| 摘要: | 近年來溫室氣體的排放，如二氧化碳導致溫室效應愈加嚴重，也使得地球的環境遭受破壞，所以如何抑制溫室氣體的排放逐漸成為全球關注的議題。一種新型的材料—混和基質薄膜(Mixed Matrixed Membranes, MMMs)，以高分子薄膜為基底添加有機金屬框架材料(Metal Organic Frameworks, MOF)為填充劑，結合了前者對特定氣體的吸附能力以及後者對特定氣體的選擇能力。此研究使用Matrimid® 5218為高分子薄膜基底，因為它具有高的玻璃轉移溫度，有助於後端加工；使用NH2-MIL-53為填充劑，因為它具有明顯的呼吸現象，有助於氣體吸附的表現。
本研究使用分子動力學模擬(Molecular Dynamics, MD)建構Matrimid® 5218模型和混和基質薄膜模型以及密度泛函理論(Density functional theory, DFT)建構NH2-MIL-53模型並且搭配蒙地卡羅法(Monte Carlo simulations)和均方根位移(Mean square displacement)討論純高分子薄膜、NH2-MIL-53以及混和基質薄膜的氣體輸送機制。
在純高分子薄膜部分，我們將Matrimid® 5218分成3個不同的系統做討論，分別是長鏈系統、中鏈系統以及短鏈系統，並且比較兩種分子動力學模擬的手法，分別是NPT-NVT步驟以及31步驟，結果顯示，長鏈的系統充分展現高分子扭轉的特性，隨著鏈長縮短，此扭轉特性會逐漸消失，導致自由體積(Free Volume)減少，氣體吸附表現降低，而在氣體擴散方面，由於自由體積(Free Volume)的減少，增加氣體與高分子鏈之間碰撞的機會，使氣體具有更多的能量可以發生擴散行為，導致氣體擴散表現上升，並且綜合以上氣體輸送的表現，我們認為使用31步驟所建構的模型較接近實驗的結果。
在NH2-MIL-53部分，我們建構大孔(Large Pore)以及窄孔(Narrow Pore)的模型，結果顯示，大孔吸附的氣體總量較窄孔高，而其最主要的影響因素就是孔洞的大小的變化而產生的呼吸現象。此外，我們還有建構用來當作填充劑的團簇(Cluster)模型，並且使用兩種電荷分配方式。
在混和基質薄膜的部分，我們摻雜不同重量百分濃度的團簇來建構混和基質薄膜，結果顯示，氣體分子無法有效的吸附在團簇的孔洞位置，使得氣體吸附表現不如預期，我們推測是因為團簇的電荷分布無法讓團簇保持原本NH2-MIL-53的特性。在本研究中，我們曾經使用NH2-MIL-53的電荷分布直接應用在團簇來建構混和基質薄膜，雖然會導致整體團簇無法維持電中性，卻可以觀察到添加填充劑後使得氣體吸附能力增強。
儘管本研究的數據與實驗的數據不太吻合，但我們還是可以從中觀察到一定的趨勢，並且系統性的探討薄膜對於氣體吸附之行為與機制。
;In recent years, carbon dioxide is one of the main greenhouse gases which lead to serious damage to our environment. Therefore, how to capture CO2 has become a global issue. A new type of material, mixed matrixed membranes (MMMs), which is composed of metal-organic framework (MOF) as filler embedded in a polymeric matrix and it can combine the advantages of both components. In this study, Matrimid® 5218 is used as polymeric matrix becaused of its high glass transition temperature, and NH2-MIL-53 is used as filler because of its significant breathing behavior.
In this work, molecular dynamics (MD) and density functional theory (DFT) are applied to construct the pure Matrimid models and MMMs models, and NH2-MIL-53 models, respectively. Additionally, Monte Carlo (MC) simulations and mean square displacement (MSD) are used to analysis the gas transport mechanism.
The pure Matrimid models are divided into three different systems for discussion, which are long, medium, and short chain system. Each system would adopt two different MD procedures to construct the models, which are NPT-NVT loop procedure and 31 MD step procedure. The results show that the long chain system fully exhibits the characteristics of polymer torsion, and as the chain length decrease, this characteristic will gradually disappear, resulting in decrease of free volume, further decreasing the gases adsorption ability. For gases diffusion, due to the gradual reduction of the free volume, the collision between the gases and polymer chain increase, resulting in an increase in gases diffusion. Furthermore, we found that the pure membrane adopting 31 MD step procedure are more reasonable and more in line with the experimental value, and is a more efficient MD method in this study.
The narrow pore (NP) and large pore (LP) types of NH2-MIL-53 models are constructed. The results sohw that the amount of gases that can be adsorbed by the LP is higher than that of NP, and the most important influencing factor is the breathing behavior caused by the change of the pore size. Besides, the cluster models of NH2-MIL-53 adoping two different kinds of charge assignment are constructed as the filler to embedded into polymeric matrix.
The MMMs models are constructed within different weight loading of NH2-MIL-53. The results show that the gas molecules cannot effectively adsorb in the pores of the clusters, so the gas adsorption performance is not as expected. We speculate that the charge distribution of the clusters makes it cannot retain the original characteristics of NH2-MIL-53. However, we have tried to use the original charge distribution from NH2-MIL-53 to apply on the cluster models. The results show that although the cluster model cannot maintain the electrical neutrality, it can enhance the sorption ability inside the MMMs.
Despite the results in this study are not in agreement with the experiment, we can still observe certain trends and systematically discuss the mechanism of film adsorption on gases. |
