Swelling of neutralized polyelectrolyte gels

Measurements are reported of the swelling pressure ω and of the collective diffusion coefficient Dc in two series of salt-free neutralized polyelectrolyte gels, one a poly(acrylamide–acrylic acid) copolymer, the other cross-linked potassium polyacrylic acid. The values of Dc measured by macroscopic deswelling are in reasonable agreement with those measured by dynamic light scattering. Dc increases as the degree of swelling of the gel increases, with similar behaviour being found for the elastic modulus G. The osmotic pressure, Π=ω+G, displays a power law behaviour over the concentration range explored, the exponent σ depending on the counter-ion concentration: σ≈1 for the sample of lower charge density, while for the sample with a high charge density, σ≈1.5. The latter behaviour, unexpected at low polymer concentrations, is interpreted in terms of the inequivalence between static and dynamic concentration fluctuations.