蒸氣誘導式相分離法中高分子富相和高分子貧相之合併速率對於薄膜結構的影響

Abstract

在製備多孔性薄膜的前提下，本研究希望能夠深入地探究液-液相分離後之合併現象，唯此方能有效地控制出預期之薄膜結構。文獻曾提及，影響合併速率最主要之兩項因素分別為介質黏度和界面張力，因此本研究便欲藉著探討這兩項因素來了解合併速率之於薄膜結構的影響。 在以DMF(N,N-dimethylformamide)和2P(2-Pyrrolidinone)作為溶劑(或共溶劑)之鑄膜液系統中，PU和PSf薄膜雖均能藉蒸氣誘導式相分離法順利製得微米級之雙連續薄膜型態，不過卻於後續之蕾絲結構維持能力上有著顯著的差異。於同一鑄膜液濃度下，使用2P作為溶劑組成可提高鑄膜液之黏度，且高黏度之性質能有效地抗拒合併現象的產生，進而使蕾絲結構得以於非溶劑蒸氣環境中長時間地保持住；相反地，以DMF作為溶劑組成時，由於無法明顯地提升鑄膜液之黏度，因此產生了較高之合併速率，使蕾絲結構很快地便消失。然而，於相同溶劑(或共溶劑)使用下，雖然提高鑄膜液之濃度也可增加黏度值，但經FTIR量測後發現，提高鑄膜液濃度同時亦會拉升相分離後之兩相組成濃度差，因此產生較大之界面張力值，故高濃度之鑄膜液系統反而無法如低濃度鑄膜液系統，展現較緩慢之合併速率。 在不同高分子之成膜系統中，高分子間的差異也會為最終薄膜結構帶來不同的型態。如本研究中之PU和PSf系統，我們觀察到PU薄膜結構整體上是較PSf薄膜結構更具多孔性；故經三成份相圖之分析後發現，PU系統可藉稀釋效應、tie line型態等現象來延緩合併現象，因而較易展現多孔之薄膜結構。除此之外，成膜環境之相對濕度亦會對薄膜結構產生影響；我們觀測到低濕度之非溶劑蒸氣環境可降緩合併及固化速率，故相較於高濕度環境而言，其薄膜結構合併(成長)得較慢但最終結構尺寸卻較大。因此，若欲藉蒸氣誘導式相分離法製備預期之薄膜型態，黏度、濃度、高分子和濕度皆係為考量重點。

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.