CHANGES IN MORPHOLOGY AND TRANSPORT CHARACTERISTICS OF POLYSULFONE MEMBRANES PREPARED BY DIFFERENT DEMIXING CONDITIONS

A polymer solution film, which is cast from a homogeneous polysulfone (PS) solution (1.5 wt%) in dimethylformamide (DMF), demixes by liquid- liquid phase separation due to nucleation of polymer-poor phase with s orption of water vapor from atmosphere. The separated two liquid phase s continue to grow until the polymer-rich phase solidifies. However, t he polymer-rich phase in the liquid-liquid phase-separated solution ca n demix again before precipitation of the phase. The demixing of the p olymer-rich phase on the top layer of the film is induced by kinetical ly fast demixing conditions, such as rapid mass transfer between nonso lvent (water) and solvent (DMF) in a nonsolvent bath. Morphology of so lidified membranes shows that when a membrane structure is established by water vapor sorption alone, the liquid-liquid phase separated solu tion film forms closed cell-like structures in a whole cross section i ncluding surface region and has round pores on the top surface. Even t hough pore sizes on the surface of the membrane are more than 1 mu m, those pores do not work as active pores for membrane performance. A sk in structure of a membrane precipitated by fast or instantaneous demix ing in a water bath includes nodules or polymer aggregates which can b e distinguished from the cell-like structures formed by the liquid-liq uid phase separation due to nucleation of the polymer-poor phase. The membrane including nodules in the skin region has higher surface area and broader pore size distribution than the membrane which consists of the cell-like structures. The crack-like pores on the surface of the former contribute to the permeation characteristics of the membrane.