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Copolymer-enabled stretchable conductive polymer fibers
Polymer 177 (2019) 189–195
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Copolymer-enabled stretchable conductive polymer fibers Guoqiang Tian
a,b
c
a
a
, Jian Zhou , Yangyang Xin , Ran Tao , Gang Jin
b,∗
, Gilles Lubineau
T a,∗∗
a
King Abdullah University of Science and Technology (KAUST), Physical Science and Engineering Division, COHMAS Laboratory, Thuwal, 23955-6900, Saudi Arabia The Key Laboratory of Polymer Processing Engineering of Ministry of Education, National Engineering Research Center of Novel Equipment for Polymer Processing, South China University of Technology (SCUT), Guangzhou, 510640, China c Sun Yat-Sen University, School of Materials Science and Engineering, Guangzhou, 510215, China b
H I GH L IG H T S
introduce a novel method to produce a copolymer-enabled stretchable conductive PEDOT/PSS fiber, using wet-spinning followed by hot-drawing. • We • An optimum fraction of PBP has been identified to maximize electrical and mechanical performances.
A R T I C LE I N FO
A B S T R A C T
Keywords: Conductive polymer Stretchable conductive fiber PEDOT:PSS
Next-generation stretchable electronics, such as wearable electronics and implantable sensors, require stretchable conductive fibers. Despite their great popularity for wearable electronics, conducting polymers do not sustain deformation very well because of their rigid conjugated backbone. Here, we report the production of stretchable conductive poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonate) (PEDOT/PSS)- conjugated polymer fibers, using optimal wet-spinning, followed by hot drawing. We engineer the fibers by introducing a copolymer, polyethylene-block-poly (ethylene glycol) (PBP), that modifies both the electrical and mechanical properties of the raw PEDOT/PSS. We then systematically investigate the effects of the PBP fraction (fs) on the properties of the PEDOT/PSS by analyzing the changes in the conductivity, morphology, stretchability, and conformation of the PEDOT chains. We find that the conductivity of PEDOT/PSS increases from 311 ± 8 S/cm to 415 ± 12 S/cm (133% increase), when fs = 0.4, and that the strain of the fibers, at failure, is as high as (ε = 36%) for fs = 0.7, eq. 3x the value of as-spun PEDOT/PSS fibers. Raman and XRD analyses show that the conformational changes from benzoid to quinoid structures, in the PEDOT chains, significantly enhance the conductivity of the fibers. This conformational change facilitate the switch from a coil structure of PEDOT/PSS into a linear or an extended-coil conformation that increases interchain interaction.
1. Introduction Fiber-shaped conductive materials have been increasingly used in several applications, such as wearable devices [1,2], artificial electronic skin [3,4], and supercapacitors [5,6]. Stretchable electronics require both mechanical stretchability and electronic performance from the fiber-shaped conductive materials they consist of. Two main design principles are currently used in order to engineer stretchable conductive fibers: 1) designing stretchable structures or 2) relying on the properties of nanocomposites. In the first approach, non-stretchable conductive materials, such as carbon nanotubes (CNTs), graphene, silver wires (AgNWs), and conductive polymers, are coated onto elastic or pre-stretched elastic fibers or substrates [7–9]. Engineering the
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microstructure of both the substrate and the coating (by introducing specific patterns or buckling) enables the fibers to sustain large displacements while, at the same time, preserving locally small strains. However, the fabrication methods involved in this process are usually complicated. The second approach, which uses nanocomposites, allows the embedding of conductive fillers (e.g. multi-walled carbon nanotubes (MWCNTs), silver nanowires (AgNWs)), onto insulating elastomeric matrices to form conductive nanocomposites. The stretchable conductive fibers can be prepared by wet-spinning, electro-spinning, or other easily scalable method [10–12]. To obtain a highly conductive fiber, a high loading of well-dispersed fillers is required, so that the percolated network experiences only minor changes during the stretching. These are strong limitations, still today, that do impact on
Corresponding author. Corresponding author. E-mail addresses: [email protected] (G. Jin), [email protected] (G. Lubineau).
∗∗
https://doi.org/10.1016/j.polymer.2019.06.002 Received 10 March 2019; Received in revised form 30 May 2019; Accepted 1 June 2019 Available online 03 June 2019 0032-3861/ © 2019 Published by Elsevier Ltd.
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Fig. 1. (a) Chemical structures of PEDOT/PSS and PBP. (b) A schematic of the wet-spinning setup with the vertical hot-drawing apparatus used in this study.
application of hot drawing assisted by wet spinning and repeated doping of ethylene glycol (EG). We observed an increase of the strain at break from 15% to 21% [29]. The present work aims to produce wearable strain sensors by fabricating stretchable conductive PEDOT/ PSS fibers, using the methods of sequential wet spinning and hot drawing, together with the use of different chemical treatments. To do so, we modify the electrical conductivity and stretchability of the PEDOT/PSS fibers by blending an excess of a selected copolymer, (polyethylene-block-poly (ethylene glycol) (PBP), in our case) that contains polyethylene glycol and polyethylene segments. For comparison, PEDOT/PSS/Triton X-100 fibers are also fabricated to check the plasticizing effect in the fiber structure. A systematic study of the effects of the spinning parameters on the properties of the fiber shows that the hot-drawing and flow rate of the syringe pump have a significant influence on the properties of the fiber. The optimum content of PBP is obtained when the fraction of PBP, fs, is 0.7, the conductivity of the PEDOT/PSS/PBP fiber is 199 S/cm, and the strain at break of the fiber shows a maximum value, ε, of 36%. In addition, we characterize the microstructure of the fibers and the interaction between PEDOT/PSS and PBP using Raman spectroscopy and X-ray diffraction (XRD) measurements.
