Heterodyne and nonergodic approach to dynamic light scattering of polymer gels: Aqueous xanthan in the presence of metal ions (aluminum(III))

Solutions of xanthan gum form a strong gel in the presence of metal ions. The gelation process of a low concentration xanthan solution in the presence of aqueous aluminum ions (AI(III)) was followed as a function of temperature. Thermal gelation was observed at 44 +/- 2 degreesC by the onset of heterodyne and nonergodic behavior and by the divergence of the observed fast motion. The paper deals extensively with the main problem of light scattering from systems where long relaxation processes are occurring and the time average no longer coincides with the ensemble average. The two approaches suggested in the literature, i.e., the heterodyne and the nonergodic approach, developed by Geissler and Pusey, respectively, are tested. Two modes of motion are observed in the solution state as well as in the gel. The fast motions can be assigned to the collective diffusion of the gel modes. The relaxation time passes through a sharp maximum at the gel point and then decreases. The slow motions arise from clusters or heterogeneities. In the gel state the collective diffusion is superimposed by marked internal network fluctuations.