High-Performance Stretchable Conductive Composite Fibers from

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High-Performance Stretchable Conductive Composite Fibers from Surface Modified Silver Nanowires and Thermoplastic Polyurethane by Wet Spinning Ying Lu, Jianwei Jiang, Sungho Yoon, Kyung-Shik Kim, JaeHyun Kim, Sanghyuk Park, Sang-Ho Kim, and Longhai Piao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16022 • Publication Date (Web): 26 Dec 2017 Downloaded from http://pubs.acs.org on December 26, 2017

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ACS Applied Materials & Interfaces

High-Performance Stretchable Conductive Composite Fibers from Surface Modified Silver Nanowires and Thermoplastic Polyurethane by Wet Spinning Ying Lu†, Jianwei Jiang‡, Sungho Yoon‡, Kyung-Shik Kim§, Jae-Hyun Kim§, Sanghyuk Park†, Sang-Ho Kim†*, Longhai Piao†* †

Department of Chemistry, Kongju National University, Chungnam, 32588, Korea

‡

Department of Bio & Nano Chemistry, Kookmin University, Seoul 02707, Korea

§

Nano-Mechanical Systems Research Division, Korea Institute of Machinery & Materials (KIMM), Daejeon 34103, Korea KEYWORDS: AgNWs, Ag nanoplates, elastic polyurethane, surface modification, stretchable conductive fibers

ABSTRACT: Highly stretchable and conductive fibers have attracted great interest as a fundamental building block for the next generation of textile-based electronics. Because of its high conductivity and high aspect ratio, Ag nanowire (AgNW) has been considered one of the most promising conducting materials for the percolation-network-based conductive films and composites. However, the poor dispersibility of AgNWs in hydrophobic polymers has hindered its application to stretchable conductive composite fibers. In this paper, we present a highly stretchable and conductive composite fiber from the co-spinning of surface modified AgNWs and thermoplastic polyurethane (PU). The surface modification of AgNWs with a polyethylene glycol (PEG) derivative improved the compatibility of PU and AgNWs, which allowed the NWs to disperse homogeneously in the elastomeric matrix, forming effective percolation networks and causing the composite fiber to show enhanced electrical and mechanical performance. The maximum AgNW mass fraction in the composite fiber was 75.9 wt%, and its initial electrical conductivity was as high as 14,205 S/cm. The composite fibers also exhibited superior stretchability: the maximum rupture strain of the composite fiber with 14.6 wt% AgNW was 786%, and the composite fiber was also conductive even when it was stretched up to 200%. In addition, 2-dimensional (2-D) Ag nanoplates were added to the AgNW/PU composite fibers to increase the stability of the conductive network under repeated stretching and releasing. The Ag nanoplates acted as a bridge to effectively prevent the AgNWs from slippage and greatly improved the stability of the conductive network.

▪ INTRODUCTION

as the basic substrate for the fabrication of electronic components on the flexible conductive fiber.11

Textile-based electronics have attracted growing interests as a promising technology for the next generation of smart wearable devices.1,2 With the advancement of nanotechnology, it is technically feasible to build various electronic functionalities or devices on a single fiber, yarn or fabric while retaining the characteristics of textiles such as light weight, flexibility and stretchability.3,4 To date, textilebased sensors,5,6 energy harvesting/storage systems7-9 and communication devices10 have been fabricated successfully. To realize high-performance textile-based electronic devices, the development of highly conductive flexible fibers is of prime importance. These flexible conductive fibers work as one of the essential building blocks for these textile-based devices, which not only interconnect the functional elements and supply electricity but can also be used

Metal wires, conducting polymer threads,12 and carbonbased fibers such as carbon fibers,13-15 carbon nanotubes (CNTs)16,17 and graphene fibers18,19 have been explored as potential conductive fiber candidates for textile-based devices. However, the direct use of these fibers has been limited by their inherent imbalance of electrical and mechanical properties. For example, metal wires have limited elasticity and can break easily during the weaving and knitting process. Both conducting polymer threads and carbonbased fibers have relatively low electrical conductivities and poor mechanical ductility.12 However, the hybrid composite fibers consisting of elastic polymers and conductive components, where the former enables the reversible deformation and the latter creates conducting networks,20 are expected to simultaneously possess both high flexibility

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and conductivity, making them more suitable for the textile-based electronic applications. Coating or growing conductive layers on a polymer fiber to form a core/sheath structure is one of the most commonly used methods to fabricate hybrid conductive fibers. Specifically, the conductive sheath layer can be constructed by dip-coating,21 spray-coating or growing/deposition5 of conductive components such as CNT,22 graphenes,18,23 Au nanoparticles (NPs),24 AgNPs,25 AgNWs,26,27 PtNPs,28 CuNWs,29 ZnO,30 etc., on nylon,26 polyester,26 cotton,26,31 PU,32 poly(m-phenylene isophthalamide) (PMIA),20 poly(styrene-b-butadiene-b-styrene) (SBS)5,25 and Kevlar33 fiber surfaces. The reported conductivities of these fibers are in the range of 20 S/cm to 500 S/cm for the carbonbased nanomaterials16,18,31 and 24 S/cm to 1,950 S/cm34 for the metal nanomaterials. However, the conductivities of these fibers would be drastically reduced under high strain and could not be recovered due to the generation of microcracks on the conductive sheath layer caused by the asynchronous deformation of the conductive layer and the polymer substrate.20 As a result, such unstable and irreversible electrical performance has limited the widespread application of these conductive fibers. For the realization of stable and reversible electrical conductivity under deformation, the conducting networks should deform synchronously with the polymer substrate. This synchronous deformation can be achieved by constructing embedded composites of conductive components in elastic polymers. Co-spinning of elastic polymer with conductive fillers is one of the most effective strategies to fabricate stretchable conductive composite fibers because it is a well-established, cost-effective method with a wide range of polymer materials available. Elastic polymers such as PU,12 SBS35 and poly(vinylidene fluoride-cohexafluoropropylene) (PVDF-HFP)36 have been employed for the fabrication of stretchable conductive fibers by wetspinning. In the typical wet-spinning process, the conductive fillers are homogeneously dispersed in these polymer solutions (referred to as spinning dope) and spun into a coagulant to produce composite fibers. The conductive components embedded in the polymer matrix create conductive networks. Therefore, the conductivity of the composite fiber is dependent primarily on both the initial conductivity of the conductive materials and the effectiveness of the constructed conducting networks. With regard to conductive components, various electrically conducting fillers have been co-spun with the elastic polymers, including carbon black, CNT,37,38 graphene,39 Ag flake,40 AgNWs, AgNPs and hybrids of these materials.36 Among these materials, CNT has been studied extensively due to its superior mechanical and electrical properties and the characteristic 1-D structure. However, the reported conductivities of the CNT-based composite fibers are in the range of 0.4 S/cm ~210.7 S/cm. Such low conductivity might be caused by the high contact resistance of the CNT networks and the poor dispersibility of the CNT in the spinning dope and the polymer matrix.

