The formation mechanism of EVAL membranes prepared with or without the nonsolvent absorption process

In this work, the effect of the nonsolvent absorption from the atmosphere into the casting solution on the membrane structure in the poly (ethylene-co-vinyl alcohol) (EVAL)–DMSO–water system was studied via scanning electron microscopy. If the casting solution was directly immersed into a water bath, the membrane showed a typically asymmetric structure dominated by liquid–liquid demixing process. As the water vapor as a coagulant, the membrane showed another asymmetric structure: porous surface with cellular pores supported by a homogeneous particles-bonded sublayer. The particle structure is representative of crystallization-induced solid–liquid demixing from a homogeneous mixture. If the water absorption time was decreased, a progressive change in the membrane morphologies containing features from both liquid–liquid demixing and solid–liquid demixing phase separations could be observed. This suggests that different types of membranes controlled by different phase separations should have different composition paths in the phase diagram even though they had the same phase diagram. Therefore, this difference in membrane structure prepared with or without nonsolvent absorption was discussed by taking into account the effect of the diffusion kinetics of the membrane formation system.