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

Summary Ionized networks of statistical copolymers of N-vinylcaprolactam (VCL), 1-vinyl-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 chloride (mole fractions 0, 0.005, 0.01 and 0.02) and a crosslinker, 1,1'-divinyl-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/ethanol mixture (H₂O/EtOH = 0.5/0.5 by vol). Their swelling and mechanical behaviour was investigated in water at various temperatures. For all copolymers, a continuous transition was found from the expanded (at low temperatures) to collapsed (at high temperatures) state; the transition temperature T _tr increases 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 with increasing temperature is accompanied by an increase in equilibrium modulus, so that mechanical behaviour is predominantly determined by swelling. Received: 31 October 2000/Revised version: 18 December 2000/Accepted: 18 December 2000