Thermally Reversible Ion Gels with Photohealing ... - ACS Publications

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Thermally Reversible Ion Gels with Photohealing Properties Based on Triblock Copolymer Self-Assembly Takeshi Ueki,† Ryoji Usui,‡ Yuzo Kitazawa,‡ Timothy P. Lodge,§ and Masayoshi Watanabe*,‡ †

Department of Materials Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ‡ Department of Chemistry & Biotechnology, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Kanagawa 240-8501, Japan § Departments of Chemistry and Chemical Engineering & Materials Science, University of Minnesota, 207 Pleasant Street, SE, Minneapolis, Minnesota 55455, United States S Supporting Information *

ABSTRACT: We describe a functional soft material that can spontaneously repair damage by straightforward application of light illumination. The composite material is composed of a common ionic liquid (IL), 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim]PF6), and a well-defined ABA triblock copolymer consisting of the IL-compatible poly(ethylene oxide) (PEO) middle block with thermo- and photosensitive random copolymers combining N-isopropylacrylamide (NIPAm) and 4-phenylazophenyl methacrylate (AzoMA) including azobenzene chromophore as terminal A blocks. The composite shows a sol−gel transition under UV light (366 nm, 8 mW cm−2) irradiation at 47 °C, whereas that observed under visible light (437 nm, 4 mW cm−2) is 55 °C, due to the difference in photochromic states of the azobenzene unit. The ABA triblock copolymer undergoes a reversible gel−sol−gel transition cycle at the bistable temperature (53 °C), with a reversible association/fragmentation of the polymer network resulting from the photoinduced self-assembly of the ABA triblock copolymer in [C4mim]PF6. A damaged ABA ion gel shows a remarkable photohealing ability based on drastic changes in the fluidity of the polymer−IL composite triggered by light illumination. The damaged part is successfully repaired by shining UV light resulting in solubilization to fill the crack, followed by gelation to fix the crack triggered by visible light illumination. Tensile tests confirmed the excellent recovery efficiency of the resultant photohealed ABA ion gel, which was as high as 81% fracture energy relative to the original sample. The flexible, selfsupported ABA ion gel is designed for various applications to exhibit not only photohealing ability to improve operating lifetime of the material but also specific functionalities imparted by the IL, such as high ion conductivity, thermal stability, and (electro)chemical stability.

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molecular self-assembly,16 since it can be directly applied to an object in a contactless manner, is easy to control (on−off switching), and can be localized. Such optical switches have also been utilized as an on-demand trigger for self-healing materials.17 Photohealable materials, especially in structurally dynamic polymers, are based on photochromic molecules that can reversibly change structure under illumination. Well-known photochromic compounds such as coumarin,18,19 cinnamoyl,20 and disulfide bond groups21−24 are able to form or break a covalent bond reversibly by light illumination.25−29 On the other hand, supramolecular approaches, for example metal coordination bonds induced by light stimuli30 or inclusion compounds involving host−guest interactions with photochromic compounds31−34 (e.g., azobenzene and cyclodextrin),

INTRODUCTION Self-healable materials that can be repaired after being damaged are crucial to improve reliability, functionality, and operating lifetime for many industrial products. In polymeric materials, a traditional approach to repair damage involves liquefying the material by heating it to a temperature above its glass transition temperature (Tg) or melting point (Tm).1−4 The contacting polymer interface then allows for surface rearrangement in the cracked area, followed by wetting between surfaces, and finally diffusion and re-entanglement of polymer chains. In more recent advances, there have been many strategies aimed at establishing self-healing materials based on precisely controlled molecular building blocks that involved noncovalent linkages such as hydrogen bonds,5−8 Coulombic interactions,9 π−π interactions,10 hydrophobic hydration,11 and metal−ligand coordination.12−15 Although such methodologies allow damage recovery by straightforward physical contact, it can be difficult to control the repair precisely in terms of timing or position. A photo stimulus can be an effective external trigger to induce © XXXX American Chemical Society

