Phase transition in swollen gels 27. Effect of negative charge concentration on swelling and mechanical behaviour of poly(N-vinylcaprolactam) gels

Ionized networks of copolymers of N-vinylcaprolactam with charged comonomer , sodium itaconate (mole fraction of ionic comonomer x(s), = 0-0.1) and of a crosslinker, 3,3'-(ethane-1,1 -diyl)bis(1-vinyl-2-pyrrolidone) were prepa red by radiation polymerization in water/ethanol mixture (H2O/Et = 0.3/0.7 by vol.) and their swelling; mechanical behaviour in water was investigated as a function of temperature. On heating, a decrease in the swelling degre e was observed. Increasing x(s), shifts the volume transition temperature T t, from the swollen to shrinked state to higher temperatures; in all cases, this transition is continuous. The observed decrease in swelling with temp erature is accompanied by an increase in equilibrium modulus, so that mecha nical behaviour is predominantly determined by swelling. The swelling of hy drogels can be in the first approximation described by the theory of polyel ectrolyte networks in which repulsion of charges on the chain and finite ch ain extensibility were considered. In accord with this theory, as the Flory -Huggins interaction parameter chi = 0.522 for the uncharged network swolle n in water, no jumpwise change in volume with increasing temperature was ob served.