多层氧化石墨烯的三维弹性常数及氧化度对力学性能的影响

宏观氧化石墨烯膜由多层石墨烯组成，其法向拉伸和层间剪切性能远比面内性能低。本文视多层氧化石墨烯为一种特殊的三维正交各向异性材料——横观各向同性材料，通过建立羟基和环氧基在石墨烯表面随机分布的多层氧化石墨烯三维模型，采用分子动力学方法模拟多层氧化石墨烯的面内拉伸、法向拉伸和层间剪切行为，分别得到了多层氧化石墨烯材料的全部五个独立弹性常数E2、E3、μ12、μ32和G23，进而确定了三维弹性矩阵（柔度矩阵和刚度矩阵），并进一步分析了氧化度对弹性常数和强度的影响规律。结果表明：随着氧化度R逐步增大，多层氧化石墨烯面内杨氏模量E2和拉伸强度σ2max逐步降低，法向杨氏模量E3和拉伸强度σ3max、层间剪切模量G23和剪切强度τ23max均逐步增大，而对泊松比的影响较小；拉伸和剪切断裂破坏位置由氧化基团(羟基和羧基)与碳原子结合键能大小所决定。

Three-Dimensional Elastic Constants and Influence of Oxidation Degree on Mechanical Properties of Multilayer Graphene Oxide

Macroscopic graphene oxide film is composed of multilayer graphene oxide.The mechanical properties of multilayer graphene oxide can be described from three aspects : in-plane tensile, normal tensile and interlayer shear properties. The in-plane tensile properties are related to the strength of carbon-carbon bonds, and the transformation of sp2 to sp3 hybrid forms is the main reason for affecting the binding energy of carbon atoms. Normal tensile and interlaminar shear properties are related to interlayer interaction, and the strength of hydrogen bond and van der Waals interaction are two factors affecting interlayer interaction. Here we treat multilayer graphene oxide as a special three-dimensional orthotropic materials--transversely isotropic material, formulate a three-dimensional model of multilayer graphene oxide with hydroxyl and epoxy groups randomly distributed on the surface of graphene, and the in-plane tensile, normal tensile and interlaminar shear behaviors of multilayer graphene oxide are studied by molecular dynamics method. All five independent elastic constants E2, E3, μ12, μ32 and G23 of multilayer graphene oxide are obtained. Then, the three-dimensional elastic matrix (flexibility matrix and stiffness matrix) is determined, and the influence of oxidation degree on the elastic constants and strength is further analyzed. It is found that normal tensile and interlaminar shear properties are much lower than in-plane tensile properties. With the increase of oxidation degree R, the in-plane Young's modulus E2 and tensile strength σ2max of multilayer graphene oxide decrease gradually, and the normal Young's modulus E3 and tensile strength σ3max, interlayer shear modulus G23 and shear strength τ23max increase gradually, but the influence on Poisson's ratio is small. The in-plane tensile fracture position is determined by the bond energy between the oxidation groups (hydroxyl and carboxyl) and carbon atoms. The number of hydrogen bonds is an important factor affecting the interlayer properties.