was added to the coagulation bath. (d−f) SEM images of fibers spun into 10 vol % DMSO in IPA with total draw ratios of 1.58, 1.97, and 2.36, respectively. Scale bars are 10 μm except in insets that are 3 μm.

by the second powered roller. At this point, the filament was strong enough to release from the roller without breakage and, therefore, drying the fiber between the DMSO bath and the second roller was not necessary. Finally, the filament was dried by passing through a keyhole-cylinder-shaped oven before being taken-up on a spool (see Figure 1d). Figure 2a shows the diameters of the fibers a function of the total draw ratio. As expected, the diameter of the fibers decreased with increasing draw from 10 to 12 μm for the coagulation bath samples to 6.7−7 μm for the fibers with the highest applied draws. It must be noted that these diameters were measured in vacuum (from SEM imaging), and thus, the fibers were presumably dehydrated. Since the ambient atmospheric diameters are unknown, it has been assumed that the change in diameter because of dehydration is negligible. No difference in diameter was observed between the fibers spun into IPA and 10 vol % DMSO in IPA. However, a difference in the cross-sectional shape of the fibers was observed. Fibers spun into a pure IPA coagulation bath showed a noncircular cross-section, while fibers spun into 10 vol % DMSO in IPA were all circular (see Figure 2b and c). Our group has investigated, in the past, the change from noncircular to circular cross-section of wet-spun polyacrylonitrile precursor fibers.38 It was found that in coagulation baths with high nonsolvent concentrations (in this case IPA), the coagulation rate is high and a dense skin forms in the outer perimeter of the filament impeding diffusion in and out of the fiber, which subsequently collapses locking-in the circumference of the dense skin layer, causing noncircular cross sections. However, diluting the coagulation bath (in this case by adding 10 vol % DMSO) decreases the coagulation rate, and little or no skin is formed resulting in circular shaped cross sections. Figure 2d−f shows SEM images of fibers spun into 10 vol % DMSO in IPA and taken at different total draw ratios, where the smooth surface and cylindrical shape of the fibers can be observed. In all cases, high quality fibers with the absence of voids could be

filament occurred in the coagulation bath. After careful observation of the breakage events, it was determined that particulates, which were unable to stretch at the same rate that the rest of the jet, were the cause of the frequent breakages. A sintered disk to filter particles down to 5 μm was included in the setup and continuous spinning without breakage was achieved. Figure 1a shows an image of the polymer jet exiting the spinneret in the coagulation bath and forming a solid fiber that can then be taken out of the coagulation bath by a rotating roller. IPA was chosen as coagulation bath since it has been previously reported that fibers coagulated in IPA were less porous than fibers coagulated in acetone and thus had superior electrical and mechanical properties.36 When the PEDOT:PSS dope entered in the coagulation bath, water diffused from the nascent fiber into the coagulation bath and IPA diffused into it. This caused a fast destabilization of the dispersion as PSS loses its surfactant effect resulting in the formation of a solid filament. In this work, the jet draw was kept constant at 1.50. Additionally, a coagulation bath with 10 vol % DMSO in IPA was also investigated. Following the coagulation bath, the fiber was dried by a vertical heater. Initial tests did not include a heater at this point. However, when the wet fiber touched the first roller, the surface tension between the IPA and the roller stuck the fiber to the roller and breakage would occur in any attempt to take the fiber further down the spinning line. Thus, completely drying the filament before touching the first roller was necessary. Then, the dried filament (see Figure 1b) entered the draw bath. DMSO is a polar solvent that can screen to some extend the Coulombic interactions between PEDOT and PSS leading to an enhancement in the local order of PEDOT chains and partial removal of excess PSS.27,37 With this screening effect in mind, we decided to test DMSO as the media to further draw the fibers. In the draw bath, DMSO rapidly swelled the fiber, which was clearly visible by the increase in size of the filament (see Figure 1c). DMSO acted as a plasticizer allowing for the application of high draw to the fiber. Afterward, the filament was taken out of the DMSO bath D


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Figure 3. WAXS analysis of wet-spun PEDOT:PSS fibers. 2D WAXS pattern of (a) PEDOT:PSS film, (b) coagulation bath fibers spun into 10 vol % DMSO in IPA (draw 1.58), and (c) fibers spun into 10 vol % DMSO in IPA and drawn through a DMSO bath at 2.36 draw ratio. (d) Normalized intensity (with respect to the PSS broad hump) versus 2θ showing the (100) and (020) peaks for PEDOT crystals and the PSS amorphous broad hump. (e) Scheme of the PEDOT:PSS crystal structure showing the π−π stacking of PEDOT and lamella stacking of PEDOT and PSS. PSS chain exact positions are not drawn and instead represented with a blank box, nevertheless, this does not affect the WAXS analysis performed. Intensity as a function of azimuthal angle for (f) (100) reflections and (g) (020) reflections showing the increase in intensity of the peaks at 0 and 180°, which indicates preferred orientation of these planes in the fiber axis direction.

