Investigating membrane selectivity based on polymer swelling

Nanofiltration has many potential applications as a separation technology for processes that use mixtures of aqueous and organic solvents, for example alcohol/water mixtures. Membrane systems are well established for separations carried out in aqueous media, however they have seen a much slower rate of uptake in non-aqueous processes or with aqueous/organic mixtures. This is because the interaction between membrane and solvent(s) dictates both the permeability and selectivity, and there is currently limited criterion for identifying the theoretical performance of a membrane based on the properties of a bulk polymeric material. The small numbers of commercial successes to date have arisen from empirical findings, with no agreed methodology by which new candidate membrane materials can be identified. New membrane materials are required to exhibit a high permeability along with the selectivity demanded by the application. Permeability can be relatively easily manipulated using engineering solutions such as large surface areas, or very thin active separation layers. Selectivity, however cannot be manipulated in such an intuitive fashion, with the mixture type and composition, pressure and polymer characteristics all reported to be major factors. This work investigates the factors which influence the inherent selectivity of polymeric materials, and the link to nanofiltration processes. The aims of this study are to investigate the effectiveness of current theoretical and predictive tools, and to establish a technique to evaluate polymeric materials without having to fabricate a membrane. Two polymers were chosen for study which are at opposing ends of the permeability/selectivity spectrum. Polydimethylsiloxane (PDMS) and poly (vinyl alcohol) (PVA) membranes have been previously investigated in several separations involving organic-water mixtures. The materials were characterized using GPC and ATR-FTIR techniques, with ATR-FTIR further used to quantify the crosslinking content of the polymers. The total swelling degree and the inherent separation that occurs upon swelling with solvent mixtures was studied for a range of model and industrially-relevant systems, using polymer materials fabricated under different conditions. It was found that the selectivity of the polymer was a highly non-linear function of mixture type and concentration. PDMS and PVA were shown to change their affinity toward mixture components depending on the concentration, and it was hypothesized that this is due to competing mechanisms based on both molecular size and polarity. Selectivity was shown to be less dependent on the applied pressure and the degree of crosslinking, with the polymer type and mixture composition the two most dominant factors. The Flory-Huggins model was evaluated and found to give an extremely poor prediction of the selectivity in all the polymer-solvent systems studied. Further analysis was carried out using chemical potential and activity coefficient models in order to establish the sorption coefficient for future comparison with membrane filtration data. One of the key outcomes of this work is the measurement of sorption coefficients at varying composition and pressure, which can subsequently be used with existing Solution-Diffusion and Pore-Flow filtration models with greater confidence than has been possible to date.