利用分子動力模擬探討石墨烯奈米複合材料機械性質

Abstract

本研究主要目的藉由分子動力模擬石墨烯奈米複合材料機械性質，探討聚□亞胺與樹脂兩種高分子基材中添加三種不同形式之石墨烯，形成群聚、分散與分散表面改質石墨烯奈米複合材料，並研究不同形式下石墨烯奈米複合材料的機械性質，其中機械性質包含了楊氏係數、玻璃轉換溫度以及熱膨脹係數。除了機械性質之外，石墨烯對高分子基材的密度分布及分子結構排列方式的影響亦為本研究重點。結果顯示，當高分子靠近石墨烯周圍時密度較高，遠離石墨烯時密度下降至高分子平均密度，亦觀察到當高分子鏈靠近石墨烯介面時形成緊密壓縮且貼附於石墨烯之情形。此外，在機械與熱性質中，分散石墨烯奈米複合材料相對於群聚石墨烯奈米複合材料呈現較高的楊氏係數、玻璃轉換溫度以及較低的熱膨脹係數。主要原因由分散石墨烯使奈米複合材料中高分子呈現高度的秩序，進而提高了奈米複合材料的綜合性質。此外在三種石墨烯奈米複合材料中，分散表面改質石墨烯為最佳的補強材料，藉著互動能量的計算了解表面改質石墨烯可有效的提高其互動能量，而互動能量的增強可能改善表面改質石墨烯奈米複合材料的機械性質。

The research aims to investigate the mechanical properties of graphene nanocomposites using molecular dynamics (MD) simulation. Three different formats of graphene, i.e., graphene flakes, intercalated graphene and intercalated graphene oxide, were incorporated respectively in polymer matrix to form the graphene nanocomposites. Both polymer systems, i.e. polyimide and epoxy, were considered respectively as matrix in the nanocomposites. The mechanical properties of the nanocomposites including Young’s modulus, glass transition temperature (Tg) and coefficient of thermal expansion (CTE), in terms of the different formats of graphene were characterized in this study. In addition to the mechanical properties, the influences of graphene on the morphology, density and order parameter of the polymers were also examined. Results illustrated that the local density in the vicinity of the graphene is relatively high and then decreases to the bulk value as the region is away from the interface. Furthermore, it was found that the polymer chains near the graphene are densely compacted and flattened down parallel to the graphene interface. On the other hand, for the mechanical and thermal properties, the nanocomposites with dispersed graphene exhibit higher Young’s modulus, higher glass transition temperature and lower thermal expansion coefficient than those with graphene flakes. This is because the dispersed graphene leads to high degree of ordered polymer in the nanocomposite and thus enhances the overall properties of the nanocomposite. In addition, the interacted graphene oxide provides the best reinforcement among the three cases of nanocomposites. Based on the calculation of interaction energy, it was validated that the oxide modification on graphene surface can effectively enhance the interaction energy, and such enhancement in interaction energy may be responsible for the improvement of mechanical properties of graphene oxide nanocomposites.