高分子分離薄膜微結構分析：
正電子湮滅光譜技術結合分子動態模擬

本研究製備化學結構相異的聚醯亞胺薄膜、聚醯胺複合膜以及聚電解質錯合物複合膜，應用於滲透蒸發分離乙醇水溶液，探討高分子化學結構變化對分離效能之影響。為探討進料溶液膨潤效應對高分子薄膜微結構與分離效能的影響，利用正電子湮滅光譜(Positron annihilation spectroscopy，PAS)技術，探討高分子薄膜微觀結構，包含自由體積的分率、尺寸、數量及分佈等，對滲透蒸發分離效能的影響。同時，以分子動態模擬(Molecular dynamics simulation，MD simulation)技術，預測及分析高分子薄膜之微結構特徵，包含自由體積分率(FFV)、容通體積分率(FAV)、自由體積形態、自由體積相當直徑(Deq)分佈圖與自由體積形狀分佈圖等，並以徑向分布函數圖、扭曲角分佈圖及均方位移圖來探討高分子鏈擾動行為。結合理論預測與實驗分析的結果，與滲透蒸發分離效能進行關聯。首先，針對三種化學結構相異之聚醯亞胺薄膜，探討其微結構變化對滲透蒸發分離效能之影響。實驗結果顯示，在乾燥環境下，隨著懸垂基團尺寸的增加，高分子主鏈的擾動性受到抑制，使分子鏈不易堆疊排列，形成較大的自由體積空間，同時，懸垂基團上的側基及末端基團，在空間中擾動及旋轉能力提升，使得較小鏈段可在大的空間中排列堆疊，最終造成整體自由體積尺寸下降。薄膜受進料溶液膨潤後，其微結構變化甚鉅，實驗結果指出，薄膜之膨潤程度由大至小依序為：9Ph-6FDA>TPA-6FDA>ODA-6FDA。其中9Ph-6FDA薄膜受進料影響最大的是主鏈結構，而TPA-6FDA薄膜則為二胺部分之氮原子連結鏈段。由滲透蒸發實驗結果可知，ODA-6FDA薄膜因膨潤程度最小，故有最高的透過水濃度。為改善緻密薄膜低透過量的缺點，將薄膜之緻密結構型態轉變為複合結構型態。以界面聚合法，製備化學結構相異聚醯胺複合膜，探討聚醯胺聚合層微結構變化對滲透蒸發分離效能之影響。實驗結果顯示，在乾燥環境下，DAPE-tNBDC聚醯胺聚合層結構之懸垂基團norbornylene的埋入效應，使分子鏈堆疊更緊密，故其自由體積尺寸較DAPE-SCC/mPAN複合膜小。在濕潤環境下，由於DAPE-tNBDC聚合層之norbonylene的擾動程度高於DPAE-SCC聚合層的分子鏈，造成DAPE-tNBDC聚合層有較高的膨潤程度。關聯聚醯胺複合膜微結構分析結果與其滲透蒸發效能發現，DAPE-tNBDC聚合層在進料溶液膨潤狀態下，分子鏈擾動程度上升，使得自由體積尺寸增加，造成DAPE-tNBDC/mPAN複合膜之透過水濃度(91.2 wt%)較DAPE-SCC/mPAN複合膜(96.3wt%)低。由自由體積形狀因子分析結果可知，DAPE-tNBDC聚合層之自由體積形狀的連通性較DAPE-SCC聚合層低，造成兩者間之透過量差異不大，約550 g/m2h。為提升滲透蒸發程序之透過水濃度，以含有離子交聯結構之聚電解質錯合物複合膜，應用於滲透蒸發分離程序，探討取代基團碳鏈長度相異之聚電解質錯合物複合膜微結構變化對滲透蒸發分離效能的影響。實驗結果顯示，在乾燥環境下，在立體障礙與分子鏈擾動伴隨埋入效應的競爭下，隨著聚陽離子電解質取代基碳數的增加，自由體積尺寸有先上升而後下降的趨勢。而在濕潤環境下，當取代基團愈大時，聚陽離子電解質主鏈的擾動性受到抑制，堆疊排列效率降低，但同時，側鏈的擾動性則呈現上升的趨勢，最終造成自由體積尺寸隨聚陽離子電解質取代基碳數的增加而增加。不論是在乾燥或濕潤環境下，具有強陰電性及含有許多未配對電子對之聚電解質錯合物，在進行正電子湮滅光譜量測時，會產生捕捉(quenching)及抑制(inhibition)效應，導致整體的自由體積數量有減少之趨勢。關聯聚電解質錯合物複合膜微結構分析結果與其滲透蒸發效能發現，隨著聚陽離子電解質取代基碳數的增加，透過量有逐漸上升的趨勢，而透過水能度則些微下降。本研究成功的以正電子湮滅光譜技術結合分子動態模擬的方法，探討高分子薄膜之微結構變化對滲透蒸發分離效能之影響，研究成果將有利於薄膜結構設計與分離效能之預測。

