Conductive CoreâShell Aramid Nanofibrils: Compromising

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Applications of Polymer, Composite, and Coating Materials

Conductive Core-shell Aramid Nanofibrils: Compromising Conductivity with Mechanical Robustness for Organic Wearable Sensing Xiangsheng Han, Lili Lv, Daoyong Yu, Xiaochen Wu, and Chaoxu Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18472 • Publication Date (Web): 28 Dec 2018 Downloaded from http://pubs.acs.org on January 1, 2019

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ACS Applied Materials & Interfaces

Conductive Core-shell Aramid Nanofibrils: Compromising Conductivity with Mechanical Robustness for Organic Wearable Sensing

Xiangsheng Han,1,2 Lili Lv,1,2 Daoyong Yu,3 Xiaochen Wu1,* and Chaoxu Li1,2,*

1CAS

Key Lab of Bio-based materials, Qingdao Institute of Bioenergy and Bioprocess

Technology, Chinese Academy of Sciences, Songling Road 189, Qingdao 266101, P.R. China. 2University

3Center

of Chinese Academy of Sciences, 19A Yuquan Road, Beijing 100049, P.R. China.

for Bioengineering and Biotechnology, China University of Petroleum (East China),

Qingdao 266580, P.R. China. *Corresponding Authors: C. Li ([email protected]); X. Wu ([email protected]).

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ABSTRACT One-dimensional organic nanomaterials with the combination of electric conductivity, flexibility and mechanical robustness have been highly in demand in a variety of flexible electronic devices. Herein conducting polymers were combined with robust Kevlar nanofibrils (aramid nanofibrils, abbreviated as ANFs) via in-situ polymerization. Owing to strong interactions between ANFs and conjugated polymers, the resultant core-shell ANFs showed high electric conductivity in combination with flexibility, robustness, physical stability and endurance to bending and solvents, in sharp contrast to many inorganic conductive nanomaterials. Due to their responsivity of conductivity to different stimuli (e.g. humidity and strain), their membranes were capable not only of sensing human motions and speech words, but also of showing high sensitivity to variation of environmental humidity. In such way, these core-shell ANFs may pave a way of combining both conductivity and mechanical properties applicable for diverse wearable devices. KEYWORDS: Core-shell, aramid nanofibril, conductive polymer, polymerization, wearable devices


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Introduction Conductive low-dimensional nanomaterials with the combination of exceptional mechanical performance have been in pursuit as efficient conductive components for flexible electronics of wearable devices, sensors, supercapacitors and optoelectronic devices.1 In comparison with metallic (e.g. Ag nanowires2) and carbon nanomaterials (e.g. graphene3 and carbon nanotubes4), one-dimensional (1D) nanostructures of conducting polymers seemed to be promising to offer appropriate conductivity in addition to flexibility, lightness, low-cost, easy-processibility, biologic and environmental-friendliness.5 Though having been broadly employed in flexible electronics, most 1D nanostructures of conductive polymers still lack sufficient mechanical robustness and bending endurance owing to their conjugating and/or crystalline structures,6 which, whereas, play a vital role in diffusing external stress and avoiding conductivity failure during distortion and deformation. Nanofibrils with  100 nm in diameter, in principle, are adopted universally by living organisms for signal transmission and structure reinforcements, e.g. nanofibrils of actin, silk and cellulose.7,8 And thus fully conjugating nanowires were widely produced by crystallization of

conducting

polymers

(e.g.

