Extraordinarily High Conductivity of Stretchable Fibers of Polyurethane

Rujun Ma,† Byeongguk Kang,‡ Suik Cho,‡ Minjun Choi,‡ and Seunghyun Baik*,†,‡

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Extraordinarily High Conductivity of Stretchable Fibers of Polyurethane and Silver Nanoflowers †

Center for Integrated Nanostructure Physics, Institute for Basic Science (IBS), and ‡School of Mechanical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea

ABSTRACT Stretchable conductive composites have received considerable attention recently,

and they should have high conductivity and mechanical strength. Here we report highly conductive stretchable fibers synthesized by the scalable wet spinning process using flower-shaped silver nanoparticles with nanodisc-shaped petals (Ag nanoflowers) and polyurethane. An extraordinarily high conductivity (41 245 S cm
1) was obtained by Ag nanoflowers, which is 2 orders of magnitude greater than that of fibers synthesized using spherical Ag nanoparticles. This was due to the enhanced surface area and vigorous coalescence of nanodisc-shaped petals during the curing process. There was a trade-off relationship between conductivity and stretchability, and the maximum rupture strain was 776%. An analytical model revealed that the enhanced adhesion between Ag nanoflowers and polyurethane provided a high Young's modulus (731.5 MPa) and ultimate strength (39.6 MPa) of the fibers. The fibers exhibited an elastic property after prestretching, and the resistance change of weft-knitted fabric was negligible up to 200% strain. The fibers with extraordinarily high conductivity, stretchability, and mechanical strength may be useful for wearable electronics applications. KEYWORDS: silver nanoflowers . polyurethane . stretchable conductive fibers . elasticity . strength

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tretchable conductive composites have received considerable attention for metallic parts of stretchable electronic devices.1
10 In order to achieve both conductivity and stretchability, conductive solid fillers were typically embedded in a stretchable polymer matrix in the form of bulk composites,6,9 thin films,1
3,5,7,8,10 and fibers.4 Different types of conductive fillers were investigated to enhance conductivity. Carbon nanotubes (CNTs) and graphene fillers provided conductivities (2.8
125 S cm
1)1,6,8
11 lower than metallic (5450
11 000 S cm
1)2,5 or carbon
metal hybrid fillers (3700
17 460 S cm
1).3,4,7 However, carbon-based fillers are of interest because some electrolytes react with metals, which is problematic for energy conversion or storage devices. On the other hand, a variety of polymers were investigated to enhance adhesion with conductive fillers and stretchability up to several hundred percentages. These include polyurethane (PU),2,8 poly(dimethylsiloxane) (PDMS),9,10 poly(styrene-block-butadiene-block-styrene) MA ET AL.

(SBS),5 and polystyrene
polyisoprene
polystyrene (SIS).7 Ionic liquid was also mixed with polymer to further enhance conductivity and stretchability.1,3,4,7,8,12 The types of conductive fillers and polymers, maximum conductivity, and stretchability in the literatures are summarized in Supporting Table S1. There has been significant improvement in conductivity and stretchability during the past decade, although these properties exhibited a trade-off relationship with regard to the conductive filler concentration.2
5,7 The remaining challenges for practical applications of stretchable conductive composites can be discussed as follows. First, the ultimate goal of high conductivity would be that of copper (5.96  105 S cm
1),13 which is widely used in conventional rigid electronic devices, although successful operation of light-emitting diodes,1,3,4 robotic sensors,4 and electronic devices1,12 was demonstrated in the literature with lower conductivities. A higher conductivity would enable faster operation and lower energy VOL. XXX

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* Address correspondence to [email protected]. Received for review June 24, 2015 and accepted October 15, 2015. Published online 10.1021/acsnano.5b03864 C XXXX American Chemical Society

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consumption of electronic devices. Therefore, the conductivity of stretchable composites needs to be further improved. Second, the current stretchable composites provide more than enough stretchability for wearable electronic devices given that the typical maximum strain induced by human motion is about ∼55%.14 However, it is more challenging to maintain high conductivity over a wide strain region.3,4 Third, the mostimperative but least-investigated property is mechanical strength.15
17 Only a small number of studies reported first cyclic stress
strain behavior.2,5,7,8 High Young's modulus, ultimate tensile strength, and elasticity need to be achieved for practical applications. Finally, a scalable fabrication process with environmentally friendly chemical elements would be important. Here we report highly conductive stretchable fibers synthesized by the scalable wet spinning process using flower-shaped silver nanoparticles with nanodisc-shaped petals (Ag nanoflowers) and PU (Ag-PU fibers). Silver-filled PU nanocomposites18 and the elastic property of nanocomposites11 were previously reported. We also previously investigated conductive fibers using the combination of CNTs and spherical Ag nanoparticles.4 However, this work is the first report of employing Ag nanoflowers for conductive composites. This single-type filler was simpler than the combination of multiple-type fillers in the fabrication process. In addition, the electrical conductivity and mechanical properties were excellent. An extraordinarily high initial conductivity of 41 245 S cm
1 (curing temperature = 155 °C, Ag concentration = 39.5 vol %, rupture strain = 90%) was obtained by Ag nanoflowers, which was 2 orders of magnitude greater than that of fibers with spherical Ag nanoparticles. This was achieved due to the active coalescence of nanodiscshaped petals (thickness ≈ 8 nm) during the curing process. There was a trade-off between maximum initial conductivity and stretchability with regard to the curing temperature and Ag concentration. The maximum rupture strain could be increased by lowering the curing temperature (209% at 115 °C, Ag concentration = 39.5 vol %) or decreasing the filler fraction (776% at 135 °C, Ag concentration = 17.9 vol %). The Young's modulus was as high as 731.5 MPa, and the maximum tensile strength was 39.6 MPa at 776% strain. The fiber exhibited elastic behavior at strains lower than the prestretching strain (120%). The resistance change of weft-knitted fabric wove using Ag-PU fibers was negligible up to 200% strain with excellent cyclability. High mechanical strength was achieved by PU, which was explained by an analytical model revealing enhanced adhesion between Ag nanoflowers and PU. We previously reported stretchable conductive fibers synthesized by the wet spinning process using spherical Ag nanoparticles, CNTs, and PVDF-HFP

