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

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

宏观氧化石墨烯膜由多层氧化石墨烯组成,其法向拉伸和层间剪切性能远比面内性能低.论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,得到了多层氧化石墨烯材料的全部五个独立弹性常数E、E、μ、μ和G,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E和拉伸强度σ逐步降低,法向杨氏模量E和拉伸强度σ、层间剪切模量G和剪切强度τ均逐步增大,而对泊松比的影响较小;面内拉伸断裂位置由氧化基团(羟基和羧基)与碳原子结合键能大小所决定,氢键数量是影响层间性能的重要原因.

The 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 interlaminar shear properties. The in-plane tensile properties are related to the strength of carbon-carbon bonds, and the transformation of sp~2 to sp~3 hybrid forms is the main factor 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 material: a 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 study the in-plane tensile, normal tensile and interlaminar shear behaviors of multilayer graphene oxide using the molecular dynamics method. All five independent elastic constants E, E, μ, μand Gof multilayer graphene oxide are obtained. Then, the three-dimensional elastic matrix(the flexibility matrix and the 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 Eand tensile strength σof multilayer graphene oxide decrease gradually, and the normal Young’s modulus Eand tensile strength σ, as well as the interlayer shear modulus Gand shear strength τincrease 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.