SELF-DIFFUSION COEFFICIENTS OF SOL-GEL INTERMEDIATES

Sol-gel techniques have shown great potential for producing ceramic ma terials of designed composition and properties. A typical sol-gel proc ess involues the hydrolysis and polymerization of one or more metal al koxides in an alcohol solution in a batch reactor to form a homogeneou s gel, which is then further treated to provide the type of ceramic de sired (such as aerogel, glass, crystal, and coating). Several authors have suggested that diffusion effects may influence the development of the gel microstructure. We investigate what governs the diffusivity o f the precursors using the pulsed-gradient-spin-echo NMR technique. Th is initial report focuses on the early stages of the batch reaction. T he system chosen is the acid catalyzed hydrolysis and condensation of tetraethyl orthosilicate in ethanol. H-1 PGSE NMR and Si-29 NMR show t hat the diffusivities of the silicate precursors are strong functions both of the molecular weight and of hydrogen bonding between the polar functional groups and the solvent. Moreover, since hydrolysis increas es and polymerization reduces the concentration of these polar groups, the mobility of the reactive oligomers changes in a complex manner. C ombining our results from PGSE NMR and gas chromatography, we estimate the extent of hydrogen bonding effect on the mobility of various olig omers, providing a model to predict diffusivity trends throughout the batch reaction.