Physicochemical Characterization of a Photoinduced Sol–Gel

Article pubs.acs.org/Macromolecules

Physicochemical Characterization of a Photoinduced Sol−Gel Transition of an Azobenzene-Containing ABA Triblock Copolymer/ Ionic Liquid System Xiaofeng Ma,‡ Ryoji Usui,‡ Yuzo Kitazawa,‡ Ryota Tamate, Hisashi Kokubo, and Masayoshi Watanabe* Department of Chemistry & Biotechnology, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan S Supporting Information *

ABSTRACT: We have previously reported that an ABA triblock copolymer with poly(ethylene oxide) as the B block and a thermo-/photoresponsive random copolymer of 4-phenylazophenyl methacrylate (AzoMA) and N-isopropylacrylamide (NIPAm) (P(AzoMA-r-NIPAm)) as the A block exhibited a reversible photoinduced sol−gel transition in a hydrophobic ionic liquid. Such reversible sol−gel transitions can be used to realize photohealable ion gels. This study contributes to a deeper physicochemical understanding of the photoinduced sol−gel transitions observed in the ion gel, showing that the glassy state of P(AzoMA-r-NIPAm) block domains is responsible for the long relaxation times. Importantly, the controlled photoisomerization of azobenzene moieties allows the glass transition temperature of P(AzoMA-r-NIPAm) block domains to be varied by choosing irradiation conditions. The photoinduced sol−gel transition can be understood as an elastic solid−viscoelastic liquid or a viscoelastic solid−viscoelastic liquid transition, depending on the dissimilarity in the magnitude of relaxation times under visible and UV light.

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midblock-selective ionic liquids (ILs)11−18 and have attracted considerable attention in the past decade due to allowing the unique properties of ILs (e.g., high thermal and chemical stability, negligible vapor pressure, low flammability, and high ionic conductivity)19−21 and the mechanical consistency of ion gels to be preserved. Additionally, the structure and physical properties of these gels can be readily controlled by variation of copolymer block length, architecture, and composition,22 allowing abundant applications such as gas separation,23 lowvoltage flexible electrochemiluminescent devices,24−26 electromechanical actuators,27 organic thin-film transistors,28−31 and photohealable materials.32,33 Recently, we have developed a thermo-/photoresponsive ion gel by gelation of a triblock copolymer comprising IL-philic poly(ethylene oxide) (PEO) as the midblock and a thermo-/ photoresponsive random copolymer of 4-phenylazophenyl methacrylate (AzoMA) and N-isopropylacrylamide (NIPAm) (P(AzoMA-r-NIPAm)) as end blocks in a hydrophobic IL, 1butyl-3-methylimidazolium hexafluorophosphate ([C4mim]PF6).16 The prepared ion gel exhibited a high-temperature sol state and a low-temperature gel state due to the upper critical solution temperature (UCST)-type phase transition behavior of P(AzoMA-r-NIPAm). In addition, the ion gel undergoes photoinduced reversible sol−gel transition at certain temper-

INTRODUCTION Physically associating ABA triblock copolymer gels are produced by dissolving these copolymers in a midblockselective solvent (aqueous or organic) at a sufficiently high concentration and an appropriate temperature.1−3 Thermoreversible gels formed by ABA triblock copolymers are of technological interest for a number of applications, e.g., bioerodible/biocompatible scaffold networks for drug delivery,4 wound healing,5 microfluidic substrates,6 and high-performance dielectric elastomers.7 The thermal behavior of such triblock copolymer gels is characterized by three transition temperatures: the critical micelle temperature (CMT, onset temperature of end block aggregation, i.e., phase separation temperature of end blocks), the gelation temperature (Tgel, temperature at which the relaxation time is comparable to the experimental time scale), and the glass transition temperature of the cross-linking domain (Tg, temperature at which the gel undergoes a transition from a viscoelastic state to an elastic one).8−10 In strongly midblock-selective and end-block-nonselective solvents, the above gels behave as elastic materials, since the cross-linking domains consist of almost neat end blocks, the Tg of which is higher than experimental temperatures. However, cross-linking domains are plasticized by less midblock-selective solvents, which results in Tg < CMT and thus affords a viscoelastic response between Tg and CMT,10 with detailed discussion of these transitions found elsewhere.8,9 Physically associating ABA triblock copolymer ion gels can be formed by the self-assembly of these copolymers in © XXXX American Chemical Society

