Relationships Between the Sol to Gel and Gel to Glass Conversions: 2. Structure of Gels-During Densification

摘要

Our results show that, for metal alkoxide derived gels, the network structure is established as a product of condensation reactions which occur in solution prior to gelation and during the gel to glass conversion. In solution these reactions lead to metastable, hydroxylated, open networks which may be retained (at least in part) to elevated temperatures where they dramatically influence viscosity and hence densification kinetics. Condensation reactions at elevated temperatures lead to further changes in the intermediate range order of the network. Depending on the heating rate, there are many pathways by which gels can be converted to glass. The energetics associated with this conversion are presented schematically. Higher heating rates retain the metastable structure to higher temperatures ultimately providing a very large driving force for densification and, in some cases, reducing the densification temperature. Surface area, excess free volume, and the formation of metastable surface species all contribute to this high energy configuration. By comparison colloidal gels generally have lower surface areas and reduced hydroxyl contents and less excess free volume reducing their associated energy. This distribution (between colloidal and alkoxide derived gels) may be exploited by intelligent processing to produce dense glasses (especially films) at the lowest possible temperatures. (ERA citation 10:021232)