Simulation Of Mechanical Properties Of Polyethylene And Its Composites Based On Molecular Dynamics

Polyethylene can effectively slow down fast neutrons due to its high hydrogen content,while boron carbide has a large thermal neutron absorption cross section and can effectively absorb thermal neutrons.Therefore,polyethylene/boron carbide nanocomposite is an ideal neutron shielding material,which can be widely used in nuclear power,aerospace,national defense and military industry,medical and health fields.However,due to the poor mechanical properties of polyethylene/boron carbide composites,the tensile strength is 10-40 MPa,which greatly limits the application of the materials.Therefore,it is necessary to study its mechanical properties.Because the mechanical properties of composite materials are affected by many factors and the structure is complex,it is difficult for traditional experimental methods to investigate the influence of a single factor,and characterize the microstructure of materials at the molecular level.The performance of composite materials is largely determined by the microstructure,while computer simulation can monitor the internal microstructure changes,and easily realize the control of single factor variables.Molecular dynamics（MD）simulation builds a bridge between microstructure and macroscopic mechanical properties.In this paper,MD simulation was used to study the relationship between the mechanical properties and microstructure of pure polyethylene with a chain length of80000C during the tensile process.The mechanical properties of polyethylene/boron carbide composites in the tensile process were further studied.The main work of this paper is as follows:（1）The all-atom model is used to simulate the mechanical properties and microstructure evolution of pure polyethylene with a chain length of 80000C in the tensile process.Young’s modulus,yield strength and tensile elongation at break are used as the measurement indicators of mechanical properties,and the changes of microstructure are characterized by porosity,chain orientation,crystallinity.The driving force in the tensile process is analyzed by energy evolution.Research results show that the tensile behavior of pure polyethylene with chain length of 80000C is typical,which consists of four stages:elastic deformation,yield,strain softening and strain hardening.In the elastic deformation stage,the molecular chains in the amorphous region are oriented and crystallized.Because there is no chain segment movement at this stage,there is no obvious micropore formation,and the porosity does not change much;At the yield stage,the chain segment began to move,the degree of orientation further increased,and the crystallinity also continued to increase.Since the formation of micropores began,it can be observed that the porosity began to rise;In the strain-softening stage,the crystal region begins to break,the crystallinity decreases,and the number of micropores increases.However,due to the stretching along the x-axis,the degree of orientation is still increasing,and at the same time,the micropores gradually become larger.From the pore evolution diagram,we can observe the process of the micropores gradually growing larger;In the strain-hardening stage,the molecular chain is oriented along the x-axis and the polyethylene chain is recrystallized.At this time,the micropores continue to grow.From the energy evolution of the tensile process,it can be seen that the main driving force before the strain hardening stage is the van der Waals interaction,while the main driving force in the strain hardening stage is the van der Waals interaction and the dihedral torsional energy.（2）The effects of different boron carbide contents on the mechanical properties and microstructure changes of polyethylene/boron carbide composites during the tensile process were studied.The microstructure changes were characterized by the interaction energy,glass transition temperature,porosity and local shear strain distribution.The results show that the tensile curve of polyethylene/boron carbide composite is a typical tensile behavior,and the shape of the stress-strain curve is determined by the polyethylene matrix material.Compared with pure polyethylene,doping 1.87wt%boron carbide hardly affects the yield strength of the material,but the elongation at break is significantly reduced;Doping 5.40wt%boron carbide,the yield strength of the material is significantly increased,and the Young’s modulus is also enhanced,but the tensile elongation at break is significantly reduced.The reason is that the addition of boron carbide will produce two kinds of interactions on the composite system.One of interactions is that there is a strong interaction between the boron carbide particles and the polyethylene matrix material,which limits the movement of polyethylene segments around the boron carbide;Another interaction is that the addition of boron carbide destroys the stable structure of the system,which makes the composite system have small defects,enhances the chain segment movement ability of polyethylene,but causes the material to break more easily.The mechanical properties of polyethylene/boron carbide nanocomposites are strongly dependent on temperature.With the increase of temperature,the yield strength and Young’s modulus of the system decrease.The reason is that the temperature rises,the atomic movement intensifies,and the motility of the chain segments increases The mechanical properties of polyethylene/boron carbide nanocomposites are strongly dependent on the tensile rate.The tensile curves with strain rates of 5×10^-6/ps,1×10^-5/ps have the same trend,showing a typical tensile process;However,the tensile curve with strain rate of 5×10^-5/ps,1×10^-4/ps has no strain-softening and strain-hardening stages.