spun for hours without breakage. The density of the fibers is estimated to be around 1.46 g/cm3 (see Figure S2). Figure 3a−c shows 2D WAXS patterns of a PEDOT:PSS film, coagulation bath fibers spun into 10 vol % DMSO in IPA and fibers stretched through the DMSO bath (draw ratio of 2.36). As expected, the 2D WAXS pattern of the PEDOT:PSS film does not show any signs of preferred orientation, indicating random orientation of the polymer chains. However, in the 2D WAXS patterns of the fibers, the characteristic arcs indicating preferred orientation of crystalline planes can be observed and become more evident at higher draw ratios (see marked arcs in Figure 3c). For the WAXS characterization, the fibers were aligned vertically; therefore, the appearance of horizontal arcs indicates the preferred orientation of crystal planes in the fiber axis direction. The integrated intensity versus 2θ is presented in Figure 3d. One broad hump and two peaks were observed. The broad hump peaking at 17.7° for all samples is largely attributed to amorphous and randomly oriented PSS.39,40 The peak that appears for the film at 4.8° shifted to 3.4° for all the fiber samples, which corresponds with a shift in d-spacings from 18.4 to 26.3 Å. This peak is attributed to the (100) lamella stacking of alternating PEDOT and PSS as shown in Figure 3e, where the exact position of PSS chains is not shown but does not affect the analysis.41 The (100) peak for the coagulation bath fiber spun into IPA increases in intensity with respect to the film, indicating that higher crystalline order was induced during fiber formation. Additionally, adding 10 vol % DMSO to the coagulation bath further increases the intensity of this peak. The screening effects of DMSO diminished the strong Coulombic interactions between PEDOT and PSS and allowed for a better organization of the polymer chains. The intensity increases even further at higher draw ratios, which could be indicative of

drawing-induced ordering of the polymer chains42,43 or of partial removal of PSS in the DMSO bath (see Figures S3−S4) or a combination of both. A peak also appears at 25.8° for the PEDOT:PSS film and the coagulation bath fibers spun into pure IPA. However, the peak shifts to 26.2° for the fibers spun into a coagulation bath with DMSO and the fibers stretched in DMSO regardless of the coagulation bath. This peak is assigned to the (020) reflection corresponding to the π−π stacking of PEDOT.41 As shown in Figure 3e, PEDOT chains crystallize stacking face-to-face by overlapping the π-orbitals and keeping a planar structure. 39 The shift observed corresponds to a reduction in the π−π stacking distance from 3.5 to 3.4 Å. The shorter distance indicates stronger π−π interactions which should lead to enhanced interchain charge carrier transport in the b-axis direction. A π−π stacking distance of 3.4 Å has been previously reported for PEDOT: tosylate crystals41 and PEDOT:PSS crystals.39,40,44 Figure 3f and g presents the intensity along the azimuthal angle for the (100) and (020) reflection, respectively. The film sample shows flat patterns indicating no preferential orientation, while the peaks observed at 0 and 180° for the fiber samples indicate a preferred orientation of the (100) and (020) planes parallel to the fiber axis direction. The peaks also increase in intensity as the drawing is increased indicating a more pronounced orientation at higher applied draws. Figure 4a shows the electrical conductivity as a function of the total draw ratio. Adding 10 vol % DMSO to the coagulation bath, while keeping all else equal increased the electrical conductivity of the coagulation bath fibers by an order of magnitude from ∼125 to ∼1030 S cm−1. This increase in electrical conductivity is attributed to the secondary doping effect of DMSO in PEDOT:PSS. Secondary doping refers to the addition of an apparently inert material that induces E


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constant at 3.4 Å; thus, the increase in electrical conductivity cannot be explained by stronger orbital overlap between PEDOT stacks. Instead, the increase in electrical conductivity can be attributed to the drawing-induced orientation of (100) and (020) planes, effectively aligning the PEDOT backbone (caxis in Figure 3e) parallel to the fiber axis direction combined with the partial removal of PSS in the DMSO bath (see Figures S3−S5). The electrical conductivity is likely to be the highest along the conjugated polymer backbone,41 thus aligning the chains improves the charge carrier transport in the fiber axis direction. It must be noted that DMSO induced stronger πorbital overlap resulting in better interchain transport in the baxis direction, while drawing the fibers aligned the polymer backbones parallel to the fibers’ axis. Interestingly, this resulted in a synergistic effect where the enhanced interchain transport occurs perpendicular to the fiber axis direction and the higher mobility intrachain transport occurs parallel to the fiber axis direction yielding the high electrical conductivities observed. Similar trends showing an increase in electrical conductivity with increasing draw or stretch have been previously reported for other conducting polymer fibers, such as poly(3alkylthiophenes)18,19 and polyaniline20 fibers. To test the possibility of the increase in electrical conductivity observed being solely due to removal of PSS, coagulation bath samples were washed in DMSO without applying any tension (for different washing times between 1 and 60 min). The electrical conductivity of the DMSO washed fibers was similar to that of the coagulation bath samples (see Figure S5), which supports the idea that the increase in electrical conductivity observed in DMSO-stretched fibers is not solely due to the removal of PSS. Moreover, the electrical conductivity as a function of strain was tested for a coagulation bath sample spun into 10 vol % DMSO in IPA (see Figure S6). The electrical conductivity was observed to increase with strain up to +13% at failure (17.2% strain). This increase in electrical conductivity must have been purely associated with induced chain orientation since the test was made in air without any PSS removal or secondary doping. Much larger increases in electrical conductivity were observed upon drawing the filament through the DMSO bath, which indicated the synergistic effect of secondary doping and simultaneous drawing. On the other hand, the Seebeck coefficient remained practically constant across all the samples studied (see Figure 4b). The Seebeck coefficient depends strongly on the charge carrier concentration of the polymer chains and, in general, decreases with increased doping.46 Thus, drawing the polymer chains did not seem to affect the charge carrier concentration but increased the mobility of the charge carriers in the fiber axis direction because of the preferred orientation of crystal planes and had little to no effect on the Seebeck coefficient. As a result of the increase in electrical conductivity and the constant Seebeck coefficient, the thermoelectric power factor, α2σ, increased following the same trend observed for the electrical conductivity and yielded maximum power factors in the range of 40−50 μW m−1 K−2 (see Figure 4c). The thermal conductivity of the fibers spun into 10 vol % DMSO in IPA was also investigated. The level of desired thermal conduction of an electrically conducting fiber for textile electronics is application dependent. For instance, from the point of view of electrical interconnections, high thermal conductivity is preferred to enhance heat dissipation and avoid hot spots that can ultimately lead to the interconnection failure. On the other hand, for applications such as thermo-

