PLASMA POLYMERIZATION OF SILICON ORGANIC MEMBRANES FOR GAS SEPARATION

Silicon organic vapours were plasma polymerized under low ion bombardment conditions at 300 K on the anode of a capacitively coupled parallel plate reactor working at 13.56 MHz. The plasma process and the film properties were characterized with respect to the deposition rate, the film density and the C, H, N content analysed by the combustion method. The qualification of the films for gas-selective membranes is tested on a microporous support and compared with silicone rubber membranes. Permeation measurements were carried out with N2, O2, CO2, He, C4H10 and CH4. Films prepared from hexamethyldisiloxane vapour at a low power input of 10 W exhibit 20 at.% C and 62 at.% H and a film density of 1.4 g cm-3. At a film thickness of 500 nm high gas separation ratios were achieved, e.g. CO2/N2 = 8 and C4H10/N2 = 21. By using hexamethyldisilazane, a decrease in gas solution capacity is observed. Increasing the plasma power or the admixture of oxygen to the process has the same effect, since it results in higher crosslinking and less flexibility of the film material. In addition, the higher deposition rate at elevated plasma power or oxygen admixture can result in microporous films. The existence of micropores is proven by temperature-dependent permeation measurements, a simple leak-sealing method and indirectly by a decrease in the film density.