Molecular Simulations Of Stretch-induced Crystallization Behavior Of Polyethylene Systems

Polymer crystallization has always been a hot topic in the field of polymer physics and polymer materials.Compared with polymer materials without crystal structure,crystal structure can help polymer materials to bear the greater load and give them higher mechanical properties,so the physical properties of semi-crystalline polymer materials can be improved by controlling the morphology of polymer crystals.Generally speaking,there are two effective ways to regulate the morphology of polymer crystals: firstly,the orientation crystal morphology is induced by shear or tensile action,which can significantly improve the mechanical properties of semi-crystalline polymer materials;Secondly,filling the polymer with nano fillers,such as carbon nanotubes(CNT)or graphene,can also lead to the improvement of the polymer crystallinity or the change of crystal morphology,to achieve a significant improvement in the mechanical properties of polymer materials.This article will revolve around the stretch of pure polyethylene(PE)system and the crystallization of CNT/PE nanocomposite system process,using molecular dynamics simulation method to explore the molecular scale drawing under the influence of the evolution of microstructure in polymer system,especially the formation process of the precursor and crystallization nucleation process,and elucidate the structure formation mechanism,It lays a solid foundation for the design and preparation of polymer nanocomposites with high strength and toughness.On the other hand,the related research results can also enrich people's understanding of the complex and unique non-equilibrium phase transition.Specific research includes the following aspects:1.Molecular dynamics simulation of tensile induced crystallization of pure PE system.At 600 K,the molten pure PE system was stretched to different strain states(strain 1,2,3,and4,respectively),and then the polymer systems with different strains were quenched to low temperature for isothermal crystallization.The system was composed of 100 PE chains,each containing 200 methylene(C200),and was studied by computer simulation.It is found that stretching causes the whole PE chain to be oriented along the stretching direction,and the local conformation to be transformed from the Gauche conformation to the trans conformation.The segments of the uniformly oriented trans conformation are then further aggregated locally to form precursors.During the crystallization process,most of the grains formed by early crystallization nucleation came from the precursors.Besides,the size of the strain directly affects the nucleation process.At low strain,the nucleation mode is more inclined to intramolecular folded chain nucleation.At high strains,parallel arrangements between molecules are preferred for nucleation.The crystallinity is 0.44 when the system strain is 4,which is about twice as high as that at strain 1.The simulation results in this chapter can provide some new understanding of the formation mechanism of tensile-induced polymer crystallization nucleation and provide relevant theoretical guidance for the regulation of polymer crystal morphology.2.Molecular dynamics simulation of tensile induced CNT/PE nanocomposites.The composite system was stretched to different strain states(0,1,2,and 3,respectively)at 600 K,and then the composite systems with different strains were quenched to low temperature for isothermal crystallization.The system was composed of 200 PE chains(C200)and 4 CNTs(7,7)with an A length of 36.89 ?.It was found that,in addition to the polymer chain segments,CNTs also Orient along the stretching direction during the tensile process,and the degree of orientation can reach 0.95.In this case,the polymer in the region near the CNT surface is affected by two induced orientations: segment orientations at the CNT-induced interface and tensile induced segment orientations,which results in higher orientations of the segment near the CNT and higher content of trans conformation.Therefore,the chain segments in the interface region of the composite system are more likely to evolve into precursors and crystallize nucleation.When the final strain is 3,the crystallinity of the composite system is about 0.55,while that of the unstretched system is only 0.3.The simulation results in this chapter reveal the strengthening and toughening mechanism of polymer nanocomposites and provide relevant theoretical guidance for the regulation of the crystal morphology of polymer nanocomposites.3.Molecular dynamics simulation of the effect of CNT content on tensile induced crystallization behavior in CNT/PE nanocomposites.In this part,A total of five groups of initial models with different CNT contents(1,2,4,8,and 16 roots,respectively,and the corresponding mass fractions were 1.8%,3.5%,6.7%,12.6%,and 22.3%,respectively)were established,and the length of CNT was 73.78?.,found the more CNT after stretching process in the system after the formation of the precursor is,the more,the early stage,resulting in low-temperature isothermal crystallization rate will be increased with the increase of CNT content showed a rising trend,but the crystallization system after the completion of the crystallinity is increased with the increase of CNT content present first increases then decreases.At the same time,it was found that the crystal size of the system with more CNT content was smaller.The simulation results in this chapter reveal the nucleation mechanism of polymers in the composite system with different contents of nano-fillers and provide theoretical guidance for the preparation of high-performance nano-polymer composites in the future.