Phase transition in swollen gels - 29. Temperature dependences of swellingand mechanical behaviour of poly(N-vinylcaprolactam-co-1-vinyl-2-pyrrolidone) gels in water

Ionized networks of statistical copolymers of N-vinylcaprolactam (VCL), 1-v inyl-2-pyrrolidone (VP) (mole ratios of VCL/VP = 1/0, 0.9/0.1. 0.8/0.2, 0.7 /0.3 and 0.5/0.5), an ionic comonomer, N,N-diallyl-(N,N-dimethyl)ammonium c hloride (mole fractions 0, 0.005, 0.01 and 0.02) and a crosslinker, 1,1'-di vinyl-3,3'-(ethane-1,1-diyl)di(2-pyrrolidone) (mole fractions 0.016, 0.024, 0.032 and 0.048) were prepared by radiation polymerization in water/ethano l mixture (H2O/EtOH = 0.5/0.5 by vol). Their swelling and mechanical behavi our was investigated in water at various temperatures. For all copolymers, a continuous transition was found from the expanded (at low temperatures) t o collapsed (at high temperatures) state; the transition temperature T-tr i ncreases with increasing contents of VP and the ammonium salt in gels. The shift of T-tr to higher temperatures is caused by hydrophilicity of network chains increasing with increasing VP contents. The decrease in swelling wi th increasing temperature is accompanied bq an increase in equilibrium modu lus, so that mechanical behaviour is predominantly determined by swelling.