Effect of Low-Temperature Plasma on the Structure of Surface Layers and Gas-Separation Properties of Poly(Vinyltrimethylsilane) Membranes

New results of studying the one-sided surface modification of polymer films and flat-sheet composite membranes based on poly(vinyltrimethylsilane) using low-temperature plasma are presented. Treatment is carried out by direct current discharge at a cathode and anode, air is used as a working medium, the exposure time is from 10 to 60 s, and the working pressure in a chamber is 15-20 Pa. The structure of the surface layers is analyzed by XPS, AFM, and SEM, and the contact properties of the surface are studied. For cathode-treated PVTMS films the effective permeability coefficients for O-2, N-2, CH4, CO2, He, and H-2, as well as the effective gas diffusion coefficients, are measured experimentally and the effective gas solubility coefficients are calculated. The permeability coefficients of the studied gases for cathode- and anode-modified composite membranes with a selective PVTMS layer are determined. It is shown that the choice of electrode significantly affects not only the chemical structure of surface and near-surface PVTMS layers but also the gas-transport parameters of the modified samples. It is found that, in the case of cathode-modified homogeneous films, the values of permeability, diffusion, and solubility coefficients of gases are higher while the values of selectivity are lower compared with the anode-modified films. At the same time, the treatment of PVTMS films at the cathode for 30 s makes it possible to increase O-2/N-2 selectivity by more than two times relative to the initial values. The results of modification of the composite membranes differ from those attained for the homogeneous films, and, what is more, for the composite membrane treated at the cathode the O-2/N-2 selectivity is higher by a factor of 2.5 than the initial value. The potential of using surface modification of polymer films and membranes by low-temperature plasma to improve their gas-separation properties is demonstrated.