Effect of the hard segment domains on the permeation and separation ability of the polyurethane-based membranes in benzene/cyclohexane separation by pervaporation

A series of membrane materials made of the poly(oxytetramethylene) (PTMO)-based polyurethanes was studied in pervaporation separation of benzene/cyclohexane mixtures. Polyurethanes with a domain morphology synthesised by using different aromatic diols as chain extenders were investigated and their transport properties were compared to those of pure copolymers of 2,4-tolylene diisocyanate (TDI) and PTMO. The chain extended polyurethanes (PUs) showed generally better permselectivity towards benzene and lower pervaporation flux than the respective TDI-based ones. They also exhibited different transport behaviour with respect to the variations in the feed mixture composition that was attributed to the hard segments capability to control membrane swelling. The general trend of increasing permselectivity accompanied by decreasing permeability was observed for both PUs with increasing hard segment length and PUs with decreasing soft segment length. However, the effect of the hard segment aromatic content was found to be more complex and to depend on the length of hard segments. These results were discussed with respect to the microphase-separated structure of segmented PUs and the differences in its nature being a consequence of the PU molecular structure. It was also postulated that the microphase-separated structure may undergo transformation in contact with a feed mixture, the evidence of which was an abrupt change in the transport parameters of the membranes applied in pervaporation of the mixtures with a higher benzene concentration. For those systems, the role of the diffusion step in determining the separation efficiency was strongly diminished.