Figure 1. Synthesis of conductive stretchable Ag-PU fibers. (a) Wet spinning apparatus. (b, c) SEM images of Ag nanoflowers. (d) Fiber collected on a winding drum (length ≈ 50 m). (e) SEM image of the fiber (diameter ≈ 200 μm). (f, g) Cross-sectional SEM images of the fiber.

(Ag-CNT-PVDF-HFP fibers),4 although mechanical properties were not characterized. This new work on Ag-PU fibers provided significant enhancement compared with Ag-CNT-PVDF-HFP fibers. For simplicity, the maximum properties of each type of fiber were compared in this paper since there was a trade-off relationship and the values were not obtained using the fibers synthesized in the same conditions. The initial conductivity was improved by 394% (from 8344 to 41 245 S cm
1) without involving the hot rolling process, and the maximum strain was increased by 58.4% (from 490% to 776%).4 We also synthesized fibers using Ag nanoflowers and PVDF-HFP (Ag-PVDF-HFP fibers) and compared mechanical properties. The maximum Young's modulus of Ag-PU fibers was enhanced by 1399% (from 48.8 MPa to 731.5 MPa), and the ultimate strength was increased by 198% (from 13.3 MPa to 39.6 MPa). The wet spinning process was less toxic since CNTs, an ionic liquid, and a hexane bath were not used in the synthesis of Ag-PU fibers. However, dimethylformamide (DMF) was still used as a major solvent for PU pellets. In order to address this concern, we experimentally confirmed that dimethylacetamide (DMAC), which is milder than DMF,19 could be used to synthesize Ag-PU fibers with similar property. However, water-based PU needs to be employed in the future for better environmental friendliness. RESULTS AND DISCUSSION Synthesis of Conductive Stretchable Ag-PU Fibers. Figure 1a shows the wet spinning process of making Ag-PU fibers (see Methods for details). Wet spinning is a scalable technology widely employed in industry.4,20 Briefly, PU pellets (5 wt %) were dissolved in DMF by ultrasonication. Ag nanoflowers were then added into the PU solution, ultrasonicated, and stirred in order to form a spinnable dope without using an ionic liquid. Figure 1b and c show scanning electron microscopy (SEM) images of Ag nanoflowers with nanodisc-shaped petals (average bud diameter ≈ 800 nm, petal diameter ≈ 160 nm, petal thickness ≈ 8 nm). Additional and VOL. XXX

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Archimedes method (electronic densimeter MD-300S), and the corresponding relative density was 0.859 (see Supporting Information for details). The fibers produced by the wet spinning process typically have some void fraction.24,25 The energy dispersive X-ray (EDX) analysis at the cross-section confirmed the existence of Ag (Supporting Figure S6). Longitudinal and cross-sectional SEM images of the Ag-PU fiber before and after the curing were comparatively analyzed in Supporting Figure S7. As shown in Supporting Figure S7a, Ag nanoflowers were uniformly distributed, and there was no debonding between Ag nanoflowers and PU, indicating good adhesion. The edge of the petals was not as sharp as raw Ag nanoflowers, as shown in the magnified images, probably due to the deformation during the sonication process. The coalescence of Ag nanoflowers could be clearly observed after the curing at 135 °C even though Ag nanoflowers were embedded in the PU matrix (Supporting Figure S7b). This is consistent with a large conductivity increase of Ag-PU fibers after the curing process, as will be discussed in Figure 2a. As the Ag-PU fiber was stretched to 50% strain (Supporting Figure S7c), the distance between Ag nanoflowers was increased, and elongation of PU could be observed. This explains the decrease in conductivity of Ag-PU fibers with stretching (Figure 2a). As a control, longitudinal and cross-sectional SEM images of the spherical Ag nanoparticle-PU fiber were also analyzed in Supporting Figure S8. The spherical Ag nanoparticles (∼90 nm) were uniformly distributed in the fiber although a slight local aggregation of nanoparticles could be observed due to the small size (Supporting Figure S8a). Unlike Ag nanoflowers, the active coalescence of spherical Ag nanoparticles could not be observed after the curing at 135 °C (Supporting Figure S8b). The increased distance between spherical Ag nanoparticles could also be observed at 50% strain (Supporting Figure S8c). The thermal stability of Ag-PU fibers was investigated by DSC and thermogravimetric analysis (TGA) (see Supporting Information Figure S9). Unlike carbon-nanotube-incorporated polymer matrix composites,8,11 the thermal stability of fibers characterized by TGA did not change by the incorporation of Ag nanoflowers. The temperature corresponding to the onset of decomposition was invariant. Besides, there was no apparent change in crystallization and melting temperatures investigated by DSC. This indicates that Ag nanoflowers did not improve the thermal stability of PU fibers. Electrical and Mechanical Properties of Conductive Stretchable Fibers. Figure 2a shows the conductivity of Ag-PU fibers cured at different temperatures as a function of the tensile strain. The curing time was identical (10 min). The strain-dependent diameter of fibers was experimentally measured and fitted using a second-order VOL. XXX

