Elastic behaviour of salt-free polyelectrolyte gels

Abstract

Poly(acrylic acid) gels ionized after preparation for different preparation concentrations and degrees of cross-linking and ionization, have been studied as a function of the swelling ratio and at swelling equilibrium. Measurements performed as a function of the polymer concentration indicated that for low swelling ratios ( < 100) the shear modulus follows a scaling law with an exponent in agreement with classical theories. For a given concentration the shear modulus has been found to decrease with the degree of ionization as qualitatively predicted by a recent model. At swelling equilibrium the shear modulus varies linearly with the osmotic pressure of the counter-ions in a large concentration domain and for various cross-linking densities. The osmotic prefactor values, found both by mechanical and by light-scattering experiments, were in agreement with various independent measurements performed on poly(acrylic acid) solutions.