Artigo Revisado por pares

Highly Stretchable, Elastic, Healable, and Ultra-Durable Polyvinyl Alcohol-Based Ionic Conductors Capable of Safe Disposal

2021; Chinese Chemical Society; Volume: 4; Issue: 9 Linguagem: Inglês

10.31635/ccschem.021.202101360

ISSN

2096-5745

Autores

Siheng Li, Yixuan Li, Yuting Wang, Hongyu Pan, Junqi Sun,

Tópico(s)

Ionic liquids properties and applications

Resumo

Open AccessCCS ChemistryRESEARCH ARTICLE5 Sep 2022Highly Stretchable, Elastic, Healable, and Ultra-Durable Polyvinyl Alcohol-Based Ionic Conductors Capable of Safe Disposal Siheng Li, Yixuan Li, Yuting Wang, Hongyu Pan and Junqi Sun Siheng Li State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 , Yixuan Li State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 , Yuting Wang State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 , Hongyu Pan State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 and Junqi Sun *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 https://doi.org/10.31635/ccschem.021.202101360 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Highly durable and stretchable ionic conductors are indispensable components of flexible electronics. However, fabricating such ionic conductors that are also non-toxic and biodegradable remains a challenge. In this study, highly stretchable, elastic, healable, and ultra-durable ionic conductors capable of non-hazardous disposal are conveniently fabricated by complexation of vanillin-grafted polyvinyl alcohol (VPVA) and ionic liquids (ILs) (denoted as VPVA-IL). No leakage of the loaded ILs occurs after repeatedly heating and pressing the ionic conductors. The VPVA-IL ionic conductor has an ionic conductivity of ∼0.67 S m−1, a tensile strength of ∼2.01 MPa, and a strain at break of ∼1231%. Meanwhile, the VPVA-IL ionic conductor shows a highly reproducible electrical response during 1100 uninterrupted extension-release cycles at a strain of 400%, demonstrating its excellent fatigue resistance and durability. Because of the reversibility of hydrogen bonds, the fractured ionic conductors can be easily healed at ∼70 °C, restoring their original conductivity, stretchability, elasticity, and durability. Highly pure ILs can be easily separated from the VPVA-IL ionic conductors for reconstruction into new ionic conductors. The VPVA matrices, which are capable of completely degrading into non-toxic substances in soil, can be safely discarded without polluting the environment. Download figure Download PowerPoint Introduction Flexible electronics have experienced a development boom over the past decade, bringing about large changes in society.1–5 Stretchable ionic conductors play an important role in various flexible electronics and are widely used in flexible batteries, soft robots, and flexible sensors.4–9 With the pursuit of superb flexible electronics to better serve users, there is increasing demand for stretchable ionic conductors with high stretchability, outstanding elasticity, and satisfactory ionic conductivity even in highly stretched states.9–12 However, large deformations can cause fatigue and mechanical damage of stretchable ionic conductors in practical applications, severely reducing their reliability and shortening their service life. Therefore, stretchable ionic conductors need to simultaneously possess excellent fatigue resistance and self-healing properties to improve their durability for long-term service.13–15 Healing of ionic conductors can be realized through the reversibility of non-covalent interactions of polymer chains containing ionic liquids (ILs), deep eutectic solvents, and electrolyte salts.14–19 Although various stretchable ionic conductors have been fabricated, these conductors still need better elasticity, fatigue resistance, and durability, especially under uninterrupted large deformations (>200%) (see Supporting Information Table S1).11–14,18–32 Until now, the most durable ionic conductors reported in the literature contain reproducible electrical responses at 100% strain over 13,000 uninterrupted