Swelling equilibrium properties of partially charged gels: the effect of salt on the shear modulus

Abstract

We studied the elastic modulus of poly(acrylic acid) gels having the same initial polymer concentration and various ionization degrees by uniaxial compression. The measurements were either performed in a salt-free solvent as a function of the swelling ratio and for various cross-link densities, or at swelling equilibrium in brines at different concentrations, with the cross-link density being constant. The dependence of the elastic modulus with the concentration in the salt-free case is affected by the cross-linking ratio. As previously reported the ionization degree lowers the value of the modulus at a given concentration, and this effect disappears at high salt concentrations. At swelling equilibrium the elastic moduli provide an estimate of the internal osmotic pressure and the comparison with the calculated values of the latter suggests the presence of an osmotic prefactor that depends on the ionization degree and the polymer and salt concentrations.