PHASE-TRANSITION IN SWOLLEN GELS XXIII - EFFECT OF THE POSITIVE CHARGE CONCENTRATION ON THE COLLAPSE AND MECHANICAL-BEHAVIOR OF POLY(1-VINYL-2-PYRROLIDONE) GELS

The swelling and mechanical behavior of ionized networks of copolymers of 1-vinyl-2-pyrrolidone, ionic comonomer, N,N-dimethyl-N,N-diallylam monium chloride (molar fraction of salt x(s)=0-0.27) and crosslinker, 3,3'-ethylidenebis(1-vinyl-2-pyrrolidone) was investigated in water-ac etone mixtures and aqueous NaCl solutions. The networks were prepared by a radiation copolymerization method. At x(s)greater than or equal t o 0.038, the first-order phase transition (collapse) was observed. Whi le the critical acetone concentration in the mixture at collapse, a(c) = 76 vol%, is independent of x(s) the extent of the collapse (jumpwis e change in the gel volume), Delta, increases with increasing salt con centration. The jumpwise change in the gel volume is accompanied by a similar change in equilibrium modulus. The swelling and mechanical beh avior of ionized networks in aqueous NaCl solutions were also measured . The expected decrease in the swelling and the increase in the modulu s with increasing electrolyte concentration c(NaCl) was found. The the ory of swelling equilibria of polyelectrolyte networks, in which the e ffect of electrostatic interactions of the charges on the chain and a limited chain extensibility were included, semiquantitatively describe s the swelling data provided an effective concentration of the charges (lower than x(s)) was introduced. Mechanical behavior of networks is predominantly determined by their degree of swelling.