polythiophene,

polyaniline9

and

poly(3,4-

ethylenedioxythiophene)) or polymerization within porous hard template (e.g. porous membrane of anodic aluminum oxide)10 for flexible electronics. In order to achieve higher modulus, conductive 1D hybrids were designed by polymerization of conductive polymers onto inorganic nanowires (e.g. V2O5, TiO2 and carbon).11 To depress mechanical rigidity resulted from fully conjugating microstructures and inorganic templates, soft templates (e.g. electro-spun nanofibers,12 amyloid fibrils,13 cellulose nanofibrils14 and DNA15) were also employed for in-situ polymerization to produce flexible and conductive 1D hybrids.16 For example, amyloid nanofibers templated the polymerization of polyaniline to form conducting nanowires;13 polypyrrole decorated carbon textile formed by bobbin winder technique showed



both supercapacitor and strain sensor properties;17 a sensitive pressure sensing electrode was constructed via self-stabilized dispersion polymerization of polyaniline on nanofibrous electrospun polyacrylonitrile.18 Kevlar fibers (i.e. poly(paraphenylene terephthalamide)) are considered as one of the strongest polymeric materials (e.g. Young’s modulus of ~ 90 GPa and tensile strength of ~ 3.8 GPa) because of their high molecular orientation and strong inter-chain interactions (e.g. π−π stacking and H-bonding).19,20 When weakening these inter-chain interactions in base-saturated dimethyl sulfoxide (DMSO), aramid nanofibrils (ANFs) with 3-15 nm in diameter were able to be exfoliated from Kevlar fibers.20 ANFs inherit the high chemical/thermal stability and exceptional mechanical performance of Kevlar, their production is easy to scale, and the surface amide groups on ANFs enables further incorporation of other components.21, 22 For example, the nanoscale sizes of ANFs allowed for high loading of Au nanoparticles for outstanding strength flexibility and effective plasmon-based optoelectronic applications;21 their robustness also enabled them to serve as a unique type of reinforcing fillers for highperformance nanocomposites (e.g. in silk fibroin,8 and conductive matrix of PEDOT and poly(styrene sulfonate)23). In addition, the super thermal stability of ANFs also enabled them to maintain mechanical properties without weight loss under high temperature.24 In order to produce conductive organic one-dimensional nanomaterials with mechanical robustness, taking advantage of both high strength nanoscale materials and high conductive organic polymer, ANFs were combined with polypyrrole by in-situ polymerization. ANFs were selected as soft template for polypyrrole polymerization, not only due to their high mechanical performance, but also due to their strong interactions (π−π stacking and H-bonding) with polypyrrole. The synthesized core-shell aramid nanofibrils showed bending endurance and robustness while without sacrificing their conductivity. These combinational merits posed an ideal opportunity to produce fibrous sensing membranes applicable for wearable devices,


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which could respond instantly not only to human motions (e.g. knee and finger), but also to humidity variation.

MATERIALS AND METHODS Materials. Kevlar 49 yarns (density: 1.44 g/cm3, tenacity: 2.08 N/tex) were obtained from DuPont. Pyrrole was bought from Shanghai Macklin Biochemical Co., Ltd. Aniline was from Tianjin Damao Chemical Reagent Factory, and 1,2-diaminobenzene was bought from Aladdin Industrial Corporation. Other reagents were obtained from Sinopharm Chemical Reagent Co. Ltd., China. Ultra-pure water (Resistivity: 18.2 MΩ cm−1) was used throughout the experiment. Liquid exfoliation of Kevlar. Typically,8, 20 bulk Kevlar yarns (1.0 g) were homogenized vigorously in dimethyl sulfoxide (DMSO, 500 mL) at 25 °C for 9 days in the presence of KOH (1.5 g), followed by filtering, washing and re-dispersing in water. Production of conductive core-shell ANFs. Pyrrole as a monomer was purified by distillation before use. 20 mL of ANFs (1 mg/mL) suspension was mixed with pyrrole (10-40 mg) and stirred for 1 h at 25 °C. After adding ammonium peroxy disulfate (mole ratio to pyrrole = 1:1, dissolving in 20 mL of 1 M HCl), the mixture was kept for stirring overnight at 25 °C. The final product was obtained by vacuum filtrating through a 0.2 µm filter membrane, washing with HCl (1M), methanol and H2O successively. Aniline and o-phenylenediamine were used as the monomers as the following: 20 mL of 1 mg/mL ANFs suspension were mixed with the monomers (10-40 mg) and stirred for 1 h at 25 ˚C to allow the efficient absorption of monomers onto ANFs. Ammonium peroxy disulfate (1:1 mole ratio to monomer) in 20 mL of 1 M HCl was quickly added. The mixture was then kept stirring overnight at 25 ˚C. Strain-sensing ability. A rectangular core-shell ANFs membrane (10 mm × 5 mm × 0.025 mm) was connected to the electrochemical workstation (CHI660E) on both sides using adhesive tape



