AbstractThin films were deposited onto porous substrates by plasma polymerization using three kinds of organosilicic compounds, tetramethylsilane (TMS), hexamethyldisiloxane (M2), and octamethylcyclotetrasiloxane (D4). Those composite membranes showed different characteristics of gas permeability. When D4was plasma‐deposited onto a porous substrate, the composites membrane showed the highest oxygen permeability and the lowest oxygen‐to‐nitrogen permeability ratio. The composite membrane prepared from TMS showed the permeability characteristics opposite to the membrane obtained from D4. Infrared spectrum of the polymer from D4resembles that of dimethylpolysiloxane. The plasma polymers from TMS and M2showed different profiles in SiO absorption bands in the range 1100–1000 cm‐1or in absorption bands of SiCH3groups in the range 850–750 cm‐1from respective monomers. X‐ray photoelectron spectroscopic observation indicated that all the plasma polymers contained more than two species of Si atom with different oxidation states. The greater part of Si atoms in plasma polymers took the same oxidation states in corresponding monomer. The gas permeability characteristics were closely related to the oxidation states of Si atom in th
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