Coarsening of polymer-rich phase and polymer-poor phase on membrane structures in VIPS process
Date Issued
2007
Date
2007
Author(s)
Wu, Cheng-Ting
DOI
zh-TW
Abstract
In order to control the porous membrane structures in preparation processes, the coarsening behavior after liquid-liquid phase separation must be deeply understood. As previous literature mentioned, the medium viscosity and interfacial tension between two phases are two main factors affecting coarsening rate, therefore, this research is to realize the effects of coarsening on membrane morphology by analyzing these two important factors.
In the VIPS process, although the bicontinuous structures were both found in PU and PSf membranes by using DMF and 2P as solvent (or cosolvent), these membranes still demonstrated different ability in keeping their own lacy structures. At the same polymer concentration of cast solution, it was observed that increasing the viscosity by using 2P as solvent composition strengthened the resistance to coarsening, while using DMF as solvent composition didn't help with enhancing viscosity, and low resistance to coarsening was shown. However, viscosity wasn't the only factor influencing coarsening. In the same couple of polymer and solvent, although the higher polymer concentration of cast solution resulted in high viscosity, higher interfacial tension, which was resulted from larger concentration difference between two phases after phase separation, was also caused by increasing concentration of cast solution from FTIR analysis. Therefore, merely increasing the viscosity of cast solution couldn't completely reduce the coarsening rate, the interfacial tension arising from the concentration of cast solution should also be taken into account.
Besides, the difference between polymers might cause different membrane structures, just like what PU and PSf were showed in this research. From the analysis of ternary phase diagram, PU membranes had higher porosity and coarsening-resisting ability than PSf membranes due to dilution effect in polymer concentration and tie line type, etc., and all these factors exactly originated in difference between polymers.
The last discussed parameter affected membranes structure in this research was relative humidity. It was found that low humidity environment could decelerate coarsening and solidifying rates, so in low humidity, the structures grew slowly but were larger in final pore size. Overall, in the VIPS process, the following parameters such as viscosity, concentration, polymer characteristics and humidity environment, must all be concerned to prepare expected membrane structures.
Subjects
PU
PSf
合併速率
concentration
viscosity
coarsening rate
Type
thesis
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