Silica-Polymers for Processing Gas Separation Membranes: High Temperature Growth of Fractal Structure

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A study is performed to investigate the structure and shape of silicate molecules, grown from TEOS (tetra-ethyl-ortho-silicate) at 65°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 Df and a radius of gyration Rg.

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 Rg and Df 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 Rg and Df, respectively.

The values of Rg and Df have large effects on gas separation performance of the resulting membranes made from these sols. With relatively small deviations in the values of Rg and Df a 2-decade difference in the permeation of helium through the membrane was observed.

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