Towards a Molecular Interpretation of Material Transport in Polymers

The successful development of processes of separation by permeation through polymeric membranes requires an understanding of the simultaneous transport of two or more substances. Although the permeation of single substances has been widely studied the systematic examination of mixtures has only just begun. This paper discusses the ways in which simultaneous permeants may mutually influence one another's fluxes and thereby determine separation factors. The separation of a binary organic liquid mixture by hyperfiltration is considered in detail. It is shown that the system may be characterized by four composition-dependent friction coefficients of which only three are probably independent. The friction coefficients may be determined from a properly designed series of interdiffusion and hyperfiltration experiments. The procedural requirements for doing this are fully explained. The friction coefficients characterize pair-wise molecular encounters and it is probable that they will prove to be interpretable in terms of the same concepts as have been used successfully with the concentration dependent diffusion of single substances in polymers. INTRODUCTION The successful large scale operation of a molecular separation process based on the use of selective polymer membranes has been achieved only relatively recently. The first such process was electrodialysis for the desalination of water. It was soon followed by a second process, reverse osmosis, which had at the outset the same objective1. The subsequent development of these processes has stimulated an interest in the possibilities of separating other mixtures by membrane permeation2. Much has already been written extolling the potential advantages of such processes over conventional techniques but at the present time only very few have approached commercial viability. The versatility of polymers as a source of thin coherent films which can have a wide range of chemical and physical properties has made them the natural choice in the search for permselective membranes3. In order to be able to separate molecules a membrane must be devoid of deliberately