Modification of asymmetric polysulfone membranes by mild surface fluorination Part II. Characterizationof the fluorinated surface

Abstract

Integrally-skinned asymmetric polysulfone (PSF) membranes were surface fluorinated under mild treatment conditions, according to one of two strategies: at fluorination times of 1 to 10 min with a constant fluorine feed concentration of 0.02% F 2 (by volume), or at concentrations ranging from 0.02 to 0.15% F 2 for a fixed treatment time of 2 min. The fluorinated region was characterized using dynamic contact angle analysis and X-ray photoelectron spectroscopy. It was shown that increasing fluorination times from 1to 5 min resulted in increased F/C and O/C ratios in the fluorinated subsurface region and also increased the polar surface energy contribution (λ P a at the surface. Longer fluorination times resulted in an apparent decrease in polarity, possibly due to surface damage. For increasing fluorine concentrations at a constant treatment time, the F/C and O/C atomic ratios in the subsurface region increased up to 0.04% F 2 and effectively leveled off at higher feed concentrations. At the surface, λ s p decreased after reaching a maximum at 0.06 F 2. For both treatment strategies the ideal selectivities for certain gas pairs (He or H 2 with N 2 or CH 4) follow trends similar to that of λ p s . Decreases in selectivity appear to be associated with fluorination damage (surface roughness and damage to the polymer structure), rather than with a change in the chemical nature of the fluorinated skin region.