Received: July 19, 2017 Revised: August 14, 2017

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used under both UV (366 nm, 8 mW cm−2) and visible (437 nm, 4 mW cm−2) light irradiation. A 500-W high-pressure mercury lamp (Ushio Optical Modulex, BA-H50) was used as a light source, with the wavelength of emitted light adjusted using color filters. A heatabsorbing filter was used to reduce the heat generated by the above lamp.

atures and thus being suitable for realizing photohealable materials.32 However, if the relaxation time in the dark/under visible light irradiation is too short at the photohealing temperature, the ion gel quickly loses its solid behavior even after healing. In view of this, previous photohealing experiments were conducted at 36 °C (i.e., below the gelation temperatures (Tgel) under visible (55 °C) and UV light (47 °C) irradiation), because the ion gel could not retain its self-supporting solid-like consistency during healing at 47−55 °C due to the short relaxation times observed under these conditions.32 Conversely, under UV light irradiation, overly long relaxation times at the photohealing temperature result in very slow photohealing due to the longer time required for the photoinduced gel−sol transition (e.g., the above healing process took ∼64 h to complete at 36 °C under UV light irradiation). Therefore, further ion gel optimization is required for its application in photohealable materials, which, in turn, can be achieved by further understanding the parameters controlling the reversible photoinduced sol−gel transition. In this study, the mechanism of the above transition was closely investigated by differential scanning calorimetry (DSC) and rheological measurements. DSC characterization demonstrated that P(AzoMA-r-NIPAm) block domains exhibit a glass−rubber transition in an ion gel containing 20 wt % of P(AzoMA-r-NIPAm)-b-PEO-b-P(AzoMA-r-NIPAm). Subsequently, the Tgs of P(AzoMA-r-NIPAm) block domains in the ion gel under visible and UV light irradiation were determined by rheological measurements, and the behavior of the ion gel was explored at various temperatures and under different irradiation conditions. Finally, the photoinduced sol− gel transition was explained by the different relaxation behavior of the ion gel under UV and visible light irradiation in a certain temperature range.

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RESULTS AND DISCUSSION Thermo-/Photosensitivity of the Ion Gel. Figure 1 shows the temperature dependences of G′ and G″ of 20 wt %

Figure 1. Temperature dependences of G′ and G″ for 20 wt % ABA triblock copolymer in [C4mim]PF6 under UV (blue circles) and visible (red diamonds) light irradiation at a frequency of ω = 0.1 rad s−1, a strain amplitude of γ = 1%, and a cooling rate of 0.2 °C min−1.

ABA triblock copolymer in [C4mim]PF6 under visible and UV light irradiation in a cooling process. Under visible light irradiation, most azobenzene molecules exist in the trans-form, whereas the cis-form prevails under UV light irradiation.36 At high temperatures, G″ values exceeded those of G′ at the observation time scale (τ = 2π/ω = 62.8 s) under both irradiation conditions, indicating that the investigated solutions were in a sol state. Conversely, the above trend was reversed at low temperatures, indicating the formation of a gel state. The sol−gel transition temperature (Tgel) was defined as the crossover of G′ and G″, equaling 45 and 37 °C for visible and UV light irradiation, respectively, and thus being dependent on the cis/trans configuration of azobenzene units in the polymer. It should be noted that the Tgel is dependent on the observation time scale, i.e., the experimental frequency. Bistable temperature is defined to be limited by Tgel values for UV and visible light irradiation, equaling 37−45 °C in this particular case, whereas a range of 47−55 °C was reported in our previous study. 32 This difference was ascribed to the different observation times used in these investigations (previous τ = 1 s, current τ = 62.8 s). Since the ion gel presented herein was a viscoelastic material, the corresponding sol−gel transition phenomena were time- and temperature-dependent. Figure 2 shows the photoinduced gel−sol−gel transition observed at a temperature of 40 °C (i.e., within the bistable temperature range). Initially, the sample was irradiated by visible light to form the gel state (G′ > G″), with a switch to UV light performed at time =0 s resulting in a drastic decrease of G′ to values below those of G″ within 600 s, indicating the occurrence of a photoinduced gel−sol transition. Subsequently, the illumination was switched back to visible light at time =2600 s, resulting in a sharp increase of G′ to values above those of G″ within 200 s, demonstrating the occurrence of a reversible photoinduced sol−gel transition.