to record its conductivity. The experiments were performed in an ambient atmosphere at room temperature (25 ± 5°C). Humidity sensing. A 25 cm × 20 cm × 15 cm glass box (volume: 7500 cm3) was used as the testing chamber. Different levels of humidity inside the chamber was controlled by pumping dry nitrogen gas or wet air (flux ~ 0.15 L/min), set to vary between ~ 30% and ~ 90%. A rectangular core-shell ANFs membrane (10 mm × 5 mm × 0.025 mm) was placed inside in the testing chamber, with both sides connected to a multimeter by adhesive tape. A ThermoHygrometer was also put inside the chamber to monitor the RH and the operating temperature. All the experiments were carried out at normal atmosphere and a constant temperature of 25 °C. Characterization. Morphologies of the samples were characterized by Hitachi H-7650 transmission electron microscopy (TEM, Japan) and JEOL 7401 emission scanning electron microscopy (FESEM, Japan, acceleration voltage of 10 kV). Atomic force microscope (AFM) in tapping mode was performed on Agilent 5400 at a scan rate of 1 Hz equipped with silicon nitride cantilevers (Bruker). X-ray diffraction (XRD) measurements were taken on a Bruker D8 ADVANCE X-ray diffractometer (Germany) using Cu Kα (λ = 1.5406 Å) radiation. UV-Vis spectrophotometric analyses were performed on a DU800 UV-vis spectrophotometer. Fourier transform infrared (FTIR) analyses were performed on a Nicolet 6700 FT-IR spectrometer (American). Raman spectra was obtained on a Thermo Fisher Scientific DXR Raman microscope (America) with 532 nm laser. Thermogravimetric analysis (TGA) was carried out on an TA SDT Q600 apparatus (America). The sample was placed in a platinum pan and heated from 25 to 1000 °C at a rate of 10 °C/min under air atmosphere. Electrical conductivity was measured at 25 °C by a RTS-8 4-point probes resistivity measurement system. Mechanical properties of the composites were performed on a CMT 6503 electromechanical


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universal testing machine (MTS systems China Co. Ltd), with the gauge length of 15 mm and the tensile speed of 2 mm/min, and each sample was tested for at least 3 parallel samples. The rectangular samples (5 mm × 10 mm) were equilibrated at ~ 25 °C and relative humidity of ~ 50% for 24 h for further measurements. The resistances of the humidity sensor and stress sensor were recorded by a MS8265 digital multimeter. The current differences and the I-V characteristics for the sensors were recorded by a CHI660E modular electrochemical workstation.

Results and Discussion Kevlar fibers were normally produced though liquid crystal spinning processes,25 in which polyaramide molecules orientated and bound together via inter-chain interactions (e.g. Hbonding and π−π interactions). In DMSO saturated with KOH,20 polyaramide molecules might be deprotonated and thus weakened their inter-chain interactions, leading to exfoliation of Kevlar fibers (Kevlar 49, ~ 12.5 μm in diameter in Fig. 1A and S1A) into ANFs upon vigorously homogenizing. As shown in Fig. 1B & S1, the obtained ANFs showed a diameter within 3-15 nm and a length of  1 m, forming dispersion with high colloidal stability. These ANFs had high aspect ratios and mechanical properties, being frequently used as highperformance building blocks for polymer composites.26 When exchanging the DMSO solvent to H2O, ANFs could template the oxidative polymerization of pyrrole in the presence of an oxidizing agent (i.e. ammonium peroxy disulfate). Polypyrrole has been preferred to be used in electrical conductive textiles due to its high conductivity, good environmental stability, ease of synthesis and non-toxicity.27 Addition with its high affinity with different substrates, polypyrrole was also frequently used to form composites with a number of non-conductive fibers or fabrics to prepare electrical conductive textiles. For example, polypyrrole-coated conductive microribbon meshes have been employed to construct actuators responding to current, pH, and temperature;28 polypyrrole-coated fabrics



