Study on the Membrane Structure Formation during Nonsolvent-Induced Phase Separation
Date Issued
2016
Date
2016
Author(s)
Hung, Wei-Lun
Abstract
The major aim of the present work is to discuss and develop the mechanism of macrovoids and other structures formation during nonsolvent induced phase separation, NIPS process. In this study, the dissimilar macrovoids and structures in the polysulfone (PSf) membrane are formed during NIPS process, which individually used N-methyl-2-pyrrolidon (NMP) or 2-pyrrolidone (2P) as solvent. Compare to PSf/NMP membrane which is easier to show macrovoids initiated from top surface and cellular-like structure, the PSf/2P membrane is easier to show lacy structure, and the position of macrovoids initiation will become deeper in the casting solution when higher PSf concentration or viscosity of casting solution is used. By observation and analysis of optical microscopy, when using rhodamine B water solution as nonsolvent, we verified that the macrovoid which initiated from top surface was formed through the nonosolvent directly flowed into casting solution. When PSf concentration was higher than a entanglement concentration, interfacial nonsolvent flow would be inhibited because the time for solution to maintain its solid-like (elasticity) is long enough to block instability flow. Furthermore, because comparing with NMP, 2P is poorer solvent for dissolving PSf, the PSf chains will theoretically perform much more entanglement and closer to gelation state at relative low PSf concentration when using 2P as solvent. That is why compare to PSf/NMP solution, PSf/2P solution is easier to show characteristic of easy-to-gel and have low entanglement concentration. Moreover, when concentration of casting solution was higher than entanglement concentration, although the interfacial flow was inhibition, the macrovoids still occurred at deep position of casting solution. Again, on the basis of the results and the observation of the membrane-formation processes with optical microscopes, new insight into the macrovoids which occur at deep position of membrane was obtained. It was observed that, during the formation of the membranes, the polymer-rich phase in the phase separation region would shrink toward the solution-coagulant interface and induced the fluid flow into the casting solution to initiate macrovoids. We also found a competitive relation between phase separation front velocity and shrinkage velocity of polymer-rich phase. When phase separation front velocity was faster than the shrinkage velocity of polymer-rich phase, the macrovoids didn’t occur. Then we used a few time scales included time for phase-separation occurrence and relaxation time of polymer chains in casting solution to construct a preliminary model. Not only we clearly knew that the entanglement concentration which affected initiation of macrovoids is strongly related to the entanglement of polymer chains, but also using this model we can predict where the macrovoids will be initiated in the membrane, and further design the membrane without any macrovoids. In fact, because this kind of macrovoids initiation through the competitive mechanism, and growth of macrovoids are all strongly related to the formation of morphology after occurrence of phase separation, the relation between behaviour of phase separation and formation of morphology is basically used to investigation in this study. By using technique of FTIR-microscopy, we got the composition path in the different position of casting solution during wet immersion process, and further analyzed the time for solution composition stayed in meta-stable region, tm. Using the tm in the different position of casting solution to relate with evolution of morphology, a critical time for solution composition stayed in meta-stable region, tm,c, was determined to imply the stability of system when solution composition stayed in meta-stable region. From the result, we found that when casting solution was closer to gelation state, the tm,c would become longer. It meant that the solution composition stayed in meta-stable region had more stability and tended to occurrence of spinodal decomposition. That is why the lacy structure and the macrovoids which initiated by competitive mechanism is always shown in PSf/2P membrane.
Subjects
Nonsolvent induced phase separation
Macrovoids formation
Competition between phase separation front velocity and shrinkage velocity of polymer-rich phase
Composition path
Nucleation and growth
Spinodal decomposition
Type
thesis
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