Figure 4. Electrical and thermoelectric properties of PEDOT:PSS fibers as a function of total draw. (a) Electrical conductivity, (b) Seebeck coefficient, and (c) power factor. The electrical conductivity increased with increasing draw ratio, while the Seebeck coefficient remained practically constant resulting in the power factor, following a similar trend as the electrical conductivity. Electrical conductivity values are the average of 5 specimens per sample. Seebeck coefficients are the average of 3 specimens per sample. The power factor was calculated using the average values. The error bars represent standard deviation between specimens within the same sample.

structural changes in the organization of the polymer chains leading to conductivity increases up to several orders of magnitude.45 As discussed above, DMSO induced stronger π−π interactions between PEDOT chains, which resulted in enhanced interchain carrier transport in the b-axis direction resulting in the overall increase of the electrical conductivity. Furthermore, the electrical conductivity of the fibers increased with increasing draw and saturated around 2000 S cm−1 for total draws higher than 2. The π−π stacking distance remained F


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large electrical conductivities observed. However, it must be noted here that there are precedents where the Lorenz value L deviates from L0 for PEDOT samples,49 and therefore, whether the thermal conductivity observed is dominated by phonon or electronic transport is still unclear and is under further investigation. The large thermal conductivities observed negatively affect the thermoelectric performance. For instance, using a thermoelectric power factor of 50 μW m−1 K−2 and a thermal conductivity of 5 W m−1 K−1, a thermoelectric figure

electric textiles, a low thermal conductivity is preferable. Determining the thermal conductivity of fibers with diameters less than 12 μm is challenging. Here, we used a self-heating technique34,35 that takes advantage of the electrically conducting nature of the fibers to determine the thermal conductivity at liquid nitrogen temperatures (more details about the technique and analysis can be found in the Supporting Information). The results are shown in Figure 5.

α 2σ

of merit, ZT = κ T , of 0.003 at 300 K can be calculated. It must be noted, however, that the thermal conductivities at room temperature are unknown and, therefore, the calculated ZT represents a mere estimation. A typical temperature dependence of the resistance for fibers spun into 10 vol % DMSO in IPA is shown in the inset to Figure 5. The temperature dependence was similar for all samples at all the different total draws studied, with R(78K)/ R(300K) varying between 1.32 and 1.38 in all cases. Note that the slope dR/dT becomes small near room temperature and, in fact, changes sign at higher temperatures (not shown), where the resistance value becomes history dependent. The temperature dependence of the resistance was fitted using a onedimensional (1D) variable range hopping (VRH) model, which is described as50 É ÄÅ ÅÅi T y1/2 ÑÑÑ ÅÅjj 0 zz ÑÑ R ∝ expÅÅj z ÑÑ ÅÅk T { ÑÑ (2) ÖÑ ÇÅ Figure 5. Average values of the calculated thermal conductivities from samples spun into 10 vol % DMSO in IPA at different total draws at T = 78 K. Sample with total draw of 1.58 is a coagulation bath sample while the rest were stretched through the DMSO bath. The error bars include uncertainties from thermal radiation and specimen length but are dominated by the range of values measured for 3−4 specimens per sample. Also shown are the values of L0σT, where L0 is the Lorentz number and σ the electrical conductivity; for these, the error bars mostly reflect the uncertainties in specimen length. Inset: blue crosses: typical temperature dependence of the resistance normalized to its (vacuum) room temperature value; red curve: fit to 1D VRH with hopping parameter T0 = 32 K.

