Effect of degree of cross-linking on spatial inhomogeneity in charged gels. I. Theoretical predictions and light scattering study

Analysis of the Rabin-Panyukov theory of scattering from polyelectrolyte gels shows that the theory predicts an anomalous cross-link density (CD) dependence of amplitude of spatial inhomogeneity in charged polymer networks. By increasing the interaction parameter or by decreasing the degree of ionization, one can pass from a regime in which the calculated structure factor is a monotonically increasing function of CD, to one in which an inflection in the CD dependence is predicted. These theoretical predictions were experimentally verified by light scattering experiments on a series of weakly charged polymer gels of N-isopropylacrylamide-acrylic acid copolymer. The ensemble average scattered intensity, a measure of the spatial inhomogeneity, increased with CD for χ<χ_c and decreased with CD for χ>χ_c, where χ and χ_c are the interaction parameter and the critical value of χ, respectively.