Thermoresponsive Hydrogels\nfrom Symmetrical Triblock\nCopolymers Poly(styrene-<i>block</i>-(methoxy diethylene\nglycol acrylate)-<i>block</i>-styrene)

Abstract

A series of symmetrical, thermo-responsive triblock copolymers\nwas prepared by reversible addition–fragmentation chain transfer\n(RAFT) polymerization, and studied in aqueous solution with respect\nto their ability to form hydrogels. Triblock copolymers were composed\nof two identical, permanently hydrophobic outer blocks, made of low\nmolar mass polystyrene, and of a hydrophilic inner block of variable\nlength, consisting of poly­(methoxy diethylene glycol acrylate) PMDEGA.\nThe polymers exhibited a LCST-type phase transition in the range of\n20–40 °C, which markedly depended on molar mass and concentration.\nAccordingly, the triblock copolymers behaved as amphiphiles at low\ntemperatures, but became water-insoluble at high temperatures. The\ntemperature dependent self-assembly of the amphiphilic block copolymers\nin aqueous solution was studied by turbidimetry and rheology at concentrations\nup to 30 wt %, to elucidate the impact of the inner thermoresponsive\nblock on the gel properties. Additionally, small-angle X-ray scattering\n(SAXS) was performed to access the structural changes in the gel with\ntemperature. For all polymers a gel phase was obtained at low temperatures,\nwhich underwent a gel–sol transition at intermediate temperatures,\nwell below the cloud point where phase separation occurred. With increasing\nlength of the PMDEGA inner block, the gel–sol transition shifts\nto markedly lower concentrations, as well as to higher transition\ntemperatures. For the longest PMDEGA block studied (DP_<i>n</i> about 450), gels had already formed at 3.5 wt %\nat low temperatures. The gel–sol transition of the hydrogels\nand the LCST-type phase transition of the hydrophilic inner block\nwere found to be independent of each other.