were used as sensing devices for the detection of strain to enable the measurement and control of various movements of human body.29 For the hybridization of polypyrrole with ANFs, in contrast to granular polypyrrole polymerized without the presence of ANFs (Fig. S2), polypyrrole seemingly preferred to polymerize on ANFs. The π−π interactions between pyrrole and benzyl groups of aramid molecules, in combination with the H-bonding between secondary amine groups of pyrrole and carbonyl groups of ANFs might be responsible for the combination of pyrrole on ANFs. The final product in Fig. 1C maintained fibrous morphologies and colloidal stability (e.g. up to  40 days). And conspicuous core-shell fibrils were zoomed in Fig 1D, which showed a diameter of 45-55 nm and shell thickness within 15-20 nm. The successful presence of core-shell fibrils was also confirmed by tuning the pyrrole amounts for polymerization in Fig. S3, in which the fibrils diameter increased due to the thickening polypyrrole shells. The presence of polypyrrole shells could be confirmed by thermogravimetric analysis (TGA) and Fourier-transform infrared (FTIR) spectra. Because of weaker thermal stability of polypyrrole than Kevlar, the core-shell ANFs started to give weight loss at lower temperatures than ANFs in Fig. 2A. In Fig. 2B, ANFs showed the C=O stretching vibrations at 1651 cm-1, N–H deformation vibrations at 1544 cm-1, C=C stretching vibrations of aromatic ring at 1514 cm-1 and Ph-N vibrations at 1318 cm-1.20 The characteristic peaks of polypyrrole were present for the core-shell ANFs, such as the widened pyrrole ring peak (1541 cm-1), the C–N vibration (1177 cm−1), the C–H in-plane vibration (1038 cm−1) and the C–H out-plane bend (915 cm−1).30 The UV-Vis absorption of ANFs in Fig. S4 showed characteristic absorption at 345 nm.19 With the polypyrrole shells, the absorption at 460 nm appeared corresponding to the π−π* transition of polypyrrole chains.31 In Fig. 2C, the Raman spectra of core-shell ANFs exhibited two bands at 1565 and 1354 cm-1, being characteristic of C-C backbone stretching and ring stretching vibration of polypyrrole, respectively.32


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Polypyrrole preferred to polymerize on ANFs surfaces, which could be attributed to strong interactions between polypyrrole and aramid molecules. Besides possible π−π interactions between conjugated polypyrrole and benzyl groups of aramid molecules, there might also be H-bonding between secondary amine groups of polypyrrole and carbonyl groups of ANFs,33 as shown in Fig. 2D. These strong interactions not only enabled the successful generation of polypyrrole shells, but also offered high endurance against solvents, mechanical and thermal treatments. When filtering this colloidal core-shell ANFs, they could deposit gradually into layered microstructures during the filtration process. Flexible and nanofibrous membranes were produced in Fig. 3A-3C with their thickness of 15-45 m controlled by the filtrating amount of core-shell ANFs. The presence of core-shell ANFs was discernible easily for both the inplane and cross-sectional SEM observations. In analogue to the membranes produced with ANFs in Fig. S5, the incorporation of polypyrrole up to 32 wt. % did not alter the flexibility of the final hybrid membranes in Fig. 3, S6 & S7. The hybrid membranes could endure various mechanical treatments such as folding, twisting, bending and stretching while without showing any structural failure. For example, hybrid membranes with 32 wt. % polypyrrole could be folded into a crane without any cracks, and were able to hold a 200 g weight steadily (Fig. 3A & 3D). The mechanical properties of hybrid membranes were evaluated in Fig. 3D and summarized in Fig. 3E. The pristine ANFs membranes possessed a Young's modulus of ~ 2.3 GPa and ultimate elongation of ~ 10.2 %. The direct mixing of ANFs and polypyrrole resulted in a heterogeneous membrane with relatively low mechanical properties (Young's modulus < 0.05 GPa, ultimate elongation < 4 %) and poor conductivity (Fig. S8). As to the core-shell membranes, though the polypyrrole hybridization seemed to decrease their Young’s moduli and ultimate stresses, the hybrid membranes with the polypyrrole content of 32 wt. % still



