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. T he gelation process of a low concentration xanthan solution in the presence of aqueous aluminum ions (AI(III)) was followed as a function of temperatu re. Thermal gelation was observed at 44 +/- 2 degreesC by the onset of hete rodyne and nonergodic behavior and by the divergence of the observed fast m otion. The paper deals extensively with the main problem of light scatterin g from systems where long relaxation processes are occurring and the time a verage no longer coincides with the ensemble average. The two approaches su ggested in the literature, i.e., the heterodyne and the nonergodic approach , developed by Geissler and Pusey, respectively, are tested. Two modes of m otion are observed in the solution state as well as in the gel. The fast mo tions can be assigned to the collective diffusion of the gel modes. The rel axation time passes through a sharp maximum at the gel point and then decre ases. The slow motions arise from clusters or heterogeneities. In the gel s tate the collective diffusion is superimposed by marked internal network fl uctuations.