關鍵字

聚醯亞胺薄膜、聚醯胺複合膜、聚電解質錯合物複合膜、滲透蒸發、正電子湮滅光譜分析儀、分子動態模擬

並列摘要

In this study, various membranes of different chemical structures were fabricated: polyimide membrane, polyamide composite membrane, and polyelectrolyte complex composite membrane. The objective was to investigate the effect of varying polymeric membrane microstructures on its pervaporation performance in dehydrating a 90 wt% aqueous ethanol mixture. Positron annihilation spectroscopy was adopted to explore the relationship between the microstructure and the separation performance of the membrane under the swelling effect. The polymeric membrane microstructural properties were analyzed: fractional free volume, free volume size, free volume intensity, and free volume distribution. Each of these properties was correlated with the separation performance. By molecular dynamics simulation, the polymeric membrane microstructural properties were predicted and analyzed: fractional free volume, fractional accessible volume, free volume morphology, free volume equivalent diameter, and free volume shape factor. The polymer chain mobility or flexibility was analyzed by radial distribution function (RDF), dihedral distribution, and mean square displacement. To investigate the effect of varying the microstructure on the pervaporation separation performance, we focused on a polyimide membrane with different chemical structures. When the size of the substituted group of the membrane in the dry state was increased, the main chain flexibility decreased, the packing efficiency decreased, and large free volume space formed. In the substituted group, however, the side chain and the end group mobility increased, causing the small polymer chain to be packed in the as-prepared large free volume space; thus, the free volume size declined. The microstructure of the polyimide membranes under the swelling effect changed dramatically. The degree of the membrane swelling was in the following order: 9Ph-6FDA > TPA-6FDA > ODA-6FDA. In the 9Ph-6FDA membrane, the main chain mobility was dominant, and the TPA-6FDA membrane was affected by the nitrogen linkage in the diamine. The pervaporation separation performance results indicated that the ODA-6FDA membrane had the lowest swelling degree; as a result, it showed the highest water concentration in the permeate. To improve the low permeation flux of a dense membrane, a polyamide composite membrane with different chemical structures was prepared by interfacial polymerization. The effect of varying the polyamide active layer microstructure on the pervaporation separation performance was also investigated. Swollen polyamide active layers indicated a longer o-Ps lifetime than dry polyamide active layers did. The RDF of atom pairs suggested that the side chain fluctuation in the swollen polyamide active layer of DAPE-tNBDC was greater than that in the DAPE-SCC. This greater fluctuation led to the formation of a more effective free volume in the former than in the latter. Furthermore, the FVSD analysis suggested that the active layer of the former had a larger free volume size than that of the latter. From the free volume shape analysis, the DAPE-SCC polyamide active layer had a larger Eeq value (0.8 to 1.0) than the DAPE-tNBDC polyamide active layer had. The results of free volume shape factor analysis indicated that the connected shape of free volume in the DAPE-tNBDC active layer was lower than that in the DAPE-SCC active layer. As such, the permeation fluxes for these active layers were close to each other; each flux was about 550 g/m2h. Polyelectrolyte complex materials with high hydrophilic content and high ionic-crosslinking structure were used. The objectives were to investigate the effect of varying the microstructure of novel polyelectrolyte complex membranes on the pervaporation separation performance and to improve the water concentration in the permeate. The number of carbon in the polycation in the dry state was increased; as a result, the competition between the steric hindrance and the polymer chain mobility led first to the free volume size increase and then to its decrease. For the polycation in the wet state, the main chain mobility was inhibited by the long side chains, which reduced the packing efficiency. When the side chain mobility was increased, the number of carbon increased. As a result, the free volume size increased with the side chain length. Whether in the dry or the wet state, quenching and inhibition effects on the formation and annihilation of positronium occurred in the polyelectrolyte complex membrane because of the higher oxygen atomic concentration, which was associated with higher electron affinity, resulting in decreased free volume intensity. The pervaporation separation performance showed that the permeation flux increased and that the side chain length increased. However, the water concentration in the permeate slightly declined. Through PALS and MD techniques, the apparent feasibility and the potential ability of conducting a microscale structure analysis of polymeric membranes showed good correlation with the pervaporation performance in dehydrating an aqueous ethanol solution.

並列關鍵字

polyelectrolyte complex membrane ； polyamdie composite membrane ； polyimide membrane ； positron annihilation spectroscopy ； pervaporation ； molecular dynamics simulation

參考文獻

M.H.V. Mulder, Basic Principles of Membrane Technology, Kluer Academic Publisher, The Netherlands (1996).

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高分子分離薄膜微結構分析：正電子湮滅光譜技術結合分子動態模擬

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