the use of nanocomposites for stretchable electronic devices. Our main goal, here, is to generate intrinsically stretchable and conductive materials using simple solution processes. The numerous, important applications of conductive polymer materials has attracted increased interest from the research community, particularly due to the electrical and mechanical properties of these materials. Poly(3,4-ethylene dioxythiophene)/poly(styrene sulfonate) (PEDOT/PSS) is the most widely used conducting polymer, due to its tunable conductivity, easy processability in aqueous solutions, and thermal stability [13]. The bandgap (Eg), defined as the energy difference between the top of the valence band (HOMO) and the bottom of the valence band (LUMO), is much higher for PEDOT/PSS (about 2.8–3.1 eV) than for PEDOT (1.6 eV) because of the ionic bonding with an insulating polymer (PSS). The large band gap of PEDOT/PSS makes the polymer normally at semiconductive state. However, upon doping, usually carried out by oxidation (more common) or reduction, the polymer may be rendered electrically conductive. For example, upon oxidative doping of polyaniline, new states are produced with reduced energy gap and this gives rise to higher electrical conductivity [13–17]. Systematic studies have reported that the conductivity of PEDOT/PSS films and fibers can be significantly improved by the application of polar solvents, surfactants, salts, acids, and ionic liquids [14–20]. For example, Shi et al. [21] detailed the application of several chemical treatments. Oh et al. [22] improved the electrical conductivity of PEDOT/PSS films by blending an excess of Triton X-100 (C14H22O (C2H4O)n) (n = 9–10), a nonvolatile surfactant, resulting in an increase of the films’ conductivity from 0.24 S/cm to ∼100 S/cm, yet improved the stretchability of the film from 5% to 80% strain. Wang et al. [23] systematically investigated the effect of polyethylene glycol (PEG), at various concentrations, on the conductivity of PEDOT/PSS, and significantly improved the conductivities of PEDOT/PSS/PEG hybrid films conductivities (e.g. from 0.1 S/cm to ∼17.7 S/cm with 4.06 × 10−2 mol/L PEG), in comparison with regular PEDOT/PSS pristine films. Mengistie et al. [24] also obtained improvements of the PEDOT/PSS conductivities, e.g. from 0.3 to 805 S/cm following the application of 2% v/v PEG. Numerous studies on the mechanical robustness of PEDOT/PSS films or fibers have revealed significant improvements in the stretchability of PEDOT/PSS films, following the incorporation of elastomeric materials such as polyurethane and polydimethylsiloxane (PDMS) [9,12,25,26], surfactants [15,27], and soft polymers [28]. However, an excessive use of nonconductive components (in the composites) dramatically reduces the conductivity of PEDOT/PSS materials. In previous work, we reported an improvement of the electrical conductivity of PEDOT/PSS fibers, up to 2800 S/cm, following the
2. Experimental 2.1. Materials Fig. 1a represent the chemical structure of the PEDOT/PSS and polyethylene-block-poly (ethylene glycol) materials, with an average Mn of ∼2250 (abbreviated as PBP). The PEDOT/PSS aqueous dispersion (Clevios™ PH1000) is obtained from HC Starck, Inc. The pristine solution exhibits a solid content of 1.1 wt%, and PEDOT to PSS exhibits a weight ratio of 1:2.5. PBP, Triton X-100 (TX), isopropyl alcohol (IPA) and acetone are obtained from Sigma-Aldrich. 2.2. Preparation of highly spinable PEDOT/PSS inks 10 mL of water was evaporated from 20 mL of the PEDOT/PSS dispersion (1.1 wt% in water) at 50 °C to increase the ink's viscosity, after which different fractions of PBP are added to the concentrated PH1000 dispersion (2.2 wt% in water), and stirred for 2 h using a magnetic stirrers. The surfactant exhibits weight fractions of 0, 0.3, 0.4, 0.5, 0.6, and 0.7, based on the equation fs = Ws/(Ws + WPEDOT/PSS), where Ws and WPEDOT/PSS represent the weights of the surfactant and the PEDOT/PSS solute (2.2 wt% in water), respectively. The maximum fraction of PBP is 0.7. When the fraction of PBP is more than 0.7, the 190
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Fig. 2. Electrical and mechanical properties of the conductive fibers. (a) Electrical conductivity of PEDOT/PSS fibers at different PBP fractions (fs = 0, 0.3, 0.4, 0.5, 0.6, and 0.7). (b) Tensile stress vs strain curve of PEDOT/PSS/PBP fibers with different PBP fractions (fs = 0, 0.3, 0.4, 0.5, 0.6, and 0.7).
Raman spectra of the fibers are collected using a LabRAM Aramis Raman spectrometer (Horiba, Ltd.), with a laser, at 633 nm. Continuous XRD tests are performed from 2 °C to 35 °C, using a Bruker D8 Advance powder X-ray diffractometer, with Cu Ka radiation (λ = 1.54) at 40 kV and 40 mA, and by slow increments of 0.02°, with a slow scan speed of 12 s per step to produce relatively high-intensity peaks.
spinning solution is too viscous to be wet-spun into fibers. The samples are then homogenized, for 20-min, at room temperature, in a sonication bath (Brason 8510 sonicator 250 W, Thomas Scientific). Finally, the dispersion is degassed in a vacuum oven, at room temperature (21 °C), before undergoing wet spinning. 2.3. Preparation of PEDOT/PSS fibers
3. Results and discussion
PEDOT/PSS/PBP conductive fibers were prepared by wet-spinning, combined with hot stretching. The spinning solution is loaded into a 6 mL plastic syringe and spun through a metal needle into a (1:1 volumetric ratio) acetone/IPA coagulation bath (see Fig. 1b). Different sizes of needles are selected for spinning. Experimental results show that fibers possess the best properties when the 27G needle is used for spinning. The ink-flow rate is adjusted, using a Fusion 200 syringe pump (Chemyx Inc.), at a spinning rate of 6 μL/min. Fibers are collected vertically onto a 50 mm winding spool, at a linear speed of 2–4 m/min. Two hot plates are mounted vertically and monitored by a thermometer to maintain the air temperature along the path of the fiber, at 90 °C.