maintained a Young's modulus of  1.4 GPa and an ultimate elongation of  9.8 %. With the polypyrrole content of 67 wt. %, the hybrid membrane became much weaker and would be invalid to withstand heavy weights for applications. The presence of polypyrrole shells offered electric conductivity for these core-shell ANFs and their membranes. The membrane conductivity increased from insulation up to ~ 25 S/m with the polypyrrole content of 67 wt. %, and was able to turn on a 3 V LED light in Fig. 4A. Because of strong interactions between polypyrrole shells and ANFs, the membrane conductivity could also endure mechanical treatments such as twisting and bending, their conductivity decreased only ~ 2 % after 5000 cycles of folding-unfolding (Fig. 4B, measured at the same unfolded state). The SEM observation of the membranes surfaces after long-term folding-unfolding cycles exhibited no obvious damage or plastic deformation on the folding part (Fig. S9). Additionally, the membrane conductivity well maintained during 40000 cycles of rubbing with a polytetrafluoroethylene stick in Fig. 4C. Besides these, the interfacial strong interactions offered super wear resistance and endurance of the hybrid membranes to organic solvents. As shown in Fig. 4D, the incubation in various frequently-used organic solvents for  5 days resulted in little change of the membrane conductivity. To be noted, the electric conductivity of polypyrrole alone was vulnerable to thermal treatments (see Fig. S10), high temperatures (> 70 °C) would decrease its conductivity due to the dopant loss.34 Impressively, the hybrid membranes of core-shell ANFs had their conductivity nearly constant at higher temperatures (at 80 °C for 4 days), and remained flexible after treatment at 120 °C for 4 days in Fig. S11. This high thermal stability was also in sharp contrast to that of conductive ANFs membranes which were produced by incubating ANFs membranes in a pyrrole solution for polymerization, and lost ~ 95 % of the conductivity after 4 days incubation at 80 °C (see Fig. S12). This instability might be because of most polypyrrole


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molecules exposed to the external environment, the heterogeneous polymerization, and the lack of intermolecular interactions between polypyrrole and ANFs. Besides polypyrrole, other conjugated polymers such as polyaniline and poly-ophenylenediamine could also be used to produce conductive core-shell ANFs through a similar procedure. The core-shell ANFs were further capable of producing fibrous hybrid membranes with the compromise of mechanical properties (i.e., Young's modulus of ~ 1.3 GPa for 22 wt. % polyaniline decorated ANFs membranes, and of ~ 1.4 GPa for 22 wt. % poly-ophenylenediamine decorated ANFs membranes), flexibility and conductivity (see Fig. 5A-5D & S13-S14). The membranes were well conductive while compromising favorable toughness, being comparable to those of conductive papers based on conductive dopants (e.g. carbon nanotubes, graphene, graphene oxide and polyaniline in Fig. 5E and Table S1. The combination of high conductivity, flexibility and endurance enabled the conductive hybrid membranes as perfect candidates for flexible conductors. For example, the fibrous hybrid membranes could serve as a sensor of pressure and strain for wearable applications. Membranes with 32 wt. % of polypyrrole were chosen to characterize the sensing ability due to their compromise of conductivity (~ 10 S/m) and mechanical robustness (ultimate stress of 57 MPa). The application of mechanical stresses, such as the tapping and bending pressure, could be readily observed via the piezoresistive responses of the core-shell ANFs membranes. The pressure applied by the finger tips and joints translated to a compressive stress applied onto the membranes, leading to the formation of a more conductive network with an increased contact points, resulting in an increased observed current (Fig. 6A). Fig. 6B & S15 depicted the piezoresistive response of the core-shell ANFs membranes to pressure, the membranes showed high stability and durability for tens of repeated cycles of pressure. The recovery of current was time-dependent, attributed to the compressive strain recovery of the membrane. It had a fast response time of < 0.2 s (Fig. S15A) and nearly 100 % recovery for  20 cycles.



