RELAXATION PHENOMENA NEAR THE SOL-GEL TRANSITION

Near the sol-gel transition, relaxation phenomena exhibit a form of critical slowing down that is manifest in a variety of microscopic and macroscopic dynamical phenomena. For example, quasielastic light scattering studies (QELS) of the relaxation of thermally induced density fluctuations in critical silica sol-gels reveal a self-similar dynamics wherein decay processes occur on all time scales (fractal time), and average decay times diverge. Likewise, time- and frequency-dependent viscoelastic phenomena of critical gels are described by power laws and both the viscosity and recoverable equilibrium creep compliance diverge. These unusual relaxation phenomena appear to be associated with the connectivity divergence that occurs at the sol-gel transition. Light scattering studies of this sol-gel transition reveal fractal clusters whose size diverges as a power law, in reasonable accord with the predictions of percolation theory. Based on these structural observations, we have developed a scaling description of the dynamics that relies on a length-scale-dependent viscosity and the geometrical self-similarity of the sol-gel transition. When percolation exponents are used in these expressions, a good description of the viscoelastic phenomena and the relaxation of desntiy fluctuations near the gel point is obtained. Finally, it is shown that the dynamic scaling treatment of the viscoelastic properties can be summarized in a Time-Cure Superposition principle, which predicts certain universal axes for frequency- or time-dependent properties near the gel point.