Nonergodicity and light scattering from polymer gels

Dynamic light scattering from polymer gels is complicated by the fact that the scattering intensity and its time correlation function change as different parts of a sample are explored. This results from the nonergodicity of the sample---time averages are not the same as ensemble averages, the result of a finite rigidity that leads to constrained inhomogeneities. We demonstrate a direct technique for ensemble averaging (by moving the sample), present the experimental correlation function containing correlations that do not decay with time, and show that the light scattering results from a superposition of static scattering from immobile inhomogeneities and dynamic density fluctuations from ``gel modes.''