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In contrast, the fibers with metallic fillers have shown much higher conductivities than CNT/polymer composite fibers because of the high intrinsic electrical conductivities of the metallic fillers. For example, Wakuda et al.40 fabricated a stretchable fine fiber with high conductivity by an injection formed of silicone conductive adhesive containing Ag flake fillers. The highest conductivity was 876 S/cm with the rupture strain of 13.8%. The fine fiber maintained the conductivity of more than 90 S/cm under a cyclic tensile test with 10% strain. In 2014, Ma et al.36 reported a highly conductive stretchable PVDF-HFP composite fiber with the mixture of 100~150 nm AgNPs and multi-walled CNTs decorated with 3~5 nm AgNPs as the conductive components by wet-spinning. The maximum initial conductivity was 17,460 S/cm with the rupture tensile strain of 50%. The maximum rupture strain reached 350% when the conductivity was 236 S/cm. Recently, Ma et al. reported another stretchable conductive fiber based on the composite of PU and Ag nanoflowers. The conductivity of the composite fiber could be as high as 41,245 S/cm with the rupture strain of 90%.41 To achieve high conductivity, the embedded conductive component with high initial conductivity should create an effective conductive network inside the composite fiber. The loading of the conductive fillers should exceed the percolation threshold, which is determined by the initial aspect ratios and the dispersion state of the fillers. A number of simulations and experimental studies have proved that the fillers with high aspect ratio have a lower percolation threshold; for example, the CNT-epoxy nanocomposites showed a percolation threshold as low as 0.0025 wt% when the CNTs were well-dispersed.42 Consequently, composite fibers consisting of well-dispersed nanofillers with high initial conductivity and high aspect ratio should be ideal for the realization of high-performance conductive fibers. In this context, AgNWs might be one of the best candidates for preparing conductive composite fibers, because AgNWs possess high conductivity (6.3×105 S/cm), high aspect ratio (up to more than 1000) and is capable of better dispersion due to the lack of entanglement such as CNTs. Unfortunately, to date, only one pioneering research study employed AgNW as a conducting component for a conducting composite fiber. In 2015, Lee et al.35 reported the fabrication of stretchable conductive fibers by wet-spinning SBS elastomer and AgNW fillers followed by AgNPs formation inside the AgNW-embedded fiber. The AgNWs were critical to connect the conductive network of AgNPs under strain to improve the electrical conductivity of the composite fiber. However, the AgNWs were just an assistant with the highest concentration of 1.68 wt%. For higher content, the AgNWs would be aggregated with each other, resulting in poor dispersion in the polymer substrate, which lowers the electrical conductivity of the composite fiber. In this paper, we present the fabrication of a stretchable conductive composite fiber by co-spinning surface modified AgNWs and elastic PU. The modification of the AgNW


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surface with PEG derivative greatly improves the compatibility between the PU matrix and the AgNW fillers, leading to a high filler loading (up to 75.9 wt%) and an effective dispersion of the AgNWs in the PU matrix, causing the composite fibers to have enhanced electrical and mechanical performance.

▪ RESULTS AND DISCUSSION The conductivity of the AgNW/polymer composite relies mainly on the distribution of the nanofillers throughout the polymer matrix. The homogenous dispersion of AgNWs in the composite favors the formation of an effective conducting network and is critical to the development of high-performance conductive fibers. To attain homogeneous dispersion, the nanofillers should have appropriate interfacial interaction with the polymer matrix. The AgNWs synthesized from the polyol process are covered by polyvinylpyrrolidone (PVP) surfactant, and the dispersion state of the AgNWs in the composite is determined mainly by the miscibility of PVP and the polymer matrix. The Hildebrand and Hansen solubility parameter (δ)43 provides a numerical estimate of the degree of interaction between materials and the polymers; materials with similar values of δ are likely to be miscible. The solubility parameter of PVP, calculated in terms of the repeating units, is 26.3 MPa1/2 (supporting information, Table S1), quite larger than the solubility parameter of PU (20.0-21.1 MPa1/2),44 leading to the agglomeration of AgNWs. To achieve better dispersion, the PVP surfactants must be exchanged with other molecules that have better compatibility with PU. In this study, polyethylene glycol methyl ether appended to thioctic acid (TA-mPEG) was employed as a surface modification agent because the solubility parameter of mPEG is 19.4 MPa1/2 (Table S2), quite similar to the solubility parameter of PU, and the disulfide group has a high affinity for AgNWs. Consequently, the TA-mPEG molecules can anchor onto the surface of AgNW and promote its homogeneous dispersion in the PU matrix. Previously, TA-mPEG has been widely used for the surface modification of spherical NPs such as AuNPs45,46 AgNPs47 and quantum dots.48 However, this is the first time TA-mPEG has been used to modify the surface of the AgNWs for the preparation of a conductive composite. The synthetic procedure of TA-mPEG was depicted schematically in Figure 1(a) and performed with a modified method from Tsukruk.49 The esterification reaction of the hydroxyl group of mPEG and TA was mediated by dicyclohexylcarbodiimide (DCC) and catalyzed by 4-dimethylaminopyridine (4-DMAP). The structure of the synthesized TA-mPEG was analyzed by 1H-NMR spectroscopy (Figure 1(b)). The characteristic resonances of the protons from the TA moiety were shown at 1.45-1.60, 1.65 -1.75, 1.84-1.96, 2.35, 2.41-2.52, 3.18 and 3.57 ppm, and the peaks at 3.38 and 3.433.90 ppm were assigned to the protons of the mPEG segment, respectively. The methylene protons next to the hydroxyl group of mPEG at 3.81 ppm shifted to 4.23 ppm after

