Thermal effects in the synthesis of thermoresponsive hydrogels of poly(N-isopropylacrylamide-co-acrylic acid)

Abstract

N-isopropylacrylamide (NIPA) has been copolymerised with acrylic acid (AA) in the presence of N,N′-methylenebisacrylamide (BIS) as crosslinker. Incorporation of an activator with the free radical initiator enabled the reaction to be conducted to high conversion at 25°C. Use of a thin mould as reaction vessel yielded optically clear hydrogels of high swelling ratio in water, whereas opaque macroporous products of reduced swellability were obtained by polymerisation in a glass vial. Exothermicity was verified by monitoring the actual (as opposed to nominal) temperature within the reaction vessel as a function of time. It was found that the heat could be dissipated adequately when the reaction vessel has a high surface to volume ratio (mould). In contrast, the heat generated in a vial was sufficient to raise the temperature of the reaction medium to the critical deswelling temperature and thereby produce an opaque macroporous gel. Separate experiments in which the contents of NIPA, AA and BIS in a vial were varied showed that the dominant role was played by NIPA.