Thermodynamic analysis of polymer solutions for the production of polymeric membranes

Abstract

Phase separation is a commonly used mechanism for the preparation of porous filtration membranes. In order to control the membrane structures for obtaining required membrane characteristics and performances, the examination of the thermodynamics of the membrane formation mechanisms is essential. This is why several studies have already been conducted to determine the phase diagrams of polymeric casting solution systems. However, most of the commonly used methods have certain limitations. This is the reason why a new method for the investigation of ternary polymeric systems was developed and evaluated in this study. The new method provides reproducible data which does not only provide information on the position of the binodal curve but also on the compositions of the phases which are formed after the phase separation. The tie-lines of the ternary system polyethersulfone/N-methyl-2-pyrrolidon/water were determined at different temperatures and compared to cloud point titrations conducted under the same experimental conditions. It could be shown that the location of the miscibility gap of the examined system is not visibly dependent on the phase separation temperature within the examined temperature range, especially in the region of the polymer-poor phases. This finding was comparable in all experiments, irrespectively of the method which was used. However, in the region of the polymer-rich phases within the phase diagram, the results of both methods differ from each other as the binodal determined by the tie-line method showed a temperature-dependent shift which cannot be found for the binodal curves extrapolated from cloud point measurements. Apart from determining the binodal curves, the molecular weight distributions of the polymer in the polymer-poor and the polymer-rich phases were determined in the frame of the tie-line determination. Hereby it was found that the distribution in the polymer-poor phase shows a temperature-dependence, as the average molecular weight raised from around 6 kDa at 10 °C to above 10 kDa at 40 °C.