PH-THERMOREVERSIBLE HYDROGELS - I - SYNTHESIS AND SWELLING BEHAVIORS OF THE SOPROPYLACRYLAMIDE-CO-ACRYLAMIDE-CO-2-HYDROXYETHYL METHACRYLATE) COPOLYMERIC HYDROGELS

A series of pH-thermoreversible hydrogels that exhibited volume phase transition was synthesized by various molar ratios of N-isopropylacryl amide (NIPAAm), acrylamide (AAm), and 2-hydroxyethyl methacrylate (HEM A). The influence of environmental conditions such as temperature and pH value on the swelling behavior of these copolymeric gels was invest igated. Results showed that the hydrogels exhibited different equilibr ium swelling ratios in different pH solutions. Amide groups could be h ydrolyzed to form negatively charged carboxylate ion groups in their h ydrophilic polymeric network in response to an external pH variation. The pH sensitivities of these gels also depended on the AAm content in the copolymeric gels; thus the greater the AAm content, the higher th e pH sensitivity. These hydrogels, based on a temperature-sensitive hy drogel, demonstrated a significant change of equilibrium swelling in a queous media between a highly solvated, swollen gel state and a dehydr ated network response to small variations of temperature. pH-thermorev ersible hydrogels were used for a study of the release of a model drug , caffeine, with changes in temperature. (C) 1999 John Wiley & Sons, I nc.