A light and X-ray scattering study of the acid catalyzed silica synthesis in the presence of polyethylene glycol

Abstract

Acid catalyzed silica sol–gel reactions were studied by means of small angle X-ray scattering (SAXS) and dynamic light scattering (DLS). Emphasis was given as to how different concentrations and molecular weights of polyethylene glycol (PEG) influence the fractal dimension ( d f) and particle size distribution during and after the sol–gel reaction. From these measurements a reaction mechanism was determined for both stable and phase separated samples. The structure the material attained was dependent of the reaction mechanism. Both high and low concentrations of PEG produced stable reactions. At higher concentrations steric stabilization of the sol occurred. The mechanism for the stable samples was determined to be diffusion limited cluster–cluster aggregation (DLCCA), reaction limited cluster–cluster aggregation (RLCCA) and reaction limited cluster–cluster aggregation with rearrangements, depending on the molecular weight and concentration of PEG and the time of reaction. Bridging flocculation occurred between the organic and inorganic polymers in the phase separated samples, but only after the particles had attained a specific size. Lower fractal dimensions were observed for these samples, because the reaction did not follow a percolation growth mechanism.