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false-colored SEM images of Ag nanoflowers are provided in Supporting Figure S1. Ag nanoflowers provided 2 orders of magnitude greater conductivity than other spherical Ag nanoparticles at the identical concentration (see Supporting Figure S2 and Table S2). The surface-roughened, flower-like Ag nanostructures were previously investigated for surfaceenhanced Raman scattering substrates.21,22 However, we are the first to use Ag nanoflowers for conductive composites. The specific surface area of Ag nanoflowers (6.69 m2/g) characterized by Brunauer
Emmett
Teller (BET, BEL Japan, BELSORP-mini II) analysis was greater than the theoretical specific surface area (0.71 m2/g) of spherical Ag nanoparticles with an identical diameter (∼800 nm) due to the enhanced surface roughness. The changes in structure of Ag nanoflowers (∼800 nm) and spherical Ag nanoparticles (∼90 nm) during the curing process were compared in Supporting Figure S3. The specific surface area (5.17 m2/g) of spherical Ag nanoparticles measured by BET analysis was slightly smaller than that of Ag nanoflowers, although the diameter was significantly smaller. However, Ag nanoflowers exhibited much more vigorous coalescence. The nanodisc-shaped petals were coalesced at 135 °C due to the nanometer thickness (∼8 nm). The active surface atoms of nanoparticles decrease the coalescence temperature.23 The sintering temperatures of Ag nanoflowers (124.6 and 195.3 °C), investigated by differential scanning calorimetry (DSC) analysis, were also significantly lower than the melting temperature of bulk silver (961.8 °C), demonstrating active coalescence among Ag nanoflowers (see Supporting Figure S4).23 In the next step, the spinnable dope was extruded through a nozzle (diameter = 310 μm) into a deionized (DI) water bath using a syringe pump (10 mL/h) (Figure 1a inset). The spinning rate (10 mL/h or 2.2 m/min) was smaller than the typical spinning rate (>1000 m/min) used in industry. This was because a precise syringe pump (KDScientific, KDS-100) with a maximum spinning rate of 33 m/min was used in this study. As shown in Supporting Figure S5, the effect of different spinning rate on the conductivity of fibers was very small within the investigated range (2.2
33 m/min). This demonstrates the possibility of scalable fabrication, although the property of fibers synthesized at an industrial spinning rate (>1000 m/min) needs to be investigated in the future. The remnant DMF solvent in the fiber was absorbed in DI water, and wet spun fiber was wound in a drum (Figure 1d, fiber length ≈ 50 m). The fiber was further cured at an elevated temperature, as will be discussed in Figure 2a. The diameter of the fiber was about ∼200 μm (Figure 1e). The diameter of the fibers could be controlled using different size nozzles.4 Crosssectional SEM images confirmed a uniform distribution of Ag nanoflowers (Figure 1f and g). The density of the wet-spun Ag-PU fiber was 4.15 g cm
3, measured by the

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ARTICLE Figure 2. Electrical and mechanical properties of conductive stretchable fibers. (a, b) The conductivity and stress of Ag-PU fibers are shown as a function of strain. The curing temperature was varied from 115 to 155 °C (Ag = 39.5 vol %). The conductivity calculated using a three-dimensional percolation theory is also shown. (c) Initial conductivities and rupture strains of fibers synthesized at different temperatures in this study (Ag-PU fibers: filled symbols; Ag-PVDF-HFP fibers and spherical Ag nanoparticle-PU fibers: half-filled symbols; Ag = 39.5 vol %) are compared with those of control materials (open symbols). Magenta open diamond, magenta open pentagon, magenta open down triangle, magenta open circle, magenta open circle with cross, polymer/CNT composites;1,6,8,12,28 dark yellow open hexagon, dark yellow open right triangle, polymer/metal composites;2,5 orange open left triangle, orange open up triangle, orange open star, orange open square, polymer/Ag/CNT composites.3,4,7 (d) Effect of Ag nanoflower concentration on stretchability and conductivity of Ag-PU and Ag-PVDF-HFP fibers. The curing temperature was fixed at 135 °C. (e) Stress
strain relationship of Ag-PU and Ag-PVDF-HFP fibers (curing temperature = 135 °C). (f) Stress
strain data of Ag-PU and Ag-PVDF-HFP fibers are compared with the modified filler
matrix interfacial bonding model.