EXPERIMENTAL SECTION

Materials. PNIPAm (Mn = 39 kDa, determined by 1H NMR; polydispersity index (PDI) = 1.22, determined by gel permeation chromatography) was prepared and characterized as described elsewhere.34 The utilized ABA triblock copolymer (A block = thermo-/photoresponsive P(AzoMA-r-NIPAm) with Mn = 15 kDa, B block = IL-compatible PEO with Mn = 33 kDa; PDI = 1.41; AzoMA content = 10.4 mol %) was identical to that used in our previous work.32 [C4mim]PF6 and [C2mim][NTf2] were synthesized and characterized according to procedures described elsewhere,35 exhibiting water contents below 10 ppm (determined by Karl Fischer titration). Preparation of Polymer Solutions and Ion Gels. PNIPAm/ [C4mim]PF6 and PNIPAm/[C2mim][NTf2] solutions and an ABA triblock copolymer/[C4mim]PF6 ion gel were prepared using a cosolvent method. The polymer was dissolved in THF, followed by the addition of an appropriate amount of IL, and the reaction mixture was stirred for a minimum of 3 h until a transparent solution was obtained. Finally, THF was removed in vacuo at 80 °C over a period of 24 h. DSC Measurements. DSC measurements were performed at temperatures between 0 and 100 °C on a Seiko Instruments DSC 6220 machine in an atmosphere of nitrogen, with temperature calibration performed using the melting point of indium. Samples were tightly sealed in Al pans and aged at 25 °C for different times, being subsequently heated at a rate of 10 °C min−1. Rheological Measurements. Oscillatory shear measurements were conducted on an Anton Paar Physica MCR 301 rheometer using a 25 mm parallel plate geometry. A gap spacing of ∼0.2 mm was used for all measurements. Dynamic shear moduli (G′ and G″) were examined in the linear viscoelastic regime. Standard cells for UV measurements (P-PTD200/GL and H-PTD200, Anton Paar) were B

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S1), which was obviously higher than the above value of 45 °C. This behavior was explained by the presence of physical crosslinking domains containing small amounts of [C4mim]PF6 as a plasticizer, which dramatically decreased the Tg of P(AzoMA-rNIPAm) blocks. A similar Tg reduction induced by the plasticizing effect of IL was also observed for physical crosslinks in other triblock copolymer−based ion gel systems.12 Moreover, this effect was also observed for PNIPAm, supporting the hypothesis that physical cross-linking domains contained small IL amounts. DSC curves of the PNIPAm/ [C2mim][NTf2] system comprised both the low-temperature endothermic peak corresponding to Tg and the high-temperature endothermic peak corresponding to UCST (Figure 3b and Table S1). However, the latter peak was not observed for P(AzoMA-r-NIPAm) blocks (Figure 3a), probably because their UCST exceeded the upper limit of the measuring temperature range, i.e., the ion gel could not be characterized at temperatures above 100 °C due to the noticeable degradation of PEO chains in [C4mim]PF6 above 120 °C.41 We have previously reported that 3 wt % P(AzoMA-r-NIPAm) in [C4mim]PF6 had a UCST of ∼80 °C, which increased with increasing polymer concentration.42,43 Since the content of P(AzoMA-r-NIPAm) blocks in the present ion gel was comparatively high (9.5 wt %), their UCST was probably outside the measuring temperature range, as stated above. This hypothesis was supported by the absence of a UCST-related endothermic peak in the DSC curve of 20 wt % PNIPAm in [C4mim]PF6 (Figure S2), which was also ascribed to high polymer content. Thus, it was concluded that P(AzoMA-rNIPAm) blocks of the triblock copolymer underwent both UCST-type and glass−rubber transitions in [C4mim]PF6. Since the Tgs of P(AzoMA-r-NIPAm) block domains of the ion gel under UV and visible light irradiation were difficult to determine by DSC owing to the use of a hermetic cell, we determined these values based on the results of rheological measurements using the principle of time−temperature superposition. Determining the Tg of P(AzoMA-r-NIPAm) Blocks in the Ion Gel under Dark Conditions Using Rheological Measurements. Figure 4 shows the master curve of the time− temperature superposition of the storage modulus (G′), recorded under dark conditions (where azobenzene exists in the trans-state (≈ 100%))36 at 46 °C as a reference temperature (data taken from Figure S3). The above data well obeyed the principle of time−temperature superposition, suggesting that ion gel elasticity (G′) was dominated by primary relaxation attributable to the micro-Brownian motion of amorphous segments in the measurement temperature range. The master curve of the time−temperature superposition of the loss modulus (G″) at 46 °C as the reference temperature is shown in Figure S4, with deviations from the above principle observed at high frequencies attributed to the motion of bridging PEO blocks related to the viscosity variation of [C4mim]PF6.12 However, as shown in Figure S5, these deviations could be taken into account by using a shift factor (aT) calculated from the above viscosity variation (aT = η/ηref, where η and ηref are viscosities of [C4mim]PF6 at temperature T and at the reference temperature, respectively). At the reference temperature, the terminal relaxation time (τref) of the ion gel was estimated from the crossover frequency (ωc) at which the G′ and G″ values were equal (inset in Figure 4) as τref = 2π/ωc, with relaxation times at other temperatures calculated as τ = τref aT. Figure 5a shows the Arrhenius plots of the ion gel relaxation

