Pervaporation membranes for ethanol-water mixture prepared by plasma polymerization of fluorocarbons. II. Perfluoropropane membranes

Abstract

Pervaporation membranes for ethanol-water binary mixtures were prepared by plasma polymerization of perfluoropropane (PFP). The plasma-polymerized thin films were deposited onto porous polysulfone (PS) filters as substrates with an average pore size from 0.1 to 0.45 μm. By adding argon carrier to PFP system, fluorine/carbon elemental ratios (F/C) of the produced membranes evaluated by X-ray photoelectron spectroscopy (XPS), showed a maximum value, then slightly decreased at higher partial pressure of Ar. This tendency was recognized more clearly by comparing the summation of −CF 3 and −CF 2-peak area percentages based on a whole C 18 peak area. As a measure of hydrophobicity, this value is more intelligible than the direct F/C ratio. The influence of substrate pore-size, plasma-treatment time and hydrophobicity of the membranes on the separation capability were studied. The ethanol-separation coefficient, (α EtOH), of PFP membranes increased slightly with decrease of the average pore-size of the substrates, but treatment time did not apparently affect the α EtOH. We classified the prepared membranes into two classes, i.e. the membranes showing higher and lower permeation fluxes than 0.5 kg/m 2-hr. The α EtOH was more evidently observed to increase with membrane hydrophobicity for the former class of PFP membranes. We suggest that, at least, three separation schemes might be necessary to understand the correlations found in each group.