Structural characterisation of organosiloxane membranes

A sol-gel polymerisation/pyrolysis route has been used to produce hybrid organic-inorganic membranes. This route involved the copolymerisation of two organo-functional precursors, methyltrimethoxysilane and phenyltrimethoxysilane or diphenyldimethoxysilane. The thermal stability and structural evolution with temperature of the resulting xerogels were investigated using multinuclear solid state NMR (¹³C and ²⁹Si), thermal analysis (TGA and DTA), and FTIR. It is shown that during pyrolysis the methyl ligand is replaced with hydroxyl groups, some of which condense into siloxane linkages. The retained hydroxyl groups are thought to provide adsorption sites throughout the material onto which polar gas molecules (such as CO₂) can adsorb, and diffuse under a pressure gradient. This hypothesis is supported by the results of gas permeation trials on the membranes, and thus a direct link is established between membrane performance and material structure.