以正電子湮滅光譜技術探討聚醯胺複合薄膜自由體積特性及微結構變化

Probing Characteristic Free Volumes and Fine Structure of Polyamide Composite Membrane by Positron Annihilation Spectroscopy

洪維松(Wei-Song Hung)

指導教授 ： 賴君義 ； 李魁然

中原大學/工學院/化學工程研究所/博士(2011年)

https://doi.org/10.6840/cycu201100167

摘要

本研究使用新穎的可變單一能量慢速正電子束測試儀(variable monoenergy slow positron beam , VMSPB)，探討聚醯胺複合薄膜自由體積尺寸、分佈、多層結構及化學組成變化，進而了解自由體積與分離效能之間的關聯性。實驗規劃，首先擬解決絕緣複合膜在正電子量測時所存在的電荷累積效應，以提升正電子湮滅光譜(positron annihilation spectroscopy, PAS)的量測效率與靈敏度；並由正電子湮滅光譜以自由體積的角度探討界面聚合過程薄膜之成長方向；進一步開發可於薄膜濕潤環境下操作之正電子分析技術，此將有助於探究溶液與高分子間的相互作用力對自由體積之影響；最後，擬建構二維正電子湮滅壽命-動量關聯测量(two-dimentional age-momentum correlation, 2D-AMOC)技術，此法可由原本一維的正電子湮滅壽命光譜中分離出動量譜，藉由湮滅壽命與動量之間的關聯，對特定動量區間 ( 3γor 2γ-annihilation ) 的湮滅壽命進行分析，可定量的獲得複合膜各層間化學組成與自由體積變化的關係。
研究結果指出利用白金濺鍍技術，將白金粒子均勻的覆蓋於絕緣高分子薄膜表面，藉由此極薄的白金層(~1 nm)可有效的移除VMSPB在高真空環境下，正電子於膜表面所累積的電荷效應，以利於之後研究的順利進行。此外，聚醯胺複合薄膜應用於滲透蒸發程序分離醇類水溶液時，透過端水濃度會隨著聚醯胺層厚度的增加而增加，此一極薄之界面聚合層結構變化與滲透蒸發分離效能之關聯性也進一步由正電子湮滅光譜分析結果予以印証。並由正電子湮滅光譜以自由體積變化的角度驗證界面聚合層成膜方向，發現當進行界面聚合反應時，有機、水相接觸的瞬間會形成緻密的聚醯胺層，而此緻密選擇層將阻礙水相單體繼續擴散至油相進行反應，故導致其後隨著時間的增加所形成的聚酰胺結構趨於鬆散。由於薄膜應用在分離液態混合溶液時，溶液對複合膜會產生膨潤(swelling)效應，如何克服VMSPB需在高真空(10-8torr)環境下方能操作的要求，以正確量測在此濕潤環境下薄膜自由體積變化是最具挑戰性的難題。研究中結合電漿沉積技術，於聚醯胺複合膜表面披覆一極薄之SiOxCyHz保護層，實驗中證實此法可有效分隔真空與薄膜濕潤區，因此可對於複合膜不同層間自由體積變化進行非破壞性的量測，結果發現不同的進料溶液對於複合膜會有程度不一的膨潤(swelling)效應，其對薄膜的膨潤度分別為IPAl＞70 wt% IPA/H2O＞water，並且發現相較於改質後的聚丙烯晴(modified polyacrylonitrile, mPAN)基材膜，聚醯胺層有較緻密的結構與較小的膨潤度。最後，藉由2D-AMOC的技術，界定電子偶素湮滅壽命(o-positronium annihiliation lifetime)與動量之間的關聯，並對特定動量區間的湮滅壽命進行分析，發現聚醯胺複合膜各層間自由體積半徑界於2.1-3.4 Å而奈米孔半徑界於36-196 Å之間，並結合TEM結果驗證聚醯胺層是由於連續的緻密聚醯胺結構與不連續的奈米孔結構所組成。並利用二維都卜勒展寬能量關聯譜(Doppler broadening of annihilation radiation, 2D-DBAR)探究複合膜各層間化學元素組成對自由體積的影響。
本研究有助於完整的了解超薄複合薄膜微結構變化，同時結合滲透蒸發分離效能，則可了解滲透蒸發分離程序的傳輸機制與透過原理，這將有利於在薄膜結構設計與薄膜效能預測領域上，提供重要的資訊。

