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

The swelling and mechanical behaviour of ionized networks of copolymers of N-vinylcaprolactam with an ionic comonomer, N,N-diallyl-N,N-dimethylammoniu m 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 temperat ure. 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 expecte d decrease in the swelling degree, Q(*), of charged hydrogels in aqueous Na Cl solutions was observed and two shrinking regions in ionic poly(N-vinylca prolactam) gels were found at 20 degrees C. While the transition at low NaC l concentrations (c(NaCl) < 0.1) is due to Donnan-equilibrium effect, the t ransition at c(NaCl) greater than or equal to 0.1 M) is due to suppression of the transition temperature T-tr by NaCl. The observed decrease in e in w ater with increasing temperature and the decrease in Q(*) in aqueous NaCl s olutions at 20 degrees C with increasing c(NaCl),cl are accompanied by an i ncrease in equilibrium modulus of gels, so that mechanical behavior of hydr ogels is predominantly determined by their degree of swelling. The swelling behavior can be in the first approximation described by the theory of poly electrolyte networks in which repulsion of charges on the chain and finite chain extensibility were considered. (C) 1999 Elsevier Science Ltd. All rig hts reserved.