Computational chemistry & molecular modeling
Fan Zhang; Jieqi Wang; Yangyang Zhao; Xuelian He
Abstract
The effect of long chain content (XL ) on the static crystallization and tensile deformation mechanisms of bimodal HDPE/UHMWPE was investigated by molecular dynamics simulations. The crystallization of HDPE/ UHMWPE undergoes three stages: nucleation, rapid growth of lamellar crystals, and stabilization. ...
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The effect of long chain content (XL ) on the static crystallization and tensile deformation mechanisms of bimodal HDPE/UHMWPE was investigated by molecular dynamics simulations. The crystallization of HDPE/ UHMWPE undergoes three stages: nucleation, rapid growth of lamellar crystals, and stabilization. The increase of XL leads to the formation of more nucleation sites, which promotes nucleation, but at the same time leads to an increase of entanglement sites, which is not conducive to the movement of the long chains to the growth front to fold and form lamellar crystals. Tensile deformation is performed on the crystallized models and the systems exhibit three stages: elastic deformation, plastic deformation and stress hardening. During deformation, the increase of XL improves the orientation nucleation and crystallinity (Xc), but when XL exceeds 4 wt.%, the entanglement effect becomes more pronounced, leading to a decrease in Xc. The effect of temperature is also taken into account: at low temperatures, a suitable range (2-4 wt.%) exists to optimize the mechanical properties of the material. At high temperatures, there is almost no stress-hardening phenomenon, but the addition of long chains has an impeding effect on the melting of the lamellar crystals, and when XL is greater than 8 wt.%, stress-induced melting is more likely to occur, accelerating the melting of the crystals.
Simulation & Modeling
Jieqi Wang; Li Zhao; Minju Song; Fenge Hu; Xuelian He
Abstract
The influence of long branches on crystallization behavior has been studied by means of molecular dynamics simulations. Using two systems: polyethylene (PE) with long branches (LCB-PE) and PE without long branches (linear-PE) with the same molecular weight, we have examined the crystallization behavior ...
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The influence of long branches on crystallization behavior has been studied by means of molecular dynamics simulations. Using two systems: polyethylene (PE) with long branches (LCB-PE) and PE without long branches (linear-PE) with the same molecular weight, we have examined the crystallization behavior of the two systems by molecular dynamics simulation. This paper explains the influence of long branches on the isothermal crystallization process and the non-isothermal crystallization process with similar initial interchain contact fraction (ICF) in terms of final ICF, crystal regions, crystallinity, concentration of tie chains and energy. It is found that the crystallization process is classified as two stages: the nucleation stage and the crystal growth stage. The existence of long branches is favorable for the first stage while unfavorable for the second stage. Knots that act as crystalline defects are excluded from the lamella, resulting in decreasing in regularity and crystallinity of molecular chains. From the perspective of potential energy and non-bond energy, LCB-PE has lower energy than linear-PE in the nucleation stage while the energy of linear-PE is lower than that of LCB-PE in the second stage. In short, the long branched chains inhibit the crystallization process.
