Gas diffusion and sorption properties of polysiloxane membranes prepared by PECVD

Thin a-SiOxCyHz film membranes were deposited from two different cyclic and linear organosilicon precursors in a low-frequency plasma polymerization process. The qualification of the films for effective gas permeation membranes was tested on the ester of cellulose substrates using N2, H2, CO2 and CH4. The gas permeability and diffusion coefficients were determined by using the constant-volume pressure-increase method. The solubility coefficients (S) were obtained by using a gravimetric method with a quartz microbalance. Correlations between the composite plasma parameter (V/FM) (V: input voltage, F: monomer flow rate and M: monomer molecular weight), which describes the energetic character of the plasma and the permeation performances of plasma polysiloxane membranes were established. Permeation measurements reveal that a wide range of gas transport properties can be obtained according to the applied plasma energy. The global mass transfer could be controlled by diffusion or sorption mechanism according to the relative influence of the different structural factors (chain packing density, chain flexibility and chemical composition) of plasma polysiloxane films directly related to deposition parameters.