Sorption and diffusion of volatile organic components in a membrane made by deposition of tetramethyl disiloxane in cold remote-plasma

A silicone-like membrane for vapor permeation was synthesized by deposition of tetramethyl disiloxane in a cold remote-plasma reactor onto a support. Although the chemical nature of the deposited polymer is similar to that of a conventional polydimethyl siloxane (PDMS) elastomer, except its higher inorganic silicon content, the morphology of the deposited layer is quite different. Polymer columns perpendicular to the surface were observed. Due to this, the dense plasma-deposited polymer, unlike the other elastomers, showed BET II-type sorption isotherms for organic and water vapors in the vapor-sorption measurements by microgravimetry. The first part of all isotherms can be well fitted with the dual-mode model for gas sorption, with a value of the Langmuir site capacity equal to 0.018 (volume fraction). The diffusion rates in the thin active layer were high compared with the vapor transport kinetics to the sample, so that the diffusion coefficient cannot be determined by vapor-sorption. An estimation of the diffusion coefficient from the steady permeation flux using the Long model confirmed the fast diffusion of vapors in the active layer (10−6 to 10−7 cm2/s). The sorption shows an anomalous behavior, which is reminiscent of a relaxation of the material structure during the course of sorption. The anomalous behavior, as well as the presence of Langmuir sorption sites in the elastomer, is attributed to the highly crosslinked materials in the zones at the boundary of polymer columns.