Micro-porous copolymer hydrogels were prepared by gamma-ray irradiation of mixed solutions of N-isopropylacrylamide (NIPAAm) and acrylic acid (AAc) above the lower critical solution temperature (LCST). From CryoSEM observations, the gels were found to consist of three-dimensional fibrous micro-gels and micro-pores. The copolymer gels swelled at temperatures below the LCST and shrunk at temperatures above it, and they showed rapid volume transitions on a time scale on the order of a minute when experiencing temperature changes between 10 and 40degreesC. The transition times for thermal shrinking were almost the same regardless of AAc composition, but the transition times for thermal swelling were increased with increasing AAc contents. The copolymer gels also showed rapid volume transitions with time constants on the order of an hour on experiencing pH changes between 2 and 12. The transition times for pH volume change at, 10degreesC were within one hour, except for the gels containing only small amounts of AAc. On the other hand, the transition times for pH-dependent volume change at 40degreesC were increased with increasing AAc content. The lower responsiveness of the transition results from an increase in hydrophobicity arising from the formation of inter- and intra-molecular hydrogen bonds between the non-ionized carboxylic acid groups and the amide groups. (C) 2003 Wiley Periodicals, Inc.