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

Abstract

The swelling and mechanical behavior of ionized networks of copolymers of 1-vinyl-2-pyrrolidone, ionic comonomer, N,N-dimethyl-N,N-diallylammonium chloride (molar fraction of salt xs=0–0.27) and crosslinker, 3,3′-ethylidenebis(1-vinyl-2-pyrrolidone) was investigated in water-acetone mixtures and aqueous NaCl solutions. The networks were prepared by a radiation copolymerization method. At xs≥0.038, the first-order phase transition (collapse) was observed. While the critical acetone concentration in the mixture at collapse, ac=76 vol%, is independent of xs the extent of the collapse (jumpwise change in the gel volume), Δ, increases with increasing salt concentration. The jumpwise change in the gel volume is accompanied by a similar change in equilibrium modulus. The swelling and mechanical behavior of ionized networks in aqueous NaCl solutions were also measured. The expected decrease in the swelling and the increase in the modulus with increasing electrolyte concentration cNaCl was found. The theory of swelling equilibria of polyelectrolyte networks, in which the effect of electrostatic interactions of the charges on the chain and a limited chain extensibility were included, semiquantitatively describes the swelling data provided an effective concentration of the charges (lower than xs) was introduced. Mechanical behavior of networks is predominantly determined by their degree of swelling.