ELASTIC BEHAVIOR OF SALT-FREE POLYELECTROLYTE GELS

Poly(acrylic acid) gels ionized after preparation for different prepar ation concentrations and degrees of cross-linking and ionization, have been studied as a function of the swelling ratio and at swelling equi librium. Measurements performed as a function of the polymer concentra tion indicated that for low swelling ratios (<100) the shear modulus f ollows a scaling law with an exponent in agreement with classical theo ries. For a given concentration the shear modulus has been found to de crease with the degree of ionization as qualitatively predicted by a r ecent model. At swelling equilibrium the shear modulus varies linearly with the osmotic pressure of the counter-ions in a large concentratio n domain and for various cross-linking densities. The osmotic prefacto r values, found both by mechanical and by light-scattering experiments , were in agreement with various independent measurements performed on poly(acrylic acid) solutions.