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 i nto 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 sh owed another asymmetric structure: porous surface with cellular pores suppo rted by a homogeneous particles-bonded sublayer. The particle structure is representative of crystallization-induced solid-liquid demixing from a homo geneous mixture. If the water absorption rime was decreased, a progressive change in the membrane morphologies containing features from both liquid-li quid demixing and solid-liquid demixing phase separations could be observed . This suggests that different types of membranes controlled by different p hase separations should have different composition paths in the phase diagr am even though they had the same phase diagram. Therefore, this difference in membrane structure prepared with or without nonsolvent absorption was di scussed by taking into account the effect of the diffusion kinetics of the membrane formation system. (C) 2000 Elsevier Science B.V. All rights reserv ed.