Phase transition in swollen gels: 25. Effect of the anionic comonomer concentration on the first-order phase transition of poly(1-vinyl-2-pyrrolidone) hydrogels

The swelling and mechanical behavior of ionized networks prepared by radiat ion polymerization of 1-vinyl-2-pyrrolidone, sodium itaconate and of a cros slinker, 3,3'-ethylidenebis(1-vinyl-2-pyrrolidone), was investigated in wat er/acetone mixtures and aqueous NaCl solutions. In the molar fraction range of the ionic comonomer x(s) = 0.022-0.15, the first-order phase transition (collapse) was found. The extent of the collapse (stepwise change in the g el volume) and the critical acetone concentration in the mixture at collaps e slightly increase with increasing x(s). The collapse is accompanied by a stepwise change in equilibrium modulus. The expected decrease in the swelli ng and increase in the modulus with increasing concentration of NaCl in aqu eous solutions were found. The earlier suggested theory describing the swel ling equilibrium of polyelectrolyte networks was applied to description of the collapse phenomenon in water/acetone mixtures and swelling in dependenc e on NaCl concentration: satisfactory agreement was obtained when an effect ive degree of ionization i (i < 2x(s)) was introduced. (C) 1998 Elsevier Sc ience Ltd. All rights reserved.