Experimental evidence of universal behavior in ion-induced volume phase transition in polyelectrolyte gels

Introduction of high valence counterions into polyelectrolyte solutions and gels results in a reversible volume phase transition. The volume change can be induced by a small change in the concentration of the equilibrium salt solution. In the present work new results are reported for the volume transition induced by calcium-sodium ion exchange in sodium polyacrylate gels made by osmotic swelling pressure and small angle neutron scattering measurements. We show that the threshold Ca²⁺ ion concentration at which the transition occurs increases with increasing NaCl concentration in the surrounding bath, decreases with increasing the concentration of the ionized groups on the polymer backbone and the temperature, and is practically unaffected by changing the crosslink density of the network. It is demonstrated that the normalized swelling data fall on a master curve, indicating that the volume transition exhibits universal behavior. These findings imply that gel volume transition is primarily governed by changes in the electrostatic interactions as a consequence of the divalent-monovalent ion exchange.