Fig. 1b shows the fabrication process for the conductive fibers. The technique follows a two-step method – wet spinning followed immediately by hot drawing. First, we discussed the effects of the wetspinning parameters on the properties of the as-spun PEDOT/PSS fibers. Details on the optimum wet-spinning parameters for high-performance PEDOT/PSS fibers are described in the supporting information (Section 1). According to the experimental results, the optimum fiber performance is achieved at a spinning rate of 6 μL/min (Figs. S1 and S2, supporting information), using a 27G spinning needle (Figs. S3 and S4, supporting information). In addition, we find that hot drawing can effectively increase the electrical conductivity of the fiber (314 ± 13 S/ cm), by a factor of 2, compared with the value obtained without hotdrawing (Table S1, supporting information). These results reflect the alignment of the fiber molecular chains in the fiber direction, which is significantly influenced by the vertical hot-drawing process [29]. Fig. 2a presents the volume electrical conductivity as a function of the PBP fraction (fs). The conductivity values are calculated using the resistance and diameter of the PEDOT/PSS fiber (Supporting Information, Table S2). The addition of non-conductive PBP first increases the conductivity of PEDOT/PSS fibers and shows a maximum conductivity of 415 ± 12 S/cm at fs = 0.4. When the PBP fraction increases (above 0.4), the electrical conductivity of the fiber decreases to minimum value of 152 ± 10 S/cm at fs = 0.7. Even at such a high fraction, the value of the conductivity is still twice the one obtained with polyurethane (PU)/ PEDOT/PSS (∼9.4 S/cm) fibers [9]. Fig. 2b shows the stress-strain curves of the PEDOT/PSS fibers, for different fractions (fs) of PBP. The properties of the PEDOT/PSS/PBP fibers show a good reproducibility (as shown by Fig. S6, Supporting Information). All fibers exhibit a bi-linear stress-strain curve that follows a strain-hardening behavior. When the PBP fraction increases, the stretchability of the fibers also increases significantly, compared with the stretchability of the PEDOT/PSS fibers reported previously [1,29]. PEDOT/PSS specimens with fs = 0 are brittle and break at relatively small tensile strains (ε = 12%). A PBP fraction of fs = 0.5 presents a significantly lower modulus, and exhibits an apparent yielding at ε = 3%, although failure is observed at ε = 23%. At fs = 0.7, the
2.4. Characterizations The electrical resistance of the fibers is measured using an Agilent 1252B multimeter. A copper wire is mounted onto the fiber surface, with silver epoxy, to produce an electrical contact between the wire and the fiber, with a distance of approximately 20 mm between the two contacts. Each type of fiber is measured at least five times. The diameter of each fiber is measured using an X61 optical microscope (Olympus Corporation). The volume electrical conductivity (σ) of the fiber is defined as σ = L/AR, where L, A, and R represent the length, cross-sectional area, and resistance of the fibers, respectively. A 5944 Instron Universal Testing Machine with a 5 N loading cell is used, at a strain rate of 5% min−1, to measure the stress-strain curve of 2-cm-long fibers, fixed on a paper card. Measurements are based on at least 10 tests for each formulation. Similarly, a U1252B digital multimeter is used to measure the electrical resistance of the fiber as a function of the tensile strain. A cyclic loading/unloading program is applied to the fiber, with an incremental extension of 0.2 mm at each cycle, after which the fibers are released at a load of 1 mN. The resistances of the fibers are measured, and captured every second during the test. Copper wires are used to connect the two ends of the samples, which we paint with silver epoxy, before sealing the silver epoxy area with an epoxy glue. The conductive polymer fibers are observed by scanning electron microscopy (SEM, FEI Magellan), at an accelerating voltage of 3 kV. The 191
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PEDOT/PSS/PBP fiber exhibits up to ε = 36% of stretching, eq. to 3x the value obtained for a as-spun PEDOT/PSS fiber. The Young's modulus of the PEDOT/PSS/PBP fiber decreases very significantly, as the fs increases from 5.6 GPa (fs = 0) to 0.67 GPa (fs = 0.7) (Table S2, Supporting Information). Triton X-100, a small-molecule nonionic surfactant, is added to the PEDOT/PSS solution, after which the electrical and mechanical performances of PEDOT/PSS/TX fiber are evaluated to compare the effects of the surfactant on the fibers. Fig. S7 (Section 3, Supporting Information) presents the electrical conductivity of the PEDOT/PSS/TX fibers as a function of the TX fraction. Similarly, the conductivity is first increased, and then decreased, with the concentration of TX. Surprisingly, a maximum value (550 ± 13 S/cm) of the PEDOT/PSS/TX fiber conductivity is observed at fs = 0.5, suggesting that TX could better improve the conductivity of the PEDOT/PSS fibers, compared with PBP. However, TX does not improve the stretchability of the PEDOT/PSS fiber, as the mechanical response remains unchanged (Fig. S8, Supporting Information). Motivated by these excellent mechanical and electrical properties, we further investigated the electrical properties of the PEDOT/PSS/PBP fibers under monotonic mechanical stretching (Fig. 3a), and under incremental cyclic loading/unloading with progressive extension (see Fig. 3b–d). Fig. 3a presents the relative change in resistance, defined as ΔR/R0 = R(ε) - R0/R0, where R(ε) is the resistance at different strains, and R0 is the initial resistance. The relative change in resistance is applied as a function of the fiber tensile strains at different PBP fractions (fs = 0, 0.3, 0.4, 0.5, 0.6, and 0.7, respectively). We find that the resistance of the fibers increases gradually, as the applied strain increases. When the applied strain is less than 5%, he resistance of the fiber remains almost unchanged. In addition, the relative change in resistance decreases with an increase of the PBP fraction, under the same tensile strain condition, in comparison with the as-spun PEDOT/ PSS fiber. At fs = 0.7, the PEDOT/PSS/PBP fiber exhibits a ΔR/R0 of
0.24, at a 36% strain (Fig. 3a and d). In comparison, the as-spun PEDOT/PSS fiber exhibits a ΔR/R0 of 0.25, at a 12.5% strain (Fig. 2a and b). The pure geometric factor is then calculated and subtracted from the measured resistance to calculate the change in intrinsic conductivity (Supporting Information, Section 4). Changes in the resistance of the PEDOT/PSS fiber depend on two factors: (i) conductivity changes in the material, which may also be dependent on the strain; and (ii) changes in the geometry of the fibers (i.e., elongation and reduction of the diameter). Change in the resistance the as-spun PEDOT/PSS fibers are mostly attributed to changes in the geometry of the sample. The Δσ/ σ of the PEDOT/PSS/PBP fibers (PBP fractions from 0.3 to 0.7) are estimated as 0.01, 0.09, 0.13, 0.20, and 0.24 when the geometrical contributions of the total resistance changes were excluded. The microstructures of the pristine PEDOT/PSS and PBP-modified fibers are investigated and compared by scanning electron microscopy (SEM) (Fig. 4). By increasing the fraction of PBP, the diameter of the fiber is gradually increased from 10.07 ± 0.54 μm, for as-spun PEDOT/PSS, to 24.4 ± 1.4 μm, for the fiber with the PBP fraction at fs = 0.7 (See Table S2, Supporting Information). When fs = 0, the fibers exhibit a remarkably smooth surface (Fig. 4a and Fig. S5a). As the fraction of PBP increases, the surface of the fibers becomes increasingly rough. When fs = 0.7, the fiber gets deformed, and the cross-section of the fiber becomes irregular, as shown in Fig. 4e and Fig. S5b. PBP is a copolymer with typical thermoplastic polymer characteristics due to the existence of polyethylene phase. We observe an uneven local dispersion of the spinning solution, following ink spinning into a coagulation bath, using a spinning needle, which results in an extrusion instability and deforms the fiber. In addition, when fs = 0.3 and 0.5, the surface of the fiber exhibits a discontinuous whitish part (Fig. 4). In fact, the PEDOT/PSS dispersion is a dark blue solution, and the as-spun PEDOT/PSS fiber is black. At the same time, the PBP appears as white dots in the PEDOT/PSS matrix, displaying a “sea-island” structure on the surface of the PEDOT/PSS fiber. When fs = 0.7, the discontinuous
Fig. 3. Mechanical behavior and change in the electrical resistance of the conductive polymer fibers following stretching/unstretching. (a) Relative changes in the resistance versus the applied strain on the fiber. (b, c, and d) The resistance changes to the incremental cyclic loading/unloading of PEDOT/PSS/PBP fibers with the PBP fraction at fs = 0, 0.4, and 0.7. 192
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Fig. 4. SEM images showing the surface of PEDOT/PSS/PBP fiber. (a), (b), (c) and (e) SEM images of PEDOT/PSS fibers with PBP fractions of fs = 0, 0.3, 0.5, and 0.7, with scales of 10 μm, 10 μm, 20 μm and 20 μm respectively. (d) and (f) SEM images of PEDOT/PSS/PBP fiber (0.5 and 0.7) at scale of 5 μm.
