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

Abstract

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<χc and decreased with CD for χ>χc, where χ and χc are the interaction parameter and the critical value of χ, respectively.