On the water swelling behaviour of poly(N-isopropylacrylamide) [P(N-iPAAm)], poly(methacrylic acid) [P(MAA)], their random copolymers and sequential interpenetrating polymer networks (IPNs)

Thermo- and pH-responsive stimuli hydrogels based on N-isopropylacrylamide (N-iPAAm) and methacrylic acid (MAA) have been synthesized and their swelling behaviour studied as a function of composition, pH and temperature. Copolymers varying in composition have been obtained by copolymerizing these two monomers and interpenetrating polymer networks (IPNs) of P(MAA) and P(N-iPAAm) by the sequential method. Temperature and pH have been changed in the ranges from 25 to 40 °C and from 2 to 9, respectively. The swelling behaviour of the hydrogels depends on the nature of the polymer and the environmental conditions, namely pH and temperature. Copolymer gels under basic conditions exhibit higher degree of swelling than the homopolymer ones. The disruption of the complexes dominates the kinetic swelling of MAA enriched gels under basic conditions. The hydrogen bond formation between carboxyl and amide groups has been made clear through the dynamic swelling behaviour of copolymers under acidic conditions. IPNs reduce their ability to swell in water with increasing P(N-iPAAm) content because of the formation of hydrophobic interpolymer complexes through hydrogen bonding. Lower critical solution temperature occurs only in the enriched N-iPAAm copolymers under acidic conditions when the MAA carboxyl groups are unionized.