polynomial regression equation. The conductivity was calculated using the true cross-sectional area of fibers. The Ag nanoflower concentration was fixed at 39.5 vol %. The initial conductivity increased dramatically, but maximum tensile strain decreased as the fiber was MA ET AL.

cured at a higher temperature. The maximum initial conductivity was 41 245 S cm
1 with a rupture strain of 90% for the fiber cured at 155 °C. The fiber without curing became insulating when it was stretched more than 130% and ruptured at 272%. In all cases, the VOL. XXX

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σ ¼ σ0 (Vf
Vc )s

(1)

where σ is the electrical conductivity of fibers, σ0 is the conductivity of bulk silver, Vf is the volumetric fraction of Ag nanoflowers, Vc is the volumetric fraction at the percolation threshold, and s is the fitting exponent. Vc (0.18) was experimentally obtained from Figure 2d. The volume of the fiber (Vmatrix) increased upon stretching, whereas the volume of Ag nanoflowers (Vsilver) was fixed, decreasing Vf = Vsilver/Vmatrix. This resulted in a decrease in conductivity upon stretching. The experimentally measured Poisson's ratio (0.09) was used to calculate the volume expansion of the Ag-PU fiber (see Supporting equation S7). There was a reasonable agreement between the data and theory, although the model slightly overpredicted the data. This could be due to the poor contact among Ag nanoflowers at the high strain region. The electrical resistance change of a fiber was monitored with time as the strain was increased stepwise by 20% from 0 to 100% (see Supporting Figure S10). The normalized resistance slightly decreased with time at each strain. However, the decrease was small, and the normalized resistance was between 0.96 and 0.99 after 60 min. The conductivity of Ag-PVDF-HFP fibers and spherical Ag nanoparticle-PU fibers as a function of the strain is also provided in Supporting Figure S11. The trend was similar to that of Ag-PU fibers. The initial conductivity at 0% strain increased as the curing temperature increased, and the conductivity decreased with increasing strain. However, the conductivity of Ag-PVDF-HFP fibers was smaller than that of Ag-PU fibers. The conductivity of spherical Ag nanoparticlePU fibers was smaller than that of Ag-PVDF-HFP fibers. Figure 2b shows the true stress
strain behavior of Ag-PU fibers cured at different temperatures. The apparent stress calculated using the fixed initial crosssectional area is also provided for comparison (see Supporting Figure S12). The fiber was disconnected at the center at rupture strain (see Supporting Figure S13). The stress
strain behavior exhibited two distinct regimes. The Young's modulus in the first low-strain regime (0
4%) increased from 96.2 MPa to 731.5 MPa as the curing temperature increased from room temperature to 155 °C (see Supporting Figure S14). However, the change in stress
strain behavior of pure PU fibers was small below 300% strain before and after the curing at 135 °C. This indicates that the large increase in Young's modulus of Ag-PU fibers was dominated by sintering among Ag nanoflowers at higher MA ET AL.

temperatures. The achieved maximum Young's modulus was similar to that of high-density polyethylene (∼800 MPa), which is widely used for drain pipes.27 The critical stress at the onset of the second high-strain regime (>4%) slightly increased as the curing temperature increased. In the second regime, the tangent modulus was significantly smaller than the Young's modulus due to the debonding between Ag fillers and PU matrix increasing interfacial fail and frictional sliding.15
17 The increase in loss modulus of polymer composites after the critical stress, which is a measure of energy dissipation during deformation, was also previously reported in polymer matrix composites.15
17 The ultimate strength increased from 17.6 MPa to 31.2 MPa and rupture strain decreased from 272% to 90% as the curing temperature increased from room temperature to 155 °C. Figure 2c compares the maximum initial conductivity and rupture strain of fibers cured at different temperatures in this study (filled and half-filled symbols) with those of control materials in the literature (open symbols).1
7,12,28 In general, there was a trade-off between initial conductivity and maximum rupture strain. Carbon nanotube-incorporated polymer composites provided higher stretchability but lower conductivity. These include CNT-vinylidene fluoridehexafluoropropylene film,1 CNT-vinylidene fluoridetetrafluoroethylene-hexafluoropropylene film,12 CNTcotton textiles,6 and CNT-PU film.8,28 Greater conductivities could be achieved by metallic nanoparticles or metal
carbon hybrids with the sacrifice of stretchability. The Au-PU film,2 Ag-poly(styrene-block-butadieneblock-styrene) mat,5 Ag-CNT-PVDF-HFP film,3 Ag-CNTSIS film,7 and Ag-CNT-PVDF-HFP fiber4 belong to this category. The Ag-PU fiber cured at 155 °C provided an extraordinarily high conductivity of 41 245 S cm
1 with a rupture strain of 90%, which is high enough to be applied for wearable electronics.14 The rupture strain could be increased to 209% and initial conductivity decreased to 3934 S cm
1 by decreasing the curing temperature to 115 °C. The initial conductivities and maximum tensile strains of Ag-PVDF-HFP and spherical Ag nanoparticle-PU fibers were smaller than those of Ag-PU fibers cured at the identical temperature. For further parametric investigations in this study, the curing temperature was fixed at 135 °C considering the optimum electrical conductivity, tensile strength, and stretchability. The Ag concentration (39.5 vol %) in Ag-PU fibers was higher than the concentration of carbon nanofillers (∼1% or lower) in carbon-incorporated conductive composites.9,10 However, the achieved conductivity of Ag-PU fibers (41245 S/cm) was significantly higher than that (2.8
108 S/cm) of the carbon nanofiller-incorporated composites.9,10 Epoxy-based commercial Ag pastes also employ a high concentration (∼80 wt %) of Ag particles to achieve high conductivity.23 The achievement of high conductivity while VOL. XXX