where T0 can be interpreted as an effective energy separation between localized states. The 1D VRH model has been previously used to describe transport in PEDOT:PSS pristine films and treated with DMSO,51 as well as treated with ethylene glycol.52 In the latter case, a value for T0 of 360 K was obtained. We obtain a value for T0 of 32 K, which is an order of magnitude smaller, implying much lower energy barrier for hopping between conducting grains. The weak overall temperature dependence suggests that the electrical conduction mechanism is dominantly metallic conductivity in the heavily doped, crystalline PEDOT domains moderated by hopping between domains through thin insulating PSS barriers.53,54 We emphasize, however, that the temperature dependent measurements are for samples in vacuum, for which the sample is presumably dehydrated. In fact, the room temperature resistances of the specimens (reversibly) increased by between 5% and 11% between ambient atmosphere and vacuum. Next, we investigated the single-filament tensile properties of the fibers. To fully understand the behavior observed in the mechanical properties, the difference in molecular weights between PEDOT and PSS must be considered. In the commercial product used in this study, the molecular weight of PEDOT ranges between 1000 and 2500 and the molecular weight of PSS is approximately 400 000.39,55 Moreover, the content of PEDOT in the fibers is estimated to be around 28.5 wt % for coagulation bath samples and 34 wt % for DMSO drawn fibers (see Figures S3 and S4). Since PSS chains are much longer than PEDOT chains and the fibers are also richer in PSS than in PEDOT, we believe that PSS bears an immensely larger fraction of the applied mechanical stress. Having these facts in mind, we can proceed to analyze the mechanical behavior of the fibers. Typical stress−strain curves of the fibers at different draw ratios are presented in Figure S7.

Here, the error bars include uncertainties due to thermal radiation and length uncertainties (∼±10%) but are dominated by deviation from specimens within the same sample. The most likely cause of these deviations are damages in the specimens, possibly caused during mounting, in which case the largest value (i.e., ∼top of the error bar) for each sample may be the best estimate. The measured thermal conductivities at liquid nitrogen temperature (2−6 W m−1 K−1) are an order of magnitude larger than conventionally found for PEDOT:PSS films at room temperature (typically between 0.2 and 0.6 W m−1 K−1).27,32,47,48 These results reflect the preferred orientation of both the PEDOT crystallites and PSS chains in the fiber axis direction as opposed to the random orientation typically found in films. Like the electrical conductivities, the coagulation bath sample had the lowest thermal conductivity (∼2.4 W m−1 K−1), while the drawn fibers all had thermal conductivities between 3 and 6 W m−1 K−1. For all samples, the measured total thermal conductivity was about a factor of 20 larger than the electronic thermal conductivity calculated from the Wiedemann−Franz law using the Sommerfeld value for L0 (see Figure 5), which could be indicating that the lattice contribution dominated despite the G


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reported wet-spun PEDOT:PSS fibers,16 and to the best of our knowledge, is the highest Young’s modulus reported for a PEDOT:PSS material. The increase in Young’s modulus came accompanied by the characteristic decrease in elongation at break that is usually observed for oriented polymer materials (see Figure 6b). The break stress or tensile strength followed a similar trend to that of the Young’s modulus reaching values as high as 425 MPa, however, with a larger dispersion (see Figure 6c). The break stress is a function of the Young’s modulus and the elongation at break and, thus, the dispersion in the latter properties gets magnified in the break stress. From the discussions on the effect of increasing applied draw on the electrical conductivity and Young’s modulus and by comparing Figures 4a and 6a, it was evident that the electrical conductivity and the Young’s modulus follow a very similar trend. The electrical properties and mechanical properties are strongly correlated because they both are affected by inter- and intrachain interactions.19 In general, the mechanical properties of polymeric materials are highest along the polymer backbone, while in conducting polymeric materials, the electrical conductivity is also highest along the polymer backbone. Thus, alignment of the polymer chains along the axis of the fiber benefits both electrical and mechanical properties.3 In this study, the applied draw resulted in the orientation of (100) and (020) planes parallel to the axis of the fiber, thus aligning both PEDOT and PSS backbones in the fiber axis direction. To quantify the degree of orientation introduced by drawing the fibers and correlate it to the mechanical and electrical properties, we calculated the Hermans orientation factor for the (100) reflection using

The Young’s modulus as a function of total draw is presented in Figure 6a. The coagulation bath samples had a Young’s

π

2

⟨cos ψc , Z⟩ =

fc =

∫0 I(ψ ) sin ψ cos2 ψ dψ π

∫0 I(ψ ) sin ψ dψ

(3)

3⟨cos2 ψc , Z⟩−1 2

(4)

