Phase transition in swollen gels Part 32. Temperature transition in charged poly(N-isopropylmethacrylamide) hydrogels in water and aqueous NaCl solutions

Abstract

Swelling and mechanical behavior of ionised networks of copolymers of N-isopropylmethacrylamide with an ionic comonomer, sodium methacrylate (mole fractions x_S = 0–0.1), and a crosslinker, methylenebisacrylamide, was investigated in water as a function of temperature and in aqueous NaCl solutions (c_NaCl = 10⁻⁵–1 M) at 23 °C; small-angle neutron scattering (SANS) was used for structure investigation. On heating, a continuous decrease in the swelling degree in water, Q, was observed; increasing x_S shifts the temperature of transition from the swollen to collapsed state, T_tr, to higher values. On subsequent cooling, the temperature T_tr was independent of x_S. This means that in the collapsed state clusters of ions are formed and these microsegregates remain stable at low temperatures; the presence of clusters is supported by SANS results. The expected decrease in the swelling in aqueous NaCl solutions, Q_s, with increasing NaCl concentrations, c_NaCl, was observed. The decrease in Q with T and the decrease in Q_s with c_NaCl are accompanied by an increase in equilibrium shear modulus of gels. The experimental swelling behavior was analyzed using the theory of polyelectrolyte networks in which repulsion of charges on the chain and finite chain extensibility were considered.