the esterification reaction (Figure S1). The broad peak at 10.5-12 ppm from the carboxylic acid of TA (Figure S2) was absent after esterification, indicating the successful esterification reaction between the carboxylic acid and the hydroxyl group of mPEG. Comparing the integral ratio of the proton signals at 3.38 and 4.23 ppm for the mPEG block with the proton signals at 2.35 and 3.18 ppm for the TA moiety, the end-capping efficiency was estimated to be 87.2%. The synthesized TA-mPEG was used to functionalize the surface of the AgNWs prepared from the typical polyol method with PVP as the capping agent (Figure 1(c) and (d)). Briefly, the AgNWs approximately 80 nm in diameter and 20 μm in length (Figure S3) were dispersed in methanol and reacted with excess TA-mPEG for 21 h in a dark environment. After separation and purification, the surface-modified AgNWs (S-AgNWs) were characterized with scanning electron microscopy (SEM), X-ray diffraction (XRD), UV-visible spectrometer (UV-vis) and thermogravimetric analysis (TGA). The shapes of the AgNWs had almost no change after the surface modification process, and the distributions of the length and the diameter maintained nearly the same (Figure S4). The XRD analysis indicated that the surface modification with TA-mPEG did not alter the crystalline structure of the AgNWs (Figure S5). However, the UV-vis absorbance peak of the AgNWs at 376 nm shifted slightly to 378 nm after surface modification (Figure S6). The organic surfactant content also changed from 3.72% to 2.87% according to the TGA weight loss curves (Figure S7), indicating the PVP surfactant was exchanged with TA-mPEG. The introduction of the mPEG segments to the surface of the AgNWs would alter the surface properties of the AgNWs and thus influence its colloidal stability in solvents. Figure 1(e) showed the dispersion stability of AgNWs (left) and S-AgNWs (right) in various solvents with different solubility parameters, including methanol (δ = 29.7 MPa1/2), N, N-dimethylformamide (DMF) (δ = 24.8 MPa1/2), acetone (δ = 20.3 MPa1/2), tetrahydrofuran (THF) (δ = 18.6 MPa1/2), and toluene (δ = 18.2 MPa1/2). The AgNWs with PVP surfactant could disperse only in methanol and DMF and would flocculate and finally precipitate shortly from the solvents with lower δ values such as acetone, THF and toluene. By contrast, the S-AgNWs maintained stable colloidal dispersions for a long time in all the solvents used, even in a nonpolar solvent such as toluene because PEG has a δ value similar to the δ value of acetone, THF and toluene, and the oxygen atoms of PEG can also form hydrogen bonds with methanol molecules. In the case of DMF, the differences in δ values with PVP and PEG are not so large, and both AgNW and S-AgNW could be dispersed in DMF. From the sedimentation test, one could envision that the exchange of PVP surfactant with TA-mPEG would modify the interfacial interaction of AgNWs with the PU matrix and improve the compatibility of the nanofillers with the polymer substrate.



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Figure 1. (a) Schematic illustration of the synthetic procedure of TA-mPEG. (b) 1H-NMR spectroscopy of TA-mPEG. “*” designates the residual water. (c) SEM image of AgNWs. (d) Schematic illustration of the surface modification of AgNW. (e) Photographs of the dispersion stabilities of AgNWs (left) and S-AgNWs (right) in different organic solvents (numbers 1 to 5 correspond to DMF, THF, acetone, toluene and methanol, respectively) with 1 min, 5 min, 10 min and 60 min, respectively.

The AgNW/PU conductive composite fibers were fabricated by a solvent/non-solvent wet-spinning method schematically illustrated in Figure 2(a). Briefly, the spinning dope (the DMF solution of PU and AgNWs or S-AgNWs) was extruded through a syringe needle into the water coagulation bath by using a syringe pump. Then, the spinning dope was solidified via the countercurrent diffusion of DMF and water in the coagulation bath. Finally, the asspun composite fiber was collected on a stainless spool and dried in air. The concentration of the spinning dope was critical to the ability to spin the dope and had an important role in the morphology and mechanical properties of the spun fiber. The optimized dope concentration was 10 wt%; when the dope concentration was too low (e.g., 5 wt%), very weak fibers with irregular diameters were formed, and when the concentration was high (e.g., 15 and 20 wt%), the fibers obtained were fragile, which might be a consequence of the increased die swell due to the increased extrusion pressure for viscous dope. Figure 2(b) was a photograph of the S-AgNWs/PU composite fiber collected on the stainless-steel spool. Longer than 10 m composite fiber could be obtained from 1 mL

