Network inhomogeneities in polymer gels

Spatial inhomogeneities present in ionic and nonionic hydrogels depend systematically on network density and on the degree of swelling. To obtain a quantitative measure, the mean-square fluctuation of the refractive index, <eta(2)>, and the correlation length, xi, were determined by static light scattering. Upon increase of the network density, <eta(2)> rises markedly while xi remains essentially unchanged. When ionic gels are swollen in aqueous solutions containing various salt concentrations, a series of different degrees of swelling is obtained. In such a series, xi rises with increasing swelling while <eta(2)> decreases, as expected due to three-dimensional expansion and dilution. The data determined in the state of formation of the network, however, deviate strongly from this tendency with <eta(2)> being some decades lower than in the swollen state. (C) 2003 American Chemical Society.