In these equations, ψ is the azimuthal angle, I(ψ) represents the azimuthal intensities, and ⟨cos2 ψc,Z⟩ is the average cosine square of the angle that the c-plane made with the draw direction, Z.56 fc takes values of 0 for an isotropic material with no orientation, −0.5 when the crystal planes are oriented perpendicular to the draw direction, and 1 for fully oriented planes parallel to the draw direction. As was expected from simple observation of Figure 3a, the PEDOT:PSS film showed no orientation with a calculated f100 value of −0.01, while all fiber samples had some degree of orientation with values ranging from 0.30 to 0.70. Figure 7a and b shows the electrical conductivity and Young’s modulus as a function of f100. On one hand, the DMSO-induced shortening of the π−π stacking distance of PEDOT increased the electrical conductivity but did not increase orientation in the coagulation bath samples, as demonstrated by the constant (and even smaller) values of f100 (see bottom left corner in Figure 7a). On the other hand, the drawing-induced orientation effectively aligned both PEDOT and PSS chains along the fiber axis, resulting in a rather linear increase of both electrical conductivity and Young’s modulus. It must be noted that the increase in Young’s modulus seems purely due to the drawing-induced alignment of the polymer chains as can be inferred from the absence of a step in Figure 7b as compared to Figure 7a where the enhanced π−π interactions caused a step jump in electrical conductivity. This

Figure 6. Single-filament tensile properties of PEDOT:PSS fibers as a function of total draw: (a) Young’s modulus, (b) elongation at break, and (c) break stress. An increase in Young’s modulus and break stress and a decrease in elongation at break with increasing draw ratio was observed. This behavior is typical for oriented polymeric materials. Values are the average of 5 specimens per sample. The error bars represent standard deviation between specimens within the same sample.

modulus of around 6 GPa, regardless if DMSO was present or not. It was discussed before that DMSO in the coagulation bath only induced stronger π−π interactions between PEDOT stacks, but did not modify the tensile stress bearing PSS chains. However, applying draw induced orientation of the PEDOT and PSS backbones along the fiber axis which resulted in higher Young’s moduli at higher draw ratios reaching values as high as 15.6 GPa. This is higher than all the previously H


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Figure 7. Correlation between polymer chain orientation and electrical and mechanical properties. (a) Electrical conductivity versus (100) Hermans orientation factor, f100. (b) Young’s modulus versus f100 and (c) electrical conductivity versus Young’s modulus. Both the electrical conductivity and Young’s modulus showed a strong correlation with the polymer chain orientation, demonstrated by the linear increase in both properties. In general, the highest conducting fibers were also the stiffest. Values are the average of 5 specimens per sample. The error bars represent standard deviation between specimens within the same sample.

performance PEDOT:PSS fibers that could serve as the fundamental building blocks for future electronic textiles.

interesting result supports the idea that the larger molecular weight PSS chains bear practically all the mechanical stress of the fibers. All the previous analysis is very well summarized in Figure 7c that plots the electrical conductivity versus the Young’s modulus showing that, in general, the highest conducting fibers were also the stiffest in tension.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsapm.9b00425. Details on self-heating technique for the measurement of the thermal conductivity; estimation of the bulk density of the fibers; details on the removal of PSS from the fibers in the DMSO stretch bath; effect of DMSO removal on the electrical conductivity; electrical conductivity−strain curve; and stress−strain curves (PDF)

4. CONCLUSIONS The field of conducting polymer fibers is still in its infancy when compared to the established fields of textile fibers or carbon fibers. However, electrically conducting and mechanically robust fibers and yarns are needed as fundamental building blocks for electronic textiles. In this work, we have developed a continuous and scalable wet-spinning process for the production of PEDOT:PSS fibers that have high electrical conductivity, high thermal conductivity, excellent mechanical properties, and moderate thermoelectric performance by including a DMSO drawing step following coagulation. On one hand, DMSO induced stronger π−π interactions between PEDOT chains, while on the other hand, the applied draw aligned the backbones of both PEDOT and PSS chains in the fiber axis direction. This synergistic effect resulted in room temperature electrical conductivities of approximately 2000 S cm−1 and Young’s moduli around 15.5 GPa at high applied draws. In fact, to the best of our knowledge, these Young’s moduli are the highest for a PEDOT:PSS material reported to date. Additionally, the Seebeck coefficients were found rather constant with draw and moderate thermoelectric power factors around 40−50 μW m−1 K−2 were obtained at high draws. The high thermal conductivities of the fibers, measured at approximately 4−5 W m−1 K−1 at liquid nitrogen temperature, although undesired for thermoelectrics, may be beneficial for other applications such as textile interconnections. Furthermore, we investigated the degree of orientation of the crystalline planes by WAXS and found a strong correlation between the orientation of the polymer chains along the fiber axis and the trends observed in the fibers’ properties. In general, the fibers with the highest orientation were also the stiffest and the most conducting fibers. We believe that these are important steps toward continuous fabrication of high

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ruben Sarabia-Riquelme: 0000-0002-6444-9531 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors would like to gratefully acknowledge the support of the University of Kentucky Center for Applied Energy Research. The authors would like to thank Marie Faraj for her help during the initial stage of this work and Yuxin He for her help with WAXS measurements.