strain cycles.12 However, these ionic conductors had a tensile strength of ∼200 kPa, a strain at break of ∼850%, and were fabricated by swelling poly(methyl methacrylate-ran-butyl acrylate) with 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. Despite the excellent durability and elasticity of these ionic conductors, which can be used as skin-type strain sensors, their ionic conductivity of 0.031 S m−1 is still too low for extended applications. The high stretchability, excellent elasticity, high ionic conductivity under large strains, and satisfactory self-healing capacities of ionic conductors are based on different or even mutually exclusive design principles. As a result, it is technically difficult to integrate all these individual properties into one ionic conductor. Because the market need for flexible electronics continuously grows, the annual production of stretchable ionic conductors is expected to significantly increase in the future.3–5,33,34 Therefore, the disposal of out-of-service stretchable ionic conductors is a major concern for the general public. Ionogels, which are three-dimensional polymer networks impregnated with ILs, retain the key properties of ILs and are suitable for the fabrication of stretchable ionic conductors.12,14,34–38 In order for ionogel-based ionic conductors to achieve high ionic conductivity under large deformations, they need to be loaded with large amounts of ILs. Unfortunately, many commonly used ILs are toxic.39 When subjected to compression, stretching, and high temperature and humid environments, the loaded ILs in ionic conductors can leak out.35,40 Without proper treatment, discarded ionic conductors can cause serious environmental pollution. Recovery of ILs from out-of-service ionic conductors through a cost-effective and environment-friendly method can thus solve the issues of environmental pollution and enable the recycling of ILs while reducing waste. However, it is challenging to recover high purity ILs from ionic conductors because ILs can dissolve many polymers and organic molecules.41 Meanwhile, there is a need to fabricate ionic conductors with degradable polymer host matrices in order to reduce the accumulation of polymeric waste. Therefore, to ensure sustainability and environmental protection, future ionic conductors should possess excellent healing ability, stretchability, elasticity, and conductivity and must be capable of safe disposal. However, ionic conductors that integrate all the above-mentioned properties have never been reported in the literature. Polyvinyl alcohol (PVA) is a cost-effective, non-toxic, and degradable polymer.42,43 Due to the hydrogen bonds between hydroxyl groups and the crystallized domains, PVA-based polymer materials exhibit well-tailored mechanical properties.43,44 In this study, highly stretchable, elastic, healable, and ultra-durable ionic conductors capable of safe disposal are fabricated by complexation of vanillin-grafted PVA (denoted as VPVA) with 1-ethyl-3-methylimidazolium dicyanamide ([EMIM][DCA], denoted as IL). The resulting ionic conductors have a tensile strength of ∼2.01 MPa and a strain at break of ∼1231%. More importantly, the ionic conductors show highly reproducible electrical responses during 1100 uninterrupted extension-release cycles at a strain of 400%. The ionic conductors can also fully heal a physical cut at ∼70 °C to restore their original conductivity, stretchability, and durability. Moreover, high purity ILs can be conveniently recovered for reconstruction into new ionic conductors. The VPVA matrices, which have the ability to fully degrade in soil into non-toxic substances, can thus be safely discarded without causing environmental pollution. Experimental Methods Materials PVA (MW ∼146,000–186,000; ∼99%+ hydrolyzed) was purchased from Sigma-Aldrich (Shanghai, China). Vanillin was purchased from Energy Chemical (Shanghai, China), while [EMIM][DCA] was purchased from Lanzhou Institute of Chemical Physics (Lanzhou, China). HCl and dimethylsulfoxide (DMSO) were