SynopsisAn improved explanation of the mechanism of the porous membrane formation with asymmetric structure has been proposed. It emerged from the correlation of the results obtained by the light-scattering measurements of the ternary membrane casting solutions and by the reverse osmosis and ultrafiltration testing of the asymmetric membranes prepared from these solutions. The results of anisotropic light-scattering measurements indicated the extent of supermolecular structurization of the casting solution which affected the porous structure formation in the membrane surface region. The variation of the casting solution composition and, particularly, the role of the CA/F ratio, the formamide content, and their influence on the asymmetric membrane properties were investigated and explained using the improved concept of the membrane-making mechanism. INTRODUCTIONLoeb-Sourirajan's discovery1 of the porous asymmetric membrane fabrication method opened a new field in separation technology: pressure-driven membrane separation and concentration processes. A substantial progress has since been accomplished in the membrane science and in the application of the membrane separation processes (reverse osmosis and ultrafiltration) in industry. Notwithstanding appreciable improvements in the membrane performance, developments of a number of new membrane materials, and the utilization of new fabrication methods, the initially introduced cellulose acetate porous asymmetric film is still, in an improved form, the most often used membrane in actual separation processes. Cellulose acetate has also been the most convenient model polymer for the membrane studies which aim at producing better, more productive or more selective membranes andlor shedding more light on the process of the asymmetric membrane formation.The first coherent picture of the mechanism of the porous asymmetric membrane formation was proposed by K e~t i n g .~.~ It has further been improved by the solution structure-evaporation rate concept4-10 and by the in- vestigation of the dynamics of the membrane formation process.11-16 All the studies mentioned have contributed to a better understanding of the relevant processes taking place during the membrane formation, and some of them have even directly led to membranes of better performance. However, there are still uncertainties in the explanation of the sequence of events occurring from the casting of the polymer solution layer to the finally formed porous asymmetric membrane. One of such unresolved problems is the effect of the casting solution structure on the phase inversion process in the cast solution layer. There is a general agreement2*5*6J3~17~*8 that the morphology of a membrane surface layer originates in the structure of the polymer solution. There have also been attempts5 to define the extent of solution structurization, but as yet neither the effective means of expressing the solution structure nor a satisfactory correlation of this structure to the membrane properties has been elaborated.The aim of this...

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