Received: June 23, 2015 Revised: August 7, 2015

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DOI: 10.1021/acs.macromol.5b01366 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Synthetic procedure for P(AzoMA-r-NIPAm)-b-PEO-b-P(AzoMA-r-NIPAm) ABA triblock copolymer. (PEO) with hydroxyl end groups and Aliquat 336 were purchased from Sigma-Aldrich. NIPAm was generously provided by the Kojin Corporation and purified two times by recrystallization with toluene as good solvent and n-hexane as poor solvent. AIBN was recrystallized with methanol two times prior to use. PEO was dried by azeotropic distillation with toluene at 80 °C prior to use. The water content of [C4mim]PF6 was determined by Karl Fischer titration to be below 10 ppm. The chloride content in the aqueous phase contacted with [C4mim]PF6 could not be detected by the addition of AgNO3, suggesting that the chloride content in [C4mim]PF6 remained below the solubility limit of AgCl in water (1.4 mg L−1). Preparation of Block Copolymer. The ABA triblock copolymer was synthesized following the procedure shown in Figure 1.64−66 First, the PEO−CTA macro-chain-transfer agent was prepared as follows: oxalyl chloride (0.37 mL, 4.32 mmol) and CTA (1.05 g, 2.88 mmol) were mixed in dichloromethane (anhydrous, 7 mL) under an Ar atmosphere and stirred at room temperature until gas evolution ceased (∼2 h). Excess reagents were then removed under vacuum, and the residue was redissolved in dichloromethane (anhydrous, 20 mL), followed by the addition of PEO (6.0 g, 0.17 mmol). The reaction was allowed to proceed for 8 h at 30 °C, after which the contents were reprecipitated three times using THF as a good solvent and cold diethyl ether as a poor solvent, and dried under vacuum at 50 °C. Next, a round-bottom flask was charged with PEO−CTA (5.24 g, 0.159 mmol), AzoMA (4.76 g, 17.9 mmol), NIPAm (42.9 g, 379 mmol) ([AzoMA]/[NIPAm] = 4.5/95.5 by mol %), and 1,4-dioxane (130 mL) and purged with Ar for 30 min at 45 °C to avoid crystallization of NIPAm monomer during bubbling. AIBN (20 mg, 0.122 mmol) was separately dissolved in 10 mL of 1,4-dioxane in another round-bottom flask and purged with Ar for 30 min at room temperature. An aliquot of the AIBN solution (5.21 mL, 0.0636 mmol) was then added to the monomer solution. Reversible addition−fragmentation chain transfer (RAFT) polymerization was carried out at 65 °C for 95 h. The products were concentrated under reduced pressure and purified by conducting reprecipitation three times using acetone as a good solvent and cold diethyl ether as a poor solvent. Following the RAFT polymerization and purification, the dodecyl trithiocarbonate residue derived from the CTAs attached to the ABA polymer termini were removed according to the following procedure: ABA triblock copolymer (5.5 g, 0.157 mmol) and AIBN (1.04 g, 6.33 mmol) were dissolved in 80 mL of 2-propanol, followed by purging of the solution with Ar. The cleavage reaction was carried out at 80 °C for 8 h under an Ar atmosphere. The product was concentrated under reduced pressure; the resulting residue was purified by reprecipitation