purchased from the Beijing Chemical Reagent Company (Beijing, China). All chemical reagents were used without further purification. Synthesis of VPVA Vanillin-grafted PVA is denoted as VPVAx%, where x represents the grafting ratio of the vanillin on the VPVA chains. VPVA40% was synthesized according to our previously reported method.43 In brief, the PVA (5.00 g) was dissolved in DMSO (70 mL) under stirring at ∼90 °C for 5 h. Subsequently, vanillin (3.45 g, 22.7 mmol) was dissolved in the PVA solution under stirring at 80 °C. After adding HCl (3.3 mL, 1 mol L−1) into the PVA and vanillin solution, the reaction was carried out at ∼80 °C for ∼8 h. Vanillin was grafted onto the PVA chains via an acid-catalyzed acetal reaction between the hydroxyl groups of PVA and the aldehyde groups of vanillin. The dried VPVA40% (20 mg) was dissolved in deuterated DMSO (0.5 mL) for 1H NMR analysis. Following the same methods used for VPVA40%, other types of VPVAx% were fabricated by varying the ratios of vanillin to hydroxyl groups of PVA. Fabrication of the VPVA40%-IL ionic conductor ILs were added dropwise into the DMSO/VPVA40% solution with a concentration of 10 wt % under vigorous stirring at ∼50 °C to obtain a homogeneous VPVA40%-IL solution. The mass ratio of VPVA40% to ILs was 1∶2. The VPVA40%-IL solution was slowly poured onto a glass plate and dried at ∼50 °C for 48 h. After drying in a vacuum oven at ∼70 °C for 4 h, the VPVA40%-IL ionic conductor was obtained. The VPVA40%-IL ionic conductor was then manually peeled off the glass plate. Characterization of the VPVA40%-IL ionic conductors The real-time I-t curves of the ionic conductors were recorded using an electrochemical workstation (CHI660E, Chenhua, Shanghai, China) at a constant voltage of 0.3 V. The electrical resistance (ΔR/R0) of the stretched VPVA40%-IL ionic conductors was calculated as follows: Δ R R 0 = R − R 0 R 0 (1)where R0 and R are the resistances of the original and stretched ionic conductors, respectively. The ionic conductivity of the ionic conductors was measured by impedance measurements using a Solartron 1260/1287 Electrochemical Impedance System (Solartron Metrology, UK) under ambient conditions (∼30% relative humidity (RH), ∼25 °C). The impedance data were collected over a frequency range of 1 Hz–1 MHz, with amplitude of 5 mV. The ionic conductivity of the ionic conductors was calculated as follows: σ = L R S (2)where σ is the ionic conductivity (S m−1) of the ionic conductors, and S and L are the area (m2) and thickness (m) of the ionic conductors, respectively. R is the bulk Ohmic resistance from the impedance data (Ω). Non-hazardous disposal of the VPVA40%-IL ionic conductors Water soluble ILs were extracted from the VPVA40%-IL ionic conductors (5 × 5 cm2) with a thickness of 350 ± 100 μm by dialyzing them in water of 300 mL for 24 h under stirring at room temperature. Pure ILs were obtained by removing water in the aqueous solution of ILs under reduced pressure at 50 °C. The VPVA40% films without the loaded ILs were buried in soil of Jilin University (City of Changchun, 43°53′N, 125°19′E) with an average depth of 10 cm. The residual weight of the films as a function of time in soil was monitored and the corresponding digital images were also taken. Instruments and characterization Fourier-transform infrared (FTIR) spectra were measured using a Bruker VERTEX 80V FTIR spectrometer (Bruker, Germany). 1H NMR spectra were recorded using a 500-MHz Bruker AVANCE III instrument (Bruker, Germany). X-ray diffraction (XRD) measurements were obtained at room temperature on a Rigaku SmartLab XRD system (Rigaku, Japan) using Cu Kα1 radiation with a wavelength of 0.154 nm. All tensile tests were carried out on an Instron 5944 testing machine (AG-I 100N, Instron, USA) under ambient conditions (∼25 °C). For uniaxial tensile measurements, the ionic conductors were cut into dumbbell shapes (overall length: 12.5 mm, width: 2 mm, and thickness: 0.35 ± 0.15 mm). The stretching speed was 50 mm min−1 when characterizing the mechanical