Study On The Mechanical Properties Of Helical Graphenes Nanocomposities

Polymer materials are widely used in aerospace,transportation and other fields because of their lightweight and corrosion resistance,but their low strength also hinders their further development.So it is very essential to study mechanical properties of polymer matrix nanocomposites,especially when new carbon nanomaterials are used as the reinforcement.In this paper,the mechanical properties of helical graphenes(marked as the HGs),a novel carbon nanomaterial,were studied by molecular dynamics(MD)and theoretical analysis.On this basis,the mechanical properties of helical graphenes/epoxy resin nanocomposites(marked as the HGs/epoxy)were investigated.Considering the great tensile property of HGs,this paper also analyzed the tensile behavior of helical graphenes/polyethylene nanocomposites(marked as the HGs/PE)under large deformation.The main contents were as follows:(1)The tensile properties of HGs were studied by MD based on the AIREBO potential function.It was found that the van der Waals(vd W)interaction between adjacent free coils played a key role in the initial deformation and stable delamination stage,while the tensile load was mainly borne by carbon-carbon(C-C)bond in the elastic deformation stage.Then,according to continuum mechanics theory,a simplified theoretical model based on the vd W interaction was established to predict the tensile properties of HGs with different sizes in the initial deformation and stable delamination stage.It was found that the critical displacement of HGs in the initial deformation stage was linear with the number of free coils(within the scope of effective coil(26)or less),independent of the internal and external diameters,while the critical tensile force was related to the size of HGs.The separation force in the stable delamination stage had nothing to do with the number of free coils,but had a positive correlation with the coil width of HGs when the width was small.The theoretical results were in good agreement with MD simulation results.(2)The mechanical properties of HGs/epoxy nanocomposites were studied by MD based on the CVFF potential function.It was found that HGs/epoxy system had higher Young’s modulus and yield strength than epoxy system with the same crosslinking degree.The enhancement effect of HGs on epoxy resin was more obvious when the crosslinking degree was lower.The Young’s modulus of HGs/epoxy system with 82% crosslinking degree along X,Y and Z directions increased by 18.87%,20.98% and 24.07%,and the yield strength increased by18.46%,23.51% and 18.42%,respectively,compared with the epoxy system.The mechanical properties of HGs/epoxy system were mainly attributed to the mechanical entanglement between molecular chain and HGs,especially when the entanglement between molecular chain and adjacent coils of HGs occurred,the binding energy between matrix and reinforcement was greater,the mechanical properties were better,and the mechanical anisotropy was more obvious.Internal and external diameters of HGs also had significant effects on mechanical properties and anisotropy of HGs/epoxy system.Due to the interaction of hydroxyl groups,there was a stronger interfacial binding energy between the epoxy resin matrix and oxidized HGs(20%oxidized degree),the mechanical entanglement between adjacent helical coils and epoxy molecular chains was easier to form,which was beneficial to improve the overall mechanical properties of HGs/epoxy nanocomposites.(3)The mechanical properties of HGs/PE nanocomposites were studied by MD based on the PCFF potential function.It was found that the intramolecular interaction of polyethylene and the interlaminar vd W interaction of HGs played an important role in the tensile process,the molecular chain of polyethylene had torsional deformation and changed to straight chain direction(i.e.the degree of orientation increased),while the interlayer slip of HGs occurred.Compared with the multilayer stacked graphenes/polyethylene system,HGs did not produce obvious interlayer slip in the process of resisting tensile deformation.This was because that HGs not only had the vd W interaction,but also the atomic bonds connection due to the special three-dimensional dtructure,so HGs/PE system had a stronger bearing capacity under the condition of large deformation.The results of this paper not only provide a theoretical model for the calculation of mechanical properties of novel nanomaterials with helical structure,but also provide a theoretical basis for the preparation of helical graphenes nanocomposites.