Gas separation properties of organosilicon plasma polymerized membranes

Thin films were polymerized from different organosilicon compounds in a rad io-frequency plasma deposition process. The properties of the layers were c haracterized with respect to the deposition rate, the density, the refracti ve index, and the chemical structure determined by FTIR and XPS analysis. T he qualification of the films for gas-selective membranes was tested on dif ferent porous substrates using N-2, H-2, O-2, CO2, and CH4. Both structure and permeation performances of the synthesized films were correlated with t he composite plasma parameter V/F . M (V: input voltage; F: monomer flow ra re; M: monomer molecular weight). At low V/F . M ratio, the thin layers are mainly constituted of the [(CH3)(2) -Si(-O)(2)] environment (monomer and p olydimethylsiloxane one). Increasing the V/F . M results in a more "inorgan ic" chemical structure, higher O/Si ratio, refractive index, and density, t he materials tend toward a silicalike structure. Concurrently the prepared membranes have solution-diffusion-controlled or Knudsen-like separation fac tors, depending on whether plasma conditions are soft or hard.