Ion gels are composed of ionic liquids (ILs) immobilized within a three-dimensional molecular network. Such gels preserve the attractive physicochemical properties of ILs, such as nonvolatility, thermal stability, electrochemical stability, and high ionic conductivity, while maintaining a soft solid consistency. Ion gels are a novel platform for many applications such as electrolyte membranes, actuators, gas-separation membranes, flexible electrochemiluminescent gels, and organic thin-film transistors.

We present here the reversible micellization and sol-gel transition of block copolymer solutions in an IL triggered by a photostimulus. The ABA triblock copolymer employed consists of a random copolymer A block, including thermosensitive N-isopropylacrylamide (NIPAm) units and a methacrylate incorporating an azobenzene chromophore into the side chain (AzoMA); the B block is composed of an IL-soluble poly(ethylene oxide) (PEO) (P(AzoMA-r-NIPAm)-b-PEO-b-P(AzoMA-r-NIPAm)). We observed a reversible unimer-to-micelle transition of a dilute ABA triblock copolymer (1 wt%) in an IL, 1-butyl-3-methylimidazolium hexafluorophosphate ([C₄mim]PF₆) induced by photostimuli at an intermediate, “bistable” temperature. We demonstrate a reversible sol-gel transition cycle of the ion gel at the bistable temperature, with reversible association and fragmentation of the polymer network structure consisting of the self-assembled ABA triblock copolymer (20 wt%) in [C₄mim]PF₆. Such sol-gel transition has been utilized to design photo-healable materials. Damaged part of materials is UV-irradiated to convert gel to sol, followed by irradiated by visible light in order to convert again sol to gel. By this procedure, the damaged part was demonstrated to be healed to the original state (Figure 1).

Furthermore, we propose moving the azobenzene groups from the polymer chains to the IL structure as a molecular-level photoswitchable IL. Photoisomerization of an azobenzene IL at the molecular level can evoke a macroscopic response (light-controlled mechanical sol-gel transitions) for ABA triblock copolymer solutions. Therefore, an azobenzene IL as a molecular switch can tune the self-assembly of a thermoresponsive polymer, leading to macroscopic light-controlled sol-gel transitions. Surprisingly, a sight change in chemical structures of thermosensitive A segments induces opposite photo responsiveness, unimer/micelle transition and sol/gel transition, against UV/Vis-irradiation.

Figure 1. Photographs showing photo-healing of a star shaped ABA ion gel (20 wt%) cut at the center: (Left) damage before applying light stimuli, (Right) 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.