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REFERENCES

(1) Pan, C.; Zhang, L.; Pan, Z.; Chen, M.; Liu, Y.; Huang, G.; Na, H.; Wang, W.; Qiu, H.; Gao, J. A simple strategy to fabricate polyaniline/expanded graphite composites with improved power factor. Mater. Chem. Phys. 2015, 167, 315−319. (2) Kaushik, V.; Lee, J.; Hong, J.; Lee, S.; Lee, S.; Seo, J.; Mahata, C.; Lee, T. Textile-Based Electronic Components for Energy Applications: Principles, Problems, and Perspective. Nanomaterials 2015, 5 (3), 1493. (3) Lund, A.; van der Velden, N. M.; Persson, N.-K.; Hamedi, M. M.; Müller, C. Electrically conducting fibres for e-textiles: An open

I


Article

ACS Applied Polymer Materials playground for conjugated polymers and carbon nanomaterials. Mater. Sci. Eng., R 2018, 126, 1−29. (4) Ryan, J. D.; Lund, A.; Hofmann, A. I.; Kroon, R.; SarabiaRiquelme, R.; Weisenberger, M. C.; Müller, C. All-Organic Textile Thermoelectrics with Carbon-Nanotube-Coated n-Type Yarns. ACS Applied Energy Materials 2018, 1 (6), 2934−2941. (5) Culebras, M.; Gómez, C. M.; Cantarero, A. Review on Polymers for Thermoelectric Applications. Materials 2014, 7 (9), 6701. (6) Xia, Y.; Yun, L. Fabrication and properties of conductive conjugated polymers/silk fibroin composite fibers. Compos. Sci. Technol. 2008, 68 (6), 1471−1479. (7) Varesano, A.; Belluati, A.; Sanchez Ramirez, D. O.; Carletto, R. A.; Vineis, C.; Tonetti, C.; Bianchetto Songia, M.; Mazzuchetti, G. A systematic study on the effects of doping agents on polypyrrole coating of fabrics. J. Appl. Polym. Sci. 2016, 133 (1), 42831. (8) Garg, S.; Hurren, C.; Kaynak, A. Improvement of adhesion of conductive polypyrrole coating on wool and polyester fabrics using atmospheric plasma treatment. Synth. Met. 2007, 157 (1), 41−47. (9) Molina, J.; Fernández, J.; del Río, A. I.; Lapuente, R.; Bonastre, J.; Cases, F. Stability of conducting polyester/polypyrrole fabrics in different pH solutions. Chemical and electrochemical characterization. Polym. Degrad. Stab. 2010, 95 (12), 2574−2583. (10) Finefrock, S. W.; Zhu, X.; Sun, Y.; Wu, Y. Flexible prototype thermoelectric devices based on Ag2Te and PEDOT:PSS coated nylon fibre. Nanoscale 2015, 7 (13), 5598−5602. (11) Du, Y.; Cai, K.; Chen, S.; Wang, H.; Shen, S. Z.; Donelson, R.; Lin, T. Thermoelectric Fabrics: Toward Power Generating Clothing. Sci. Rep. 2015, 5, 6411. (12) Kirihara, K.; Wei, Q.; Mukaida, M.; Ishida, T. Thermoelectric power generation using nonwoven fabric module impregnated with conducting polymer PEDOT:PSS. Synth. Met. 2017, 225, 41−48. (13) Ryan, J. D.; Mengistie, D. A.; Gabrielsson, R.; Lund, A.; Müller, C. Machine-Washable PEDOT:PSS Dyed Silk Yarns for Electronic Textiles. ACS Appl. Mater. Interfaces 2017, 9 (10), 9045−9050. (14) Okuzaki, H.; Ishihara, M. Spinning and Characterization of Conducting Microfibers. Macromol. Rapid Commun. 2003, 24 (3), 261−264. (15) Okuzaki, H.; Harashina, Y.; Yan, H. Highly conductive PEDOT/PSS microfibers fabricated by wet-spinning and diptreatment in ethylene glycol. Eur. Polym. J. 2009, 45 (1), 256−261. (16) Zhou, J.; Li, E. Q.; Li, R.; Xu, X.; Ventura, I. A.; Moussawi, A.; Anjum, D. H.; Hedhili, M. N.; Smilgies, D.-M.; Lubineau, G.; Thoroddsen, S. T. Semi-metallic, strong and stretchable wet-spun conjugated polymer microfibers. J. Mater. Chem. C 2015, 3 (11), 2528−2538. (17) Zhang, J.; Seyedin, S.; Qin, S.; Lynch, P. A.; Wang, Z.; Yang, W.; Wang, X.; Razal, Joselito M. Fast and scalable wet-spinning of highly conductive PEDOT:PSS fibers enables versatile applications. J. Mater. Chem. A 2019, 7 (11), 6401−6410. (18) Moulton, J.; Smith, P. Electrical and mechanical properties of oriented poly(3-alkylthiophenes) I. Doping-enhanced stiffness of poly(3-octylthiophene). Synth. Met. 1991, 40 (1), 13−22. (19) Moulton, J.; Smith, P. Electrical and mechanical properties of oriented poly(3-alkylthiophenes): 2. Effect of side-chain length. Polymer 1992, 33 (11), 2340−2347. (20) Pomfret, S. J.; Adams, P. N.; Comfort, N. P.; Monkman, A. P. Electrical and mechanical properties of polyaniline fibres produced by a one-step wet spinning process. Polymer 2000, 41 (6), 2265−2269. (21) Poulaert, B.; Chielens, J. C.; Vandenhende, C.; Issi, J.-P.; Legras, R. Thermal conductivity of highly oriented polyethylene fibres. Polymer Communications Guildford 1990, 31, 148−151. (22) Zhu, B.; Liu, J.; Wang, T.; Han, M.; Valloppilly, S.; Xu, S.; Wang, X. Novel Polyethylene Fibers of Very High Thermal Conductivity Enabled by Amorphous Restructuring. ACS Omega 2017, 2 (7), 3931−3944. (23) Pukacki, W.; Płocharski, J.; Roth, S. Anisotropy of thermoelectric power of stretch-oriented new polyacetylene. Synth. Met. 1994, 62 (3), 253−256.

