Modelling of gas permeability for membranes prepared by PECVD

Plasma polymerization is a very powerful technique for preparing very thin highly cross-linked polymeric films. As in the case of classical polymers, the gas permeation performances of plasma polymers are strongly related to their chemical composition and structural properties. These properties depend directly on the precursor nature and on the operating conditions of the polymerization process like temperature, monomer flow rate, discharge power, frequency, etc. Previous results obtained by our group show that it is difficult to apply the classical Fick's correlations between the permeability, sorption and diffusion to this type of polymer. In addition, their structure, which is not very well known, is very cross-linked, high disordered and behaves like a complex material with two components: a rubber and an inorganic part. Indeed, these polymers cannot be represented as a regular repeat of monomer units. A mathematical model inspired from the free volume concept and Bondi's contributions group has been proposed for the DEDMS, HMCTSO, HMDSO and OMTSO polymers. This model is based on mesoscopic parameters like the density of the polymer and factor T = number of (Si-C) bonds/number of (Si-O) bonds. The model has displayed good correlations factors for the studied polymers.