Study on the Lacy Structure Formation and Coarsening during Vapor-Induced Phase Separation
|關鍵字:||蒸氣誘導式相分離;傅立葉轉換紅外光顯微鏡;組成路徑;合併速度;膠化速度;離相分離;vapor-induced phase separation;FTIR microscopy;composition path;coarsening rate;gelation rate;spinodal decomposition||公開日期:||2010||摘要:||本研究分別以N-甲基吡咯酮(N-methyl pyrrolidinone, NMP)與吡咯酮(2-Pyrrolidinone, 2P)為溶劑，探討聚碸(polysulfone, PSf)、聚甲基丙烯酸甲酯(polymethylmethacrylate, PMMA)及聚醚碸(polyethersulfone, PES)所配製之高分子鑄膜液於蒸氣誘導式相分離(Vapor-induced phase separation, VIPS)過程的結構變化。鑄膜液在靠近水蒸氣表層的位置，於蒸氣誘導式相分離初期，都能先以離相分解(spinodal decomposition, SD)產生高分子貧相與高分子富相皆為連續相的雙連續(bi-continuous)結構，後續系統為減少界面能而產生結構合併(coarsening)。
為進一步了解蒸氣誘導式相分離過程中，各成份質傳行為、鑄膜液黏度、流變性質與結構合併的關係：本研究以傅立葉轉換紅外光顯微鏡的技術(Fourier transform infrared microscopy, FITR microscopy)分析成膜過程中各成份的濃度變化，並以流變儀量測溶液黏性模式(G”)、彈性模式(G’)及其比值(tanδ=G”/G’)作為黏度與流變性質的探討。由於NMP與2P皆為低揮發性溶劑，所以在蒸氣誘導相分離過程中，相分離的發生主要來自於水氣的吸入，使得組成路徑進入相分離區。研究結果發現，在靠近氣-液界面的鑄膜液因水氣吸入趨使高分子鏈往深處移動，造成表面附近高分子濃度低於初始濃度。然而，PSf/2P和PMMA/2P系統，溶液擁有高黏度，高分子鏈於其中移動不易，得以維持住初始高分子濃度。由於不同深度位置皆能維持相同濃度進入相分離區，而能製備出對稱性的薄膜。
當溶液產生相分離形成高分子貧相與高分子富相後，結構合併行為即開始發生，其合併速度與高分子貧富相界面張力成正比與富相黏度成反比。本研究以組成路徑搭配相圖上的結線(tie line)，計算其貧富相組成，並量測富相黏彈性作為合併行為分析。在合併過程，富相組成中的溶劑會持續被貧相置換出，而達到膠化狀態，使結構被固定下來不再產生合併。薄膜最終結構即主導於合併速度與膠化速度的競爭：若合併速度慢、而膠化很快的發生，將能有效抑制結構合併，快速固定結構。比較以NMP與2P為溶劑的高分子溶液系統，以2P為溶劑皆能有效提升溶液的黏度而減緩合併速度；同時，溶液也較趨向膠化狀態，而能在相分離後快速的膠化以固定住結構。在12 wt.% PMMA/2P的系統，由於相分離後的富相組成立即達膠化態而能固定住SD相分離初期的絲狀尺度(~0.05 μm)，得到不論表面或截面都為微小蕾絲結構之薄膜。PSf/2P系統，截面蕾絲結構隨著吸水時間增加由0.05 μm成長至0.2 μm，但仍能維持住雙連續連通結構，然而，表面絲狀結構卻因合併形成獨立的孔洞。分析不同高分子濃度的PSf/2P系統探討濃度對於截面與表面結構合併行為的影響：其結果發現截面絲狀的合併速度不隨濃度明顯改變，但由於隨著濃度增加系統也越接近膠化狀態，有助於結構能越快被固定住，維持較小的絲狀尺度。然而，薄膜表面結構因合併過程中除了貧富相的接觸面外，有一面朝向空氣面，造成結構合併的趨動力增加，使得薄膜表面結構隨著高分子濃度增加而越為緻密。
In the present work, N-methyl pyrrolidinone (NMP) and 2-pyrrolidinone (2P) were used as solvents to investigate membrane morphology evolution during vapor-induced phase separation for polysulfone (PSf), polymethylmethacrylate (PMMA) and polyethersulfone (PES) polymer solutions. Casting solutions near air-solution interface occurred phase separation via spinodal decomposition (SD) to form continuous polymer-rich phase and polymer-poor phase structure called bi-continuous structure and further the phase domains coarsened due to the interfacial tension. In order to understand the relationship between mass transfer, viscosity, rheology of solutions and morphology coarsening during VIPS process, the FTIR microscopy was used to analysis the composition change, and the rheologic instrument was used to measure viscous modulus (G”), elastic modulus (G’) and their ratio (tanδ=G”/G’) to get the viscosity and rheology of solutions. The FTIR microscopy was performed to establish the compostion paths on the phase diagram showing how the solution composition changed during the contact of casting solutions with humid air. The mass transfer was dominated by the intake of nonsolvent from the air because the solvent removal rate was low due to its low volatility for NMP and 2P as solvents. The intake of nonsolvent drove the polymer moving away from the air-soltuion interface to the deeper position. Therefore, the polymer concentration at the position near the film surface was lower than the initial concentration. However, the polymer concentration maintained with the water absorption at different positions for PSf/2P and PMMA/2P solutions. The reason should be that when the polymer chains with high viscosity