respectively) (Fig. 5b). The two peaks of interest at 2θ = ∼ 4.7° and ∼26° correspond to an alternate stacking of lamella in the PEDOT/PSS fibers, with a lattice spacing of ∼23 Å, and an interchain π-π stacking in the PEDOT segments [15,18,31], with a lattice spacing of ∼3.4 Å. The PBP-modified fiber exhibits a clear increase in peak intensity at 2θ = ∼26°, suggesting a better crystalline packing in the PEDOT segment, resulting in an improvement of the electrical conductivity following the addition of PBP in the PEDOT/PSS fibers. This is in a good agreement with the conversion of benzoid structure to quinoid structure as analyzed by Raman spectra. The planar nature of quinoid structure will facilitate the packing of PEDOT segments. Fig. 5c shows a schematic of the change in microstructure of the fibers without and with PBP as manifested by Raman and XRD results. Though it is not the scope of this study to quantify the ratio between the quinoid and benzoid structures in the fibers, we could still remark that a more favorable quinoid structure in the fiber leads to planarization of the conductive PEDOT chains, resulting in the reduction of π-π stacking distance. This reduces the electron path-ways on the chains and between the chains, thus results in higher conductivity of the fibers.
whitish part disappears (Fig. 4f). It is reasonable to think that a cocontinuous structure, formed inside the fiber, contributes to the improvement of the stretchability of the fiber. In contrast, there is no significant change in the diameter and in the surface of PEDOT/PSS/TX fibers (see Fig. S9, Supporting Information). A comparative analysis of these observations and the results suggest that the PBP improves the electrical conductivity and stretchability of the PEDOT/PSS/PBP fibers. For PBP fractions fs > 0.4, the fiber conductivity is significantly dependent on: (i) the small volumetric density of the PEDOT nanofibrils, which is a result of an excess of PBP, and (ii) the PBP covering the PEDOT nanofibril, thus reducing the density of the conductive network. The PBP is an excellent insulator; it forms a continuous structure that covers the PEDOT nanofibril, resulting in a decrease of the overall conductivity of the PEDOT/PSS fiber. The conductivity of the PEDOT/PSS was significantly dependent on the length and quality of the PEDOT nanofibrils. PEG can increase the conductivity of PEDOT/PSS effectively [23]. PBP contains two segments, i.e. polyethylene segments and polyethylene glycol segments. It is inferred that the increase in conductivity with PBP fractions is mainly due to the presence of polyethylene glycol segments in the PBP. The microstructure characterization of the samples, with and without PBP, are then examined, using Raman spectroscopy, to investigate the role of PBP in the significant improvement of the electrical and mechanical properties of PEDOT/PSS fibers. Quyang et al. reported that the conformational changes in the PEDOT chains, from a benzoid-type to a quinoid-type structure, improve the interchain interactions, consequently enhancing the conductivity of the PEDOT/PSS fibers [30,31]. A strong peak was observed at 1421 cm−1, which indicates the Cα = Cβ symmetric stretching of the thiophene ring in the PEDOT chains of the pure PEDOT/PSS fibers (Fig. 5a) [32,33] The Raman spectrum exhibits a red shift, following the addition of PBP, from that obtained with pure PEDOT/PSS fibers. Peak shifts from 1421 cm−1 to 1414 cm−1 and 1416 cm−1, can be observed for fraction values of fs = 0.5 and 0.7, respectively. In addition, shoulder signals observed at 1445 cm−1 [33], which corresponds to benzoid structure are weaker for PBP-containing fibers. This is an indication of a conversion from a benzoid structure to a quinoid structure that favors the inter- and intra-chain charge transport of the PEDOT, and generate an increase in the conductivity of the PEDOT/PSS fibers. In addition, the XRD analysis presents the XRD patterns of the PEDOT/PSS fibers, with or without PBP (fs = 0, 0.4, and 0.7,
4. Conclusions This work introduces a novel method to produce a copolymer-enabled stretchable conductive PEDOT/PSS fiber, using wet-spinning followed by hot-drawing. An examination of the effects of the wetspinning parameters on the properties of the wet-spun PEDOT/PSS fibers reveals a significant influence of both the hot drawing and the flow rate on the fiber properties. For example, we observe an increase in the fiber's electrical conductivity from 151 ± 21 S/cm to 311 ± 8 S/cm, following hot drawing. The effects of the PBP fractions on the electrical conductivity and mechanical properties of the PEDOT/PSS fibers are then systematically investigated. An optimum fraction of PBP has been observed to maximize electrical performance. In addition, we find that the strain at the break increases from 12.5% to 36%, following an increase in the PBP fraction. The PEDOT/PSS/PBP fiber exhibits a maximum conductivity (415 ± 12 S/cm) at fs = 0.4 and a strain break at 23%. At fs = 0.7, the strain first increases to levels as high as 36%, which represents a three-fold improvement, compared with the pure PEDOT/PSS fiber. Raman and XRD spectra suggest that the addition of the PBP produces conformational changes in the PEDOT chains, as the structure undergoes a structure conversion, from a benzoid to a 193
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Fig. 5. Effects of PBP fraction on the electrical properties of the PEDOT/PSS fibers. (a) Raman spectra for the PEDOT/PSS/PBP fiber, for different PBP fractions (fs = 0, 0.4, 0.7). (b) XRD patterns of the PEDOT/PSS/PBP fiber, for different PBP fractions (fs = 0, 0.4, 0.7). (c) A schematic of the Benzoid and quinoid microstructures for PEDOT chains.
quinoid, resulting in an increase in interchain interactions, which, in turn improves the conductivity of the PEDOT/PSS fibers.