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conductivity decreased with increasing strain. The three-dimensional percolation model prediction for the Ag-PU fiber cured at 135 °C is shown in Figure 2a. A detailed description of the model was published previously, and a brief summary is provided below (see Supporting Information for details).3,4,26 The conductivity of composites follows the power law relationship.

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relationship was investigated using a modified filler
matrix interfacial bonding model (Figure 2f).29
31 φ ¼

1
Vp φ eβVp 1 þ 2:5Vp m

(2)

where φ is the stress of the composite fiber, φm is the stress of the pure polymer fiber, Vp is the volume fraction of Ag nanoflowers, and β is the adhesion coefficient between Ag nanoflowers and the polymer matrix. The stress of Ag-PU and Ag-PVDF-HFP fibers was analyzed as a function of Vp at different strains in the second regime since the strain range of the first regime was small (0
4%). The comparison between the theory and experimental data at 30%, 70%, and 140% strains is shown in Figure 2f, and the analysis at a wider strain range is provided in Supporting Figures S17 and S18. The analysis revealed that the greater strength of Ag-PU fibers was obtained for two major reasons. First, φm of the pure PU fiber was greater than φm of the pure PVDF-HFP fiber, as shown in Figure 2e. This indicates that the matrix polymer should be strong to achieve the high strength of composite fibers. Second, β was higher for Ag-PU fibers, indicating that the adhesion between Ag nanoflowers and PU was greater than that between Ag nanoflowers and PVDF-HFP. The good adhesion between polyurethane and conductive fillers was also demonstrated in a previous study.8 We developed the strain (ε)-dependent β by fitting the experimental data. β ¼ βo
Cε

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decreasing Ag concentration would be an important research topic in the future. Figure 2d compares the maximum initial conductivity and rupture strain of fibers synthesized using two different polymers: PU and PVDF-HFP (see Methods for detailed synthesis process). The conductivity increased but stretchability decreased as the volume fraction of Ag nanoflowers increased. The Ag-PU fibers provided significantly higher conductivity than Ag-PVDF-HFP fibers at the equivalent Ag nanoflower concentration. At the highest concentration of Ag nanoflowers (∼46.5 vol %), the conductivity of the Ag-PU fiber (24 480 S cm
1) was 463% greater than that of the Ag-PVDF-HFP fiber (4351 S cm
1). The stretchability was also dependent on the Ag nanoflower concentration. At the lowest or highest Ag concentrations, Ag-PU fibers provided greater stretchability than Ag-PVDF-HFP fibers. However, Ag-PVDF-HFP fibers provided greater stretchability when Ag concentration was 30
38%. The maximum stretchability of Ag-PU fiber was 776% at Ag = 17.9 vol %. Figure 2e compares the true stress
strain relationship of Ag-PU and Ag-PVDF-HFP fibers. The pure PU fiber provided a high rupture strain of 995% with an ultimate strength of 66.1 MPa. However, the slope between stress and strain was very small below 300%. The rupture strain decreased and two distinct regimes discussed in Figure 2b became more apparent as the concentration of Ag nanoflowers increased. The Young's modulus and ultimate strength of Ag-PU and Ag-PVDF-HFP fibers are shown as a function of the Ag nanoflower concentration in Supporting Figure S15. The ultimate strength initially decreased with the addition of Ag nanoflowers but increased again when the Ag concentration was 39.5 vol %. The Young's modulus increased with the addition of Ag nanoflowers, resulting in a maximum of 549.6 MPa with an ultimate strength of 24.9 MPa at Ag = 39.5 vol %. A further addition of Ag nanoflowers decreased both the Young's modulus and ultimate strength. Apparently, the Ag-PVDF-HFP fibers provided a smaller Young's modulus and ultimate strength than Ag-PU fibers. The maximum ultimate strength of Ag-PVDF-HFP fibers was 13.3 MPa (Ag = 32.1 vol %), which is much smaller than that of Ag-PU fibers (39.6 MPa at Ag = 17.9%). The mechanical stress
strain relationship of a single, nonconductive, stretchable, commercial fiber is also provided in Supporting Figure S16. The ultimate tensile strength was 46.7 MPa, which was 18% greater than that (39.6 MPa) of the Ag-PU fiber (Ag = 17.9 vol %). The similar order in tensile strength demonstrates the potential of Ag-PU fibers for practical applications. The mechanical strength could be further enhanced by making ropes, as will be discussed in Figure 4f. As shown in Figure 2e, Ag-PU fibers were stronger than Ag-PVDF-HFP fibers. The mechanical stress
strain

