Silica-polymers for processing gas separation membranes: High temperature growth of fractal structure

A study is performed to investigate the structure and shape of silicate molecules, grown from TEOS (tetra-ethyl-ortho-silicate) at 65 degrees C, as building bricks for the preparation of gas separation membranes. The study is mainly carried out with in situ Small Angle X-ray Scattering (SAXS). The structure of the resulting silicate polymers could be interpreted in terms of fractals with a fractal dimension D-f and a radius of gyration R-g. During the first minutes of reaction of TEOS with water in the presence of protons as catalysts fast hydrolysis take place with a heat effect of about 15-30 kJ/mol. Condensation (growth) of hydrolyzed species follows according to a diffusion-limited cluster-cluster aggregation (DLCCA). Growth is faster with higher amount of water and acid but a plateau-value is reached for both R-g and D-f on a 60-hour timescale when a moderate amount of water or protons are present. These plateau-values are 15 nm and 1.8 for R-g and D-f, respectively. The values of R-g and D-f have large effects on gas separation performance of the resulting membranes made from these sols. With relatively small deviations in the values of R-g and D-f a 2-decade difference in the permeation of helium through the membrane was observed.