利用傅立葉轉換紅外光顯微鏡探討聚醚碸薄膜之成膜機制

Abstract

本研究藉由傅立葉紅外光顯微鏡(FTIR Microscope)分析高分子溶液在成膜過程中的組成路徑，再將組成對應到三成分熱力學相圖中，得到溶液組成於不同深度位置滯留於介穩區的時間(tm)，tm與質傳速率有關，質傳速率越快，tm越小。對應到其不同位置下的膜結構變化，可以定義出溶液於介穩區的臨界滯留時間(tmc)，tmc與系統的穩定度有關，當tmc越大，表示系統的穩定性越高。可以藉由tmc分辨系統的相分離機制。當tm大於tmc時，系統會以成核成長(Nucleation and growth)的機制相分離成蜂窩狀結構。反之，當tm小於tmc時，系統則會以Spinodal decomposition機制行相分離，形成孔洞連通的雙連續結構。 本研究使用分子量為25000及75000的聚醚碸(Polyethersulfone, PES)高分子，溶於2-吡咯酮(2-pyrrolidone , 2P)及N,N-二甲基乙醯胺(N,N-Dimethyl Acetamide, DMAc)兩種溶劑中，觀察不同系統的膜結構後可以發現，2P系統較容易得到雙連續的結構。然而，DMAc系統則皆會成核成長相分離形成蜂窩狀的結構。因此本研究同樣利用FTIR Microscope分析不同系統中高分子溶液的組成路徑變化，並利用tm及tmc分析不同溶劑系統的相分離機制，同時也會探討改變濃度及改變分子量對於膜結構的影響。從先前的研究中得知，tmc和高分子溶液的黏度有關，當黏度提高時，同時提升了系統的穩定度，因此tmc也會隨著增加，此時系統形成雙連續結構的機會應該較大。然而，當提升高分子溶液的黏度時，同時也會降低質傳速率，使tm變大，最終形成蜂窩狀的結構。本研究將會更進一步討論，質傳速率及系統穩定度兩者對於最終膜結構的影響程度。除此之外，由於聚碸(Polysulfone, PSf)高分子之分子結構與PES十分相似，本研究也探討了使用不同高分子系統之薄膜結構變化，而結果發現由於PSf系統的黏度比PES系統高，質傳阻力也較大，導致質傳速率較慢，較容易形成蜂窩狀的結構。

According to the previous research, the composition path of polymer solution during its membrane formation process can be analyzed by FTIR microscope. If we further corresponds each composition to the ternary phase diagram, we can obtain its residence time in the metastable region (tm) at different depth. tm is related to the mass transfer rate, when the mass transfer rate gets higher, tm will get smaller. Corresponding tm to the transition of the membrane structure along different position, the critical residence time in meta-stable region (tmc) can be defined. tmc is related to the stability of the system, the higher tmc indicates the higher system stability. tmc provides a criterion to determine which phase separation mechanism will happen. When tm is larger than tmc, the system will undergo nucleation and growth and phase separate into cellular structure. On the other hand, when tm is smaller than tmc, the system will phase separate via the spinodal decomposition and form the pore-connective bi-continuous structure. In this study, Polyethersulfone (PES) polymer with molecular weight of 25000 and 75000 were solved in 2-pyrrolidone (2P) and N,N-dimethylacetamide (N,N-Dimethyl Acetamide, DMAc). It is observed that 2P system is easier to form bi-continuous structure while DMAc system will all form the cellular structure by nucleation and growth. Therefore, this study also used FTIR microscope to analyze the change of composition path for polymer solution in different systems, and the phase separation mechanism of different solvent systems can then be analyzed by tm and tmc. Meanwhile, we will also discuss the effect of concentration and polymer molecular weight on the membrane structure. From the previous research, we can know that tmc is related to the viscosity of polymer solution. When the viscosity increases, the stability of the system is also improved and thus results in the higher tmc, which will have larger chance to form bi-continuous structure. However, the mass transfer rate will also decrease when enhancing the solution viscosity, which will lead to the larger tm and thus the cellular structure. As a result, this study will have further discussion about the competitive effect of mass transfer rate and system stability on the membrane structure. In addition, because the molecular structure of polysulfone (PSf) polymer is similar to that of PES, this study will also discuss between the membrane structure of different polymer systems.