properties of the ionic conductors. To save time, the stretching speed was adjusted to 200 mm min−1 during durability characterization of the VPVA40%-IL ionic conductors. Results and Discussion Fabrication of VPVA-IL ionogels Vanillin is widely used as an edible flavoring agent and is extracted from natural vanilla beans. VPVA was synthesized according to the methods described in our previous work.43 For simplicity, vanillin-grafted PVA is denoted as VPVAx%, where x represents the grafting ratio of the vanillin on the VPVA chains, as determined by 1H NMR spectroscopy (see Supporting Information Figure S1). As shown in Figure 1a, the ionogel-based ionic conductors are fabricated by casting a homogenous DMSO complexation solution of VPVAx% and ILs onto glass substrates, followed by solvent evaporation and drying. [EMIM][DCA] was used to fabricate ionic conductors in this study because it has a high ionic conductivity (2.8 S m−1 at 25 °C) and satisfactory compatibility with VPVAx%.36 Ionogels with a 1∶2 mass ratio of VPVAx% to ILs are denoted as VPVAx%-IL. The optimization of mass ratio of VPVAx% to ILs will be discussed later. Figure 1b shows a digital image of the VPVA40%-IL ionogel with a thickness of ∼350 μm and an area of 23 × 16 cm2. Because of the compatibility between VPVA40% and ILs, the beige VPVA40%-IL ionogel is homogenous, transparent, and free of defects. The transmittance of the ionic conductor is ∼92% at 550 nm (see Supporting Information Figure S2). Comparison of the ILs, VPVA40%-IL ionogel, dried VPVA40%, and PVA FTIR spectra indicates that hydrogen bonds are present between the aliphatic and phenolic hydroxyl groups on the VPVA40% chains in the VPVA40%-IL ionogel. Meanwhile, the loaded ILs have hydrogen-bonding interactions with VPVA40% chains in the VPVA40%-IL ionogel (see Supporting Information Figure S3).45–48 The XRD spectrum of the PVA film shows a sharp diffraction peak at 2θ = 19.6°, which is assigned to the 10 1 ¯ reflection of PVA crystals (see Supporting Information Figure S4a).44 The obvious decrease in peak intensity of the 2θ peak at ∼19.6° in the VPVA40% film indicates that the grafted vanillin significantly decreases the crystallization of the VPVA40% chains. The XRD spectrum of the VPVA40%-IL ionogel shows a broad hump at ∼23.5°, indicating that the grafting of vanillin and the introduction of ILs completely inhibit the crystallization of PVA in the VPVA40%-IL ionogel. The XRD spectra also confirm that the PVA chains are amorphous in the VPVA25%-IL, VPVA33%-IL, and VPVA45%-IL ionogels (see Supporting Information Figure S4b). The loaded ILs can weaken and partially break the hydrogen bonds between the phenolic hydroxyl and aliphatic hydroxyl groups on the VPVAx% chains, thereby largely decreasing the crystallization of VPVAx% in the ionogels. The structure of the VPVAx%-IL ionogel is schematically shown in Figure 1c. Figure 1 | (a) Illustration of the preparation process of the VPVAx%-IL ionogel. (b) Digital image of the VPVA40%-IL ionogel. (c) Schematic structure of the VPVAx%-IL ionogel. Download figure Download PowerPoint Mechanical properties of VPVAx%-IL ionogels The mechanical properties of the various VPVAx%-IL ionogels were characterized by tensile tests at ∼25 °C and ∼30% RH (Figure 2a and Supporting Information Table S2). When the mass ratio of VPVAx% to ILs is fixed at 1∶2, the tensile strength and Young's modulus of the VPVAx%-IL ionogels gradually decrease as the vanillin grafting ratio increases. VPVAx% with higher vanillin grafting ratios has smaller hydrogen bond densities, because each grafted vanillin molecule reacts with two aliphatic hydroxyl groups while generating one phenolic hydroxyl group. Therefore, VPVAx%-IL ionogels with increased vanillin grafting ratios have decreased tensile strength and Young's modulus. The VPVA40%-IL ionogel exhibits a tensile strength, strain at break, and Young's modulus of ∼2.01 MPa, ∼1231%, and ∼0.12 MPa, respectively. The elasticity of the VPVA40%-IL ionogels was investigated by performing 10 successive