When applying constant pressure, the measured current values were almost constant, signifying that the core-shell ANFs yielded similar conducting network structures with similar connections. The relative variation of resistance (△R/R0) changed linearly with the applied pressure in the range of 4-60 kPa, with detection limit of ~4 kPa (Fig. S15B). Strain sensing was conducted in a similar approach. Due to the structural deformation under strain, the hybrid membrane exhibited substantial piezoresistive response to tensile loads, and showed good durability and reproducibility within long-term stretch and release cycles (Fig. 6C). The morphology of hybrid membranes after long-term stretch/release cycles exhibited neither obvious damage of the core-shell structure, nor fiber cracks, confirming their good stability (Fig. S16A & S16B). Additionally, in order to measure the sensitivity, the performance of the strain sensor was characterized by a gauge factor, which is defined to be the slope of the normalized resistance (R)-strain (ε) curve, [ΔR(ε)/R(0)]/Δε. The resistance of the core-shell ANFs membranes started to change at a strain of 1.6 %, the gauge factor values of the strain sensors ranged from 18.8 to 6.5 when the strains increased from 1.6 % to 8.6 % (Fig. S16C), being larger than those of the metal-foil strain sensors (gauge factor of 1-5) and capacitive-type strain sensors (gauge factor of < 1),35 while comparable to sensors based on conductive carbon nanomaterial hybrids (Table S2). Considering the fact that tensile elongation at break of the core-shell membrane was < 10 %, this strain-dependent behavior could be employed to sense non-vigorous human motions such as finger bending, arm bending, and knees flexion with different bending angles (Fig. 6D-6F), in which lower current corresponded to the flat nonbent states, whereas the higher current values were related to the bent state. Humidity was suggested to be able to alter the conductivity of polypyrrole via variation of charge carrier holes.36 Thus the conductive core-shell ANFs could also be used for humidity sensing. As shown in Fig. 7A, the resistance of the hybrid membrane increased gradually when the relative humidity was higher than 55 %. The conductivity decrease with the relative


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humidity might be caused by the counterion leaching, reduction of charge holes, and the reaction of nucleophile (i.e., H2O) with polypyrrole backbone, which interrupted the conjugated structure and lowered the intrinsic conductivity of polypyrrole.36, 37 The humidity sensing behavior displayed good reversibility, with the resistance fully recover when exposing to 60-80 % relative humidity (Fig. 7B). The recovery time of this conductive ANFs sensor was measured in Fig. S17. Upon changing the relative humidity from 90 % to 50 %, the sensor's output current returned to its original value in less than 30 s, displaying good recoverability. When approaching to the mouth, this sensor was able to precisely sense the speech by measuring the variation of electric current and hereby breath humidity. Different words were spoken and the corresponding current-time curves were recorded in Fig. 7C. The word information was distinguished separately, various patterns of electric current represented different breath strengths in spoken words e.g. “nature” and “speaking”. In addition, the hybrid membranes could be used for monitoring the ambient humidity. We tested the humidity sensing performance of the core-shell ANFs membranes on Laoshan District, Qingdao for 24 hours. With the changes of environment humidity, the resistance of membranes exhibited perfect corresponding alterations in Fig. 7D, which thus made it as a fully organic monitor of ambient humidity. CONCLUSION In summary, in order to prepare conducting polymer nanowires with mechanical robustness, in-situ polymerization of pyrrole was adopted to produce conductive core-shell ANFs with 2065 nm in diameter. Owing to strong interactions between the shell and the core, these coreshell ANFs could combine the conductivity of conjugated polymers and mechanical robustness of ANFs. Upon filtering into membranes, these core-shell ANFs showed the electric conductivity up to ~ 25 S/m as well as a Young’s modulus of ~ 2.1 GPa and an ultimate elongation of ~ 13.6 %, being comparable to even or high than many membranes produced



