Hybrid plasma polymerized membranes from organosilicon precursors for gas separation

Abstract
Thin a-SiO_xC_yH_z 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 ²⁹Si NMR analysis. The qualification of the films for gas-selective membranes was tested on ester of cellulose substrates using N₂, H₂, O₂, 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).