Molecular Dynamics Simulation Of Non-isothermal Crystallization Of Polyethylene Chains Based On All Atom Model

Polyethylene is widely used in daily life,industrial production and medical equipment due to its excellent chemical stability,physical characteristics and costeffective performance.The material properties of polyethylene are mainly determined by its crystalline morphology.At present,the computer simulation of polyethylene mainly focuses on the study of crystallization kinetics at macroscopic scale,but there are still some problems in the crystallization process at the molecular scale that need to be further studied.Therefore,it is of great significance to simulate the crystallization behavior of polyethylene at molecular scale.Molecular dynamics simulation is a mature and reliable method to explore the crystallization behavior of polymers at molecular scale.Most existing molecular dynamics simulations are based on united-atom model and coarse-grained model.These two models simplify the polyethylene chain and omit or coarser some details.Although the time cost is reduced and the computational efficiency is improved,part of the information is lost due to the coarse-grained model,which leads to the lack of accuracy of conformation on the molecular scale.With the continuous upgrading of computer hardware and software,the accuracy of computer simulation has become the goal pursued by researchers.This paper is based on the all-atom model and OPLS-AA(Optimized Potential for Liquid Simulation-All Atom)force field.Molecular dynamics simulations for collapse processing at high-temperature and non-isothermal crystallization during cooling are carried out.Two systems are achieved,including single-chain polyethylene systems with different chain lengths(C1000,C2000,C3000,C4000)and multi-chain polyethylene systems with different chain numbers(2C500,4C500,6C500,8C500).The main work and conclusions of this paper are as follows:(1)Molecular dynamics simulations of collapse processing at high temperature(600K)with single-chain and multi-chain all-trans polyethylene systems are carried out.The conformational changes of the system were observed and the energy curves and corresponding crystallinity changes are analyzed.The results of computer simulation show that the chain length has an important effect on the conformational change of the system during isothermal collapse.With the increase of chain length,polyethylene chain changes from global collapse to local collapse,and potential energy and Vdw(Van der Waals)energy curve fluctuate greatly,the curves of Rg(Radius of Gyration)and RMSD(Root Mean Square Deviations)show hump.(2)Molecular dynamics simulations of non-isothermal crystallization processing with polyethylene single-chain and multi-chain systems at different cooling rates(100K/ns,50K/ns,20K/ns)after collapse processing at high temperature are carried out.The conformational changes,potential energy,Vdw energy,Rg and RMSD,as well as the crystallinity changes for single-chain and multi-chain systems under nonisothermal conditions are studied.The simulation results show that the cooling rate and the total molecular weight of the system have a great influence on the nonisothermal crystallization stage;as the cooling rate decreases,the final conformation of the system becomes more regular and the crystallinity becomes higher.In the process of cooling,a relatively obvious crystallization "platform" appeares in the Vdw energy curve.In this temperature range,the conformation of the system is rapidly adjusted from random coil to relatively ordered crystalline,which has a decisive effect on the final crystalline conformation of the system.In this paper,based on the all-atom model and the OPLS-AA force field,molecular dynamics simulations of the collapse processing at high temperature and non-isothermal crystallization processing at cooling for single-chain polyethylene systems with different chain lengths and multi-chain polyethylene systems with different chain numbers are carried out.Compared with the conclusions with the united-atom model,the polyethylene crystallization temperatures obtained in this paper is much closer to the experimental results.In addition,due to the use of all the information of molecular chain,the molecular dynamics simulation of non-isothermal crystallization process of polyethylene chain based on the whole atom model is more close to the actual simulation effect in the detail analysis of conformation.