Surface fluorination of polysulfone asymmetric membranes and films

Asymmetric polysulfone membranes were surface fluorinated using a fixed set of treatment conditions and the transport of six gases was measured. After 2 and 5 min of fluorination the membranes show a significantly reduced ratio of permeability and effective skin thickness (P/l) and an improved selectivity for certain gas pairs like hydrogen, helium, and carbon dioxide relative to methane. There is some variability in the H₂/CH₄ selectivity improvement observed but, in general, it increases 2-3 fold for membranes fluorinated for 5 min and 2-5 fold for those fluorinated for 2 min. In order to gain insight about the fraction of the asymmetric membrane thickness that is being fluorinated and to determine the chemical nature of the fluorinated layer, thick (76 μm) isotropic films of polysulfone were fluorinated from one side only for a wide range of times. The chemical composition and structure of the fluorinated layer were determined from bulk elemental analysis and Fourier transform infrared spectroscopy-attenuated total reflection (FTIR-ATR). X-ray photoelectron spectroscopy (XPS) was also used to determine the chemical composition at the surface of asymmetric membranes and the thick films. The fluorinated surface of selected membranes was examined by scanning electron microscopy.