The Study Of Ultra-high Molecular Weight Polyethylene By Molecular Dynamics Simulation

Ultra-high molecular weight polyethylene(UHMWPE)is a typical semi-crystalline polymer.Because of its unique chemical structure,it often has special functions and uses.Such as its wear,impact resistance,corrosion resistance,self-lubricating,absorbing impact energy and other special features.Therefore,UHMWPE is widely used in defense military industry,aerospace industry,chemical industry,sport equipment,textile industry,oil and gas transportation and other fields.A coarse-grained molecular dynamics simulation method is presented and used in the study of the structure change of UHMWPE under tension.On this basis,the evolution of the structure of the semi-crystalline UHMWPE micro pores structure during heat treatment was investigated.In this paper,the main results are as follows:(1)Using coarse-grained molecular dynamics to simulate the evolution of the structure of the semi-crystalline UHMWPE under tension,the influence of temperature,strain rate and relax time on the strain-stress curve is discussed,and then,by the microstructure evolution,orientation,number density,radial distribution function was used to observe the structural change during the stretching process.Finally,the main driving force of the stretching process was analyzed by the energy evolution diagram.The results show that the stress-strain curve of the semi-crystalline UHMWPE is a typical tensile behavior,ie,the elastic stage,the yield stage,the strain softening and the strain hardening process.As the stretching temperature increases,Young's modulus and yield stress gradually decrease;also the Young's modulus and yield stress will decrease with low stretching rate;but the relax time has little impact about the Young's modulus and yield stress.On the elastic deformation stage,The orientation of the molecular chains in the amorphous region of the semi-crystalline UHMWPE is increased,so that the degree of orientation increases,but also the peak value of the radial distribution function increases,Since there is no obvious micro pores formation,the Z-axis number density does not change much;on the yield phase,the lamellae of the semi-crystalline UHMWPE begins to slip,which cause the orientation degree does not change significantly,however,,to some extents,the degree of entanglement and radial distribution function distribution become increase.Some areas of the Z-axis number density have begun to decrease because of the micro pores begin to form;On the strain softening stage,the nucleation of the semi-crystalline UHMWPE begins to crack,which result in a reduction in the degree of orientation and entanglement,the long-range ordering of the radial distribution function decrease,at some time,the micro pores begin to gradually grown,which can be observed that the reduction area of number density of Z-axis become to increase;For the strain hardening phase,the UHMWPE molecular chains begins to reorient,which result in the degree of orientation,the peak value of radial distribution function and the number of peaks of radial distribution become to increase,at this time,the micro porous continue to grow into large pores,that is why the reduction area of number density continue to expand.At last,from the energy point of view,the potential energy is the main driving force for UHMWPE structure changes before the strain softening phase,however,both the bond energy and potential energy are the main driving force for structural changes in the tensile process.(2)In order to study the influence of the heat treatment method on the micro porous structure of UHMWPE,first,we measured the melting temperature of the initial structure by the V-T method.The UHMWPE micro porous structure was treated based on this temperature.In the isothermal mode,there are some areas where the Z-axis number density is reduced.It can be known that some micro porous will heal,while some micro pores will gradually grow into large pores.It can be known that UHMWPE structural undergoes slower change at low temperature,while at high temperature,UHMWPE structure change is mainly due to disordered particle motion;In the cooling process,the UHMWPE pores will heal,and some micro pores will become large pores,which can be inferred from the Z-axis number density,and then,the UHMWPE pore structure changes during cooling are mainly caused by recrystallization,which can be seen from the radial distribution function evolution.In the heating process,according to the Z-axis number density,we can conclusion that the temperature rise treatment will cause the micro pores to heal,and also the micro pores will grow into large pores.However,from the radial distribution function graph,it can be known that the main reason for the change of the UHMWPE structure during the heating process is due to the system moves toward the melt state and the disorder movement of the particles.Finally,from the energy point of view,the isothermal process,the interaction energy of the bond,and the non-bonding energy has not changed much.It is speculated that the surface energy of the hole plays a major role.For the cooling process,non-bonding can play a major role,For the heating process,rapid heating rate is the main function of kinetic energy and potential energy.While the slow heating rate process,it is speculated that the surface energy of the hole plays a major role.