loading–unloading cycles at a strain of ∼200%. As shown in Figure 2b, the first loading–unloading curve for the VPVA40%-IL ionogel exhibits a very small hysteresis and has an extremely small residual strain of ∼1.5%. Moreover, the residual strain slightly increases to ∼4.2% after five uninterrupted loading–unloading cycles but remains unchanged even with further loading–unloading cycles. After resting at room temperature for 1.5 h, the VPVA40%-IL ionogel was again subjected to 10 successive loading–unloading cycles at a strain of ∼200%. The hysteresis curves almost overlap with those of the original ionogel, indicating the excellent elasticity of the VPVA40%-IL ionogel (Figure 2c). As shown in Figure 2d and Supporting Information Figures S5a and S5b, the elasticity of the VPVA25%-IL and VPVA33%-IL ionogels are inferior to that of the VPVA40%-IL ionogel. The elasticity of the VPVA45%-IL ionogel is quite similar to that of the VPVA40%-IL ionogel (Figure 2d and Supporting Information Figure S5c). Compared with the VPVA25%-IL and VPVA33%-IL ionogels, the VPVA40%-IL and VPVA45%-IL ionogels have a lower density of hydrogen bonds because of the higher grafting ratios of vanillin, which facilitates the elastic contraction of the stretched VPVA chains after stress release. Because the tensile strength and Young's modulus of the VPVA40%-IL ionogels are higher than those of the VPVA45%-IL ionogels, the VPVA40%-IL ionogels are investigated for use as healable and stretchable ionic conductors. As shown in Figures 2e and 2f, the VPVA40%-IL ionogel also exhibits excellent elasticity when subjected to 10 successive loading–unloading cycles at 400% strain. It is worth noting that the elasticity of the VPVA40%-IL ionogels are largely improved compared with previously reported covalently and noncovalently cross-linked ionogels (see Supporting Information Table S1).12–14,18,20,22–28 Originating from the hydrophilicity of ILs, the VPVA40%-IL ionic conductors under highly humid environments of 75% and 85% RH exhibit slightly lower tensile strength than that at 30% RH (see Supporting Information Figure S6a). However, the elasticity of the ionic conductors at 75% and 85% RH are obviously enhanced (see Supporting Information Figures S6b and S6c). Figure 2 | (a) Stress–strain curves of the VPVAx%-IL ionic conductors with different vanillin grafting ratios. (b and c) Ten successive loading–unloading cycles of the original (b) and recovered (c) VPVA40%-IL ionic conductor after resting for 1.5 h at room temperature at 200% strain. (d) Residual strains of various VPVAx%-IL ionic conductors after the initial loading–unloading cycles, where the strain was 200%. (e and f) Ten successive loading–unloading cycles of the original (e) and recovered (f) VPVA40%-IL ionic conductors after resting for 24 h at 400% strain at room temperature. Download figure Download PowerPoint Durability and healing ability of the VPVA40%-IL ionic conductors The VPVA40%-IL ionic conductor with a mass ratio of VPVA40% to ILs of 1∶2 has a measured ionic conductivity of ∼0.67 S m−1. The IL content in the ionic conductors significantly influences the ionic conductivity and mechanical strength. As shown in Supporting Information Figures S7a and S7b, as the IL content in the ionicconductors increase, the corresponding conductivity gradually increases, while the mechanical strength decreases. Supporting Information Figure S7 shows that the VPVA40%-IL ionic conductor combines excellent and well-balanced conductivity, mechanical strength, and elasticity. The environmental stability of the VPVA40%-IL ionic conductors is essential for their practical applications. The VPVA40%-IL ionic conductor exhibits no mass loss when placed in a vacuum oven at a pressure of ∼9 × 10−4 Pa and temperature of ∼70 °C for 10 days (see Supporting Information Figure S8a). Meanwhile, no IL leakage occurs even after the VPVA40%-IL ionic conductors are hot-pressed at ∼2 MPa and ∼70 °C for 10 min, a total of 20 consecutive times (see Supporting Information Figure S8b). Moreover, the