Effect of Initial Monomer Concentration on Spatial Inhomogeneity in Poly(acrylamide) Gels

The spatial inhomogeneity in poly(acrylamide) (PAAm) gels has been investigated with the static light scattering technique. Three different sets of PAAm gels were prepared, each at a fixed chemical cross-link density but at various initial monomer concentrations. The gels were characterized by swelling and elasticity tests as well as by light scattering measurements at a gel state just after their preparation. A critical polymer network concentration was found where the degree of the inhomogeneity in PAAm gels attains a maximum value. This phenomenon was explained as a result of two opposite effects of the initial monomer concentration on the gel inhomogeneity. Increasing monomer concentration increases both the effective cross-link density and the polymer concentration of the hydrogels. While the inhomogeneity becomes larger due to the first effect, the latter effect decreases the apparent gel inhomogeneity. The interplay of these two opposite effects determines the spatial inhomogeneity in PAAm gels. The theory proposed by Panyukov and Rabin also predicts the appearance of a maximum degree of spatial gel inhomogeneity at a critical polymer network concentration.