Dynamic Inhomogeneities in Polymer Gels Investigated by Dynamic Light Scattering

Abstract

Position-dependent dynamic fluctuations in polymer gels have been investigated by dynamic light scattering (DLS). DLS measurements were performed over a hundred sampling points by rotating test tubes for as-prepared and swollen gels in order to probe different scattering volumes. The time-averaged scattered intensities obtained by DLS were decomposed into two contributions, i.e., thermal fluctuations, IF, and time-independent frozen inhomogeneities, IC. The two variables were analyzed as a function of sampling position, where not only IC but also IF were considered to be position dependent. As often observed in gelling systems, IC generally dominates in comparison with IF irrespective of the method of cross-linking. Here, we propose a nonergodic method to examine whether position dependent fluctuations in IF(p) observed experimentally are due to the nature of gels or to an experimental noise. For gels with high cross-linking densities, dynamic inhomogeneities are observed as a result of the difference in the molecular environment, e.g., the difference in the local gel concentration and/or cross-linking density, resulting in the emergence of position dependence of IF as well as that of the homodyne diffusion coefficient, D. We will also address the importance of a third component, i.e., the low molecular weight species, IS, in the analysis of the dynamics of polymer gels.