with graphene and carbon nanotubes. In addition, they also had super wear resistance and endurance, e.g. showing negligible conductivity variation during 5000 cycles of bending. As a novel type of soft nanofibrils with the compromise of conductivity and mechanical robustness, theses core-shell ANFs promised great potential for applications in strain and humidity sensing. For example, they were capable of distinguishing non-vigorous human motions, speech words and environmental humidity variation, being ideal for sensing garment, wearable hardware and rehabilitation, etc.

ACKNOWLEDGEMENTS National Natural Science Foundation of China (No. 21474125), Shandong Provincial Natural Science Foundation (No. ZR2016EEB25 and JQ201609), Chinese “1000 youth Talent Program” and Shandong “Taishan Youth Scholoar Program” are kindly acknowledged for the financial supports.

ASSOCIATED CONTENT Supporting Information Available: Optical images, SEM images, TEM images, UV-vis absorption, conductive measurements, thermal stability and mechanical properties of ANFs based conductive papers.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Li C.); [email protected] (Wu X.) Notes The authors declare no competing financial interest.

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Fig. 1 Procedure followed to produce core-shell ANFs. (A) Kevlar 49 yarns. (B) ANFs produced by homogenizing Kevlar yarns in DMSO saturated with KOH. (C) Core-shell ANFs produced by template polymerization of polypyrrole on ANFs. (D) Zoom-in TEM image of polypyrrole shell around ANFs. Polypyrrole content: 32 wt. %.


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Fig. 2 Spectroscopic characterization of core-shell ANFs. (A) TGA curves in air. (B) FT-IR spectra. (C) Raman spectra. Polypyrrole and ANFs were given for comparison. (D) Interactions between polypyrrole and ANFs.



Fig. 3 Morphologic and mechanic characterization of membranes of core-shell ANFs. (A) Visual observation. (B-C) SEM images of cross-section and in-plane observation. Polypyrrole content: 32 wt. %. (D) Stress-strain curves with different polypyrrole contents. The inset shows that a strip (10 × 5 mm2 and 12.2 µm in thickness) could hold a weight of 200 g. (E) Mechanic properties with different polypyrrole contents.


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Fig. 4 Electric conductivity of membranes of core-shell ANFs. (A) Conductivity dependence on polypyrrole contents. (B-C) Conductivity endurance against folding (B) and friction (C). (D) Conductivity endurance against rinsing in organic solvents. The polypyrrole content was 32 wt. % without specification.



Fig. 5 Membranes of core-shell ANFs based on different conductive polymers. (A & B) Poly-o-phenylenediamine. (C & D) Polyaniline. (E) Comparison of toughness and conductivity with different conductive membranes. References were detailed in Table S1.


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Fig. 6 Membrane of core-shell ANFs for stress and strain sensing. (A) Schematic illustration of stress sensor. (B) Current variation during finger pressing (pressure ~ 110 kPa). (C) Current variation during straining (strain ~ 3.5 %). (D-F) Current variation during finger, knee, and elbow bending. Strip size: 10 × 5 mm2 and 25 µm in thickness. Polypyrrole content: 32 wt. %.



Fig. 7 Membrane of core-shell ANFs for humidity-sensing. (A) Resistance versus relative humidity. (B) Resistance variation (ΔR/R0) cyclically exposing to 60-80 % relative humidity (%RH). (C) Current variation during monitoring speech humidity change. The inset gives schematic illustration for words sensing. (D) Evaluation of outdoor humidity and corresponding resistance variation for 24 hours. Polypyrrole content: 32 wt. %.


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