非溶劑誘導式相分離過程之膜結構生成探討

Study on the Membrane Structure Formation during Nonsolvent-Induced Phase Separation

洪偉倫(Wei-Lun Hung)

指導教授 ： 王大銘

國立臺灣大學/工學院/化學工程學研究所/博士(2016年)

https://doi.org/10.6342/NTU201610450

摘要

本研究主要工作在於探討並發展巨形孔洞(macrovoids)以及其微孔結構在非溶劑誘導式相分離(nonsolvent induced phase separation, NIPS)程序中的生成機制。所使用的高分子是聚碸(polysulfone, PSf)，並分別以N-methyl-2pyrrolidinone(NMP)和2-Pyrrolidinone(2P)為溶劑來製備高分子溶液，在經由非溶劑誘導相分離程序成膜之後，在包括鑄膜液濃(黏)度，添加劑或不同的非溶劑等在內不同的鑄膜條件下，可以發現PSf/NMP和PSf/2P膜中巨形孔洞以及微孔結構的差異；相較之下，PSf/NMP膜比較容易產生從表面開始起始的巨形孔洞和實心的細胞狀結構(cellular-like)，PSf/2P膜的巨形孔洞則會隨著鑄膜液的濃(黏)度增加往更深的位置起始，且膜結構容易呈現雙連續的蕾絲結構(lacy)。
藉由光學顯微鏡的觀察分析，利用玫瑰紅B(rhodamine B)水溶液當作非溶劑來確認從表面起始的巨形孔洞是經由非溶劑直接流入高分子溶液中起始，並且在本研究中發現當鑄膜液濃度大於糾纏濃度(entanglement concentration)時，界面非溶劑的流動行為就會被抑制，這是因為大於糾纏濃度的鑄膜液，其高分子鏈於鑄膜液當中的鬆弛時間會大幅地增加，進而呈現固態特性的鑄膜液則可以阻擋界面的不穩定流動；此外，比起溶劑NMP，2P屬於較不好溶PSf的溶劑，是故2P更容易造成PSf高分子鏈在相對低的濃度下即展現高度的高分子鏈糾纏情況，所以這種PSf/2P溶液較PSf/NMP溶液易膠化(easy-to-gel)的特性，使得PSf/2P系統擁有較低的糾纏濃度。然而，本研究亦發現到當鑄膜液濃度高於糾纏濃度時，雖然界面非溶劑的流動受到抑制，但巨形孔洞仍有機會從鑄膜液較深的位置開始起始。利用光學顯微鏡觀察並分析成膜過程後發現，鑄膜液在產生相分離後，其高分子富相會產生收縮造成在膜深處的不穩定流動，並且發現到此高分子富相的收縮速度與相分離的速度會存在一競爭關係；換言之，當相分離發生的速率較高分子富相收縮速率要來得快時，巨形孔洞即無法產生。在瞭解不穩定流動產生的主因和相分離在巨形孔洞生成中所扮演的角色後，本研究亦利用相分離所需的時間，以及能代表高分子鏈於鑄膜液中的鬆弛時間等時間尺度，除了成功地解釋了糾纏濃度對於巨形孔洞的影響與高分子鏈的糾纏情況有密切的關係外，亦進一步地發展出一個可判斷巨形孔洞會在哪個深度位置起始之模型，並藉此模型來抑制巨形孔洞生成且得到完全沒有巨形孔洞的高分子薄膜。
另外一方面，考慮到相分離後的型態結構除了與藉由競爭來起始的巨形孔洞有密切的關係外，也會對於巨形孔洞的成長產生影響。所以本研究亦從相分離行為與微孔結構生成的關聯出發，利用FTIR-microscopy技術得到在溼式程序時各個不同鑄膜液深度位置下的組成路徑變化，並進一步地分析系統組成在相分離程序時橫跨在三成分相圖中介穩區(metastable)的時間(tm)，並且藉由相對應的結構變化來決定出一個臨界的組成停留於介穩區時間(critical tm，tm,c)；換句話說，從這個tm,c可知道該系統組成位於介穩區時的穩定程度。從結果發現，隨著組成越趨近於膠化狀態，系統的組成待於介穩區時會有越高的穩定度。是故像PSf/2P這種易膠化的系統才會如此容易產生雙連續的蕾絲結構，並且產生因競爭關係在膜深處生成的巨形孔洞。

關鍵字

非溶劑誘導式相分離 ； 巨形孔洞生成 ； 相分離速度與高分子富相收縮速度的競爭關係 ； 組成路徑 ； 成核成長相分離 ； Spinodal decomposition相分離

並列摘要

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.

並列關鍵字

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

參考文獻

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