are also utilized for photohealing materials based on the reversible noncovalent bonding linker. In this paper, we report a functional photohealable polymer composite, a photohealing ion gel, composed of a well-defined photochromic ABA triblock copolymer and an ionic liquid (IL), which can spontaneously be repaired after damage by drastic changes in the fluidity of polymer network with the aid of optical trigger. An ion gel is a soft solid material consisting of a polymer network35−38 or nanoparticle dispersion39−42 swollen with a substantial amount of IL. Ion gels are now recognized as promising platforms in many applications, such as polymer electrolyte membranes,43−45 actuators,46,47 gas separation membranes,48−50 catalysis support membranes,51,52 flexible electrochemiluminescent gels,53 and gate dielectrics in organic thin-film transistors (OTFTs).54−56 Stimuli-responsive (hydrated) ion gels are also designed for switching or sensing applications.37,57−60 Key attributes of these soft materials include facile ion or solute transport, chemical and thermal robustness, tunable modulus, and negligible vapor pressure. In this study, a polymer composite consisting of a common hydrophobic IL, 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim]PF6), and a well-defined ABA triblock copolymer having an IL-compatible poly(ethylene oxide) (PEO) middle block with thermo-/photoresponsive random copolymer of N-isopropylacrylamide and 4-phenylazophenyl methacrylate (P(AzoMA-r-NIPAm)) at both ends of the block copolymer was prepared. The photohealing ion gel proposed here preserves the attractive physicochemical properties imparted by ILs such as nonvolatility, thermal stability, electrochemical stability, and high ion conductivity, while maintaining a soft solid consistency.

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EXPERIMENTAL SECTION

Materials. [C4mim]PF6,61 S-1-dodecyl-S′-(α,α′-dimethyl-α″-acetic acid)trithiocarbonate (chain transfer agent (CTA)),62 and 4-phenylazophenyl methacrylate (AzoMA)63 were synthesized and characterized according to previous reports. Dichloromethane (super dehydrated), oxalyl chloride, toluene, n-hexane, tetrahydrofuran (THF), diethyl ether, 2,2′-azobis(isobutyronitrile) (AIBN) 1,4dioxane (super dehydrated), and 2-propanol (super dehydrated) were purchased from Wako Chemical. Telechelic poly(ethylene oxide) B

DOI: 10.1021/acs.macromol.5b01366 Macromolecules XXXX, XXX, XXX−XXX

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Photohealing Treatment. Ion gel samples were molded into star or rectangle shapes with 1 mm thickness, and the center part in the ion gels was cut with a razor blade. UV light was directed to the cut portion by using a photomask for 64 h at 36 °C, and then visible light irradiation was applied to the ion gels for 30 min. Tensile Test. Tensile tests were conducted with 1 mm thick rectangular ion gel samples at room temperature and a strain rate of 6 mm min−1 on a tensile testing machine (EZ-LX, Shimadzu) with load cell for 10 N.

three times using acetone as a good solvent and cold diethyl ether as a poor solvent and, finally, dried under vacuum at room temperature overnight. The structure and average molecular weight of the polymers were characterized by a combination of 1H NMR spectroscopy and size exclusion chromatography (SEC) using dimethylformamide (DMF) containing 0.01 mol L−1 LiBr as the carrier solvent (Figure S1 in Supporting Information). The total number-average molecular weight (Mn) of the triblock copolymer was calculated from the ratio of integrated signal intensities between the PEO main chain and the end blocks (δ = 4.1 ppm from NIPAm and δ = 7.8 ppm from AzoMA). The AzoMA composition was calculated using the ratio of integrated signal intensities between NIPAm and AzoMA (Figure S2). The columns (Tosoh) used for SEC analyses were calibrated using polystyrene standards. Polymer characterization results are summarized in Table 1.

