Preparation of membranes from polyacrylonitrile—polyvinylpyrrolidone blends and the study of their behaviour in the pervaporation of water—organic liquid mixtures

Abstract

Membranes were made by blending polyacrylonitrile (PAN) and polyvinylpyrrolidone (PVP) in dimethylformamide, followed by an evaporation—coagulation phase to solidify the blends. Membranes of different structures were obtained by changing the preparation conditions: asymmetric membranes with finger-like macrovoids (low PVP content in the polymer and low solvent content in the precipitant bath), asymmetric membranes with honeycomb-like sub-structure, unskinned, granulous membranes (long precipitation in tetrahydrofuran), and dense film membranes (long evaporation time). The formation of these structures was interpreted on the basis of known mechanisms, except in the case of unskinned granulous membranes, where other hypotheses were proposed. Only dense membranes were sufficiently selective for the dehydration of water—organic liquid mixtures. Highest selectivity was obtained at a 1:1 composition of the blend. This type of membrane showed good performance with water—tetrahydrofuran mixtures. Its selectivity was lower but its permeability was higher with water—ethanol mixtures. Generally speaking, the membrane consisting of the 1:1 blend had a similar behaviour to that obtained by radiochemical grafting.