Figure 2. Photoinduced reversible gel−sol−gel transition for 20 wt % ABA triblock copolymer in [C4mim]PF6 under UV (blue circles) and visible (red diamonds) light irradiation at 40 °C, ω = 0.1 rad s−1, and γ = 1%.

DSC Traces of the ABA Triblock Copolymer/IL System. Figure 3a shows DSC curves of the 20 wt % ABA triblock

Figure 3. (a) DSC curves of 20 wt % ABA triblock copolymer in [C4mim]PF6 recorded for different aging times at a heating rate of 10 °C min−1. (b) DSC curves of 20 wt % PNIPAm in [C2mim][NTf2] with different aging times at a heating rate of 10 °C min−1.

copolymer/[C4mim]PF6 system measured at a heating rate of 10 °C min−1 for different aging times. An endothermic peak observed at ∼45 °C shifted to higher temperatures with increasing aging time, being typical of enthalpy relaxation observed for amorphous polymers.37−39 Therefore, the above peak was attributed to the glass−rubber transition of P(AzoMA-r-NIPAm) blocks, since the Tg of PEO lies well below room temperature.40 P(AzoMA-r-NIPAm) blocks in bulk ABA triblock copolymer exhibited a Tg of 92 °C (Figure C

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Figure 4. G′ master curve constructed using time−temperature superposition for 20 wt % ABA triblock copolymer in [C4mim]PF6 in the dark at a reference temperature of 46 °C. Inset shows the variation of G′ and G″ as a function of frequency in the range of 0.1 to 100 rad s−1 at 46 °C and a constant strain of γ = 1%.

