Structure–property relationships for liquid transport in modified polypropylene membranes

Abstract

AbstractIn view of the intensifying interest in the application of polymeric membranes in mixture separation processes, the permeation and permselective properties of polypropylene films toward several candidate organic liquids and vapors were investigated. Polymer films were subjected to solvent and thermal treatments, and the effects of these treatments on film morphology and transport properties were studied. Structure–property relationships for membrane permeation were then developed. Polypropylene films were found to be selective toward toluene, relative to isooctane, and p‐xylene relative to o‐xylene. Liquid flux rates were found to depend primarily upon the solubility of the permeants in the films and the absolute difference in the solubility parameters of the polymer–liquid pair provided a good basis for correlation of this effect. Considering liquids of closely similar solubility parameters, fluxes were found to be dependent upon the apparent molecular cross sections of the permeants. Films annealed in various organic solvents at temperatures of 60–100°C exhibited enhanced permeability, with up to fifteenfold increase relative to untreated membranes, but with reduced selectivity towards the permeants. A mechanism to account for these effects through consideration of the influence of treating solvent type on polymer morphology is proposed. It postulates the formation of more open or coarser spherulitic structures as a result of recrystallization in the presence of solvent during annealing. The enhanced flux rates in the treated films are attributed to the changes in the spherulite textures and to diminished intercrystalline tie chain constrainment within the spherulitic substructure.