Hydrophobic properties of plasma polymerized thin film gas selective membranes

Hydrophobic gas separation membranes were prepared by a plasma polymerization process. Thin films of about 400-500 nm thickness deposited on porous Al2O3 substrates represent the composite membranes investigated. The permeation properties of the composites were examined by the pressure increase and an isobar method. Depending on the precursor composition and the plasma polymerization parameters, it is possible to prepare membranes with Knudsen-Like or solution-diffusion controlled separation factors. Low plasma polymerization energy densities and a mixture of silico-and fluoro-organic organic precursors result in water/methane separation factors as low as alpha(CH4)(H2O) = 0.3 and high membrane permeabilities. Infrared analysis yields that the structure of the films is mainly determined by the silico-organic component. The fluoro-organic coprecursor causes a fluorination of methyl groups of the films as manifested by an infrared absorption band at 900-880 cm(-1). (C) 1998 Elsevier Science B.V.