Figure 5a shows that above 43 °C, the relaxation times well followed the VFT model (dashed line), diverging from it at lower temperatures, with the relaxation time at 43 °C being of the order of 102 s. Thus, this result indicates that the Tg of P(AzoMA-r-NIPAm) block domains equaled ∼43 °C, in agreement with DSC analysis results (Figure 3a). Increased τ values were observed for samples aged at temperatures lower than Tg (43 °C), which was attributed to the enthalpy relaxation of glassy P(AzoMA-r-NIPAm) block domains, whereas τ was independent of aging time at temperatures above Tg. This result confirms that P(AzoMA-r-NIPAm) block domains exhibit a glassy state below 43 °C, as additionally supported by the effect of aging on ion gel elasticity. Figure 5b shows the temperature dependences of G′ and G″ for aged (at 25 °C) and nonaged ion gels. At temperatures below Tg, higher G′ and decreased tan δ (tan δ = G″/G′) values were observed for aged samples, indicating that the aging-induced hardening of physical cross-links formed by P(AzoMA-r-NIPAm) blocks was caused by the enthalpy relaxation of glassy P(AzoMA-rNIPAm) block domains. On the other hand, at temperatures above Tg, almost no G′ changes were observed, suggesting that the P(AzoMA-r-NIPAm) block domains of the ion gel exist in a glassy state at temperatures below Tg and in a rubbery state at temperatures above Tg. Therefore, the behavior of this ion gel can be defined as an elastic state below Tg and a viscoelastic state between Tg and the UCST of P(AzoMA-r-NIPAm) blocks. Tg of P(AzoMA-r-NIPAm) Blocks in the Ion Gel under Visible and UV Light Irradiation. Figure 6 shows Arrhenius plots of ion gel relaxation times under visible and UV light irradiation, with the profiles of both plots resembling that observed under dark conditions (Figure 5a). The Tgs of P(AzoMA-r-NIPAm) block domains of the ion gel under visible and UV light irradiation equaled 41 and 34 °C, respectively. Previously, the photostationary contents of trans-azobenzene in a random copolymer of benzyl methacrylate and AzoMA under dark, visible, and UV light irradiation were determined as 100, 85, and 20%, respectively.36 Therefore, we can conclude that the Tg values of P(AzoMA-r-NIPAm) block domains increase with increasing trans-azobenzene content, i.e., depend on the photoisomerization state of azobenzene moieties. As noted above, P(AzoMA-r-NIPAm) block domains contained small amounts of [C4mim]PF6 that could be determined from Tg under visible and UV light irradiation using the Fox equation,49,50 assuming that the Tgs of both

Figure 5. (a) Arrhenius plots of relaxation time obtained for 20 wt % ABA triblock copolymer in [C4mim]PF6 under dark conditions using the horizontal shift factor for time−temperature superposition. (b) Variation of the temperature dependence of G′ and G″ for 20 wt % ABA triblock copolymer in [C4mim]PF6 under dark with or without the aging process for 8 h at 25 °C, a frequency of ω = 6.28 rad s−1, a stress amplitude of γ = 1%, and a heating rate of 1 °C min−1.

time vs the inverse of absolute temperature, with the fitted parameters listed in Table S2. In the above figure, the dashed line represents the Vogel−Fulcher−Tammann (VFT) equation, i.e., τ = τ0 exp(B/(T − T0)),37 where T0 is the Vogel temperature, B is the Vogel activation energy, and τ0 is a preexponential factor (Table S2). The primary (α) relaxation of amorphous polymers in the rubbery state is expressed by the VFT equation, although their rubber−glass transition induces a divergence from this equation due to frozen segmental mobility, with relaxation times at Tg reaching 100−2 s.39,44−48 D

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Figure 7. Temperature dependence of relaxation times for 20 wt % ABA triblock copolymer in [C4mim]PF6 under UV (blue circles) and visible (red diamonds) light irradiation.

Figure 6. Arrhenius plots of relaxation times obtained for 20 wt % ABA triblock copolymer in [C4mim]PF6 under UV (blue diamonds) and visible (red diamonds) light irradiation using horizontal shift factors for time−temperature superposition. Solid lines represents fits obtained using the VFT equation.