(3)

where βo (10 for Ag-PU fiber and 8.6 for Ag-PVDF-HFP fiber) and C (strain dependence, 1 for Ag-PU fiber and 0.1 for Ag-PVDF-HFP fiber) are empirical constants. βo and C were greater for Ag-PU fibers than Ag-PVDF-HFP fibers. β decreased as ε increased, indicating a decreased interfacial bonding at high strains. The fibers synthesized by wet spinning typically exhibit plastic deformation upon stretching.24 The elasticity can be significantly improved by prestretching the fibers after the wet spinning process.24 Here Ag-PU fibers (Ag = 39.5 vol %, curing temperature = 135 °C) were prestretched up to 120% strain (from 10 mm to 22 mm) and released. The fiber length after releasing was 13 mm, demonstrating plastic deformation. As shown in Figure 3a, the initial conductivity of the fiber was decreased to 4056 S cm
1 after the prestretching cycle due to the finite increase in length. The decrease in conductivity with increasing length is also explained in Figure 2a. Nevertheless, this conductivity was still greater than those of other CNT-based films and fibers.1,6,12,28 After the prestretching cycle, the fiber exhibited excellent elastic behavior during the first stretching cycle up to ∼70% strain. There was no hysteresis in conductivity during the stretching and releasing process. Figure 3b shows the first cyclic stress
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ARTICLE Figure 3. Elastic performance of Ag-PU fibers (Ag = 39.5 vol %, curing temperature = 135 °C) after the prestretching at 120% strain and releasing. (a) Conductivity
strain relationship of a Ag-PU fiber. (b) Strain of a Ag-PU fiber that was progressively increased and released successively using the same fiber. (c) Conductivities of three Ag-PU fibers shown as a function of the stretching cycles. The maximum tensile strains were 8.5%, 30%, and 70%. The inset images show a stretching cycle (maximum strain = 70%). (d) Three cyclic stress
strain tests were carried out using a Ag-PU fiber at 8.5% strain. The maximum strain was successively increased to 30% and 70% using the same fiber (3 cycles at each maximum strain).

temperature = 135 °C) after the 120% prestretching cycle. The maximum strain of the same fiber was progressively increased from 5% to 75%. The prestretched fiber demonstrated excellent mechanical elasticity up to 75% strain, and the length of the fiber returned to the initial value after releasing the tensile stress. The elastic modulus of the prestretched fiber was smaller than the Young's modulus shown in Figure 2b. This was due to the interfacial fail and frictional sliding between Ag nanoflowers and PU during the 120% prestretching cycle.15 It also caused significant dissipation of energy and strain response lag during the loading and unloading process, leading to a large hysteresis.15,32 Figure 3c shows the conductivity of prestretched Ag-PU fibers at 0% strain as a function of the stretching cycles. Three different maximum strains (8.5%, 30%, and 70%) were investigated using three fibers. There was little change in conductivity after 500 cycles when maximum strains were 8.5% and 30%. However, there was a greater decrease in conductivity when the maximum strain was 70%, although a negligible conductivity MA ET AL.

change was observed in the first stretching cycle (Figure 3a). The optical images during the stretching cycle are also provided (Figure 3c inset). Figure 3d shows cyclic stress
strain curves of a prestretched Ag-PU fiber. The maximum strain of the same fiber was progressively increased from 8.5% to 75% (three cycles at each maximum strain). Nearly complete mechanical elasticity was observed up to 30% strain. However, the fiber was degraded when the maximum strain was 75%, which is consistent with the electrical conductivity analysis in Figure 3c. It is important to note that the electrical and mechanical properties of 120% prestretched fibers were discussed in Figure 3. The properties may be different or improved if different prestretching conditions are employed. This is out of the scope of this study since a variety of different prestretching parameters can be applied.33 Ropes and Weft-Knitted Fabrics Made Using Ag-PU Fibers. The electrical and mechanical properties could be further enhanced by making ropes and fabrics.4,34 Figure 4a shows SEM images of 2-ply and 4-ply ropes. The 2-ply ropes were made by twisting and relaxing VOL. XXX

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ARTICLE Figure 4. Ropes and weft-knitted fabrics made using Ag-PU fibers (Ag = 39.5 vol %, curing temperature = 135 °C). (a) SEM images of 2-ply and 4-ply ropes. (b) Optical and SEM images of the weft-knitted fabric. (c) Optical images of the weft-knitted fabric during the stretching process. (d) Normalized resistance of the single fiber, ropes, and fabric shown as a function of the tensile strain. (e) Normalized resistance of a fabric up to 500 stretching cycles. The maximum tensile strain was 100%. (f) Load
strain relationship of the single fiber, ropes, and fabric.

