Effect of monovalent-divalent cation exchange on the swelling of polyacrylate hydrogels in physiological salt solutions

The volume transition induced by monovalent-divalent cation exchange of fully neutralized polyacrylate hydrogels was investigated in aqueous NaCl solutions. The variation of the osmotic swelling pressure, shear modulus, and mixing pressure was measured when Na+ ions were substituted by divalent or trivalent cations. Alkali metal salts move freely throughout the entirely network, and alkaline earth metal salts (CaCl2, SrCl2) promote aggregation of polyacrylate chains, but these aggregates are relatively weak. Transition metal salts (CoCl2, NiCl2) form stronger interchain associates. Rare earth cations (La3+ and Ce3+) bind practically irreversibly to the polymer. Experimental data indicate that transition metal cations modify both the elastic and mixing components of the free energy, while alkaline earth metal cations affect primarily the mixing term. The behavior of freely swollen gels was compared with similar gels subjected to uniaxial compression. In uniaxially compressed gels, volume transition occurs at lower cation concentrations than in the corresponding undeformed gels. The shift of the transition point increases with the deformation ratio and is larger for Co2+ than for Ca2+.