VPVA40%-IL ionic conductors also exhibit high stability under a highly humid environment with 85% RH (see Supporting Information Figure S9). The PVA-IL ionic conductors with a 1∶2 mass ratio of PVA to ILs were also fabricated following the same procedures for the VPVA40%-IL ionic conductors. In sharp contrast, the loaded ILs leak out of the PVA-IL ionic conductor when the ionic conductor contacts a piece of polyethylene terephthalate substrate at room temperature and ∼30% RH (see Supporting Information Figure S10). Therefore, the grafted vanillin is critical for improving the compatibility of VPVA40% with ILs and stabilizing the VPVA40%-IL ionic conductors. The improved compatibility between ILs and VPVA40% mainly originates from the formation of hydrogen bonds between ILs and vanillin groups of VPVA40% chains. As shown in Figure 3a(i), the VPVA40%-IL ionic conductor can be used as a wire to illuminate a light-emitting diode (LED) bulb, when connected to a battery. As shown in Figure 3a(ii), the bulb remains powered-on when the VPVA40%-IL ionic conductor is stretched to ∼200% strain. After stress release, the VPVA40%-IL ionic conductor can spontaneously recover from a ∼200% strain back to its original shape at room temperature. The LED bulb also reverts to its original brightness (Figure 3a(iii)). Afterward, the VPVA40%-IL ionic conductors were subjected to 12,000 successive extension–release cycles at a strain of 100% to investigate their durability. The relative changes in ΔR/R0 were recorded in real time. As shown in Figure 3b, the ΔR/R0 of the VPVA40%-IL ionic conductor is highly stable and reproducible during 12,000 uninterrupted strain cycles. Because the tests took ∼25 h, occasional data fluctuations caused by environmental disturbances were unavoidable. The digital images in Figure 3c show that the dumbbell-shaped VPVA40%-IL ionic conductor can recover its original shape after 12,000 uninterrupted strain cycles, confirming that the conductor has excellent elasticity and fatigue resistance. During 1100 uninterrupted extension-release cycles at a strain of 400%, the VPVA40%-IL ionic conductor also exhibits a highly reproducible and reliable electrical response (Figure 3d). Figure 3e shows the stretchability and durability of the recently reported hydrogel-, organohydrogel-, and ionogel-based ionic conductors.10–14,19–21,24–31,34,35,49–58 Our VPVA40%-IL ionic conductors have record levels of stretchability and durability, demonstrating their considerable potential for use in flexible electronics and devices. Figure 3 | (a) VPVA40%-IL ionic conductor serving as a wire to illuminate an LED bulb, (i) before and (ii) after the conductor was stretched to ∼200% strain, and (iii) the recovered conductor. (b) Cyclic stability tests of the VPVA40%-IL ionic conductor under 100% strain for 12,000 cycles, where the insets show the enlarged signals of the ΔR/R0 over the first and the last 10 cycles. (c) Digital images of the VPVA40%-IL ionic conductor before (i) and after (ii) repeated stretching at 100% strain for 12,000 cycles. (d) Cyclic stability tests of the VPVA40%-IL ionic conductor under 400% strain for 1100 cycles, where the insets show the enlarged ΔR/R0 signals over the first and last 10 cycles. (e) Ashby plot showing the maximum number of stretching cycles and cyclic strains for the VPVA40%-IL ionic conductor and other hydrogel-, organohydrogel-, and ionogel-based ionic conductors as reported in the literature. It should be noted that the comparison of durability is based on the experimental results obtained in the related papers. The excellent properties of the ionic conductors in the related papers might not be fully exploited. Download figure Download PowerPoint The healing ability of the VPVA40%-IL ionic conductors is critically important for improving their durability and extending their service life. The VPVA40%-IL ionic conductor can restore its original conductivity after being cut into two pieces and then healed. As shown in Figures 4a(i) and 4a(ii), the LED bulb