Micellization/Demicellization Self-Assembly Change of ABA Triblock Copolymers Induced by a Photoswitchable Ionic Liquid with a Small Molecular Trigger

To date, the demonstration of photoinduced micellization/demicellization of ABA-type triblock copolymers in ionic liquids (ILs) has been based on photoresponsive polymers. Herein, rather than the photoresponsive polymers, a small molecular trigger, an azobenzene-based IL, is employed for the first time to achieve a photocontrollable micellization. ABA-type triblock copolymers were synthesized in which the A block (either poly(2-phenylethyl methacrylate) or poly(benzyl methacrylate)) has a lower critical solution temperature (LCST) in imidazolium-based ILs, while the B block (poly(methyl methacrylate)) is compatible with ILs; these triblock copolymers are denoted as PMP and BMB, respectively. Solutions of the azobenzene-based IL containing the copolymers exhibited different micellization temperatures in the dark and under UV irradiation. For PMP, at a temperature between the two micellization temperatures, UV irradiation induced a "unimer-to-micelle" transition, while for BMB, UV irradiation induced a "micelle-to-unimer" transition. The main difference in the chemical structures of the copolymers is the number of methylene spacers (1 or 2) between the aromatic ring and ester of the A blocks. NMR analysis showed that the chemical shifts of the ILs were shifted in opposite directions on UV irradiation, indicating that azobenzene isomerization can affect the solvation interactions between the polymers and the ILs.