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

Summary

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 prepared 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 degree was observed. Increasing x_s shifts the volume transition temperature T_tr from the swollen to shrinked state to higher temperatures; in all cases, this transition is continuous. The observed decrease in swelling with temperature is accompanied by an increase in equilibrium modulus, so that mechanical behaviour is predominantly determined by swelling. The swelling of hydrogels can be in the first approximation described by the theory of polyelectrolyte networks in which repulsion of charges on the chain and finite chain extensibility were considered. In accord with this theory, as the Flory-Huggins interaction parameter χ= 0.522 for the uncharged network swollen in water, no jumpwise change in volume with increasing temperature was observed.