Elastic Moduli of Silica Gels Prepared with Tetraethoxysilane

The elastic moduli of several organometallic silica gels were measured using pulse‐superposition interferometry (PSP). A series of gels were prepared under acid‐catalyzed conditions where the molar ratio of water to tetraethoxysilane (TEOS) in the sol was varied (4:1, 12:1, 16:1, and 24:1). It was found that the elastic moduli varied systematically with the hydrolysis conditions used to prepare the gels, as well as with the relative humidity used to equilibrate the gels before and during the PSP measurement. The experimental PSP data for the gels were evaluated using a composite model (matrix plus pores) and then the elastic moduli of the gel matrix were determined by applying a correction for porosity based on a self‐consistent‐scheme (SCS) approximation. The results show that the elastic moduli of the gel matrix also exhibit a systematic dependence upon the preparation conditions, but they always fall below the corresponding values for fused silica. Even when excess water Was used to hydrolyze the TEOS, this did not lead to a fully polymerized silica matrix structure. It is suggested that the lowered matrix moduli values are due to the presence of residual hydroxy and ethoxy groups which are internal to a polymeric structure.