Hybrid plasma polymerized membranes from organosilicon precursors for gas separation

Thin a-SiOxCyHz films were deposited from different cyclic and linear organosilicon precursors in a radio-frequency plasma polymerization process. The plasma deposition and the film properties were characterized with respect to the deposition rate, the layers density and the chemical structure determined by FTIR and Si-29 NMR analysis. The qualification of the films for gas-selective membranes was tested on ester of cellulose substrates using N-2, H-2, O-2, CO2 and CH4. We are able to correlate both physico-chemical properties and permeation performances of the synthesized polymers with the composite parameter V/F.M (V: input voltage, F: monomer flow rate, M: monomer molecular weight) which describes the energetic character of the plasma. At low V/F.M ratios, the thin layers extremely preserve the monomeric structure and get close to polydimethylsiloxane; this resemblance is all the more considerable that the precursor siloxane chain is longer, and particularly pronounced using cyclic monomers. As V/F.M increases, the plasma polymers have a more "inorganic" structure and higher density: the materials tend towards a silica-like structure. Concurrently, the synthesized membranes exhibit solution-diffusion controlled or Knudsen-like separation factors depending on whether plasma conditions are soft (low V/F.M ratios) or drastic (high V/F.M ratios).