Effect Of Crystallization And Entropy Contribution To The Mechanical Response Of Polymer Fibers

Polyethylene is one of the top five general-purpose plastics in the world,with an annual production of 90 million tons,and is widely used in various fields because of its excellent mechanical properties,good processing performance and high cost performance.The properties of polyethylene are mainly influenced by the crystalline form,but due to the limitations of experimental tools and classical theoretical models,the crystallization behavior of polyethylene has not been studied yet,especially the molecular scale analysis is lacking.Molecular dynamics simulations are a very important tool for interpreting the crystallization behavior of polyethylene at the molecular scale.Studying the connection between polymer crystallization behavior and mechanical response at the molecular level will help to understand and explore the microscopic mechanism of polyethylene crystallization and reveal the kinetic law of polyethylene crystallization in essence.In this paper,four different systems of polyethylene molecular chains are designed and their isothermal and non-isothermal crystallization behaviors are investigated by means of molecular dynamics simulations.The main studies are as follows:In this paper,the effect of crystallization behavior on the mechanical response of four polyethylene molecular chain systems of the same chain length with different chain numbers subjected to chain end retraction and stretching at isothermal conditions is first investigated.The four polyethylene molecular chains with 25 nm length and 1,4,9 and 16 molecular chains were studied,and all-atom polyethylene models of the corresponding systems were established.The study of the crystallization behavior of the chain ends of the four polyethylene systems during retraction and stretching at eleven different temperatures of 200 K,250K,280 K,300K,350 K,400K,450 K,500K,600 K,800K,and 1000 K,respectively,reveals that the molecular chains appear in a folded and ordered crystalline state at low and medium temperatures,while at high temperatures they appear in a The higher the temperature,the higher the number of molecular chains.The higher the temperature,the higher the number of molecular chains and the higher the forces generated during the retraction and stretching process.Then,the effect of the crystallization behavior of four polyethylene molecular chain systems of the same chain length with different chain numbers on their mechanical response was investigated under non-isothermal conditions with continuous warming and continuous cooling of the molecular chains at equal end distances.The four generated polyethylene molecular chains with different chain numbers were subjected to a continuous heating and cooling process from 200 K to1000K at fixed chain end distances of 5nm,10 nm,15nm and 20 nm,respectively.It was found that the crystallization behavior of molecular chains changed from crystalline state to molten state correspondingly with the change of temperature in this process.Moreover,as the end distance of molecular chain increases,the restriction on the structure of molecular chain also increases,and the tension of molecular chain increases during the continuous heating up and continuous cooling down process.The paper concludes by investigating the effect of different rates on the entropic effect during the crystallization of polyethylene fibers.When four polyethylene molecular chain ends with different chain numbers were retracted to 5nm at 200 K,400K,500 K,600K and 1000 K at 0.01nm/ps and 0.001nm/ps rates,the Ramachandran plot conformation of the molecular chains during the retraction process was plotted using Veusz and the "information entropy" of the molecular chains was obtained from the image information "The results show that the molecular chains of the slower rate can be reduced to 5nm.The results showed that the borders of the bishop space distribution in the Rasch plot of the slower rate molecular chains were clearer than those in the Rasch plot of the faster rate molecular chains.And the information entropy of molecular chain conformation increases with the increase of temperature and the number of molecular chains.