Phase transition in swollen gels: 26. Effect of charge concentration on temperature dependence of swelling and mechanical behaviour of poly( N-vinylcaprolactam) gels

Abstract

The swelling and mechanical behaviour of ionized networks of copolymers of N-vinylcaprolactam with an ionic comonomer, N, N-diallyl- N, N-dimethylammonium chloride (molar fraction x s=0−0.1), in the presence of a crosslinker, 3,3′-(ethane-1,1-diyl)bis(1-vinyl-2-pyrrolidone), was investigated in water and aqueous NaCl solutions ( c NaCl=10 −5–1 M) as a function of temperature. On heating, a continuous decrease in the swelling degree in water, Q, was observed; increasing x s shifts the volume transition temperature, T tr, from the swollen to collapsed state to higher temperatures. The expected decrease in the swelling degree, Q*, of charged hydrogels in aqueous NaCl solutions was observed and two shrinking regions in ionic poly( N-vinylcaprolactam) gels were found at 20°C. While the transition at low NaCl concentrations ( c NaCl<0.1) is due to Donnan-equilibrium effect, the transition at c NaCl⩾0.1 M is due to suppression of the transition temperature T tr by NaCl. The observed decrease in Q in water with increasing temperature and the decrease in Q* in aqueous NaCl solutions at 20°C with increasing c NaCl are accompanied by an increase in equilibrium modulus of gels, so that mechanical behavior of hydrogels is predominantly determined by their degree of swelling. The swelling behavior 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.