dope. The color of the fiber turned from white to gray after the addition of AgNWs to PU and became darker when the concentration of AgNWs was increased. Figure 2(c) and (d) showed the microscope images of the composite fiber in transmission and reflection modes, respectively. Uniform fibers with a diameter of approximate 140 µm were obtained. Figure 2(e) consisted of the SEM images of the composite fiber with a knot, indicating the composite fiber was highly flexible. The surface of the composite fiber was highly folded and had many irregular longitudinal pleats. The cross-sectional SEM image of the composite fiber was also an irregular shape with some holes inside (Figure 2(f)). The shape might be caused by the mass transfer rate differences;50 the inward diffusion rate of the coagulant to spinning dope was higher than the outward diffusion rate of the solvent in the spinning solution, leading to the coagulation of the PU dopes to form a fiber with a solid skin and a relatively viscous core remained, causing the fiber to collapse into an irregular shape when the remaining solvent was extracted. Figure 2(g) presented the magnified SEM image of the composite fiber. Some S-AgNWs embedded in the PU matrix could be seen, and the S-AgNWs were uniformly dispersed in the fiber without aggregation,


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Figure 2. (a) Schematic illustration of wet spinning of composite fibers. (b) Photograph of the 38.0 wt% S-AgNWs/PU composite fiber collected on a stainless-steel spool. Optical microscope images of the composite fiber in (c) transmission and (d) reflection modes. The SEM images of the 38.0 wt% S-AgNWs/PU composite fiber: (e) with a knot, (f) cross-sectional image, (g) the surface morphology and (h) high-magnification cross-sectional image.

implying the good compatibility between the S-AgNWs and the PU matrix. In addition, the S-AgNWs were aligned along the longitudinal direction, which might be caused by the high shear force when the dope extruded through the needle by the syringe pump. Comparing with the AgNWs/PU composite fiber (Figure S8), the magnified cross-sectional SEM image of the fiber in Figure 2(h) further showed that the AgNWs dispersed uniformly inside the fiber without aggregation, and some kinds of interconnected AgNW networks were constructed. The mass fractions of the AgNWs in the composite fibers were determined from the TGA weight loss (Figure 3(a)) and could be controlled by simply changing the relative ratio of AgNW and PU in the dope. The relationship between the AgNW mass fractions in the composite fibers and the AgNW mass fractions in the dopes (defined as the relative mass fraction of AgNWs to the solid contents of the dope) were shown in Figure 3(b). Compared to the AgNWs without surface modification, S-AgNWs exhibited better spinnability because of the improved dispersion in the dope. Higher mass fractions of AgNWs in the composite fibers were obtained. For example, without surface modification, the upper limit mass fraction of AgNWs in the dope was only 39.8 wt%. When more AgNWs were added, the AgNWs could not disperse uniformly in the dope, and apparent aggregations would form, making it difficult to get continuous uniform composite fibers. The mass fraction of

AgNWs in the composite fiber was 24.1 wt%, lower than the mass fraction of AgNWs in the dope, indicating that some AgNWs were lost during the coagulation process. In contrast, the spinning dope with the mass fraction of SAgNWs as high as 74.7 wt% could be spun successfully into continuous uniform fibers. In addition, the mass fractions of S-AgNWs in the fibers and in the dopes were almost the same. For example, the composite fiber spun from the 74.7 wt% S-AgNWs dope contained 75.9 wt% S-AgNWs, implying that almost no AgNWs were lost during the spinning and coagulation process. Figure 3(c) compared the electrical conductivities of AgNW/PU and S-AgNW/PU composite fibers with different AgNW mass fractions. The electrical conductivities increased with increasing mass factions of AgNWs. The conductivities for S-AgNWs/PU composite fibers were much higher than the conductivities of AgNWs/PU fibers. For example, the conductivity of AgNWs/PU composite fiber with 24.1 wt% AgNW was only 147 S/cm while the SAgNWs/PU composite fiber with similar AgNW mass fraction (24.4 wt%) showed a conductivity of 331 S/cm, approximately twice as high as the conductivity of the AgNW/PU composite fiber. The highest electrical conductivity of SAgNWs/PU composite fiber reached 7,330 S/cm for the fiber with 75.9 wt% S-AgNWs. The extraordinarily high conductivity was the consequence of the high mass fraction and the effective conducting networks from the uniformly



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Figure 3. (a) Representative TGA curves of AgNWs/PU and S-AgNWs/PU composite fibers. (b) The relationship between the AgNW mass fractions in the composite fibers and the NW mass fractions in the dopes. (c) The electrical conductivities of AgNWs/PU and S-AgNWs/PU composite fibers as a function of AgNW mass fractions. (d) The conductivity-strain relationship of AgNWs/PU and S-AgNWs/PU composite fibers with different AgNW mass fractions.

dispersed AgNWs due to the improved compatibility of SAgNWs and the PU matrix, together with the aligned configuration of the AgNWs. Surprisingly, our AgNW-based composite fiber showed an extremely low percolation threshold compared to the composite fiber with symmetrical metal particles; our composite fiber was still conductive (σ = 10 S/cm) when the volume fraction of S-AgNW was as low as 1.8 vol% (14.6 wt%) while the reported percolation threshold of the composite fiber with symmetrical Ag nanomaterials was approximately 17.9 vol% (66.7 wt%).41 Such a low percolation threshold might be attributed to the high aspect ratio of AgNWs (~ 250). Although the embedded AgNWs were aligned along the longitude direction of the fiber, it was not perfect. Therefore, an effective conductive percolation network could be created with relatively low filler content, which would favor the composite fibers to preserve the mechanical features of the elastic polymer.

under high strain. Figure 3(d) showed the curves of electrical conductivities of our composite fibers under uniaxial strain. The conductivity was calculated according to the true cross-sectional area of the fibers measured by an optical microscope and the corresponding length of the fiber. The S-AgNWs/PU composite fibers showed a better conductivity degradation rate (σ/σ0, where σ0 is the initial conductivity, and σ is the instantaneous conductivity of the fiber under strain, respectively) than the AgNWs/PU under strain. The fiber with 38.0 wt% S-AgNWs had an initial conductivity of 1,656 S/cm with the conductivity degradation rate of σ/σ0=0.7% at 100% strain, still conductive even under a strain as large as 170%. In the case of AgNWs/PU composite fibers, the fibers with 18.7 wt% and 24.1 wt% of AgNWs lost conductivity at the strains of 20% and 100%, respectively. The S-AgNWs/PU fibers with higher mass fractions, i.e., 75.9 wt% and 50.1 wt%, exhibited flexibility to knot. However, they broke under small strain (lower than 3%).

