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

Two different types of membranes were prepared by immersion-precipitation p rocess in two nonsolvent-DMSO-poly(ethylene-co-vinyl alcohol) (EVAL) system s. The effect of nonsolvent on the resulting membrane structure was studied via scanning electron microscopy. On using 2-propanol as the nonsolvent, t he membrane showed a homogeneous particulate morphology. If the nonsolvent was changed to water, the membrane structure was a typically asymmetric str ucture while the particulate morphology was suppressed. In order to underst and the change of the obtained membrane structures, the phase diagrams of w ater-DMSO-EVAL and 2-propanol-DMSO- EVAL systems were contrasted at 25 degr ees C. Both crystallization-induced gelation and liquid-liquid demixing wer e observed and equilibrium crystallization lines are always positioned abov e the binodal boundaries. Moreover, the precipitation rate of the EVAL solu tion in 2-propanol and water were examined by the light transmission experi ments and the local composition profiles of membrane solution during membra ne formation were analyzed by using a ternary mass transfer model. Based on both thermodynamic behaviors and kinetic properties, the membrane structur es obtained were discussed in terms of the sequence and mechanism of phase transformations during membrane formation. (C) 2000 Elsevier Science Ltd. A ll rights reserved.