immediately powered-off when the VPVA40%-IL ionic conductor was cut into two pieces. After heating at ∼70 °C for 3 h, the separated VPVA40%-IL ionic conductor was fully healed and the LED bulb connected to the healed conductor was on again (Figure 4a(iii)). This indicates that the healing ability of the VPVA40%-IL conductor allows it to regain its ionic conductivity. The ionic conductivity of the healed VPVA40%-IL conductor can reach ∼0.69 S m−1 (see Supporting Information Figure S11). The healed ionic conductor can be stretched to 200% strain without fracturing while still powering an LED bulb (Figure 4a(iv)). Figure 4b shows the stress–strain curves of the damaged VPVA40%-IL ionic conductor after being healed for different times at ∼70 °C. The tensile strength of the ionic conductor gradually increases as the healing time increases. In terms of the restored tensile strength, healing efficiency reaches ∼93% after 3 h of healing time. When the ionic conductor is heated, the hydrogen bonds between VPVA40% chains are dynamically broken, endowing the VPVA40% chains with higher mobility. The interdiffusion and entanglement of the VPVA40% chains at the fracture surfaces along with the reformation of hydrogen bonds enable the complete healing of the damaged VPVA40%-IL ionic conductor. The fully healed VPVA40%-IL ionic conductor was subjected to 10 successive loading–unloading cycles at 200% strain. Figure 4c shows that the cyclic tensile curves of the healed VPVA40%-IL ionic conductor are almost identical to those of the pristine VPVA40%-IL ionic conductor, confirming that the elasticity of the healed ionic conductor is restored. More importantly, the healed VPVA40%-IL ionic conductor can retain its original ultra-durability. As shown in Figure 4d, the healed ionic conductor exhibits a highly stable, reproducible, and reliable electrical response for over 1100 cyclic tensile tests at 400% strain, which is identical to the undamaged ionic conductor. Thus, the excellent elasticity, fatigue resistance, healing ability, and durability of the VPVA40%-IL ionic conductors can significantly improve their reliability and extend their service life. As a proof-of-concept, we also demonstrated that the ultra-durable VPVA40%-IL ionic conductors can be used as wearable sensors for monitoring various human movements (see Supporting Information Figure S12). Figure 4 | (a) Healing of the VPVA40%-IL ionic conductor. (i) The original conductor, (ii) the same conductor that was cut in half, (iii) the healed conductor, and (iv) the healed conductor after being stretched to 200% strain. (b) Stress–strain curves of the VPVA40%-IL ionic conductor with different healing time. (c) Ten successive loading–unloading cycles of the healed VPVA40%-IL ionic conductor at 200% strain. (d) Cyclic stability tests of the healed VPVA40%-IL ionic conductor under 400% strain for 1100 cycles. The insets show the enlarged ΔR/R0 signals over the first and the last 10 cycles. Download figure Download PowerPoint Non-hazardous disposal of VPVA40%-IL ionic conductors Considering that stretchable ionic conductors will have huge production in the future, non-hazardous disposal is an essential property for effectively alleviating environmental pollution. Water soluble ILs can be extracted from the VPVA40%-IL ionic conductor by dialyzing the ionic conductor in water (Figure 5a). The recovery ratio of ILs is as high as ∼95%. Figure 5b shows that the 1H NMR spectrum of the recovered ILs is almost identical to that of the original ILs, indicating a high purity of the recovered ILs. Moreover, the stress–strain and cyclic tensile curves of the VPVA40%-IL ionic conductor fabricated using the recovered ILs overlap with those of the VPVA40%-IL ionic conductor using original ILs (Figure 5c and 5d). As shown in Supporting Information Figure S13, the VPVA40%-IL ionic conductor containing the recovered ILs also retained its original ionic conductivity (∼0.64 S m−1). These results indicate that the rec

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