The composite fibers of elastic polymers and conductive fillers were capable of preserving electrical conductivity


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The conductivity of the AgNW-based composite or conducting film could be improved greatly through proper heat treatment. During the heat treatment, the surfactant residues on the AgNWs surface might be decomposed or vaporized and the inter-wire junctions would be partially welded at high temperature.51 To further increase the conductivity, the composite fibers were cured at different temperatures for 30 min. Figure 4(a) revealed the influence of heating temperature on the electrical conductivity of the composite fibers as a function of the uniaxial tensile strain. As expected, the conductivity of the fibers increased with increasing temperature until 155 °C. Then, the conductivity was decreased inversely when the curing temperature was 175 °C. The decrease in conductivity at higher temperature might be caused by the destruction of the conductive network from the melting and disconnection of the AgNWs. After heat treatment at 155 °C for 30 min, the initial conductivity of the 38.0 wt% S-AgNWs/PU composite fiber increased from 1,656 S/cm to 4,962 S/cm. The highest electrical conductivity of S-AgNWs/PU fiber (75.9 wt%) reached 14,205 S/cm. These initial conductivities were higher than the reported data for the composite fibers with similar content of symmetrical Ag particles (Table S3). However, the AgNWs/PU fibers showed a limited improvement of conductivity after heat treatment; the conductivity of 24.1 wt% AgNWs/PU fiber increased slightly from 147 S/cm to 173 S/cm when it was heated at 155 °C for 30 min. Heat treatment also improved the stretchability of the composite fibers. Figure 4(b) and (c) showed the conductivities and the conductivity degradation rates of SAgNWs/PU fibers after heat treatment at 155 °C for 30 min with different AgNW mass fractions as a function of tensile strain. The conductivities degraded more rapidly with a low mass fraction of AgNWs. For example, with a lower mass fraction of AgNWs, such as 14.6 wt% and 24.4 wt%, the composite fibers lost conductivity at the strain of 50% and 70%, respectively. However, in the case of 33.1 wt% and 38.0 wt% S-AgNWs/PU composite fibers, the degradation rates were 0.3% and 2.8% at 100% strain, respectively. The 38.0 wt% S-AgNWs/PU composite fiber still had a conductivity of 17.8 S/cm even it was stretched to 200%. The conductivity degradation rate for 38.0 wt% S-AgNWs/PU composite fiber (2.8% at 100% strain) was between the conductivity degradation rate of the Ag nanoflower/PU composite fiber (approximately 1.5% at 100% strain41) and that of the AgNW/AgNP/SBS composite fiber (4.4% at 100% strain35). The mechanical properties of the composite fibers were studied by uniaxial tensile tests. All of the composite fibers exhibited a tensile behavior reminiscent of an elastomeric material. This tensile behavior derived from the unique thermoplastic PU chain composition; the randomly segmented hard and soft segments formed a two-phase microstructure, where the soft segments provided high elongation and rubber-like behavior while the hard segments provided stiffness and physical cross-linking points.52.53 The mechanical properties of the composite fibers

Figure 4. (a) The electrical conductivities of the cured composite fibers with different curing temperature as a function of the uniaxial tensile strain. (b) The electrical conductivities and (c) the conductivity degradation rates of S-AgNWs/PU fibers cured at 155 °C for 30 min as a function of tensile strain.



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contributed both from the PU matrix and the fillers. The addition of 1-D AgNWs with high modulus and strength effectively reinforced the composite fiber, and the mechanical properties of the AgNWs/PU composite fiber could be further enhanced by heat treatment because the AgNWs, in loose contact at first, were slightly sintered with one another. The typical strain-stress curves for the 38.0 wt% SAgNWs/PU fibers at different curing temperatures for 30 min were shown in Figure 5(a). Young’s modulus and the ultimate strength of the 38.0 wt% S-AgNWs/PU fiber increased from 49.7 to 78.0 MPa and 11.0 to 17.7 MPa, respectively, as the curing temperature increased to 175 °C, whereas the rupture strain of the composite fiber decreased from 533% to 350%. Therefore, considering that the composite fibers heated at 155 °C revealed an improved strength and high rupture strain along with high electrical conductivity, the following samples for mechanical property tests were all cured at 155 °C for 30 min. Even though the addition of AgNWs increased the modulus and strength of the composite fiber, that addition led to decreased rupture strain. For example, Young’s modulus of the S-AgNWs/PU fibers was increased from 6.77 to 116.5 MPa, and the ultimate strengths were in the range from 13.5 to 18.9 MPa with the mass fractions of S-AgNWs increased from 14.6 wt% to 50.1 wt%. However, the rupture strains decreased from 766% to 145% (Figure 5(b)). Figure 5(c) showed the strain-stress curves for the composite fibers with different mass fractions of AgNWs and SAgNWs. The surface modification of AgNWs with PEG derivatives also improved the mechanical performance of the composite fibers. Typically, with similar mass-fractions, the 24.4 wt% S-AgNWs/PU fiber had an ultimate strength of 18.9 MPa and rupture strain of 694%, respectively, higher than those parameters of the 24.1 wt% AgNWs/PU fiber (12.4 MPa and 546%, respectively). Notably, the highest rupture strain of S-AgNWs/PU fiber was 766% (14.6 wt% S-AgNW), only slightly lower than the rupture strain of PU fiber (798%). In addition, the composite fiber of 38.0 wt% S-AgNWs/PU still had a high rupture strain of 348%, and the composite also showed a relatively high Young’s modulus (59.8 MPa) and ultimate strength (16.7 MPa) as well. Textile-based electronics that can be worn directly on the body should endure repeated deformation of bending, twisting, abrasion, compression and stretching for daily life human actions typically in the range of 3% ~ 55% strain.3 In addition, the conductive fibers should also be capable of withstanding the strain of the weaving or knitting process for the fabrication of fabrics. As a result, the mechanical and electrical reliability and durability of the conductive fibers under repeated stretching and releasing are a crucial issue in textile-based wearable electronics. The PU used in this study is a thermoplastic elastomer. There are no chemical crosslinks, and the hard segment phase plays the role of physical crosslinks through van der Waals interactions or hydrogen bonding.54 When a deformation is applied to the physically cross-linked PU, coiled

