Photoreversible gelation of a triblock copolymer in an ionic liquid

The reversible micellization and sol-gel transition of block copolymer solutions in an ionic liquid (IL) triggered by a photostimulus is described. The ABA triblock copolymer employed, denoted P(AzoMA-r-NIPAm)-b-PEO-b-P(AzoMA-r-NIPAm)), has a B block composed of an IL-soluble poly(ethylene oxide) (PEO). The A block consists of a random copolymer including thermosensitive N-isopropylacrylamide (NIPAm) units and a methacrylate with an azobenzene chromophore in the side chain (AzoMA). A phototriggered reversible unimer-to-micelle transition of a dilute ABA triblock copolymer (1 wt%) was observed in an IL, 1-butyl-3-methylimidazolium hexafluorophosphate ([C₄mim]PF₆), at an intermediate "bistable" temperature (50°C). The system underwent a reversible sol-gel transition cycle at the bistable temperature (53°C), with reversible association/fragmentation of the polymer network resulting from the phototriggered self-assembly of the ABA triblock copolymer (20 wt%) in [C₄mim]PF₆.