非溶劑誘導相分離製備具雙連續結構微孔膜及其成膜機制之研究

Study of microporous membranes with bicontinuous structure by nonsolvent induced phase separation

郭純因(Chun-Yin Kuo)

指導教授 ： 賴君義 ； 王大銘

中原大學/工學院/化學工程學系/博士(2008年)

https://doi.org/10.6840/CYCU.2008.00016

摘要

本論文主要目的是探討非溶劑誘導相分離，包括蒸氣誘導式相分離與醇類非溶劑溼式相分離，以製備具雙連續(bicontinuous)結構之微孔膜，同時釐清形成雙連續結構之成膜機制。此外，研究中也對薄膜表面結構對膜面疏水性質的影響加以探討。
研究發現以蒸氣誘導相分離(Vapor Induced Phase Separation, VIPS)方式製備具雙連續結構薄膜成膜機制由spinodal decomposition主導，在VIPS成膜過程中，利用FTIR-Microscopy、動態光散射實驗與薄膜結構分析，證實15wt% polysulfone(PSf)/ N-methyl-2-pyrrolidinone(NMP)系統在相分離過程中(RH=70%，T=25°C)，高分子溶液表面位置組成路徑往低濃度方向進入介穩定區(metastable region)，當組成可以快速跨過介穩定區進入不穩定區(unstable region)時，會由spinodal decomposition機制主導，形成雙連續結構。將VIPS成膜運用於不同種類高分子溶液，如Poly(bisphenol A carbonate) (PC)/NMP、Poly(methyl methacrylate)(PMMA)/NMP、Poly (bisphenol A-co-4-nitrophtalic anhydride-co-1,3-phenylenediamine) (PEI)/NMP、Cellulose acetate (CA)/NMP系統(高分子溶液濃度皆為15wt%)，發現在適當成膜條件下，均可成功製備出具雙連續結構之薄膜。但在VIPS過程中，由於溶劑揮發速度慢，相分離所產生的結構會進一步合併成長(coarsening)。在PSf、PEI與PC三系統中，VIPS初期所產生的表面雙連續結構會轉變形成緻密層，使得水份受到阻礙無法順利進入高分子溶液中，在表面形成一對水接觸角接近0度之液體層，高分子溶液表面緻密層的生成會使得高分子溶液內部組成無法快速跨過介穩定區，而以nucleation and growth之機制進行相分離，使得薄膜內部型態為胞狀結構。而在PMMA與CA系統，VIPS初期所形成的雙連續結構在後續過程中得以維持，不會形成阻擋水進入的皮層及表面之液體層，所以高分子溶液表面與內部皆可以進行spinodal decomposition相分離機制，整體薄膜型態較容易維持雙連續結構。
研究亦發現以醇類非溶劑溼式法成膜時，亦可以由spinodal decomposition主導相分離而形成雙連續結構，其過程與VIPS頗為類似。在以 PMMA/1,4-dioxane溶液為鑄膜液而n-propanol為凝聚劑之成膜系統中，亦利用FTIR-Microscopy、動態光散射實驗與薄膜結構分析說明其成膜機制，證實其高分子溶液表面位置的組成路徑變化也是往降低高分子濃度方向進入相分離區，產生spinodal decomposition及雙連續結構。使用不同種類高分子溶液(如15wt% PSf/NMP、Poly(vinylidene fluoride)PVDF/NMP、PEI/NMP、PMMA/NMP、CA/NMP)以醇類非溶劑溼式成膜，也可成功製備出雙連續結構。但在溼式法中，溶劑可以被非溶劑交換出高分子溶液，所以所產生的雙連續結構是否會進一步演變，則與非溶劑與高分子之親和性有關。當所用的醇類對高分子為強非溶劑時(如PEI與 PSf系統)，相分離所產生的初始結構幾乎不隨其在醇類溶液中的時間變化；但若醇類與高分子之親和性高時(如 PMMA與CA系統)，雙連續結構會合併成緻密皮層，將導致薄膜內部進行NG相分離機制(與VIPS機制相似)。另外在PVDF/NMP/n-proanol系統，具結晶性PVDF高分子以醇類成膜過程中發生膠化(gelation)現象使得雙連續結構不受在n-propanol中滯留時間所影響。
量測所製備具雙連續結構微孔膜之水接觸角，發現薄膜會因表面結構而具有高疏水性質。PC高分子材料(緻密結構對水接觸角約70°)在以VIPS成膜後，甚至會具超疏水特性(對水接觸角約150°)。PC在成膜過程中，先經由spinodal decomposition 相分離機制，形成微米級的雙連續結構(第一層次的結構)，爾後，高分子富相再次發生固-液相分離，在孔壁上形成奈米級的結晶結構(第二層次的結構)。此多層次結構(hierachical structure)相信是超疏水性產生的原因。

