Self‐diffusion coefficients of sol‐gel intermediates

Sol‐gel techniques have shown great potential for producing ceramic materials of designed composition and properties. A typical sol‐gel process involves the hydrolysis and polymerization of one or more metal alkoxides in an alcohol solution in a batch reactor to form a homogeneous gel, which is then further treated to provide the type of ceramic desired (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 of the precursors using the pulsed‐gradient‐spin‐echo NMR technique. This initial report focuses on the early stages of the batch reaction. The system chosen is the acid catalyzed hydrolysis and condensation of tetraethyl orthosilicated in ethanol. ¹H PGSE NMR and ²⁹Si NMR show that 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 increases and polymerization reduces the concentration of these polar groups, the mobility of the reactive oligomers changes in a complex manner. Combining our results from PGSE NMR and gas chromatography, we estimate the extent of hydrogen bonding effect on the mobility of various oligomers, providing a model to predict diffusivity trends throughout the batch reaction.