The formation mechanism of membranes prepared from the nonsolvent–solvent–crystalline polymer systems

Two different types of membranes were prepared by immersion–precipitation process in two nonsolvent–DMSO–poly(ethylene-co-vinyl alcohol) (EVAL) systems. The effect of nonsolvent on the resulting membrane structure was studied via scanning electron microscopy. On using 2-propanol as the nonsolvent, the membrane showed a homogeneous particulate morphology. If the nonsolvent was changed to water, the membrane structure was a typically asymmetric structure while the particulate morphology was suppressed. In order to understand the change of the obtained membrane structures, the phase diagrams of water–DMSO–EVAL and 2-propanol–DMSO–EVAL systems were contrasted at 25°C. Both crystallization-induced gelation and liquid–liquid demixing were observed and equilibrium crystallization lines are always positioned above the binodal boundaries. Moreover, the precipitation rate of the EVAL solution in 2-propanol and water were examined by the light transmission experiments and the local composition profiles of membrane solution during membrane formation were analyzed by using a ternary mass transfer model. Based on both thermodynamic behaviors and kinetic properties, the membrane structures obtained were discussed in terms of the sequence and mechanism of phase transformations during membrane formation.