關鍵字

聚醯胺複合薄膜 ； 電子偶素 ； 正電子

並列摘要

In this research, a polyamide composite membrane was fabricated, and its free volume size and distribution and variation in the multilayer structure and chemical composition were probed using a variable monoenergy slow positron beam (VMSPB) to understand the relationship between the membrane separation performance and free volume. The experimental plan for this research is divided into four parts: (1) solving the charging effect in an insulating composite membrane during positron annihilation spectroscopic (PAS) experiments to improve the measurement sensitivity and efficiency; (2) verifying the membrane growth direction by means of positron annihilation spectroscopy from the point of view of free volume; (3) developing a positron analytical technique that can measure a membrane maintained in the wet condition by means of the membrane’s protective layer of SiOxCyHz on the vacuum side; and (4) setting up a two-dimensional age-momentum correlation (2D-AMOC) technique that can separate momentum spectroscopy from the original one-dimensional positron annihilation lifetime spectroscopy and that can quantitatively obtain the relationship between chemical composition and free volume variation at each layer in a composite membrane by means of the relationship between momentum and annihilation lifetime based on conducting an annihilation lifetime analysis for a specific momentum range (3 or 2 annihilation).
The data obtained from the first part of the experimental plan indicated that use of the platinum sputtering technique resulted in a uniform deposition of platinum particles on the insulating polymeric membrane surface. An extremely thin layer (~ 1 nm) of deposited platinum could effectively remove the charging effect associated with the accumulation of positron in the membrane surface, leading to uninterrupted measurements using a VMSPB operating at high vacuum conditions. In the second part of the experiments considered in this research, positron annihilation spectroscopy, based on the concept of free volume, was used to verifying the membrane growth direction of the interfacially polymerized polyamide layer that began to form the moment the organic and aqueous phases contacted each other. This selective layer, once formed, would hinder the diffusion of the aqueous monomer solution toward the organic phase. Hence, over time, the structure of the polyamide layer formed tended to become loose. When the polyamide composite membrane was applied for the pervaporation separation of an aqueous alcohol solution, the permeate water concentration was shown to increase with the polyamide layer thickness. The pervaporation performance and the change in the structure of the composite membrane were correlated with the findings from the positron annihilation spectral analysis. The third part of this research experimental plan is about devising means to maintain a membrane in the wet condition, despite the operation of a VMSPB at a high vacuum (10-8 torr). The challenge of using a VMSPB to measure the correct free volume in a composite membrane wetted by a feed solution was addressed by integrating a plasma deposition technique, with which a protective SiOxCyHz layer was deposited on the surface of a polyamide composite membrane. Such a nondestructive way enabled simulating the condition of a pervaporation membrane kept in direct contact with a liquid environment, thereby making it possible to measure the free volume in the different layers of a composite membrane in the wet state. Results indicated that different feed solutions had varying degrees of swelling effect, and the extent of the membrane swelling due to the feed solution was demonstrated to be in the following order: isopropanol ＞ 70 wt% isopropanol/water solution ＞ water. Compared to a modified polyacrylonitrile (mPAN) substrate, the polyamide layer was found to be comparatively denser and less swollen. The fourth and last part of experiments for this research was conducted to obtain data by means of the 2D-AMOC technique developed to define the relationship between momentum and ortho-positronium (o-Ps) annihilation lifetime, which was determined by doing an annihilation lifetime analysis for a specific momentum range. The free volume radius in the polyamide layer was found to be 2.1-3.4 Å and in the nano-scale range of 36-196 Å in the region after the polyamide layer. Combined with transmission electron microscopy (TEM), findings indicated that the polyamide layer was shown to be composed of a continuous dense structure and that the region after the polyamide layer was descriptive of a discontinuous nano-scale hole structure. A two-detector coincidence Doppler broadening of annihilation radiation (2D-DBAR) was used to investigate the effect of the chemical composition of each layer in the composite membrane on the free volume.
This research would provide a complete understanding of the microstructure variation in an ultrathin composite membrane. In regard to the membrane pervaporation performance, this research would also provide an understanding of the principle of pervaporation rate and the mechanism of pervaporation separation transport. Both of this understanding would then furnish significant information about the areas of membrane structure design and membrane performance prediction.

並列關鍵字

positron ； positronium ； polyamide composite membrane

參考文獻

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