Effect of solvent evaporation on the formation of asymmetric and symmetric membranes with crystallizable EVAL polymer

The effect of the evaporation process on the nonsolvent-induced phase inversion in the poly (ethylene-co-vinyl alcohol) (EVAL)/DMSO/water system was investigated. The duration of the evaporation process correlated with the resultant membrane morphologies via scanning electron microscope. Surprisingly, evaporation time not only played an important role in the existence of macrovoids, but also affected membrane morphology from sponge structure to particulate structure. Macrovoids were found within a membrane if the casting was immediately immersed into water bath or the evaporation time was less than 15 min, while macrovoids diminished and sponge structure appeared with evaporation time of 30 min. When the evaporation time was 45 and 60 min, a symmetric membrane consisting of a packed bed of nearly equal-diameter particles dominated by the solid–liquid demixing process was observed. This suggests that crystallization of EVAL polymer from the evaporated solution inhibited the formation of an asymmetric membrane with cellular pores dominated by the liquid–liquid demixing process. Compared with the corresponding phase diagram, such a crystallization process depends on how deep the solid–liquid demixing region is entered. The results of this study show that the structure of membranes can be well controlled by the knowledge of membrane formation mechanism.