Spatial correlations and growth in dilute gels

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Abstract
A series of dilute tetramethoxysilicon gels was made in order to study the spatial correlations that arise from the presence of solvent. Light-scattering measurements from these gels show a fractal structure on length scales as large as 1 μm. The short-length-scale fractal dimension differs from that observed for the connectivity transition at the gel point, indicating a crossover in universality class. Quasielastic light scattering measurements on the incipient gel demonstrate a critical slowing down in the relaxation of concentration fluctuations, causing frozen inhomogeneities in the final gels. Finally, the growth at early times, where the clusters are smaller than the spatial correlation length of the final gel, is shown to be exponential in time, in accord with models of aggregation. It is concluded that a kinetic model of gelation is needed to describe the short-length-scale structures observed in these gels.

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