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RESULTS AND DISCUSSION The ABA triblock copolymer was prepared by RAFT random copolymerization of NIPAm and AzoMA from a PEO-based, telechelic macro-CTA.64−66 The P(AzoMA-r-NIPAm) random copolymer end blocks of the triblock exhibit an upper critical solution temperature (UCST)-type phase transition (solid− liquid demixing on cooling)67,68 as well as a glass transition in [C4mim]PF6, irrespective of the photochromic state of azobenzene. The aggregation temperature can alter by changes in the molecular weight, concentration of polymer, and [AzoMA] monomer composition incorporated into the random copolymer segment.64,65 Thus, the composite solidifies at low temperature owing to the three-dimensional network formation inside the IL and liquefies at higher temperature due to dissolution of the P(AzoMA-r-NIPAm) domains. Figure 2a shows the temperature dependence of the dynamic moduli (G′ and G″) for a [C4mim]PF6 solution containing 20 wt % ABA triblock copolymer under different photochromic states. The sol−gel transition temperature (Tgel) of the ion gel changes depending on the photochromic state of the AzoMA. The Tgel is defined as the temperature where G′ exceeds G″ for a given frequency and strain (ω = 6.28 rad s−1, γ = 1%). Under UV light (366 nm, 8 mW cm−2) irradiation, Tgel ≈ 47 °C, whereas that observed under visible light (437 nm, 4 mW cm−2) is 55 °C. We have determined from 1H NMR spectroscopy measurements in the photostationary state that the major component (80%) under UV light irradiation was cis-AzoMA (cis-ABA triblock copolymer), while trans-AzoMA was the major component (85%) under visible light (trans-ABA triblock copolymer).69 It is reported that the dipole moment and dielectric constant of [C4mim]PF6 are μ = 5.3 D and ε = 11.4, respectively, indicating moderate polarity.70,71 It is well-known that the polarity of azobenzene varies with isomerization state; trans-azobenzene has a dipole moment of 0.5 D, while cisazobenzene has a higher polarity of 3.1 D.72 For this reason, the relatively higher polarity cis-ABA triblock copolymer is more

Table 1. Characterization of Polymers polymer

Mn/kDa

Mw/Mn

[AzoMA]/[NIPAm] (by mol)

PEO ABA

33.0 15.0−33.0−15.0

1.28 1.44

10.4/89.6

Preparation of Ion Gels. The ABA triblock copolymer was first dissolved in THF. An appropriate amount of [C4mim]PF6 was then added to the homogeneous THF solution with continuous stirring for at least 3 h until a clear solution was obtained. The volatile THF was then evaporated by heating the solution at 80 °C under reduced pressure for 24 h. Photoirradiation. A 500 W high-pressure mercury lamp (Ushio Optical Modulex USH-500SC, BA-H500) was used as the light source. The wavelength of irradiated light (UV: 366 nm with 8 mW cm−2; visible: 437 nm with 4 mW cm−2) was adjusted by using line filters. A heat-absorbing filter was also used to reduce the heat generated by the mercury lamp. Rheology under Photoirradiation. Oscillatory shear measurements were conducted on an Anton Paar Physica MCR 301 rheometer with a standard cell for UV measurements (P-PTD200/GL and HPTD200, Anton Paar). Parallel plates (25 mm diameter) were used, and the gap spacing was kept to approximately 0.2 mm to decrease the light path length. UV or visible light illuminated the sample from the bottom side for at least 1 h to reach a photostationary state prior to measurement. Temperature scanning measurements were performed with a strain of γ = 1%, which is within the linear viscoelastic regime; a frequency of ω = 6.28 rad s−1 and a cooling rate of 0.2 °C min−1 in the range from 80 to 10 °C were used under continuous UV or visible light irradiation. The photoswitching measurements were also performed with the same strain and frequency. The frequency dependence of the dynamic moduli were examined over the range from 0.1 to 100 rad s−1 with strain of γ = 1%.

Figure 2. (a) Variation of dynamic storage (G′, solid symbols) and loss (G″, open symbols) moduli of the ABA triblock copolymer solution (20 wt %) in [C4mim]PF6 as a function of temperature at frequency ω = 6.28 rad s−1 and strain amplitude γ = 1% under irradiation with UV (circles) and visible (diamonds) light. (b) Reversible gel−sol−gel transitions of the ABA triblock copolymer (20 wt %) in [C4mim]PF6 induced by switching vis− UV−vis light irradiation at a “bistable” temperature (53 °C). Irradiation period of visible and UV light is indicated by solid and dotted arrows, respectively. C

