熱重組高分子薄膜之結構特性與其氣體分離機制之分子模擬解析

Thermally Rearranged Polymeric Membranes Structure Characterization and Performance Investigation : A Molecular Simulation Study

巫振誠(Zhen-Cheng Wu)

指導教授 ： 賴君義 ； 童國倫

中原大學/工學院/化學工程研究所/碩士(2013年)

https://doi.org/10.6840/cycu201300598

摘要

本研究以分子模擬技術探討熱重組薄膜(thermal rearranged polybenzoxazole, TR-PBO)之結構特性與氣體輸送行為。首先，以分子力學原理建立二組結構穩定的薄膜分子模型：(a)具有TR-benzoxaxzole 之衍生物薄膜 (b)對熱重組之加熱溫進行度探討的 TR-poly(ether-benzoxazole)薄膜。確認分子模型適用於分析熱重組效應後，以分子動態模擬(molecular dynamics simulation, MD)方式進行動態計算分析，進而由微觀的角度了解TR-benzoxaxzole片段，對薄膜的結構特性與氣體輸送行為之影響；並以蒙地卡羅法(Monte Carlo method)進行氣體吸附行為之分析。由兩面角分佈與廣角X-ray繞射模擬中發現， 堅硬的TR-benzoxaxzole 片段，降低了高分子鏈的堆積效率，使TR-PBO呈較鬆散的結構；而在自由體積與容通體積分析中顯示，含有較多堅硬TR-benzoxaxzole 片段之薄膜，可形成更多有效之自由體積。在氣體吸附行為討論中，薄膜中堅硬的TR-benzoxaxzole 片段所堆疊出的有效自由體積，提升了薄膜的吸附與擴散能力；同時也增加了薄膜的透過率。由薄膜在不同氣體的恆溫吸附曲線中發現，當薄膜中TR-benzoxaxzole 片段之比例增加時，由 Henry’s law 主導之氣體吸附行為明顯增加，因 TR- benzoxaxzole 片段使高分子鏈堆疊出較鬆散的結構，進而提供了大量分子鏈與分子鏈間的吸附位置。針對氣體分子在薄膜中的擴散分析，因 TR-benzoxaxzole 片段堆疊出的大型自由體積，有效的提供氣體分子在薄膜中進行運動，因此在氣體位移量上有著顯著的差異。由本研究之分子模擬結果顯示，在高分子特性方面，熱重組效應主要影響高分子的堆疊能力；在薄膜形態方面，高分子經熱重組反應後形成了更多有效之自由體積，進而影響氣體吸附能力、氣體擴散行為以及氣體透過現象。在本研究之分子模擬結果與實驗結果有相同的趨勢，說明了分子模擬技術可以完整描述材料的結構特性與氣體輸送機制，可應用於材料設計與開發。

關鍵字

熱重組反應 ； 氣體分離 ； 自由體積 ； 分子動態模擬

並列摘要

The molecular simulation technique was adopted to investigate the structure and transport performance of thermally rearranged poly(benzoxazole-co-imide) membranes. A molecular dynamics (MD) technique was used to construct three models: a poly-benzoxazole (PBO) membrane with high free volume; a polyimide (PI) membrane with dense structure; and their co-polymer, PBO-PI membrane. The MD simulation was performed to characterize the membrane models to understand how the very rigid benzoxazole segments affect the micro-structure, free volume, cavity size, and gas diffusion of the membrane models. A Monte Carlo method was adopted to investigate the gas sorption behaviors in the three types of membrane. The torsional angle and wide-angle X-ray diffraction analyses suggest that the benzoxazole segments stiffened the polymeric chains, leading to the formation of a looser structure. In free-volume and cavity-size studies, the PBO membrane exhibited the highest free volume and largest cavity size, which can be attributed to the presence of the benzoxazole structure constructed by thermal rearrangement. The enlarged free volume in the membranes with benzoxazole segments provided more space for gas sorption and diffusion, which effectively enhanced the gas permeability. In addition, increasing the benzoxazole segments in the membrane structure enhances the gas sorption in accordance with Henry’s law, as the PBO membrane provides more inter-polymeric chain space and allows for the larger free volume elements.

In this study, the Poly(ether-benzoxazole) also constructed for analysis the different thermal treatment protocols. A molecular dynamics (MD) technique was used to construct four models: a poly-benzoxazole (aPBO) membrane with high free volume; a Poly(ether o-hydroxyimide) (HPEI) membrane with dense structure; and two Poly(ether-benzoxazole) (PEBO) membrane which have different thermal rearrangement degree. PEBO 450-1 with 96% conversion degree, the other one PEBO 450-2 with 100% conversion after thermal treatment.The MD simulation was performed to characterize the membrane models to understand how the thermal conversion degree affect the micro-structure, free volume, cavity size, and gas transport of the membrane models. A Monte Carlo method was adopted to investigate the gas sorption behaviors in the four types of membrane models. The torsional angle and wide-angle X-ray diffraction analyses suggest that the benzoxazole segments stiffened the polymeric chains after thermal treament, leading to the formation of a looser structure. In free-volume and cavity-size studies, the PEBO 450-2 membrane exhibited higher free volume and larger cavity size that a little lower than aPBO membrane, which can be attributed to the presence of the benzoxazole structure constructed by thermal rearrangement. The enlarged free volume in the membranes after thermal treament provided more space for gas sorption and diffusion, which effectively enhanced the gas permeability. In addition, increasing the thermal conversion degree in the membrane structure enhances the gas sorption in accordance with Henry’s law, as the PEBO and aPBO membranes provide more inter-polymeric chain space and allows for the larger free volume elements.

Fabrication of the poly(benzoxazole-co-imide) membrane with an appropriate PBO/PI composition would help optimize the gas permeability and selectivity in the gas separation process. The results from the simulation agree with the experimental data, indicating that the molecular simulation technique is a useful method in the field of materials design and development for the membrane separation process.

並列關鍵字

Membrane Gas Separation ； Free Volume ； Molecular Simulation ； Gas Transport Mechanism

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