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A series of pH-thermoreversible hydrogels that exhibited volume phase transition was synthesized by various molar ratios of N-isopropylacrylamide (NIPAAm), acrylamide (AAm), and 2-hydroxyethyl methacrylate (HEMA). The influence of environmental conditions such as temperature and pH value on the swelling behavior of these copolymeric gels was investigated. Results showed that the hydrogels exhibited different equilibrium swelling ratios in different pH solutions. Amide groups could be hydrolyzed to form negatively charged carboxylate ion groups in their hydrophilic 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 the pH sensitivity. These hydrogels, based on a temperature-sensitive hydrogel, demonstrated a significant change of equilibrium swelling in aqueous media between a highly solvated, swollen gel state and a dehydrated network response to small variations of temperature. pH-thermoreversible hydrogels were used for a study of the release of a model drug, caffeine, with changes in temperature. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 221–231, 1999

A series of pH-thermoreversible hydrogels exhibiting volume phase transition were synthesized by three degrees of neutralization (DN) of acrylic acid (AA) and N-isopropylacrylamide (NIPAAm). The influence of environmental conditions, such as temperature and pH, on the swelling behavior of these copolymeric gels is investigated in this article. Results show that the negatively charged hydrogels exhibit different equilibrium swelling ratios at different pH values depending on the ionic composition. The pH-sensitivities of these gels also strongly depend on the DN of AA in the copolymeric gels. The results show that the higher the DN, the higher the gel pH-sensitivity. These hydrogels based on a temperature-sensitive hydrogel demonstrate a larger change of equilibrium swelling in aqueous media between a highly solvated, swollen gel state and a collapsed dehydrated network in response to a variation of temperature. On the other hand, a significant phenomenon that was found in the gel swelling kinetics was an overshooting under high temperature conditions. The presented hydrogels were used for release of model drugs that occur at the changes of surrounding conditions, such as temperature and pH, in this study. It was also found that the higher the DN of AA, the higher the gel transition temperature and the larger the release in a high temperature environment and, at the same time, the larger the swelling ratios.