DOI: 10.1021/acs.macromol.5b01366 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules soluble in the IL, leading to gelation at a lower temperature. On the other hand, lower polarity trans-ABA triblock copolymer under visible light illumination shows a higher Tgel, which is consistent with the results reported earlier.64−66 Importantly, a difference of 8 °C between the Tgel of trans- and cis-ABA was observed. This provides an opportunity to demonstrate a photoinduced sol−gel transition cycle at a bistable temperature by irradiating UV and visible light alternately on the ion gel. A reversible gel−sol−gel transition cycle showing changes in the macroscopic state of the composite induced by an optical trigger is exhibited in Figure 2b. The ion gel sample was kept at a bistable temperature of 53 °C under visible light illumination before the measurements to form the trans-ABA ion gel, followed by exposing the sample to UV light at t = 0. The G′ value drastically dropped upon UV light irradiation and decreased below G″ within 200 s. The illumination was then switched back to visible light to re-form the trans-ABA ion gel at t = 1800 s, and G′ recovered to its initial level and exceeded G″, indicating reversible gelation. The result indicates that the polymer network composed of photosensitive physical crosslinking points formed inside the composites can melt and rebuilt rapidly under light illumination. The sol−gel transition temperature is controllable by photochromic state of azobenzene chromophore in the random copolymer that can drastically change the solubility of the entire polymer in the IL. Elasticity theory predicts that the network modulus is proportional to kT per network strand;73 i.e., G′ ∼ nkT, where n is the number density of network strands inside the correlation volume and k and T are the Boltzmann constant and absolute temperature, respectively. From this relation, n before and after five sol−gel transition cycles was estimated to be 5.2 × 1023 and 5.3 × 1023 m−3, respectively.66 Keeping constant n (G′) confirms an efficient reversibility of formation and breakup of network strands during photoswitching cycles, suggesting material stability without any solvent evaporation even after light exposure at 53 °C for over 5 h.66 The rapid and reversible gel−sol−gel transition cycle in [C4mim]PF6 induced by light illumination favors the possibility of a photohealing process. In order to evaluate the photohealing ability of the ion gel, we selected 36 °C, where the ion gel still retained a self-supporting solid-like consistency without any fluidity. The present ion gel formed by physical crosslinking can be categorized as a viscoelastic solid; thus, Tgel eventually decreases with increases in the observation time, i.e., decreases in frequency (see Figures S2 and S3). Interestingly, the relaxation time, i.e., the observation time, of the ion gel is greatly affected by light illumination. In other words, when UV light is irradiated (cis-ABA triblock copolymer), the relaxation time becomes much shorter than that of trans-ABA triblock copolymer (in the dark or visible light irradiation). We have achieved the relaxation time difference by 3 orders of magnitude at a constant temperature (see Figure S3) by the photoisomerization of AzoMA. Figure 3 exhibits photographs showing the time course of photohealing of a damaged part in an ion gel exposed to UV light. The star-shaped ion gel was cut at the center before the demonstration. By shining localized UV light at the slit, the damaged part gradually liquefies to form a sol, and the damaged part is completely filled and recovered after 64 h. The sol state exposed to UV light is then firmly fixed in the gel state by switching to visible light again for 30 min. This clearly indicates that the ion gel recovers from being damaged by optically triggered wetting between fractured interfaces; individual chains diffuse and re-entangle, followed by

Figure 3. Photographs showing photohealing of a star shaped ABA ion gel (20 wt %) cut at the center: (a) damage before applying light stimuli, (b) in progress of repairing under 24 h of UV light irradiation, and (c) completely repaired damaged part after 64 h UV light exposure, followed by 30 min visible light exposure. The cracked part is highlighted by white dotted square.

immobilization in physical cross-linking points by visible light illumination. We further applied tensile tests on the ion gel before and after photohealing in order to evaluate the healing efficiency. Ion gel samples with standardized size, thickness (1 mm), and polymer concentration inside the composite were fabricated for the tensile tests (Figure 4a−c). The original

