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

Full Paper: N-isopropylacrylamide (NIPA) has been copolymerised with acryli c acid (AA) in the presence of N,N'-methylenebisacrylamide (BIS) as crossli nker. Incorporation of an activator with the free radical initiator enabled the reaction to be conducted to high conversion at 25 degreesC. Use of a t hin mould as reaction vessel yielded optically clear hydrogels of high swel ling ratio in water, whereas opaque Macroporous products of reduced swell-a bility were obtained by polymerisation in a glass vial. Exothermicity was v erified by monitoring the actual (as opposed to nominal) temperature within the reaction vessel as a function of time. It was found that the heat coul d be dissipated adequately when the reaction vessel has a high surface to v olume ratio (mould). In contrast, the heat generated in a vial was sufficie nt to raise the temperature of the reaction medium to the critical deswelli ng temperature and thereby produce an opaque macroporous gel. Separate expe riments in which the contents of NIPA, AA and BIS in a vial were varied sho wed that the dominant role was played by NTPA.