(24) Hynynen, J.; Järsvall, E.; Kroon, R.; Zhang, Y.; Barlow, S.; Marder, S. R.; Kemerink, M.; Lund, A.; Müller, C. Enhanced Thermoelectric Power Factor of Tensile Drawn Poly(3-hexylthiophene). ACS Macro Lett. 2019, 8 (1), 70−76. (25) Hiroshige, Y.; Ookawa, M.; Toshima, N. High thermoelectric performance of poly(2,5-dimethoxyphenylenevinylene) and its derivatives. Synth. Met. 2006, 156 (21), 1341−1347. (26) Yan, H.; Ohta, T.; Toshima, N. Stretched Polyaniline Films Doped by (ñ)-10-Camphorsulfonic Acid: Anisotropy and Improvement of Thermoelectric Properties. Macromol. Mater. Eng. 2001, 286 (3), 139−142. (27) Kim, G. H.; Shao, L.; Zhang, K.; Pipe, K. P. Engineered doping of organic semiconductors for enhanced thermoelectric efficiency. Nat. Mater. 2013, 12, 719. (28) Park, H.; Lee, S. H.; Kim, F. S.; Choi, H. H.; Cheong, I. W.; Kim, J. H. Enhanced thermoelectric properties of PEDOT:PSS nanofilms by a chemical dedoping process. J. Mater. Chem. A 2014, 2 (18), 6532−6539. (29) Yi, C.; Wilhite, A.; Zhang, L.; Hu, R.; Chuang, S. S. C.; Zheng, J.; Gong, X. Enhanced Thermoelectric Properties of Poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) by Binary Secondary Dopants. ACS Appl. Mater. Interfaces 2015, 7 (17), 8984−8989. (30) Sarabia-Riquelme, R.; Ramos-Fernández, G.; Martin-Gullon, I.; Weisenberger, M. C. Synergistic effect of graphene oxide and wetchemical hydrazine/deionized water solution treatment on the thermoelectric properties of PEDOT:PSS sprayed films. Synth. Met. 2016, 222, 330−337. (31) Fan, Z.; Li, P.; Du, D.; Ouyang, J. Significantly Enhanced Thermoelectric Properties of PEDOT:PSS Films through Sequential Post-Treatments with Common Acids and Bases. Adv. Energy Mater. 2017, 7 (8), 1602116. (32) Li, Z.; Sun, H.; Hsiao, C.-L.; Yao, Y.; Xiao, Y.; Shahi, M.; Jin, Y.; Cruce, A.; Liu, X.; Jiang, Y.; Meng, W.; Qin, F.; Ederth, T.; Fabiano, S.; Chen, W. M.; Lu, X.; Birch, J.; Brill, J. W.; Zhou, Y.; Crispin, X.; Zhang, F. A Free-Standing High-Output Power Density Thermoelectric Device Based on Structure-Ordered PEDOT:PSS. Advanced Electronic Materials 2018, 4 (2), 1700496. (33) Wang, X.; Kyaw, A. K. K.; Yin, C.; Wang, F.; Zhu, Q.; Tang, T.; Yee, P. I.; Xu, J. Enhancement of thermoelectric performance of PEDOT:PSS films by post-treatment with a superacid. RSC Adv. 2018, 8 (33), 18334−18340. (34) Brill, J. W.; Tzou, C. P.; Verma, G.; Ong, N. P. Thermal conductivity of NbSe3. Solid State Commun. 1981, 39 (2), 233−237. (35) Li, G.; Liang, D.; Qiu, R. L. J.; Gao, X. P. A. Thermal conductivity measurement of individual Bi2Se3 nano-ribbon by selfheating three-ω method. Appl. Phys. Lett. 2013, 102 (4), 043104. (36) Jalili, R.; Razal, J. M.; Innis, P. C.; Wallace, G. G. One-Step Wet-Spinning Process of Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate) Fibers and the Origin of Higher Electrical Conductivity. Adv. Funct. Mater. 2011, 21 (17), 3363−3370. (37) Liu, S.; Deng, H.; Zhao, Y.; Ren, S.; Fu, Q. The optimization of thermoelectric properties in a PEDOT:PSS thin film through posttreatment. RSC Adv. 2015, 5 (3), 1910−1917. (38) Morris, E. A.; Weisenberger, M. C.; Rice, G. W. Properties of PAN Fibers Solution Spun into a Chilled Coagulation Bath at High Solvent Compositions. Fibers 2015, 3 (4), 560. (39) Takano, T.; Masunaga, H.; Fujiwara, A.; Okuzaki, H.; Sasaki, T. PEDOT Nanocrystal in Highly Conductive PEDOT:PSS Polymer Films. Macromolecules 2012, 45 (9), 3859−3865. (40) Wei, Q.; Mukaida, M.; Naitoh, Y.; Ishida, T. Morphological Change and Mobility Enhancement in PEDOT:PSS by Adding Cosolvents. Adv. Mater. 2013, 25 (20), 2831−2836. (41) Aasmundtveit, K. E.; Samuelsen, E. J.; Pettersson, L. A. A.; Inganäs, O.; Johansson, T.; Feidenhans’l, R. Structure of thin films of poly(3,4-ethylenedioxythiophene). Synth. Met. 1999, 101 (1), 561− 564. (42) Mahendrasingam, A.; Martin, C.; Fuller, W.; Blundell, D. J.; Oldman, R. J.; Harvie, J. L.; MacKerron, D. H.; Riekel, C.; Engström, P. Effect of draw ratio and temperature on the strain-induced J