moved slowly in the solution, the polymer was minimally involved in the mass transfer and only the exchange of solvent and water would be observed. The uniformity of the cross-sectional structure indicated that the solutions at different positions actually entered the phase separation with almost the same composition. The domain growth rate after the demixing of a polymer solution is proportion to the interfacial tension between polymer-poor phase and polymer-rich phase and inversion to viscosity of the polymer-rich phase. In this present work, the compositions of the polmer-poor and polymer-rich phases were estimated via tie line for the compostion path on the phase diagram and the viscoelasticity of polymer-rich phase was measured to analysis coarsening behavior after phase separation. The polymer-rich phase coarsens until it becomes gelation due to the solvent extracting by the polymer-poor phase. The final structure is dominated by the competition between coarsening rate and gelation rate: If the coarsening rate is slow and gelation rate is fast, the coarsening of structure will be suppressed effectively. Comparing with NMP and 2P as solvent, the polymer solutions with 2P as solvent possessed higher viscosity to reduce coarsening rate and the polymer-rich domain was easier to gel, decreasing the time allowing for the domains to coarsen at the same time. For 12 wt.% PMMA/2P solution, the polymer-rich phase gelled immediately after SD phase separation and the domain size of tiny lacy structure (~0.05 μm) was freezed not only for cross-section but also for top surface. However, the lacy domain grew from 0.05 μm to 0.2 μm with exposure time for PSf/2P solution. The pore connectivity was maintained for cross-section but it was coarsened to close pores for top surface. The concentration effect was analysed for PSf/2P system, and the results showed the coarsening rate slight influenced by the polymer concentration. But the polymer-rich domain easily gelled due to increasing polymer concentration, and domain growth was freezed quickly. However, the interfacial tension which included not only polymer-poor and polymer rich phase but also humid air for top surface enhanced the driving force for domain growth. The surface morphology could not maintain bi-continuous structure but coarsened to close pores. And the surface became denser with the increasing of polymer concentration. The composition path of polymer solution travels to the nucleation and growth region first, and then goes into SD region. Therefore, if the stable nuclei can form and grow to big enough before SD, they will not be destroyed by bi-continuous structure via SD. In this study, the macrovoid structure during VIPS process was formed for suitable polymer concentration and casting thickness for PSf/2P and PES/2P solutions. It is the evidence for the nuclei formation and growth during VIPS process. The polymer solution possesses higher viscosity by using 2P as solvent than using NMP as solvent. We propose using 2P as solvent can enhance the viscosity of polymer solutions due to poor solvency for polymers. It makes the polymer chains dispersion not well and increases the entanglement of polymer chains to form the 3D network which enhances the the viscosity of polymer solution and tends to induce gelation.
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