Figure 5. (a) The typical strain-stress curves for the 38.0 wt% S-AgNWs/PU fiber without curing and cured at different temperatures. (b) The modulus and rupture strain of the composite fibers as a function of the mass fraction of S-AgNWs. (c) The typical strain-stress curves for the composite fibers with different AgNW mass fraction cured at 155 °C for 30 min.


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Figure 6. The normalized resistance changes of (a) 38.0 wt% S-AgNWs/PU and (b) S-AgMix/PU for a cyclic tensile strain test for 20% strain. The composite fibers were pre-stretched to 20% strain.

or folded polymer chains in the soft segment will be extended. As the strain increases, stress can be transferred to the hard segment, and some kind of chain slippage between the hard segments will occur, resulting in plastic deformation.55 The polymer chains can align in the extension direction showing strain hardening that cannot be corrected after removal of stress. Therefore, some irreversible changes in strain were observed when the composite fiber was released from stretching.56 However, during elongation, polymer chains are likely to align along the extension direction, and the crystalline areas of the soft segments will develop more, yielding the formation of an ideal elastic network.57,58 Therefore, the pre-stretching can improve the elasticity of the PU, resulting in completely reversible deformation behavior within the pre-stretched strain level. To evaluate the reliability of our composite fiber under repeated deformation, the composite fiber with 38.0 wt% S-AgNWs/PU was pre-stretched to 20% strain (from 5 cm to 6 cm) and released. The length of the fiber after the prestretching process showed an increase of 2 mm, and the initial electrical conductivity decreased from 4,962 S/cm to 3,071 S/cm. Then, the composite fiber was stretched and released 50 times for 20% strain, and the normalized resistance changed, ΔR/R0 (ΔR = R - R0, where R is the resistance and R0 is the initial resistance, respectively) were monitored under stretching/releasing cycles (Figure 6(a)). In each tensile stretching/releasing cycle, ΔR/R0 increased with the strain and decreased when the fiber was released. However, the resistivity could not recover to the initial state, while the length could completely recover to the initial length. During the first 15 cycles of the 50 cyclic deformations, ΔR/R0 at 0% strain (ΔR0%/R0) increased considerably from 0 to 0.20. Then, the ratio showed a moderately increasing tendency for ΔR0%/R0 during the following repeated stretching and releasing tests. After 50 cycles of the stretching and releasing process, ΔR0%/R0 was increased to 0.37. The increase of ΔR/R0 might be caused by the poor stability of the conducting network, specifically by the irreversible slide of the AgNW contact point under

deformation. Some recent research studies have revealed that a combination of 1-D and 2-D conductive materials could enhance the mechanical and electrical stability of conductive films and composites.59 Therefore, to improve the stability of S-AgNW/PU composite fiber during the cyclic stretching/releasing test, 2-D Ag nanoplates were added to the composite fiber. Ag nanoplates with a lateral size between 0.4 and 1.2 μm and thickness of approximately 30 nm were synthesized according to the literature60 and chemically surface-modified with a procedure the same as the AgNWs (Figure 7(a), see the Supporting Information for details). A PU composite fiber with the mixture of AgNWs and Ag nanoplates (S-AgMix/PU) was obtained from the co-spinning of S-AgNWs and surface-modified Ag nanoplates at a mass ratio of 7:3. (See the Supporting Information for details) The S-AgMix/PU fiber almost kept the same surface morphology as the S-AgNWs/PU fiber, showing a folded surface of irregular longitudinal pleats with a similar average diameter of approximately 140 μm (Figure 7b). Figure 7c showed the SEM image of a magnified surface of the S-AgMix/PU fiber. Some uniformly dispersed S-AgNWs and S-Ag nanoplates could be seen in the PU matrix without agglomeration. The mass fraction of the mixed fillers (the total Ag content) in the composite fiber determined from the TGA weight loss was 54.3 wt% (Figure S9), almost the same as the relative mass fraction of Ag in the dope. After heat treatment at 155 °C for 30 min, the electrical conductivity of the S-AgMix/PU composite fiber increased from 2,247 S/cm to 5,262 S/cm and decreased to 3,652 S/cm through pre-stretching for 20%. Figure 6(b) showed the ΔR/R0 of the S-AgMix/PU composite fiber for 50 cycles of stretching and releasing with the tensile strain of 20%. Compared with the SAgNWs/PU composite fiber, the ΔR/R0 for the S-AgMix/PU composite fiber increased moderately. After 50 cycles of stretching and releasing, ΔR0%/R0 was 0.13, much lower than the ΔR0%/R0 of S-AgNWs/PU. Accordingly, the reliability of the composite fiber under cyclic tensile strain showed an improvement after adding the S-Ag nanoplate due to the synergistic effect of S-AgNWs and S-Ag nanoplates.