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Polymer journal homepage: www.elsevier.com/locate/polymer
Copolymer-enabled stretchable conductive polymer fibers Guoqiang Tian
a,b
c
a
a
, Jian Zhou , Yangyang Xin , Ran Tao , Gang Jin
b,∗
, Gilles Lubineau
T a,∗∗
a
King Abdullah University of Science and Technology (KAUST), Physical Science and Engineering Division, COHMAS Laboratory, Thuwal, 23955-6900, Saudi Arabia The Key Laboratory of Polymer Processing Engineering of Ministry of Education, National Engineering Research Center of Novel Equipment for Polymer Processing, South China University of Technology (SCUT), Guangzhou, 510640, China c Sun Yat-Sen University, School of Materials Science and Engineering, Guangzhou, 510215, China b
H I GH L IG H T S
introduce a novel method to produce a copolymer-enabled stretchable conductive PEDOT/PSS fiber, using wet-spinning followed by hot-drawing. • We • An optimum fraction of PBP has been identified to maximize electrical and mechanical performances.
A R T I C LE I N FO
A B S T R A C T
Keywords: Conductive polymer Stretchable conductive fiber PEDOT:PSS
Next-generation stretchable electronics, such as wearable electronics and implantable sensors, require stretchable conductive fibers. Despite their great popularity for wearable electronics, conducting polymers do not sustain deformation very well because of their rigid conjugated backbone. Here, we report the production of stretchable conductive poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonate) (PEDOT/PSS)- conjugated polymer fibers, using optimal wet-spinning, followed by hot drawing. We engineer the fibers by introducing a copolymer, polyethylene-block-poly (ethylene glycol) (PBP), that modifies both the electrical and mechanical properties of the raw PEDOT/PSS. We then systematically investigate the effects of the PBP fraction (fs) on the properties of the PEDOT/PSS by analyzing the changes in the conductivity, morphology, stretchability, and conformation of the PEDOT chains. We find that the conductivity of PEDOT/PSS increases from 311 ± 8 S/cm to 415 ± 12 S/cm (133% increase), when fs = 0.4, and that the strain of the fibers, at failure, is as high as (ε = 36%) for fs = 0.7, eq. 3x the value of as-spun PEDOT/PSS fibers. Raman and XRD analyses show that the conformational changes from benzoid to quinoid structures, in the PEDOT chains, significantly enhance the conductivity of the fibers. This conformational change facilitate the switch from a coil structure of PEDOT/PSS into a linear or an extended-coil conformation that increases interchain interaction.
1. Introduction Fiber-shaped conductive materials have been increasingly used in several applications, such as wearable devices [1,2], artificial electronic skin [3,4], and supercapacitors [5,6]. Stretchable electronics require both mechanical stretchability and electronic performance from the fiber-shaped conductive materials they consist of. Two main design principles are currently used in order to engineer stretchable conductive fibers: 1) designing stretchable structures or 2) relying on the properties of nanocomposites. In the first approach, non-stretchable conductive materials, such as carbon nanotubes (CNTs), graphene, silver wires (AgNWs), and conductive polymers, are coated onto elastic or pre-stretched elastic fibers or substrates [7–9]. Engineering the
∗
microstructure of both the substrate and the coating (by introducing specific patterns or buckling) enables the fibers to sustain large displacements while, at the same time, preserving locally small strains. However, the fabrication methods involved in this process are usually complicated. The second approach, which uses nanocomposites, allows the embedding of conductive fillers (e.g. multi-walled carbon nanotubes (MWCNTs), silver nanowires (AgNWs)), onto insulating elastomeric matrices to form conductive nanocomposites. The stretchable conductive fibers can be prepared by wet-spinning, electro-spinning, or other easily scalable method [10–12]. To obtain a highly conductive fiber, a high loading of well-dispersed fillers is required, so that the percolated network experiences only minor changes during the stretching. These are strong limitations, still today, that do impact on
Corresponding author. Corresponding author. E-mail addresses: [email protected] (G. Jin), [email protected] (G. Lubineau).
∗∗
https://doi.org/10.1016/j.polymer.2019.06.002 Received 10 March 2019; Received in revised form 30 May 2019; Accepted 1 June 2019 Available online 03 June 2019 0032-3861/ © 2019 Published by Elsevier Ltd.
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Fig. 1. (a) Chemical structures of PEDOT/PSS and PBP. (b) A schematic of the wet-spinning setup with the vertical hot-drawing apparatus used in this study.
application of hot drawing assisted by wet spinning and repeated doping of ethylene glycol (EG). We observed an increase of the strain at break from 15% to 21% [29]. The present work aims to produce wearable strain sensors by fabricating stretchable conductive PEDOT/ PSS fibers, using the methods of sequential wet spinning and hot drawing, together with the use of different chemical treatments. To do so, we modify the electrical conductivity and stretchability of the PEDOT/PSS fibers by blending an excess of a selected copolymer, (polyethylene-block-poly (ethylene glycol) (PBP), in our case) that contains polyethylene glycol and polyethylene segments. For comparison, PEDOT/PSS/Triton X-100 fibers are also fabricated to check the plasticizing effect in the fiber structure. A systematic study of the effects of the spinning parameters on the properties of the fiber shows that the hot-drawing and flow rate of the syringe pump have a significant influence on the properties of the fiber. The optimum content of PBP is obtained when the fraction of PBP, fs, is 0.7, the conductivity of the PEDOT/PSS/PBP fiber is 199 S/cm, and the strain at break of the fiber shows a maximum value, ε, of 36%. In addition, we characterize the microstructure of the fibers and the interaction between PEDOT/PSS and PBP using Raman spectroscopy and X-ray diffraction (XRD) measurements.
