Phase transition in swollen gels

Abstract

The swelling and mechanical behaviour of ionized networks of N-isopropylacrylamide with an ionic comonomer, (2-acrylamidoethyl)trimethylammonium chloride (mole fractions x S= 0−0.1), in the presence of a crosslinker, N,N'-methylenebisacrylamide, was investigated in water as a function of temperature and in aqueous NaCl solutions (c NaCl= 10−5− 1 M) at 23 °C. On heating, a continuous decrease in the swelling degree in water, Q w, was observed; increasing x S shifts the volume transition temperature, T tr, (from the swollen to collapsed state) to higher temperatures. The expected decrease in the swelling degree, Q, with increasing NaCl concentration in aqueous NaCl solutions was observed and two shrinking regions in ionic gels were found. The decrease in Q w with increasing temperature and the decrease in Q with increasing c NaCl are accompanied by an increase in equilibrium shear modulus of gels, so that the mechanical behaviour of gels is predominantly determined by the swelling degree. The experimental swelling behaviour could be, in the first approximation, described by the theory of polyelectrolyte networks in which repulsion of charges on the chain and finite chain extensibility were considered.