Structural Properties and Gas Permeation for PTMSP Films Treated by Elemental Fluorine in Liquid Perfluorodecalin

Abstract

A one-sided modification of homogeneous poly(1-trimethylsilylpropyne) (PTMSP) films was carried out by the method of liquid-phase fluorination with elemental fluorine in perfluorodecalin medium. The structure of the initial and modified samples was studied by X-ray photoelectron spectroscopy (XPS) and cross-sections of fluorinated films were tested by scanning electron microscopy (SEM). The differential gas chromatographic method was used to obtain the transport and separation characteristics of the studied samples for O₂, N₂, CO₂, CH₄, He, H₂. Fluorination of PTMSP films was shown to occur not only in the near-surface layer of the sample, but also to a depth of up to 50 µm. It was found that the gas permeability of the fluorinated films decreases, while the change in the flow through the membrane depends on the size of the penetrant molecule, as a result there is a significant increase in the selectivity of fluorinated PTMSP samples for H₂–CO₂, H₂–CH₄, H₂–N₂ and O₂–N₂ pairs relative to the virgin polymer. It was shown that in the process of liquid-phase fluorination PTMSP films are saturated with perfluorodecalin, which leads to a significant decrease in gas permeability. An approach is proposed to increase the flow of gases through a fluorinated membrane by removing perfluorodecalin by holding samples in hexafluorobenzene, leading to a significant increase in gas permeability while maintaining selectivity. The gas separation data for fluorinated PTMSP films after treatment in hexafluorobenzene are located significantly above the upper bound of 2008 in the permeability-selectivity plots. Thus, the method of liquid-phase fluorination followed by hexafluorobenzene treatment leads to a significant improvement in the gas separation characteristics of PTMSP.