the use of nanocomposites for stretchable electronic devices. Our main goal, here, is to generate intrinsically stretchable and conductive materials using simple solution processes. The numerous, important applications of conductive polymer materials has attracted increased interest from the research community, particularly due to the electrical and mechanical properties of these materials. Poly(3,4-ethylene dioxythiophene)/poly(styrene sulfonate) (PEDOT/PSS) is the most widely used conducting polymer, due to its tunable conductivity, easy processability in aqueous solutions, and thermal stability [13]. The bandgap (Eg), defined as the energy difference between the top of the valence band (HOMO) and the bottom of the valence band (LUMO), is much higher for PEDOT/PSS (about 2.8–3.1 eV) than for PEDOT (1.6 eV) because of the ionic bonding with an insulating polymer (PSS). The large band gap of PEDOT/PSS makes the polymer normally at semiconductive state. However, upon doping, usually carried out by oxidation (more common) or reduction, the polymer may be rendered electrically conductive. For example, upon oxidative doping of polyaniline, new states are produced with reduced energy gap and this gives rise to higher electrical conductivity [13–17]. Systematic studies have reported that the conductivity of PEDOT/PSS films and fibers can be significantly improved by the application of polar solvents, surfactants, salts, acids, and ionic liquids [14–20]. For example, Shi et al. [21] detailed the application of several chemical treatments. Oh et al. [22] improved the electrical conductivity of PEDOT/PSS films by blending an excess of Triton X-100 (C14H22O (C2H4O)n) (n = 9–10), a nonvolatile surfactant, resulting in an increase of the films’ conductivity from 0.24 S/cm to ∼100 S/cm, yet improved the stretchability of the film from 5% to 80% strain. Wang et al. [23] systematically investigated the effect of polyethylene glycol (PEG), at various concentrations, on the conductivity of PEDOT/PSS, and significantly improved the conductivities of PEDOT/PSS/PEG hybrid films conductivities (e.g. from 0.1 S/cm to ∼17.7 S/cm with 4.06 × 10−2 mol/L PEG), in comparison with regular PEDOT/PSS pristine films. Mengistie et al. [24] also obtained improvements of the PEDOT/PSS conductivities, e.g. from 0.3 to 805 S/cm following the application of 2% v/v PEG. Numerous studies on the mechanical robustness of PEDOT/PSS films or fibers have revealed significant improvements in the stretchability of PEDOT/PSS films, following the incorporation of elastomeric materials such as polyurethane and polydimethylsiloxane (PDMS) [9,12,25,26], surfactants [15,27], and soft polymers [28]. However, an excessive use of nonconductive components (in the composites) dramatically reduces the conductivity of PEDOT/PSS materials. In previous work, we reported an improvement of the electrical conductivity of PEDOT/PSS fibers, up to 2800 S/cm, following the
2. Experimental 2.1. Materials Fig. 1a represent the chemical structure of the PEDOT/PSS and polyethylene-block-poly (ethylene glycol) materials, with an average Mn of ∼2250 (abbreviated as PBP). The PEDOT/PSS aqueous dispersion (Clevios™ PH1000) is obtained from HC Starck, Inc. The pristine solution exhibits a solid content of 1.1 wt%, and PEDOT to PSS exhibits a weight ratio of 1:2.5. PBP, Triton X-100 (TX), isopropyl alcohol (IPA) and acetone are obtained from Sigma-Aldrich. 2.2. Preparation of highly spinable PEDOT/PSS inks 10 mL of water was evaporated from 20 mL of the PEDOT/PSS dispersion (1.1 wt% in water) at 50 °C to increase the ink's viscosity, after which different fractions of PBP are added to the concentrated PH1000 dispersion (2.2 wt% in water), and stirred for 2 h using a magnetic stirrers. The surfactant exhibits weight fractions of 0, 0.3, 0.4, 0.5, 0.6, and 0.7, based on the equation fs = Ws/(Ws + WPEDOT/PSS), where Ws and WPEDOT/PSS represent the weights of the surfactant and the PEDOT/PSS solute (2.2 wt% in water), respectively. The maximum fraction of PBP is 0.7. When the fraction of PBP is more than 0.7, the 190
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Fig. 2. Electrical and mechanical properties of the conductive fibers. (a) Electrical conductivity of PEDOT/PSS fibers at different PBP fractions (fs = 0, 0.3, 0.4, 0.5, 0.6, and 0.7). (b) Tensile stress vs strain curve of PEDOT/PSS/PBP fibers with different PBP fractions (fs = 0, 0.3, 0.4, 0.5, 0.6, and 0.7).
Raman spectra of the fibers are collected using a LabRAM Aramis Raman spectrometer (Horiba, Ltd.), with a laser, at 633 nm. Continuous XRD tests are performed from 2 °C to 35 °C, using a Bruker D8 Advance powder X-ray diffractometer, with Cu Ka radiation (λ = 1.54) at 40 kV and 40 mA, and by slow increments of 0.02°, with a slow scan speed of 12 s per step to produce relatively high-intensity peaks.
spinning solution is too viscous to be wet-spun into fibers. The samples are then homogenized, for 20-min, at room temperature, in a sonication bath (Brason 8510 sonicator 250 W, Thomas Scientific). Finally, the dispersion is degassed in a vacuum oven, at room temperature (21 °C), before undergoing wet spinning. 2.3. Preparation of PEDOT/PSS fibers
3. Results and discussion
PEDOT/PSS/PBP conductive fibers were prepared by wet-spinning, combined with hot stretching. The spinning solution is loaded into a 6 mL plastic syringe and spun through a metal needle into a (1:1 volumetric ratio) acetone/IPA coagulation bath (see Fig. 1b). Different sizes of needles are selected for spinning. Experimental results show that fibers possess the best properties when the 27G needle is used for spinning. The ink-flow rate is adjusted, using a Fusion 200 syringe pump (Chemyx Inc.), at a spinning rate of 6 μL/min. Fibers are collected vertically onto a 50 mm winding spool, at a linear speed of 2–4 m/min. Two hot plates are mounted vertically and monitored by a thermometer to maintain the air temperature along the path of the fiber, at 90 °C.