Article

ACS Applied Polymer Materials crystallization of poly (ethylene terephthalate) at fast draw rates. Polymer 1999, 40 (20), 5553−5565. (43) Fanous, J.; Schweizer, M.; Schawaller, D.; Buchmeiser, M. R. Crystalline and Conductive Poly(3-hexylthiophene) Fibers. Macromol. Mater. Eng. 2012, 297 (2), 123−127. (44) Massonnet, N.; Carella, A.; de Geyer, A.; Faure-Vincent, J.; Simonato, J.-P. Metallic behaviour of acid doped highly conductive polymers. Chemical Science 2015, 6 (1), 412−417. (45) MacDiarmid, A. G.; Epstein, A. J. The concept of secondary doping as applied to polyaniline. Synth. Met. 1994, 65 (2), 103−116. (46) Snyder, G. J.; Toberer, E. S. Complex thermoelectric materials. Nat. Mater. 2008, 7, 105. (47) Mengistie, D. A.; Chen, C.-H.; Boopathi, K. M.; Pranoto, F. W.; Li, L.-J.; Chu, C.-W. Enhanced Thermoelectric Performance of PEDOT:PSS Flexible Bulky Papers by Treatment with Secondary Dopants. ACS Appl. Mater. Interfaces 2015, 7 (1), 94−100. (48) Lee, S. H.; Park, H.; Kim, S.; Son, W.; Cheong, I. W.; Kim, J. H. Transparent and flexible organic semiconductor nanofilms with enhanced thermoelectric efficiency. J. Mater. Chem. A 2014, 2 (20), 7288−7294. (49) Weathers, A.; Khan, Z. U.; Brooke, R.; Evans, D.; Pettes, M. T.; Andreasen, J. W.; Crispin, X.; Shi, L. Significant Electronic Thermal Transport in the Conducting Polymer Poly(3,4-ethylenedioxythiophene). Adv. Mater. 2015, 27 (12), 2101−2106. (50) Joo, J.; Long, S. M.; Pouget, J. P.; Oh, E. J.; MacDiarmid, A. G.; Epstein, A. J. Charge transport of the mesoscopic metallic state in partially crystalline polyanilines. Phys. Rev. B: Condens. Matter Mater. Phys. 1998, 57 (16), 9567−9580. (51) Kim, J. Y.; Jung, J. H.; Lee, D. E.; Joo, J. Enhancement of electrical conductivity of poly(3,4-ethylenedioxythiophene)/poly(4styrenesulfonate) by a change of solvents. Synth. Met. 2002, 126 (2), 311−316. (52) Ashizawa, S.; Horikawa, R.; Okuzaki, H. Effects of solvent on carrier transport in poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate). Synth. Met. 2005, 153 (1), 5−8. (53) Bubnova, O.; Khan, Z. U.; Wang, H.; Braun, S.; Evans, D. R.; Fabretto, M.; Hojati-Talemi, P.; Dagnelund, D.; Arlin, J.-B.; Geerts, Y. H.; Desbief, S.; Breiby, D. W.; Andreasen, J. W.; Lazzaroni, R.; Chen, W. M.; Zozoulenko, I.; Fahlman, M.; Murphy, P. J.; Berggren, M.; Crispin, X. Semi-metallic polymers. Nat. Mater. 2014, 13, 190. (54) Ugur, A.; Katmis, F.; Li, M.; Wu, L.; Zhu, Y.; Varanasi, K. K.; Gleason, K. K. Low-Dimensional Conduction Mechanisms in Highly Conductive and Transparent Conjugated Polymers. Adv. Mater. 2015, 27 (31), 4604−4610. (55) Lang, U.; Müller, E.; Naujoks, N.; Dual, J. Microscopical Investigations of PEDOT:PSS Thin Films. Adv. Funct. Mater. 2009, 19 (8), 1215−1220. (56) Chuah, H. H.; Chang, B. T. A. Crystal orientation function of poly(trimethylene terephthalate) by wide-angle x-ray diffraction. Polym. Bull. 2001, 46 (4), 307−313.

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Effect of Drawing on the Electrical, Thermoelectrical, and Mechanical

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