transition. The slower relaxation below Tg reflects the suppressed motions of P(AzoMA-r-NIPAm) blocks within physical cross-links, which become active once experimental temperatures exceed the Tg of these cross-links. It is worth noting that the above motions can also be activated by switching from visible to UV light irradiation. The photoinduced gel−sol−gel transition depicted in Figure 2 occurred at a temperature of 40 °C, which was between the Tg of P(transAzoMA-r-NIPAm) block domains and that of P(cis-AzoMA-rNIPAm) block domains. Therefore, this transition was described as (elastic solid)-(viscoelastic liquid)-(elastic solid). More precisely, the photoinduced gel−sol transition includes three elementary steps. The first step corresponds to the transto-cis photoisomerization of azobenzene under UV light irradiation, and the second one corresponds to the transition from an elastic solid state to a viscoelastic solid state caused by the increased IL penetration into physical cross-links (see Table 1, cis-form block domains contain larger amounts of IL than trans-form ones). Finally, the third step represents the transition from a viscoelastic solid state to a viscoelastic liquid state via the relaxation of physical cross-links. Considering the fact that the relaxation time of the viscoelastic state of the ion gel is smaller than 100 s (Figure 7), and the trans-to-cis photoisomerization in glassy polymers is as fast as that in rubbery ones,52 the slowest process corresponds to the second step, i.e., IL diffusion into cross-links. This conclusion provides a possible explanation for the long photohealing time (64 h) of the ion gel at 36 °C.32 On the other hand, the sol−gel transition is that from a viscoelastic liquid to an elastic solid, featuring two elementary steps, namely the cis-to-trans photoisomerization of azobenzene under visible light irradiation and the formation of an elastic solid state due to end block aggregation. According to Figure 2, this process occurs within 200 s at 40 °C. Therefore, this sol−gel transition is fast, since the aggregation of end blocks in concentrated solutions occurs without involving the diffusion of polymer chains. This conclusion rationalizes the relatively short time (0.5 h) observed in our previous study for the recovery of the gel state from the sol state upon visible light irradiation after the damaged part of the ion gel was healed by UV light irradiation.32 In the above-mentioned study, rheological characterization of the photoinduced gel−sol−gel transition was conducted at 53 °C, i.e., above the Tgs of P(trans-AzoMA-rNIPAm) and P(cis-AzoMA-r-NIPAm) block domains. Thus, this transition should essentially correspond to a photoinduced transformation of a viscoelastic solid to a viscoelastic liquid and

P(trans-AzoMA-r-NIPAm) and P(cis-AzoMA-r-NIPAm) blocks equal those of bulk polymers 1/Tg = wP(AzoMA − r − NIPAm)/TgP(AzoMA − r − NIPAm) + w[C4mim]PF6/Tg[C4mim]PF6

where wP(AzoMA‑r‑NIPAm) and w[C4mim]PF6 are weight fractions of P(AzoMA-r-NIPAm) blocks and [C4mim]PF6 in P(AzoMA-rNIPAm) block domains, respectively, and TgP(AzoMA‑r‑NIPAm) and Tg[C4mim]PF6 represent the Tgs of P(AzoMA-r-NIPAm) blocks in the bulk triblock copolymer and [C4mim]PF6, respectively. On the basis of the data in Figure S1, TgP(AzoMA‑r‑NIPAm) was determined as 92 °C, with Tg[C4mim]PF6 previously reported as −77 °C.35 The obtained compositions are listed in Table 1, Table 1. Estimated Tgs and Weight Fractions of P(AzoMA-rNIPAm) and [C4mim]PF6 at Physical Cross-Linking Points of the Ion Gel (20 wt % ABA Triblock Copolymer/ [C4mim]PF6) under UV and Visible Light Irradiation Tg/°C wP(AzoMA‑r‑NIPAm) w[C4mim]PF6

vis

UV

41 0.81 0.19

34 0.78 0.22

which shows that weight fraction of [C4mim]PF6 under UV light irradiation was slightly higher than that under visible light irradiation, i.e., P(cis-AzoMA-r-NIPAm) blocks were more compatible with [C4mim]PF6 than P(trans-AzoMA-r-NIPAm) blocks. The polarity of azobenzene depends on its photoisomerization state, i.e., planar trans-azobenzene has a dipole moment of 0.5 D, whereas cis-azobenzene has a dipole moment of 3.0 D.51 Therefore, the higher IL compatibility of P(cisAzoMA-r-NIPAm) was ascribed to the higher polarity of cisAzoMA moieties. Physicochemical Understanding of the Photoinduced Sol−Gel Transition of the Ion Gel. Figure 7 shows the temperature dependence of relaxation times for 20 wt % ABA triblock copolymer in [C4mim]PF6 under visible and UV light irradiation, revealing that these times increased with decreasing temperature under both conditions. At the same temperature, the relaxation time under visible light irradiation exceeded that under UV light irradiation, accounting for the gel−sol E

