Gas separation membranes made by plasma polymerization of mixtures of silanes and fluoromethane

Abstract

Plasma polymerization in mixtures of tetramethylsilane (TMS), trimethylvinylsilane (TMVS), and ethynyltrimethylsilane (ETMS) with fluoromethane (CF 4) was investigated by elemental analysis, infrared spectroscopy, and electron spectroscopy for chemical analysis. TMS/CF 4, TMVS/CF 4, and ETMS/CF 4 mixtures were plasma-polymerized to yield films containing both silicon and fluorine residues. The elemental composition of the formed polymers depended on the composition of the starting mixture. The Si/C atomic ratio decreased with increasing CF 4 concentration, whereas the F/C ratio increased. Addition of 75 mol% CF 4 resulted in 44%, 46%, and 88% fluorinated carbon units such as CF 3, CF 2, and CF in the film for a TMS/CF 4 mixture, a TMVS/CF 4 mixture, and an ETMS/CF 4 mixture, respectively. The gas separation ability of these plasma films was enhanced by CF 4 addition, although permeability was diminished somewhat. The plasma films from the TMVS/CF 4 mixture with 75 mol% CF 4 had a P O 2 value of 1.5x10 −9 cm 3 (STP) cm/cm 2-sec-cmHg and a P O 2/ P N 2 ratio of 3.9.