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Figure 7. (a) SEM image of Ag nanoplates. The inset is a highly magnified SEM image that shows the thick of an arbitrary Ag nanoplate. (b) SEM image of the S-AgMix/PU composite fiber. (c) Highly magnified SEM image of the surface of the S-AgMix/PU composite fiber. (d) Schematic illustration of the evolution of (i) S-AgNW/PU and (ii) S-AgMix/PU fibers during the cyclic tensile strain tests.

The conductivity hysteresis was supposed to be caused by the instability of the AgNW network under repeated stretching and releasing. Specifically, some contact sites of the S-AgNWs in the PU matrix were disrupted because the AgNWs were unable to return to the original position even after the full recovery of the PU fiber from deformation due to the friction forces between stiff AgNWs and soft PU polymers. The slippage of AgNWs damaged the pristine electrical conductive network and resulted in conductivity deterioration. In comparison, the addition of S-Ag nanoplates enlarged the contact area of the fillers; 2-D Ag nanoplate acted as a bridge to connect the S-AgNWs that separated due to slippage. The synergistic action of the hybrid SAgNWs/S-Ag nanoplates in the composite fiber statistically reduced the degradation of the conductive network and prevented the large increase of resistance under the cyclic tensile strain test (Figure 7(d)). Collectively, our composite fibers showed relatively high electrical conductivity and excellent stretchability. All these performances stem from the AgNWs characteristic 1D structure and inherent high conductivity, as well as the highly effective conductive networks from the enhanced compatibility of S-AgNWs and PU elastomers. We also proposed a prospective solution for the hybrid fillers of AgNWs and Ag nanoplates to ameliorate the conductivity deterioration from repeated stretching and releasing due to the unrecoverable slippage of AgNWs. Apparently, this attempt enhanced the recoverability after stretching to a certain extent. However, the complete elimination of the conductivity hysteresis of AgNWs/PU composite fiber still

required further exploiting. The permanent chemical crosslinking of polymer chains could be expected to ensure complete elastic recovery of the fiber. Therefore, the reliability of the composite fiber would be enhanced through appropriate chemical crosslinking of the polymer matrix and the conductive fillers that could be achieved by employing some thermoset elastomers, such as polybutadienes, butyl rubber or silicone, etc., accompanied by surface modification agents with crosslinkable moieties. Furthermore, previous studies have proven that some yarns and fabrics with rationally designed architecture from the twisting, weaving and knitting of the conductive fibers have shown excellent stretchability and electrical stability. The studies on the chemically crosslinked composite fibers, weaving/knitting of the conductive fibers and applications of these fibers on wearable devices are in progress in our group.

▪ CONCLUSION In this study, a stretchable conductive composite fiber was prepared by co-spinning of elastic PU and surface modified AgNWs. The AgNWs after surface modification with a PEG derivative showed a wide dispersion in organic solvents and improved compatibility with the PU matrix. The effective dispersion of S-AgNWs in the PU matrix made the composite fibers show enhanced electrical and mechanical performance. The highest S-AgNWs mass fraction could be as high as 75.9 wt%, and the fiber showed the highest initial electrical conductivity of 14,204 S/cm. With the volume fraction of S-AgNWs as low as 1.8 vol% (14.6 wt%), the SAgNWs/PU composite fiber formed a conductive


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percolation network, and the fiber showed the maximum rupture strain of 766%. The 38.0 wt% S-AgNWs/PU composite fiber exhibited electrical conductivity of 4,962 S/cm and a rupture strain of 348% as well as a high conductivity degradation rate of 2.8% for 100% strain. The addition of SAg nanoplates could further improve the reliability of the S-AgNWs/PU composite fiber under the cyclic tensile strain test, and the composite fiber showed ΔR0%/R0 of 0.13 after 50 cycles of 20% stretching and releasing.

▪ MATERIALS AND METHODS Materials. Dichloromethane (DCM, 99.8%), poly (vinylpyrrolidone) (PVP, Mn = 360 000 g/mol), N, N’-dicyclohexylcarbodiimide (DCC, 99%), 4-(dimethylamino) pyridine (4-DMAP, 99%), polyethylene glycol methyl ether (mPEG, Mn = 5000 g/mol), ZnCl2, AgNO3 and thioctic acid (TA, 98%) were obtained from Sigma-Aldrich. Thermoplastic polyurethane (PU, 1185A) pellets were obtained from BASF. Acetone (99.5%), ethyl ether (99%), N, N-dimethylformamide (DMF, 99%), and methanol were purchased from Daejung Chemicals & Metals Co. Ltd., Korea. Ethylene glycol (EG, 99.5%), toluene (99.8%), and tetrahydrofuran (THF, 99%) were obtained from Samchun Pure Chemical Co., Ltd., Korea. AgNW synthesis. AgNWs were synthesized by a modified polyol process. Briefly, 3.5 g PVP and 80 mg ZnCl2 were added to 300 mL EG and heated to 170 °C with magnetic stirring to dissolve PVP. Then, 20 mL of a 0.2 mol/L AgNO3 solution in EG was added to the solution rapidly to form nucleation of Ag seeds. Next, 80 mL of a 0.2 mol/L AgNO 3 solution in EG was injected into the solution drop by drop at a rate of 1 mL/min. After all of the AgNO3 solution was injected, the solution was heated for an additional 30 min and then cooled to room temperature. The AgNWs were purified with acetone and methanol and centrifuged at 3 000 rpm for 10 min to remove EG and excess PVP. Synthesis of TA-mPEG. The synthesis was carried out with standard Schlenk techniques under N2 protection. Typically, 5.00 g mPEG, 0.268 g TA, 0.0305 g 4-DMAP and 16 mL DCM were added to a flask and degassed under N2. Next, the solution was cooled to 0 °C in an ice bath under magnetic stirring. Then, 2 mL of a 0.65 mol/L DCC solution in DCM was injected dropwise. The reaction mixture continued to react in an ice bath for 2 h and was then allowed to stir for 24 h at room temperature. The light-yellow byproduct of dicyclohexylurea was removed by filtration. Next, the solution was precipitated by adding to ethyl ether slowly to obtain crude polyethylene glycol methyl ether with thioctic acid appended (TA-mPEG). The crude TAmPEG was purified three times by dissolving in DCM and precipitated in ethyl ether. The residual solvent in the