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increasing the Tg difference between P(trans-AzoMA-rNIPAm) and P(cis-AzoMA-r-NIPAm) block domains. The structural properties such as the molar ratio of AzoMA and molecular weight are expected to affect the phase transition behavior,43,53 and the investigation to widen the Tg difference is currently under way.

again to a viscoelastic solid, which can be realized by utilizing the small difference of relaxation times between the trans-ion gel and the cis-ion gel at the experimental temperature. However, at this temperature, the self-standing ability of the ion gel was difficult to maintain for a long time, being the reason for performing photohealing at 36 °C. Considering the physicochemical properties of the ion gel obtained in this study, the Tg of P(trans-AzoMA-r-NIPAm) block domains should be higher than the healing temperature and the difference between the Tgs of trans- and cis-form block domains needs to be increased to enhance the self-standing ability and photohealing speed of this gel.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01538. DSC curves of the triblock copolymer and PNIPAm/ [C4mim]PF6, glass transition temperature, enthalpy relaxation, and phase transition temperature of 20 wt % PNIPAm in [C2mim][NTf2], G′ of 20 wt % ABA triblock copolymer in [C4mim]PF6 under dark conditions as a function of frequency, time−temperature superposition of G′ and G″ for 20 wt % ABA triblock copolymer in [C4mim]PF6 under dark conditions, time− temperature superposition for high-frequency G″ of 20 wt % ABA triblock copolymer in [C4mim]PF6 under dark conditions and fitting parameters for VFT and Arrhenius equations (PDF)

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CONCLUSION DSC analysis revealed that P(AzoMA-r-NIPAm) blocks of the ABA triblock copolymer exhibited both glass−rubber and UCST-type phase transitions in [C4mim]PF6, with the glass transition temperature being much lower than UCST. In the experimental temperature range, the behavior of the ion gel was mainly governed by the Tg of the copolymer end blocks. Upon heating, the ion gel transformed from an elastic solid (below Tg) to a viscoelastic solid and finally to a viscoelastic liquid (above Tg). The long relaxation times of the ion gel were associated with the glassy state of P(AzoMA-r-NIPAm) block domains. Rheological measurements revealed that the Tg of P(trans-AzoMA-r-NIPAm) block domains was 7 °C higher than that of P(cis-AzoMA-r-NIPAm) block domains in the ion gel. Depending on experimental temperature, the photoinduced gel−sol−gel transition of the ion gel can be understood either as a photoinduced (elastic solid)−(viscoelastic liquid)−(elastic solid) or as a photoinduced (viscoelastic solid)−(viscoelastic liquid)−(viscoelastic solid) transition (Figure 8). The relaxation times of trans- and cis-ion gel forms did not distinctly differ, with the difference between the Tgs of P(transAzoMA-r-NIPAm) and P(cis-AzoMA-r-NIPAm) block domains also being insignificant, which resulted in a limited healing temperature range and a long photohealing time (64 h). On the basis of the obtained results, this issue can be solved by

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone/Fax: +81-45-339-3955. ORCID

Ryota Tamate: 0000-0002-1704-1058 Masayoshi Watanabe: 0000-0003-4092-6150 Author Contributions ‡

X.M., R.U. and Y.K. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by Grants-in-Aid for Scientific Research (#S-15H05758) and Specially Promoted Research on Iontronics funded by MEXT, Japan. Y.K. and R.T. acknowledge Research Fellowships awarded by the Japan Society for the Promotion of Science for Young Scientists (Nos. 13J00192 and 17J00756, respectively).

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REFERENCES

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Figure 8. Schematic illustration for the photoinduced sol−gel transitions. Transition between glassy and rubbery cores (middle) corresponds to the (elastic solid)−(viscoelastic liquid)−(elastic solid) transition, whereas that between both rubbery cores (right) corresponds to the (viscoelastic solid)−(viscoelastic liquid)−(viscoelastic solid) transition. For simplicity, UCSTs, which exist under much higher temperatures, are not illustrated. F

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Macromolecules

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DOI: 10.1021/acs.macromol.7b01538 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.7b01538 Macromolecules XXXX, XXX, XXX−XXX