Phase transition in swollen gels 24

Abstract

The swelling and mechanical behaviour of ionized networks of copolymers of acrylamide (AAm), methylenebisacrylamide and sodium 2-[(3-carboxypyridine-4-carbonyl)oxy]ethyl methacrylate (PyNa, molar fraction of salt x_S=0–0.20) was investigated in water–acetone (w/a) mixtures and in aqueous NaCl solutions. In the range $\text{x}{}_{\mathrm{<mtext>S</mtext>}}\text{⩾0.01}$, a first-order phase transition (collapse) was found in w/a mixtures; with increasing x_S, both the critical acetone concentration a_c in the mixture at which collapse takes place and the extent of collapse (jump in the swelling ratio X), $\text{Δ}\text{log}\text{X}$, increases. A comparison of these results with the results obtained for charged PAAm networks with sodium 2-[(2-carboxybenzoyl)oxy]ethyl methacrylate (BeNa) as ionic comonomer led to a conclusion that substituting benzoyl for pyridyl groups in the side chain slightly decreases $\text{Δ}\text{log}\text{X}$ and the swelling degree in water, and does not change critical a_c values. On the other hand, a comparison with charged PAAm networks with sodium methacrylate (MNa) as ionic comonomer showed that at constant x_S, the swelling degree in water, the extent of collapse $\text{Δ}\text{log}\text{X}$ and the concentration of elastically active network chains strongly decreased with the increased distance of the bound charge from the main chain (length of the side chain of the ionic comonomer). The expected decrease in the swelling degree of ionized networks with increasing NaCl concentration was found. The experimental swelling data in w/a mixtures and in aqueous NaCl solutions could be described by a theory proposed previously if an effective degree of ionization i is introduced (i<x_S). The mechanical behaviour of gels in w/a mixtures is predominantly determined by the degree of swelling. On the other hand, different dependencies of the reduced modulus on the swelling ratio X found in w/a mixtures and in aqueous NaCl solutions in the high swelling region suggests that contracted gels with different microstructure were formed during gel deswelling in bad solvents or salt in water.