關鍵字

薄膜 ； 超疏水 ； 雙連續結構 ； 成膜機制

並列摘要

The main purpose of this dissertation is to study how to prepare microporous membranes with bicontinuous structure and to interpret the mechanism of membrane formation by nonsolvent induced phase separation methods, which focused on absorption of water vapor to induce phase separation (VIPS) and wet immersion precipitation with alcohol as coagulant. In addition, the effects of surface morphologies on the surface hydrophobicity of membranes were discussed.
In the VIPS process, 15wt% (polysulfone)PSf/NMP polymer solution was used to study the phase separation mechanism by exposure it to humid air (70%RH, 25°C) at varying times. The results of FTIR-Microscopy, dynamic light scattering and the observation of membrane structure proved that the composition path of polymer solution on the top layer followed a decreasing trend. When the composition path can rapidly pass through metastable region and then entered into unstable region, and which indicates the phase separation mechanism of spinodal decomposition occurred, leading to the formation of the PSf membrane with bicontinuous structure. The VIPS method can be successfully applied in various polymer systems in preparing membranes with bicontinuous structure such as Poly(methyl methacrylate)(PMMA)/NMP, Poly (bisphenol A-co-4-nitrophtalic anhydride-co-1,3-phenylenediamine) (PEI)/NMP, Poly(bisphenol A carbonate) (PC)/NMP, Cellulose acetate (CA)/NMP (all 15wt% polymer solutions). The VIPS method was a feasibility method to prepare bicontinuous structures, but, the evolutions of the bicontinuous structures in various polymeric membranes were different. A forming surface liquid layer at the interface position of polymer solution/air during the VIPS processes would be related to the membrane morphologies in various polymer systems. The forming surface liquid layer was observed by optical microscopy and analyzed by water contact angle. In PSf or PEI system, the water contact angle decreased to 0°, it meant that the liquid layer was formed which accumulated water vapor on the top surface of casting film. The nonsolvent (water vapor) was delayed to transfer to interior, resulting phase separation transforming in nucleation and growth and as a result, the final morphology is cellular. On the other hand, as the mass transfer of nonsolvent was continued, spinodal decomposition can be maintained in the interior of casting film, and as a result, the final morphology was bicontinuous (such as PMMA or CA membrane).
Based on the results of the VIPS method, microporous membrane with bicontinuous structures were further prepared by wet immersion precipitation with alcohol as coagulant. The 20wt% PMMA/1,4-dioxane polymer solution precipitated with n-propanol as nonsolvent was used to study the membrane formation. The formation mechanism of bicontinuous structure was similar with VIPS method. The composition path of PMMA polymer solution on the top layer also followed a decreasing trend. When the composition path can rapidly pass through metastable region and then entered into unstable region, the phase separation mechanism of spinodal decomposition occurred, and which leaded to the formation of the PMMA membrane with bicontinuous structure. The method also can apply in various 15wt% polymer solutions (PSf/NMP, Poly(vinylidene fluoride)PVDF/NMP, PEI/NMP, PMMA/NMP and CA/NMP) to successfully prepare microporous membranes with bicontinuous structures. The affinity between polymer and nonsolvent would be related to the evolutions of bicontinuous structure in various polymer systems. When the polymer and alcohol had higher affinity, the bicontinuous structure which formed in the earlier stage was not evolving with the immersion time in alcohol bath, due to the polymer rich phase was solidified rapidly (such as PSf or PEI). However, when the polymer and alcohol had lower affinity, the bicontinuous structure could evolve into dense during the immersion process (such as PMMA or CA system).

In addition, the membranes with bicontinuous structure can enhance hydrophobicity, even to the degree of a superhydrophobic characteristic. A super-hydrophobic surface (water contact angle of about 150°) can be obtained by coating a film with polycarbonate, when suitable porous surface structure was formed after VIPS (although the water contact angle of a dense polycarbonate film was measured to be only about 70°). The process of mechanism of membrane formation to form hierarchical porous structure can be controlled. Membrane pores were formed after spinodal decomposition of the cast film, which generated the micron scale bicontinuous structures (first-tier). The polymer-poor phase would further develop into the pores. The polymer-rich phase would crystallize and precipitate forming the nano scale pore wall (second-tier).

並列關鍵字

bicontinuous structure ； superhydrophobic ； membrane ； membrane formation

參考文獻

85. 蘇順良, 雙凝聚槽製備PMMA三層結構薄膜之成膜機制探討, 中原大學化學工程學系碩士論文, (2006).

2. M. Mulder, Basic Principles of Membrane Technology, Kluwer Academic Publishers (1996).

3. W. Eykamp, Microfiltration and Ultrafiltration in Membrane Separation Technology (ed. Noble and Stern), Elsevier, Amsterdam (1995).

4. R. Baker, Controlled Release of Biologically Active Agent, Wiley, New York (1987).

5. B. S. Kim, and D. J. Mooney, Development of biocompatible synthetic extracellular matrices for tissue engineering, Tibtech (1998) 224-230.