Section VI. Sol-gel processes STRUCTURAL EVOLUTION DURING THE GEL TO GLASS

We have used differential scanning calorimetry (DSC) to measure quantitatively enthalpic changes which accompany gel densification and have related these changes to the evolving gel structure using Raman spectroscopy, gas adsorption, and thermal analysis. We show that the network structure, which results principally from skeletal dehydration (via condensation) during gel densification, is considerably different from the melt-glass structure. A dramatic reduction in viscosity and the formation of metastable M O M bonds as a product of condensation reactions are examples of these differences. Despite the complex manner in which the gel evolves toward a glass, once the gel has been densified and heated above T~, its structure and properties, e.g. viscosity and distribution of relaxation times, become indistinguishable from those of the conventionally melted glass.