石墨烯/聚乙烯复合材料及其拉伸性能的分子动力学模拟

Molecular Dynamics Simulations of Graphene/Polyethylene and Its Tensile Properties

利用分子动力学方法,模拟石墨烯/聚乙烯复合材料的微观结构和性能,并采用单轴拉伸模拟方法研究石墨烯/聚乙烯复合材料的拉伸性能.结果表明,在石墨烯/聚乙烯复合材料平衡构型中,聚乙烯基体分子在石墨烯表面处形成多层吸附层,吸附层处于动态稳定状态,层内分子可以发生扩散迁移.吸附层内聚乙烯分子发生"吸附固化"现象,分子弯曲程度减弱,发生有序排列,且在垂直于石墨烯方向的运动性能受到抑制.拉伸模拟结果表明,石墨烯能够提高聚乙烯材料的拉伸性能.在弹性区和屈服区,石墨烯阻碍了复合材料在垂直于拉伸方向的压缩变形,聚乙烯分子"吸附固化"结构保持稳定,引起体系整体应力的迅速升高.在软化区,由于石墨烯发生剧烈弯曲,"吸附固化"结构发生破坏,最终引起体系应力迅速减小.在弹性区和屈服区,体系应变主要引起了非键相互作用的改变.在软化区之后,应变主要导致了体系内分子成键相互作用的改变.应变速率能够提高复合材料的屈服应力,而不改变复合材料应力应变的整体趋势.

Molecular dynamics simulations have been used to study the micro structures and properties of graphene/polyethylene composite, and tensile properties of the composite have also been analyzed using a uniaxial tension simulation. The results show that, different from the pure polyethylene model, multi adsorption layers are formed by polyethylene molecules on the graphene surface due to equilibrium structure of the composite model. These adsorption layers are dynamic stable, through which the polyethylene molecules can migrate during the simulation progress. ＂Adsorption curing＂ occurs in the adsorption layers, where the polyethylene molecules become more extended and ordered, and their movements are inhibited in the direction perpendicular to the graphene surface. Tension simulation suggests improvement in tensile properties of the polyethylene matrix brought by graphene. In elastic region and yield region, the graphene can inhibit compression deformation in directions perpendicular to the strain, which keeps the stability of ＂adsorption curing＂ structures and causes sharp increase of stress in the composite model before yield strain. In softening region, the graphene bends greatly in one of the perpendicular directions and the ＂adsorption curing＂ structures are damaged, which results in stress decrease after the yield strain. The strain increase leads to not only change of non-bond interactions in the first two regions but change of intramolecular bonding energy in the last region for the composite system. The yield stress of composite increases with the increase of strain rate, which, however, has no effects on the general trend of the stress-strain curves for the model.