two Ag-PU fibers, reaching a torsional balance.4,34 The 4-ply ropes were made using two 2-ply ropes. Figure 4b shows optical and SEM images of a weft-knitted fabric (purl stitch) wove using a Ag-PU fiber.4 A very fine stitching was observed. Additional optical images of ropes and fabric are provided in Supporting Figure S19. Figure 4c shows stretching of the weft-knitted fabric. The pore was first deformed, and the fiber started to rupture at 210% strain. The entire fabric was broken at ∼550% strain. As shown in Figure 4d, the normalized resistance of a single Ag-PU fiber increased rapidly with increasing tensile strain. However, the normalized resistance of the single Ag-PU fiber (R/Ro = 211) was much smaller than that of the previously reported single Ag-CNTPVDF-HFP fiber (R/Ro = 6901) at 100% strain.4 The 2-ply and 4-ply ropes provided a smaller resistance change with increased stretchability.4,34
36 The resistance change of weft-knitted fabric was negligible up to about 200% strain. This is consistent with the optical MA ET AL.

observation in Figure 4c since the fabric started to disintegrate from 210% strain. The cyclability of the fabric was excellent when the maximum strain was 100% (Figure 4e). The temperature-dependent resistance change of the woven fabric is shown in Supporting Figure S20. The normalized resistance increased from 1 to 1.12 as the temperature increased from 25 °C to 100 °C. This was due to the increase in resistivity of silver with temperature due to the electron
phonon interaction.37 The resistance change of the woven fabric at different relative humidity is shown in Supporting Figure S21. The normalized resistance slightly decreased from 1 to 0.982 as the relative humidity increased from 25% to 70%. It is possible that adsorbed water molecules created an additional conductive path, decreasing the resistance. Figure 4f compares load
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ARTICLE Figure 5. Insulating layer-coated Ag-PU fibers (Ag = 39.5 vol %, curing temperature = 135 °C). (a) Optical images of Ecoflex- or PDMS-coated Ag-PU fibers. (b) Load
strain relationship of Ag-PU fibers with and without the coating. (c) Light-emitting diode connected to two conductive 8-ply ropes. A desired letter could be selectively illuminated by the elastic ropes. Ag nanoflower images (Figure 1b and c) are shown again as false-colored inset images.

cross-sectional area. Compared with the single fiber, the applied load was higher for 2-ply and 4-ply ropes at equivalent strain. Two distinct regimes with a large change in modulus were also observed for ropes. In contrast, only one constant modulus, which was much smaller than those of single fiber and ropes, was observed for the fabric in the strain range between 0 and 200%. This indicates that the strain was generated by the deformation of pores in addition to the stretching of the fiber. There was a kink at ∼210% strain, which is consistent with the optical observation in Figure 4c. The conductive stretchable fiber needs an electrically insulating outer layer for practical applications. As shown in Figure 5a, the insulating layer coating was carried out using Ecoflex or PDMS. The thickness of the coating layer was 150
200 μm, providing an excellent insulating performance. However, the mechanical strength could not be significantly improved after the coating, as shown in the load
strain analysis (Figure 5b). This was due to the much smaller Young's modulus of Ecoflex (41 MPa)38 and PDMS (3 MPa)39 compared with that of Ag-PU fibers (549.6 MPa). A light-emitting diode (LED) was suspended by two 8-ply Ag-PU ropes as a visual demonstration (Figure 5c, 1.8 V dc supply). The excellent elastic property of the ropes enabled selective illumination of a desired spot. The LED could be returned to the original position after the illumination. The high conductivity of the ropes maintained brightness during the stretching maneuver. MA ET AL.

False-colored SEM images of Ag nanoflowers are also provided in the inset. The elastically stretchable conductive fibers and ropes may be employed in surgical operations to selectively illuminate diseased parts of patients. CONCLUSION In summary, extraordinarily high electrical conductivity and enhanced mechanical strength were obtained by the combination of Ag nanoflowers and PU. The enhanced surface area and ∼8 nm thickness of the petals enabled active coalescence of Ag nanoflowers during the curing process. There was a trade-off relationship between conductivity and stretchability with regard to the curing temperature and filler fraction. The maximum initial conductivity (41 245 S cm
1) and rupture strain (776%) of the Ag-PU fibers were significantly enhanced by 394% and 58.4% compared with those of the previously published Ag-CNT-PVDF-HFP fibers without involving the hot rolling process.4 The maximum Young's modulus and ultimate strength were 731.5 and 39.6 MPa, which were 1399% and 198% greater than those of Ag-PVDF-HFP fibers. The modified filler
matrix interfacial bonding model indicated that the high mechanical properties were achieved due to the greater strength of PU and better adhesion between Ag nanoflowers and PU. The Ag-PU fiber exhibited the elastic property in a strain region smaller than the prestretching strain. The resistance change of weftknitted fabric was negligible up to 200% strain with VOL. XXX

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and mechanical strength may be useful for wearable and stretchable electronics applications.