Fig. 1b shows the fabrication process for the conductive fibers. The technique follows a two-step method – wet spinning followed immediately by hot drawing. First, we discussed the effects of the wetspinning parameters on the properties of the as-spun PEDOT/PSS fibers. Details on the optimum wet-spinning parameters for high-performance PEDOT/PSS fibers are described in the supporting information (Section 1). According to the experimental results, the optimum fiber performance is achieved at a spinning rate of 6 μL/min (Figs. S1 and S2, supporting information), using a 27G spinning needle (Figs. S3 and S4, supporting information). In addition, we find that hot drawing can effectively increase the electrical conductivity of the fiber (314 ± 13 S/ cm), by a factor of 2, compared with the value obtained without hotdrawing (Table S1, supporting information). These results reflect the alignment of the fiber molecular chains in the fiber direction, which is significantly influenced by the vertical hot-drawing process [29]. Fig. 2a presents the volume electrical conductivity as a function of the PBP fraction (fs). The conductivity values are calculated using the resistance and diameter of the PEDOT/PSS fiber (Supporting Information, Table S2). The addition of non-conductive PBP first increases the conductivity of PEDOT/PSS fibers and shows a maximum conductivity of 415 ± 12 S/cm at fs = 0.4. When the PBP fraction increases (above 0.4), the electrical conductivity of the fiber decreases to minimum value of 152 ± 10 S/cm at fs = 0.7. Even at such a high fraction, the value of the conductivity is still twice the one obtained with polyurethane (PU)/ PEDOT/PSS (∼9.4 S/cm) fibers [9]. Fig. 2b shows the stress-strain curves of the PEDOT/PSS fibers, for different fractions (fs) of PBP. The properties of the PEDOT/PSS/PBP fibers show a good reproducibility (as shown by Fig. S6, Supporting Information). All fibers exhibit a bi-linear stress-strain curve that follows a strain-hardening behavior. When the PBP fraction increases, the stretchability of the fibers also increases significantly, compared with the stretchability of the PEDOT/PSS fibers reported previously [1,29]. PEDOT/PSS specimens with fs = 0 are brittle and break at relatively small tensile strains (ε = 12%). A PBP fraction of fs = 0.5 presents a significantly lower modulus, and exhibits an apparent yielding at ε = 3%, although failure is observed at ε = 23%. At fs = 0.7, the
2.4. Characterizations The electrical resistance of the fibers is measured using an Agilent 1252B multimeter. A copper wire is mounted onto the fiber surface, with silver epoxy, to produce an electrical contact between the wire and the fiber, with a distance of approximately 20 mm between the two contacts. Each type of fiber is measured at least five times. The diameter of each fiber is measured using an X61 optical microscope (Olympus Corporation). The volume electrical conductivity (σ) of the fiber is defined as σ = L/AR, where L, A, and R represent the length, cross-sectional area, and resistance of the fibers, respectively. A 5944 Instron Universal Testing Machine with a 5 N loading cell is used, at a strain rate of 5% min−1, to measure the stress-strain curve of 2-cm-long fibers, fixed on a paper card. Measurements are based on at least 10 tests for each formulation. Similarly, a U1252B digital multimeter is used to measure the electrical resistance of the fiber as a function of the tensile strain. A cyclic loading/unloading program is applied to the fiber, with an incremental extension of 0.2 mm at each cycle, after which the fibers are released at a load of 1 mN. The resistances of the fibers are measured, and captured every second during the test. Copper wires are used to connect the two ends of the samples, which we paint with silver epoxy, before sealing the silver epoxy area with an epoxy glue. The conductive polymer fibers are observed by scanning electron microscopy (SEM, FEI Magellan), at an accelerating voltage of 3 kV. The 191
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PEDOT/PSS/PBP fiber exhibits up to ε = 36% of stretching, eq. to 3x the value obtained for a as-spun PEDOT/PSS fiber. The Young's modulus of the PEDOT/PSS/PBP fiber decreases very significantly, as the fs increases from 5.6 GPa (fs = 0) to 0.67 GPa (fs = 0.7) (Table S2, Supporting Information). Triton X-100, a small-molecule nonionic surfactant, is added to the PEDOT/PSS solution, after which the electrical and mechanical performances of PEDOT/PSS/TX fiber are evaluated to compare the effects of the surfactant on the fibers. Fig. S7 (Section 3, Supporting Information) presents the electrical conductivity of the PEDOT/PSS/TX fibers as a function of the TX fraction. Similarly, the conductivity is first increased, and then decreased, with the concentration of TX. Surprisingly, a maximum value (550 ± 13 S/cm) of the PEDOT/PSS/TX fiber conductivity is observed at fs = 0.5, suggesting that TX could better improve the conductivity of the PEDOT/PSS fibers, compared with PBP. However, TX does not improve the stretchability of the PEDOT/PSS fiber, as the mechanical response remains unchanged (Fig. S8, Supporting Information). Motivated by these excellent mechanical and electrical properties, we further investigated the electrical properties of the PEDOT/PSS/PBP fibers under monotonic mechanical stretching (Fig. 3a), and under incremental cyclic loading/unloading with progressive extension (see Fig. 3b–d). Fig. 3a presents the relative change in resistance, defined as ΔR/R0 = R(ε) - R0/R0, where R(ε) is the resistance at different strains, and R0 is the initial resistance. The relative change in resistance is applied as a function of the fiber tensile strains at different PBP fractions (fs = 0, 0.3, 0.4, 0.5, 0.6, and 0.7, respectively). We find that the resistance of the fibers increases gradually, as the applied strain increases. When the applied strain is less than 5%, he resistance of the fiber remains almost unchanged. In addition, the relative change in resistance decreases with an increase of the PBP fraction, under the same tensile strain condition, in comparison with the as-spun PEDOT/ PSS fiber. At fs = 0.7, the PEDOT/PSS/PBP fiber exhibits a ΔR/R0 of
0.24, at a 36% strain (Fig. 3a and d). In comparison, the as-spun PEDOT/PSS fiber exhibits a ΔR/R0 of 0.25, at a 12.5% strain (Fig. 2a and b). The pure geometric factor is then calculated and subtracted from the measured resistance to calculate the change in intrinsic conductivity (Supporting Information, Section 4). Changes in the resistance of the PEDOT/PSS fiber depend on two factors: (i) conductivity changes in the material, which may also be dependent on the strain; and (ii) changes in the geometry of the fibers (i.e., elongation and reduction of the diameter). Change in the resistance the as-spun PEDOT/PSS fibers are mostly attributed to changes in the geometry of the sample. The Δσ/ σ of the PEDOT/PSS/PBP fibers (PBP fractions from 0.3 to 0.7) are estimated as 0.01, 0.09, 0.13, 0.20, and 0.24 when the geometrical contributions of the total resistance changes were excluded. The microstructures of the pristine PEDOT/PSS and PBP-modified fibers are investigated and compared by scanning electron microscopy (SEM) (Fig. 4). By increasing the fraction of PBP, the diameter of the fiber is gradually increased from 10.07 ± 0.54 μm, for as-spun PEDOT/PSS, to 24.4 ± 1.4 μm, for the fiber with the PBP fraction at fs = 0.7 (See Table S2, Supporting Information). When fs = 0, the fibers exhibit a remarkably smooth surface (Fig. 4a and Fig. S5a). As the fraction of PBP increases, the surface of the fibers becomes increasingly rough. When fs = 0.7, the fiber gets deformed, and the cross-section of the fiber becomes irregular, as shown in Fig. 4e and Fig. S5b. PBP is a copolymer with typical thermoplastic polymer characteristics due to the existence of polyethylene phase. We observe an uneven local dispersion of the spinning solution, following ink spinning into a coagulation bath, using a spinning needle, which results in an extrusion instability and deforms the fiber. In addition, when fs = 0.3 and 0.5, the surface of the fiber exhibits a discontinuous whitish part (Fig. 4). In fact, the PEDOT/PSS dispersion is a dark blue solution, and the as-spun PEDOT/PSS fiber is black. At the same time, the PBP appears as white dots in the PEDOT/PSS matrix, displaying a “sea-island” structure on the surface of the PEDOT/PSS fiber. When fs = 0.7, the discontinuous
Fig. 3. Mechanical behavior and change in the electrical resistance of the conductive polymer fibers following stretching/unstretching. (a) Relative changes in the resistance versus the applied strain on the fiber. (b, c, and d) The resistance changes to the incremental cyclic loading/unloading of PEDOT/PSS/PBP fibers with the PBP fraction at fs = 0, 0.4, and 0.7. 192
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Fig. 4. SEM images showing the surface of PEDOT/PSS/PBP fiber. (a), (b), (c) and (e) SEM images of PEDOT/PSS fibers with PBP fractions of fs = 0, 0.3, 0.5, and 0.7, with scales of 10 μm, 10 μm, 20 μm and 20 μm respectively. (d) and (f) SEM images of PEDOT/PSS/PBP fiber (0.5 and 0.7) at scale of 5 μm.
