| Polymeric membranes have been widely developed for a variety of industrial, biomedical and biotechnology applications, such as microfiltration, ultrafiltration, reverse osmosis and gas separation. Each application imposes specific requirements on the materials and structure of membrane. For microfiltration and ultrafitration membranes, the porosity and the pore sizes of the membranes determine the efficiency of filtration. To prepare these membranes, there have so various methods of which the immersion precipitation is the most popular. During the immersion process, the initial stable polymer solution is transformed into unstable two liquid phase demixing solutions, and then solidified as a porous membrane by gelation. The morphology of final membranes is greatly depended on mass transfer and phase separation processes, as well as gel structure. Therefore, it is meaningful and necessary for understanding the membrane formation mechanism, by means of studying the membrane formation processes and related phenomena under different conditions.In this thesis, membrane formation processes of PSF-DMAc binary system and PSF-DMAc-IBA, PSF-DMAc-IPA ternary system are studied. Sectional morphology of membrane can be observed directly by using our designed ultra-thin coagulation cell during coagulation. The mass transfer and phase inversion processes are investigated by coagulating the polymer dope solution in humid air. Under this condition, the phase separation is controlled to proceed slowly that make it possibleto observe the fore-and-aft course of the development of phase separation. The difference structure of the final membrane is also studied under different concentration of PSF-DMAc solution and different relative humidity (RH) of air in each system.As for PSF-DMAc binary system, when the concentration is lower than 6% (such as 2%,4%,6%), a primary mass transport just takes place at the interface of PSF solution/ humid air. Direct accumulation of polymer, mutual diffusion between solvent and nonsolvent, convection and hydrodynamic flow of components are observed. The transformation from clusters to fractals is also visualized. The fractals are accumulated underneath the skin layer in a certain extent, but they still can not change into gel to form membrane structure. The solidification of direct accumulated polymer is only exhibited. It is also observed that the droplet size become gradually bigger with time, which shows the nucleation and growth of polymer poor phase. From skin to sublayer, it is denser with the concentration increasing. When the concentration is 2%, it shows a structure of accumulated polymer, as in 4%, some channels and drop-like structures happens. And in 6%, there have a tendency to form membrane by some accumulated fractals. Under same RH, the increasing ratio of membrane thickness gradually decreases with increasing concentration.When concentration reaches 9%, formation of membrane structure can be observed obviously. A new mode of phase separation, secondary mass transport, happens during the process of membrane formation, which exhibits specifically the nucleation and growth of polymer-rich phase and polymer-lean phase, as well as coalescence of the polymer-rich and polymer-lean nucleus at the late stage. In this process, membrane structure derives mainly from the solidification of gels. The higher the concentration gets, the denser the membrane is, and the thicker the skin is. The particles in the lean-phase in diluted solution are observed as clusters; the macro voids derive from the formation of nuclei and their growth in the polymer-lean phase, and finally they would be kept in the bulk of membrane as the voids.The higher the relative humidity is, the higher the coagulation rate of membrane is. Compared with the phenomena that the increasing of concentration would resultin the decreasing of coagulation rate, the relative humidity is the main factor that determines the coagulation rate. Under different polymer concentrations, microscopy studies demonstrate that, in general, the macrovoids grow faster than the appearance gelation front. Each growth rate decreases with increasing concentration as well.It is observed that the pore size increase is determined mainly by the coarsening, especially coalescence of polymer-lean phase at the late stage of phase separation. The very good linear relationship between the average droplet diameter and the time isR-t1/3.In ternary systems, addition of IBA or IPA can quicken the precipitation rate. Skin layer of the system containing IBA is more thicker than the system containing IPA, as the coagulation rate, the former is lower than the later. Compared with the binary system in the same concentration, the coagulation rate of membrane decreases due to the rate of diffusion between the humidity of air and polymer solution.In order to characterize the gel microstructure change during the PSF membrane formation, preliminary studies on gel formation and structure of PSF membrane by LSCM are processed. A fluorescent label-Rhodamine B (RhB) is grafted on sulfonated PSF.An attempt to use Monte Carlo model to simulate the dynamic information about the instantaneous phase separation of PSF-DMAc system is also made in this thesis. By using Matlab and Maple 8.0, calculation is performed according to the method applied by Kools. |
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