Thermoreversible swelling behaviour of hydrogels based on N-isopropylacrylamide with acidic comonomers

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-methyl-2-acrylamidopropanesulfonic acid (AMPS) as comonomers. For each copolymerization three different compositions were used, and for all systems three concentrations of a tetrafunctional crosslinker were employed. Swelling ratios, r = (mass hydrogel/mass dry polymer), were measured on heating from 20 degrees C and then on cooling back to 20 degrees C, and the swelling/deswelling was shown to be thermoreversible. Lower critical swelling temperatures (T-c) were exhibited except for copolymers of medium-high content of AMPS. The influences of composition and crosslinker concentration on r and T-c are discussed. The unusual behaviour of hydrogels containing MAA as comonomer is explained on the basis of attractive hydrophobic interactions. (C) 1997 Elsevier Science Ltd.