Dynamic Inhomogeneities in Polymer Gels Investigated by Dynamic Light Scattering

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, I F, and time-independent frozen inhomogeneities, I C . The two variables were analyzed as a function of sampling position, where not only I C but also I F were considered to be position dependent. As often observed in gelling systems, I C generally dominates in comparison with I F irrespective of the method of cross-linking. Here, we propose a nonergodic method to examine whether position dependent fluctuations in I F ( 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 I F 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, I S, in the analysis of the dynamics of polymer gels.