Investigation on the Membrane Formation Mechanism of Vapor-Induced Phase Separation
Date Issued
2009
Date
2009
Author(s)
Su, Yu-Shen
Abstract
Investigated in the present work is the preparation of membranes by the vapor-induced phase separation (VIPS) of polysulfone (PSf) /N-methyl-pyrrolidinone (NMP), polyetherimide (PEI)/NMP and poly methyl methacrylate (PMMA)/NMP solutions. For the PSf/NMP and PEI/NMP systems, it was observed that the exposure of the polymer solution to humid air was able to bring about spinodal decomposition to the solution, resulting in transient bi-continuous structure that later developed into cellular pores. For the PMMA/NMP system, the initial structure was bi-continuous throughout the whole cross-section, but it became non-uniform, forming a thin dense skin, followed by nodular, bi-continuous, and cellular-like structures from top to bottom. To get insight into the mechanism of phase separation and morphology evolution associated with VIPS, we employed FT-IR microscopy to measure the composition change during membrane formation at different positions of the cast film. Because of the high affinity of NMP for water, water vapor was imbibed into the cast film when the film was exposed to humid air. As the water concentration at a certain position was high enough, the polymer solution moved from the homogeneous state to the meta-stable state or even the unstable state, resulting in phase separation. On the basis of the time dependence of the solution compositions at a certain position in a cast film, measured with FT-IR microscope, we can determine if the solution was in the meta-stable or unstable region, with the help of the polymer/NMP/water ternary phase diagram. We then defined the deepest position where the solution was in the meta-stable region as the metastability front, and the deepest position where the solution was in the unstable region as the unstability front. During membrane formation, due to the intake of water from the humid air, the metastability and unstability fronts moved toward the deeper region of the cast film. In addition, after phase separation, the formed polymer-rich and polymer-poor domains coalesced and grew. When the domains grew large enough to scatter visible light, the solution became turbid. We employed the optical microscope to observe the propagation of the turbidity in the cast film, and defined the deepest position where the solution was turbid as the turbidity front. With comparison of the movement of the metastability, unstability, and turbidity fronts, together with the observation of the morphology evolution during membrane formation, new understanding of the mechanism of VIPS was obtained. Because it took less water to reach the meta-stable region than to reach the unstable region, the solution entered first into the meta-stable region. During the initial period of membrane formation, the solution in the meta-stable region did not phase separate. It performed spinodal decomposition and resulted in bi-continuous structure as the solution entered into the unstable region, when the unstability front reached. To reduce the interfacial energy between the polymer rich and poor phases, the bi-continuous structure coarsened and turned into nodular or cellular structure, making the solution turbid. Therefore, we observed that, during the initial period, the unstability front lagged the metastability front, but led the turbidity front. But, once the turbidity front occurred, it might travel faster than the unstability front. When the turbid front cannot catch the unstability front, the cast film underwent spinodal decomposition everywhere. On the other hand, when the turbidity front caught the unstability front, the solution in the meta-stable region between the metastability and unstability fronts, was disturbed by the arrival of the turbidity front and demixed via the nucleation and growth mechanism, forming cellular structure. Under this circumstance, the turbidity front led the unstability front and might catch the metastability front. To summarize, the phase separation mechanism was spinodal decomposition before the turbidity front caught the unstability front, but after that the mechanism turned to nucleation and growth. As a result, the competition between the travel of the unstability front driven by the water mass-transfer, and that of the turbidity front, initiated by the interfacial tension between the phase rich and poor domains, would determine the thickness of the membrane region containing bi-continuous structures. The above mechanism can explain why spinodal decomposition occurred throughout the whole cast film for the cases with 10 wt% PSf/NMP and 20 wt% PMMA/NMP as the casting solution, while spinodal decomposition was confined in a small region near the film surface for other cases with 20 wt% PSf/NMP and 20 wt% PEI/NMP.
Subjects
vapor-induced phase separation
mass transfer
FT-IR microscopy
nucleation and growth
spinodal decomposition
Type
thesis
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