Figure 4. (a) Geometry of the sample (20 wt % ABA triblock copolymer in [C4mim]PF6) for tensile test. (b) A photograph showing actual view of the sample with a slit (damaged part). (c) Illustration of tensile test showing orientation of the applying stress to the sample. (d) Stress−strain curves for (A) original, (B) damaged, (C) photohealed after being damaged, and (D) aged under dark for 64 h at 36 °C without any photohealing process after being damaged.

specimen without any damage fractured at over 450% strain, while the ion gel with a slit at the center was, not surprisingly, fractured after only 200% strain (Figure 4d). The photohealed sample that had undergone the UV light irradiation for 64 h and then visible light illumination for 30 min showed a strainto-break of 420%, comparable to that of the original undamaged sample. For reference, sample C stored under dark for 64 h at 36 °C after being damaged without any photohealing procedure shows almost the same fracture strength as sample B. This suggests that diffusion, reorientation, and re-entanglement of the polymer chains inside the ion gel still occur, but very slowly, under dark conditions (healing efficiency: 6%, as mentioned below). On the other hand, light illumination at the damaged part to the sample enhances the kinetics of diffusion, reorientation, and re-entanglement of the ABA triblock copolymer in the ion gel significantly, resulting in excellent healing efficiency. Furthermore, we estimated the fracture energy from the area of the stress−strain curve for each sample (Figure 5a). The results confirm that the rupture energy of photohealed ion gel is almost comparable to that of original sample before being damaged. The tensile test is reproducible, if the thickness and size of the specimen is properly adjusted to be the same, as shown in Figure 4. The average maximum strain of the original ion gel for 5 times measurements is 451.2% with D

DOI: 10.1021/acs.macromol.5b01366 Macromolecules XXXX, XXX, XXX−XXX

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Figure 5. (a) Fracture energy calculated from the area of stress−strain curve obtained from each sample and (b) healing efficiency of photohealed sample together with that of the sample without applying any photohealing process. The healing efficiency is estimated by the difference between the fracture energy of the original to that of damaged samples, divided by the difference between the fracture energy of photohealed to damaged samples.

H NMR spectrum and size exclusion chromatography trace of P(AzoMA-r-NIPAm)-b-PEO-b-P(AzoMA-rNIPAm) ABA triblock copolymer; master curves of G′ and G″ for 20 wt % ABA triblock copolymer solution in [C4mim]PF6 under visible light and UV light irradiation; relationship between gelation temperature and observation time (τ = 2π/ω, where τ and ω are observation time and frequency, respectively) for 20 wt % ABA triblock copolymer solution in [C4mim]PF6 (PDF)

AUTHOR INFORMATION

Corresponding Author

*(M.W.) E-mail: [email protected]. Author Contributions

T.U. and R.U. equally contributed to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grants-in-Aid for Scientific Research (A/23245046 and S/15H05758 to M.W. and 15H05495 and 26620164 to T.U.) from the MEXT of Japan. Y.K. acknowledges financial support from the Research Fellowship of the Japan Society for the Promotion of Science for Young Scientists (13J00192). T.P.L. acknowledges the support of the National Science Foundation through Award DMR-1206459.

a standard deviation σ = 22.0. However, if the ion gel accidentally contains bubbles, the sample is immediately ruptured at a low strain level from the defect, leading to poor mechanical properties. The self-healing mechanism at a polymer−polymer interface for temperatures higher than Tg has been closely investigated.1−4,13 One theoretical study proposed that the rupture strength would scale with time and molecular weight as1

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σ ∼ t 1/4M −1/4 where σ is the rupture strength and t and M are contact time between two polymer interfaces and average molecular weight of the polymer, respectively. In order to further accelerate the photohealing process, polymer diffusion at the damaged interface should be improved by reducing M of the block copolymer or increasing the bistable temperature. Healing efficiency is calculated from the ratio of ruptured strength shown in Figure 5a. The efficiency of the light illuminated sample reaches as high as 81% relative to the sample before being damaged, whereas that of the sample stored under dark (without light illumination) is 6% (Figure 5b).

