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A series of pH–thermoreversible hydrogels are prepared from the three molar ratios of N-isopropylacrylamide (NIPAAm) and acrylic acid neutralized 50 mol % by sodium hydroxide (SA50) and N,N′-methylene bisacrylamide (NMBA). The influence of the environmental conditions, such as temperature and pH values, on the swelling behavior of these copolymeric gels is also investigated in this article. Results show that the hydrogels bearing negative charges exhibit different equilibrium swelling ratios under various pH media. The pH sensitivities of these gels also strongly depend on the molar ratio of SA50 in the copolymeric gels; thus, the more the SA50 content, the higher the gel pH sensitivity. These hydrogels exhibited thermosensitivity demonstrating a larger change of the equilibrium swelling ratio in aqueous media under temperature changes. An overshooting phenomenon is observed from the gel swelling kinetics under high-temperature conditions. The said hydrogels are also used to investigate the release of model drugs in this study. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1955–1967, 1999

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