Membranes with a particulate morphology prepared by a dry–wet casting process

Abstract

The effect of the evaporation step on the occurrence of particles in poly(ethylene-co-vinyl alcohol) (EVAL) membranes cast from DMSO solutions via the dry/wet process were studied. The structure of the EVAL membranes can be changed from an asymmetrical structure consisting of a dense skin layer and finger-like macrovoids in the sublayer to a skinless and symmetric structure by constituent particles bonded to each other. From the analysis of the membrane formation mechanism, the competition of different phase separation process during membrane formation is most important. The onset of the phase separation may be either liquid–liquid demixing or solid–liquid demixing, which determines the resulting membrane properties. Directly immersing the casting solution into a water bath, liquid–liquid demixing is of considerable importance during the phase separation of the solutions. As a result of the lower activation energy for nucleation, liquid–liquid demixing can precede solid–liquid demixing even in cases where solid–liquid demixing is favored thermodynamically. By using the evaporation process, the phase separation proceeds slowly via solid–liquid demixing and thus leads to a particulate morphology in the membrane. This suggests that the evaporation step cause crystallization of EVAL molecules from the casting solution to inhibit the macrovoid formation. In addition, the duration of the evaporation step is shown to have a strong influence on the disappearance of particles. The results presented here offer a qualitative basis for the development of membranes with a particulate morphology.