以分子動力學模擬探討氯化鋰對聚乙二醇/聚甲基丙烯酸羥乙酯有機凝膠之分子結構與力學性質之影響

Abstract

有機凝膠是一種以有機分子作為溶劑，高分子彼此膠聯所形成的具有良好彈性的高分子材料。他們具有高延展性、自癒合能力與高穩定性等優點，因此被廣泛應用於生物醫學、食品加工、以及軟性電子產品等領域。然而，有機凝膠的抗拉能力並不好，容易因為拉力而形變，這對有機凝膠的應用造成很大的限制。其中一種增加抗拉性質的方法，是透過添加金屬離子來加強其楊氏模數。但是，不同種類的金屬離子會對不同高分子構型與膠聯過程產生不同影響，並且必須透過反覆實驗才能確定特定原子對特定高分子材料的影響。因此，本研究使用分子動力學方法，研究以聚乙二醇(PEG)與聚甲基丙烯酸羥乙酯(PHEMA)為材料所形成的有機凝膠材料，並且以氯化鋰為金屬離子，探討該金屬離子對PEG-PHEMA有機凝膠力學性質之影響。我們首先針對有機凝膠的平衡態結構進行分析，比較加入金屬離子與未加金屬離子的差異。接著，我們進行單軸拉伸實驗，研究金屬離子對材料力學性質的影響。從平衡態分析我們發現：在添加金屬離子後，有機凝膠內的PEG高分子會因為配位鍵的而產生蜷曲，並且減少氫鍵總數。然而這樣的現象並不會造成連接的下降，而是透過鋰與氯形成的離子團加強了高分子間的連結性，也使得PHEMA高分子因為和PEG溶劑連接性的增加而更加舒展。在拉伸實驗中，我們發現：加入氯化鋰的PEG-PHEMA有機凝膠有更高的楊氏模數以及更強的最大應力，破壞應變則和未加氯化鋰的有機凝膠相近。金屬離子和高分子形成的配位鍵，能使材料在高應變的形變下避免PEG高分子彼此分開。希望透過分子模擬的分析，能幫助研究者對於凝膠材料力學性質與添加金屬離子的影響有更加深入的認識，並促進凝膠材料的研發。

Organogels are semi-solid systems with an organic liquid phase immobilized by a three-dimensional elastic or viscoelastic crosslink network composed of self-assembled, crosslinked, or entangled gelator fibers. Due to its good stretchability, self-healing properties, and high stability, they are widely used in drug delivering, food processing and flexible electronics devices. However, like most gel-like materials, organogels’ failure stress is usually low and may reduce their potential for further applications. One effective method, which may enrich organogels’ mechanical properties and denser the network structures, is to blend ions into the organic solvent. Inspired by this method, we aim to investigate organogel made up of polyethylene glycol (PEG) and poly (2-hydroxyethyl methacrylate) (PHEMA) and the mechanical influence of adding lithium chloride. However, previous study shows that different ions cause different effect to polymer materials. Therefore, we use molecular dynamics simulation to analyze the difference between the PEG-PHEMA organogel and ionized LiCl PEG-PHEMA organogel. We perform equilibrium analysis to investigate the microstructure of two materials. We also perform the tensile test simulation to study the different mechanical properties of ionized and non-ionized organogel. In equilibrium analysis, the addition of lithium ions makes the PEG chain curl. However, this does not decrease the connectivity of polymers in organogel. Instead, the lithium-chloride clusters strengthen the interactions between multiple polymer chains and stretch the PHEMA chain longer. In tensile test, we show that the addition of lithium chloride ions improves the tensile strength and elastic modulus of the organogel while maintaining a similar failure strain. Coordination bonds formed by lithium ions and polymers strengthen the inter-polymer interaction and prevent post-deformation defect in ionized PEG-PHEMA organogel. We hope this study helps reveal the mechanical enhancement mechanism behind LiCl ions and PEG-PHEMA organogels and provides a deeper look for researchers seeking to develop new functional soft material.