Molecular Dynamics Simulation Of Crystallization And Stretching Process Of Polyethylene

Most of the polymer materials are composed of homologues with different molecular weights,so the molecular weight and molecular weight distribution will become important factors affecting their structure and performance.At present,a large number of experiments have been carried out to study the effect of polymer structure on mechanical properties,but this can only be qualitatively studied from a macroscopic perspective,which is not sufficient to explain the microscopic evolution of its molecular chain.In order to study the microscopic mechanism of polymer materials,the relationship between the structure and the mechanical properties of mixed multi-chain polyethylene with different molecular weight and molecular weight distribution was investigated by computer simulation.The main research contents are as follows:Firstly,the effect of chain length on the crystallization behavior and tensile behavior of polyethylene was studied.The research object consists of three polyethylene multi-chain systems consisting of one chain length with chain lengths of 100,200,and 400,respectively,and a coarse-grained model of the corresponding system is established.In the crystallization simulation of polyethylene chain multi-chain systems with different chain lengths,it was found that the shorter the molecular chain,the faster the crystallization rate and the higher the crystallinity.Then,the three semi-crystalline polyethylenes obtained in the above process were subjected to tensile simulation,and the molecular chains of different chain length polyethylene systems were obtained by the change of crystallinity,chain entanglement and chain orientation degree with strain during the stretching process.The microscopic evolution mechanism found that the shorter the chain length,the larger the yield stress of the polyethylene system,and the smaller the stress required in the strain hardening stage.Secondly,the effects of two chain length systems with different chain ratios on the crystallization behavior and tensile behavior of polyethylene were studied.The research object is a multi-chain polyethylene mixed system of C20/C200,C50/C200 and C100/C200.Each system is set with three kinds of long-short chain mixing ratios of 1:1,1:3,1:4,and then the coarse granulation model of the corresponding system is established.The crystallization simulation of the above various long-short chain polyethylene mixed systems was carried out,and the crystallization process of the uniform chain length polyethylene system was compared.It was found that the incorporation of short chains promoted crystallization,and the shorter and more short-chains were mixed,the higher the crystallinity.Then,the above semi-crystalline polyethylene was subjected to tensile simulation,and the microscopic evolution mechanism of the molecular chain was revealed by analyzing the change curves of energy,crystallinity,chain entanglement and chain orientation degree,and it was found that the short chain was mixed in the polyethylene system.The more and the shorter,the greater the tensile yield stress and the smaller the strain hardening stress.Finally,the effects of different molecular weight distributions on the crystallization behavior and tensile behavior of polyethylene were investigated.The study consists of two multi-chain mixed polyethylene systems with different chain lengths and different dispersions.The chain lengths are 50/200/350,150/200/250,and the dispersions of the two systems are 1.225 and 1.0125,respectively.Establish a coarse-grained model of the corresponding system.The crystallization simulation of the above multi-chain mixed polyethylene systems with different molecular weight distributions showed that the initial crystallization rates of the two polyethylene systems were almost the same,but at lower temperatures,the crystallization rate of the system with high dispersion is greater and the crystallinity of the resulting semi-crystals is higher.Stretching simulation of the semi-crystalline polyethylene of the above two systems,the microscopic evolution mechanism of the molecular chain was obtained by analyzing the curves of energy,crystallinity,chain entanglement and chain orientation.It was found that the yield stress of the dispersion system was larger and the stress required in the strain hardening stage was higher.