respectively) (Fig. 5b). The two peaks of interest at 2θ = ∼ 4.7° and ∼26° correspond to an alternate stacking of lamella in the PEDOT/PSS fibers, with a lattice spacing of ∼23 Å, and an interchain π-π stacking in the PEDOT segments [15,18,31], with a lattice spacing of ∼3.4 Å. The PBP-modified fiber exhibits a clear increase in peak intensity at 2θ = ∼26°, suggesting a better crystalline packing in the PEDOT segment, resulting in an improvement of the electrical conductivity following the addition of PBP in the PEDOT/PSS fibers. This is in a good agreement with the conversion of benzoid structure to quinoid structure as analyzed by Raman spectra. The planar nature of quinoid structure will facilitate the packing of PEDOT segments. Fig. 5c shows a schematic of the change in microstructure of the fibers without and with PBP as manifested by Raman and XRD results. Though it is not the scope of this study to quantify the ratio between the quinoid and benzoid structures in the fibers, we could still remark that a more favorable quinoid structure in the fiber leads to planarization of the conductive PEDOT chains, resulting in the reduction of π-π stacking distance. This reduces the electron path-ways on the chains and between the chains, thus results in higher conductivity of the fibers.
whitish part disappears (Fig. 4f). It is reasonable to think that a cocontinuous structure, formed inside the fiber, contributes to the improvement of the stretchability of the fiber. In contrast, there is no significant change in the diameter and in the surface of PEDOT/PSS/TX fibers (see Fig. S9, Supporting Information). A comparative analysis of these observations and the results suggest that the PBP improves the electrical conductivity and stretchability of the PEDOT/PSS/PBP fibers. For PBP fractions fs > 0.4, the fiber conductivity is significantly dependent on: (i) the small volumetric density of the PEDOT nanofibrils, which is a result of an excess of PBP, and (ii) the PBP covering the PEDOT nanofibril, thus reducing the density of the conductive network. The PBP is an excellent insulator; it forms a continuous structure that covers the PEDOT nanofibril, resulting in a decrease of the overall conductivity of the PEDOT/PSS fiber. The conductivity of the PEDOT/PSS was significantly dependent on the length and quality of the PEDOT nanofibrils. PEG can increase the conductivity of PEDOT/PSS effectively [23]. PBP contains two segments, i.e. polyethylene segments and polyethylene glycol segments. It is inferred that the increase in conductivity with PBP fractions is mainly due to the presence of polyethylene glycol segments in the PBP. The microstructure characterization of the samples, with and without PBP, are then examined, using Raman spectroscopy, to investigate the role of PBP in the significant improvement of the electrical and mechanical properties of PEDOT/PSS fibers. Quyang et al. reported that the conformational changes in the PEDOT chains, from a benzoid-type to a quinoid-type structure, improve the interchain interactions, consequently enhancing the conductivity of the PEDOT/PSS fibers [30,31]. A strong peak was observed at 1421 cm−1, which indicates the Cα = Cβ symmetric stretching of the thiophene ring in the PEDOT chains of the pure PEDOT/PSS fibers (Fig. 5a) [32,33] The Raman spectrum exhibits a red shift, following the addition of PBP, from that obtained with pure PEDOT/PSS fibers. Peak shifts from 1421 cm−1 to 1414 cm−1 and 1416 cm−1, can be observed for fraction values of fs = 0.5 and 0.7, respectively. In addition, shoulder signals observed at 1445 cm−1 [33], which corresponds to benzoid structure are weaker for PBP-containing fibers. This is an indication of a conversion from a benzoid structure to a quinoid structure that favors the inter- and intra-chain charge transport of the PEDOT, and generate an increase in the conductivity of the PEDOT/PSS fibers. In addition, the XRD analysis presents the XRD patterns of the PEDOT/PSS fibers, with or without PBP (fs = 0, 0.4, and 0.7,
4. Conclusions This work introduces a novel method to produce a copolymer-enabled stretchable conductive PEDOT/PSS fiber, using wet-spinning followed by hot-drawing. An examination of the effects of the wetspinning parameters on the properties of the wet-spun PEDOT/PSS fibers reveals a significant influence of both the hot drawing and the flow rate on the fiber properties. For example, we observe an increase in the fiber's electrical conductivity from 151 ± 21 S/cm to 311 ± 8 S/cm, following hot drawing. The effects of the PBP fractions on the electrical conductivity and mechanical properties of the PEDOT/PSS fibers are then systematically investigated. An optimum fraction of PBP has been observed to maximize electrical performance. In addition, we find that the strain at the break increases from 12.5% to 36%, following an increase in the PBP fraction. The PEDOT/PSS/PBP fiber exhibits a maximum conductivity (415 ± 12 S/cm) at fs = 0.4 and a strain break at 23%. At fs = 0.7, the strain first increases to levels as high as 36%, which represents a three-fold improvement, compared with the pure PEDOT/PSS fiber. Raman and XRD spectra suggest that the addition of the PBP produces conformational changes in the PEDOT chains, as the structure undergoes a structure conversion, from a benzoid to a 193
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Fig. 5. Effects of PBP fraction on the electrical properties of the PEDOT/PSS fibers. (a) Raman spectra for the PEDOT/PSS/PBP fiber, for different PBP fractions (fs = 0, 0.4, 0.7). (b) XRD patterns of the PEDOT/PSS/PBP fiber, for different PBP fractions (fs = 0, 0.4, 0.7). (c) A schematic of the Benzoid and quinoid microstructures for PEDOT chains.
quinoid, resulting in an increase in interchain interactions, which, in turn improves the conductivity of the PEDOT/PSS fibers.
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