Effect of pH on the swelling behaviour of hydrogels based on N-isopropylacrylamide with acidic comonomers

Abstract

Hydrogels have been prepared by free radical polymerization in aqueous solution of N-isopropylacrylamide (NIPA) and of NIPA with acrylic acid (AA), methacrylic acid (MAA) and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS). For each copolymerization three comonomer feeds were used and for all systems three concentrations of a crosslinker were employed. For all of the 30 polymers previously obtained, swelling data in deionised water were extended here by corresponding data measured in aqueous media of pH = 1 and pH = 12. Starting at ca. 15 °C phase diagrams of swelling ratio r (= mass hydrogel/mass xerogel) were determined at several temperatures (T) up to ca. 70°C and reversibility in swelling/deswelling behaviour was verified on cooling back to 15 C. In some instances a lower critical swelling temperature (T c ) was located, whereas in others there was merely a continuous decrease in r with T. Swellability at pH = 1 is lower than that in pure water, the reduction in r increasing with content of acidic comonomer. At pH = 12 a similar reduction in r was observed in gels containing AA and AMPS, whereas with MAA as comonomer r is equal to or greater than the value in pure water. The findings are discussed on the basis of the influence of pH on the ionisation of carboxyl groups, hydrogen bonding, and chelation.