Interplay of mass transfer, phase separation, and membrane morphology in vapor-induced phase separation

Investigated in the present work was the formation of membranes by the water vapor-induced phase separation of polysulfone/N-methyl-pyrrolidinone solution. The morphology evolution during membrane formation was studied and the associated mass transfer behavior was also explored. It was observed that the exposure of the polymer solution to humid air was able to bring about spinodal decomposition to the solution, resulting in transient bi-continuous structure that later developed into cellular pores. The composition paths determined by FTIR microscopy revealed that if spinodal decomposition occurred was dependent on the duration that the solution stayed in the meta-stable region. The shorter the duration, the higher the probability that the solution phase separated via spinodal decomposition. It was also found that the duration in the meta-stable region was related to the amount of water needed to bring the solution from binodal to spinodal (the gap between them on the ternary phase diagram) and if skin occurred to retard water penetration into the cast film during membrane formation. With the information obtained, mechanism was proposed to explain why spinodal decomposition occurred throughout the whole cast film for the case that the polymer concentration of the casting solution was 10 wt.%, while spinodal decomposition was confined in a small region near the film surface for the other case that the polymer concentration was 20 wt.%.