Study On Formation Mechanism Of Pore Structure In Polyethersulfone Ultrafiltration Membranes Prepared By Phase Inversion

PolyethersuIfone(PES) ultrafiltration membranes were prepared by wet phase-inversion in this work. Formation mechanism of pore structure in the membranes was studied, which would be helpful to instruct the membrane preparation and application. Therefore, the investigation has a important practical significance.Firstly, the precipitation time of the nascent membrane immersed into coagulant bath was defined and determined by a simple and effective method. Based on the precipitation time and the solubility parameters, empirical rules for the membrane sublayer formation were proposed as follows: the stronger the nonsolvent for PES, the larger the precipitation rate during the membrane formation and the more the membrane sublayer trended to be full of macrovoids. For PES/DMAc/HiO and PES/DMAc/isopropanol systems, effects of nascent membrane thickness on the two typical sublayer structures were investigated. The phenomenon that Vogrin et al. had discovered wasn't detected in our work. In a word, not all wet phase-inversion systems have the phenomenon. For PES/DMAc/F^O system, the lower relative rate between moving coagulated front and diffusional front played an important role in the formation of macrovoids.Secondly, for PES/DMAc/H2O system, effects of PES concentration in the casting solution and additives on sublayer structure and properties of the membrane were studied. Both the high PES concentration in the casting solution and addition of lithium chloride or phosphoric acid, to some extent, suppressed the formation of macro-voids in membrane sublayer, but neither eliminated macrovoids. Based on Flory-Huggins theory, the equilibrium model between the polymer-poor phase and the polymer-rich phase during the membrane formation was established. Effective methods to determine the interaction parameters were adopted and the viewpoint that thepolymer concentration should be calculated in initiative two-component system was accepted. By means of the Marquardt algorithm improved by ourselves, the binodal curve and the spinodal curve of phase diagram for PES/DMAc/H2O system were obtained. The results showed that the binodal curve within the high PES concentration range could be well calculated by the improved algorithm.Finally, for the formation of macrovoids in membrane sublayer, a new three-step mechanism was proposed as follows: the step of macrovoid initiation, the step of top-outline formation and the step of macrovoid growth. Rapid phase-inversion led to the membrane sublayer full of macrovoids. Spinodal decomposition happened at the interface of membrane and bath. Consequently, the polymer-poor nuclei were immediately formed in the polymer-poor phase while the second phase separation occurred in the polymer-rich phase because of nucleation and growth. The toplayer rapidly formed so that solid-liquid demixing occurred, which resulted in initiation of macro-voids. Nonsolvent that entered the nascent membrane before the toplayer solidification promoted the initiation, growth and amalgamation of the polymer-poor nucleus. Some nuclei independently grew into macrovoids while others amalgamated to be larger nuclei that grew up to be macrovoids in the end. The growing process of macrovoids was that of continual renewal for the polymer-poor nucleus boundary. In conclusion, macrovoids in the sublayer were resulted from the initiation, growth and amalgamation of the polymer-poor nuclei.