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 (15 wt%) in dimethylformamide (DMF), demixes by liquid-liquid phase separation due to nucleation of polymer-poor phase with sorption of water vapor from atmosphere. The separated two liquid phases continue to grow until the polymer-rich phase solidifies. However, the polymer-rich phase in the liquid-liquid phase-separated solution can demix again before precipitation of the phase. The demixing of the polymer-rich phase on the top layer of the film is induced by kinetically fast demixing conditions, such as rapid mass transfer between nonsolvent (water) and solvent (DMF) in a nonsolvent bath. Morphology of solidified membranes shows that when a membrane structure is established by water vapor sorption alone, the liquid-liquid phase separated solution film forms closed cell-like structures in a whole cross section including surface region and has round pores on the top surface. Even though pore sizes on the surface of the membrane are more than 1 μm, those pores do not work as active pores for membrane performance. A skin structure of a membrane precipitated by fast or instantaneous demixing in a water bath includes nodules or polymer aggregates which can be distinguished from the cell-like structures formed by the liquid-liquid 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.