product was removed under vacuum for several hours to obtain TA-mPEG. Surface Modification of AgNWs. 2.4 g TA-mPEG was dissolved in 60 mL methanol with 5 vol% deionized water. Methanol solution (60 mL) with 0.30 g AgNWs was added to the mixture and stirred with a magnetic stirrer in the dark for 21 h. The solution was purified three times with acetone and deionized water and centrifuged at 3 000 rpm for 10 min to obtain surface-modified AgNWs with TAmPEG (S-AgNWs). Sedimentation Test. A methanol solution of AgNW or SAgNWs (20 μL) with a concentration of 0.05 g/mL were separately pipetted into 2 mL of organic solvents methanol, DMF, acetone, THF, and toluene. The solutions were mixed using ultrasonication and then rested to monitor periodically the dispensability of AgNWs in the organic solvents. Photographs of the NWs dispersions were collected after different time intervals. Fabrication of stretchable and conductive composite fibers. The high-performance stretchable conductive fibers were fabricated by wet spinning technology. First, different amounts of S-AgNWs were dispersed in 2 mL DMF solvent by ultrasonication. Then, 0.2 g PU pellets were dissolved in the S-AgNWs/DMF solutions by magnetic stirring overnight to form spinning dopes. The mass fraction of PU in the solvent was kept at 10 wt% for favorable spinning. In the next step, a spinning dope was injected into a coagulation bath of deionized water through a 23 G detachable needle of 0.33 mm inner diameter by a syringe pump (KDScientific, KDS-100). The flow rate of the injection was 15 mL/h, and the fiber was continuously collected on a stainless spool. The wet-spun fiber was immersed in deionized water for 12 h for further curing and then dried in air to obtain an S-AgNWs/PU composite fiber. The mass fraction of S-AgNWs in the dope (the relative mass fraction of S-AgNWs to the solid contents of the dope) was evaluated by thermogravimetric analysis (TGA) after complete removal of DMF. The mass fraction of S-AgNWs in the composite fiber was also assessed by TGA. The AgNWs/PU composite fibers were fabricated with the same procedures by only replacing S-AgNWs with AgNWs. Characterization. Field emission scanning electron microscopy (FE-SEM, JEOL JSM-7610F) was performed at an accelerating voltage of 10 kV after sputter coating with platinum on the sample surface. The cross-section image of the fibers was obtained by breaking the frozen fibers after immersing in liquid nitrogen for 1 min. The cross-sectional areas of the fibers were determined from SEM images using ImageJ image analysis software. X-ray diffraction (XRD, Rigaku, Mini Flex 600 diffractometer)

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measurements were performed at a voltage of 40 kV and current of 40 mA with Cu Kα radiation (λ= 1.5418 Å) with a scanning rate of 0.02°/s in 2θ ranging from 10° and 90°. UVvis absorption spectra of AgNWs were obtained using a UV-visible spectrometer (Shimadzu, UV-2600). AgNWs were dispersed in methanol and ultrasonicated for 30 seconds. The spectra were collected with quartz cuvettes with wavelength from 200 nm to 700 nm and a step of 5 nm. 1HNMR spectra were recorded on a Bruker BioSpin 400 MHz spectrometer, and the chemical shifts were reported relative to the TMS peak. The mass fractions of Ag in AgNWs, spinning dopes and composite fibers were determined by TGA using a temperature sweep from 25 to 600 °C with a heating rate of 10 °C/min under an N2 atmosphere (Netzsch, STA 409). An optical microscope (Nikon MM-400, Japan) was used for measuring surface profiles of fibers in both transmission and reflection mode. Fiber diameters under different strains were measured as the average of 10 points along the fiber length using an optical microscope (Nikon MM-400, Japan). The mechanical properties of PU and composite fibers were measured using Nano UTM (MTS Corporation). The load capacity was 500 mN, and the load resolution was 50 nN. The nominal gauge length of the fiber sample was 10 mm. The strain rate was set to 5% s-1, and all the fiber samples were stretched until failure. The test was performed in a laboratory environment, and 4 or 5 replicate samples were measured for each test condition. The electrical resistance of the composite fibers as a function of tensile strain was measured using the fourprobe method (Keithley 2400 source meter) and an inhouse-built stretching device. The conductivity of the fiber was calculated by the following equation.

𝜎=

𝑙 𝑅𝑆

where σ is the electrical conductivity of the fiber, l is the length of the fiber, R is the resistance and S is the crosssection area of the fiber. Three replicate samples were prepared and measured.

ASSOCIATED CONTENT Supporting Information. 1H-NMR spectroscopy of mPEG and TA. Size distribution, SEM image, XRD patterns, UV-vis absorptions and TGA curves of AgNWs and S-AgNWs. Crosssectional SEM image of AgNWs/PU fiber. Synthetic method and XRD patterns of Ag nanoplates with and without surface modification. Synthetic method and the TGA curve of the SAgMix/PU composite fiber. The calculation of the solubility parameters of PVP and TA-mPEG. The comparison of the electrical conductivities of composite fibers. The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

Author Contributions All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was supported by Basic Science Re-search Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education(2017035300) and the research grant of the Kongju National University in 2015.

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High-Performance Stretchable Conductive Composite Fibers from

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