Structural Modifications in Silica Sonogels Prepared with Additions of Poly(vinyl alcohol)

Abstract

Silica wet gels were prepared from acid sonohydrolysis of tetraethoxysilane (TEOS) and additions of poly(vinyl alcohol) (PVA)−water solution. Aerogels were obtained from supercritical CO2 extraction. The samples were studied by thermal gravimetric (TG) analysis, small-angle X-ray scattering (SAXS), and nitrogen adsorption. The structure of wet gels can be described as a mass fractal with dimension D equal to 2.0 on the whole length scale experimentally probed by SAXS, from ∼0.3 to ∼15 nm. Pure and low-PVA-addition wet gels exhibit an upper cutoff accounting for a finite characteristic length ξ of the mass fractal structure. Additions of PVA increase ξ without modifying D, which was attributed to a steric effect of the polymer in the structure. The pore volume fraction of the aerogels diminishes typically about 11% with respect to that of the wet gels, although nitrogen adsorption could be underestimating some porosity. The pore size distribution of the aerogels is shifted toward the mesopore region with t...