Spatial inhomogeneity and dynamic fluctuations of polymer gels

Polymer gels have two types of concentration fluctuations; liquidlike and solidlike fluctuations. The former are time-dependent (thermal) fluctuations and the latter are time-independent spatial inhomogeneity. The static structure factor, obtained by small-angle X-ray (SAXS) or neutron scattering (SANS), consists of both contributions. Several methods which allow to decompose the static structure factor into the individual contributions are reviewed. In practice, however, the decomposition is not as easy as that predicted by the theory due to the lack of information about dynamic fluctuations. The dynamic light scattering technique (DLS), on the other hand, provides the complimentary information of the dynamic fluctuations. It will be shown that DLS becomes a powerful tool to distinguish the two contributions if it is coupled with ensemble averaging. In this article, recent developments of the structure investigation of polymer gels, particularly those of environmental sensitive polymer gels, are reviewed from both experimental and theoretical points of view. The advantage to use the environmental sensitive gels is that the structure of a gel can be examined as a function of its environmental parameters, e.g., temperature, concentration, and pH. Furthermore, the characteristic features of these environmental sensitive gels, such as a volume phase transition and phase separation, are extensively discussed in terms of the thermal fluctuations and the spatial inhomogeneity.