METHODS

universal testing machine (Instron, model 3343). The diameter of the fiber was experimentally measured every 10% strain below 50% strain and every 30% strain beyond 50% strain using an optical microscope. The diameter was then fitted using a second-order polynomial regression equation. The conductivity and true stress were calculated based on the varying crosssectional area as a function of strain. Conflict of Interest: The authors declare no competing financial interest.

Wet spinning technology was employed to synthesize conductive stretchable fibers.4,20 Ag-PU fibers were synthesized using the following protocol. First, PU pellets (Sigma-Aldrich, 81367, 0.25g) were dissolved in DMF solvent (Sigma-Aldrich, 227056, 4.75g) by ultrasonication (540 W, 20 min). Ag nanoflowers (Nanopoly Co., Korea) were commercially obtained and dispersed in the PU solution (5 g) by ultrasonication (490 W, 20 min). The mixture was additionally stirred at 70 °C for 2 h to form spinnable dope. An ionic liquid was not used for the Ag-PU fibers. In the next step, the dope (∼4 mL) was extruded through a spinneret with a diameter of 310 μm into coagulant (deionized water) by aid of a syringe pump (KDScientific, KDS-100, 10 mL/h) to remove the solvent.4,20 Finally, the spun fiber was continuously collected on a winding drum. The fiber was further cured in an oven at 115
155 °C for 10 min after air-drying for 12 h. Supporting Table S3 shows both mass and volume fractions of synthesized fibers. The mass fraction of each component was experimentally measured in the dope assuming complete removal of solvent and preservation of the mixture ratio in the fibers. The volume fraction of Ag nanoflowers in the Ag-PU fibers was then calculated using eq 4. Vf ¼

VAg VAg þ VPU

l Rπr 2

(5)

where σ is the conductivity of the fiber, l is the length of the fiber, R is the resistance of the fiber, and r is the radius of the fiber. The load
strain characteristics were obtained by a

MA ET AL.

Acknowledgment. This work was supported by IBS-R011-D1, Ministry of Trade, Industry & Energy (10048884), a National Research Foundation of Korea (NRF) grant (NRF-2014R1A2A1A10050639) funded by the Korean government (MSIP), and Fundamental Technology Research Program (2014M3A7B4052200) through the NRF grants funded by the Korean government (MSIP). Supporting Information Available: The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b03864. Additional data, SEM analysis, differential scanning calorimetry analysis, thermogravimetric analysis, energy dispersive X-ray analysis, optical images of ropes and weft-knitted fabric, and theoretical analysis (PDF)

(4)

where Vf is the volume fraction of the Ag nanoflowers, VAg is the volume of the Ag nanoflowers, and VPU is the volume of PU. The Ag nanoflowers were porous, and the experimentally measured apparent density (Copley Scientific, JV 1000, 1.71 g/cm3) was smaller than the real density of silver without any void (10.49 g/cm3). As an approximation, real densities without any void (FAg = 10.49 g/cm3, FPU = 1.14 g/cm3, FPVDF‑HFP = 1.77 g/cm3) were used with the mass information on each component to calculate the volume fraction assuming that the polymer could penetrate into the porous space of Ag nanoflowers. For the insulating outer layer, the Ag-PU fiber was dip-coated using PDMS (Dow Corning) or Ecoflex (Smooth-on. Inc.). The fiber was then dried in an oven (60 °C, 15 min). Ag-PVDF-HFP fibers were synthesized by the wet spinning process using a modified previously published protocol.4 First, Ag nanoflowers were dispersed in 4-methyl-2-pentanone (10 mL) by ultrasonication (560 W, 20 min). Ionic liquid (1-butyl4-methylpyridiniumtetrafluoroborate, 300 mg) was then added to the Ag nanoflower suspension and additionally ultrasonicated (560 W, 20 min). The PVDF
HFP solution (5.875 wt % in 4-methyl2-pentanone, 10 mL) was additionally added and stirred for 2 h to form a spinnable dope. In the next step, the dope (∼2 mL) was extruded through a spinneret with a diameter of 310 μm into coagulant (hexane, Sigma-Aldrich) by aid of a syringe pump (10 mL/h) to remove the solvent. Finally, the spun fiber was continuously collected on a winding drum. The fiber was cured in an oven at 115
155 °C for 10 min after air-drying for 12 h. Ag nanoflowers were characterized by field-emission SEM (JEOL, JSM 890), DSC (SEICO INST, DSC 7020), and TGA (SEICO INST, TG/DTA 7300). The resistance of the fiber was measured as a function of tensile strain using the two-probe method (Fluke, 289 TRUE RMS multimeter) and an in-house-built stretching device. The resistance of the probing wires was subtracted from the total resistance in order to precisely measure the resistance of the stretchable conductive fiber. The conductivity of the fiber was then calculated. σ ¼

ARTICLE

excellent cyclability. This new design of Ag-PU fibers with significantly enhanced conductivity, stretchability,

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