(1) Kim, Y. H.; Wool, R. P. Macromolecules 1983, 16, 1115−1120. (2) Jud, K.; Kausch, H. H. Polym. Bull. 1979, 1, 697−707. (3) Prager, S.; Tirrell, M. J. Chem. Phys. 1981, 75, 5194−5198. (4) Wool, R. P.; O’Connor, K. M. J. Appl. Phys. 1981, 52, 5939− 5963. (5) Cordier, P.; Tournilhac, F.; Soulié-Ziakovic, C.; Leibler, L. Nature 2008, 451, 977−980. (6) Zhang, X.; Wang, C. Chem. Soc. Rev. 2011, 40, 94−101. (7) Feldman, K. E.; Kade, M. J.; Meijer, E. W.; Hawker, C. J.; Kramer, E. J. Macromolecules 2009, 42, 9072−9081. (8) Hentschel, J.; Kushner, A. M.; Ziller, J.; Guan, Z. Angew. Chem., Int. Ed. 2012, 51, 10561−10565. (9) Wang, Q.; Mynar, J. L.; Yoshida, M.; Lee, E.; Lee, M.; Okuro, K.; Kinbara, K.; Aida, T. Nature 2010, 463, 339−343. (10) Burattini, S.; Greenland, B. W.; Merino, D. H.; Weng, W.; Seppala, J.; Colquhoun, H. M.; Hayes, W.; Mackay, M. E.; Hamley, I. W.; Rowan, S. J. J. Am. Chem. Soc. 2010, 132, 12051−12058. (11) Tuncaboylu, D. C.; Sari, M.; Oppermann, W.; Okay, O. Macromolecules 2011, 44, 4997−5005. (12) Schubert, U. S.; Hofmeier, H.; Newkome, G. R. Modern Terpyridine Chemistry; Wiley-VCH: Weinheim, 2008, and references therein. (13) Cook, T. R.; Zheng, Y. − R.; Stang, P. J. Chem. Rev. 2013, 113, 734−777. (14) Beletskaya, I.; Tyurin, V. S.; Tsivadze, A. Y.; Guilard, R.; Stern, C. Chem. Rev. 2009, 109, 1659−1713. (15) Ueki, T.; Takasaki, Y.; Bundo, K.; Ueno, T.; Sakai, T.; Akagi, Y.; Yoshida, R. Soft Matter 2014, 10, 1349−1355. (16) Zhao, Y. Macromolecules 2012, 45, 3647−3657. (17) Fiore, G. L.; Rowan, S. J.; Weder, C. Chem. Soc. Rev. 2013, 42, 7278−7288. (18) Ling, J.; Rong, M. Z.; Zhang, M. Q. J. Mater. Chem. 2011, 21, 18373−18380. (19) Ling, J.; Rong, M. Z.; Zhang, M. Q. Polymer 2012, 53, 2691− 2698.

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CONCLUSION In this paper, we report a novel soft material that can spontaneously be repaired after being damaged in response to an optical trigger. The photohealing ability of the ion gel is based on the significant enhancement of the kinetics of polymer diffusion, reorientation, and re-entanglement inside the composite induced by straightforward light stimuli and recovered to 81% of the strength of the undamaged (original) sample. Although there have been many proposed self-healable materials that can recover from being damaged, more conventional composites were not designed to exhibit specific function such as high ionic conductivity. This photohealing ion gel is therefore promising from the perspective of designable soft materials, such as plastic electronics, actuators, and electrochromic gels.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01366. E

DOI: 10.1021/acs.macromol.5b01366 Macromolecules XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.macromol.5b01366 Macromolecules XXXX, XXX, XXX−XXX