All-atomic and coarse-grained molecular dynamics investigation of deformation in semi-crystalline lamellar polyethylene
All-atomic and coarse-grained molecular dynamics investigation of deformation in semi-crystalline lamellar polyethylene
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DOI：
10.1016/j.polymer.2018.07.075
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发表时间：
2018-09-26
期刊：
影响因子：
4.6
通讯作者：
Kroon, Martin
中科院分区：
文献类型：
--
作者：
Olsson, Par A. T.;in't Veld, Pieter J.;Kroon, Martin
在本工作中，我们用经典的分子动力学模型研究了不同类型的力场对半结晶层状堆积线性聚乙烯的应力-应变和屈服行为的影响。为此，本文采用全原子优化液态势模拟(OPLS-AA)和粗粒联合原子力场(UA)来模拟层状分离模式的屈服和拉伸行为。尽管所考虑的样本及其拓扑结构对于这两种方法是相同的，但结果表明它们预测的应力应变和屈服行为有很大的不同。对于所有的UA模拟，我们都得到了振荡的应力-应变曲线，伴随着向非晶区的重复链传输，以及大量的链滑移和晶体重新取向。对于OPLS-AA模型，主要观察到空化的形成，通过少量的链滑移来重新定位晶体，使链在拉伸方向上对齐。这种力场依赖性的根源在于UA方法中缺乏显式的H-H和C-H排斥，从而导致低估了理想的临界可分辨剪应力。OPLS-AA方法计算的临界分辨剪应力与密度泛函理论计算结果吻合较好，屈服机制类似于片层分离模式。不同模型中不同的链滑移能量和剪应力势垒可以解释为对该机制的本征激活率的不同预测，这最终导致了观察到的两种模拟方法的不同响应。
In the present work we have performed classical molecular dynamics modelling to investigate the effects of different types of force-fields on the stress-strain and yielding behaviours in semi-crystalline lamellar stacked linear polyethylene. To this end, specifically the all-atomic optimized potential for liquid simulations (OPLS-AA) and the coarse-grained united-atom (UA) force-fields are used to simulate the yielding and tensile behaviour for the lamellar separation mode. Despite that the considered samples and their topologies are identical for both approaches, the results show that they predict widely different stress-strain and yielding behaviours. For all UA simulations we obtain oscillating stress-strain curves accompanied by repetitive chain transport to the amorphous region, along with substantial chain slip and crystal reorientation. For the OPLS-AA modelling primarily cavitation formation is observed, with small amounts of chain slip to reorient the crystal such that the chains align in the tensile direction. This force-field dependence is rooted in the lack of explicit H-H and C-H repulsion in the UA approach, which gives rise to underestimated ideal critical resolved shear stress. The computed critical resolved shear stress for the OPLS-AA approach is in good agreement with density functional theory calculations and the yielding mechanisms resemble those of the lamellar separation mode. The disparate energy and shear stress barriers for chain slip of the different models can be interpreted as differently predicted intrinsic activation rates for the mechanism, which ultimately are responsible for the observed diverse responses of the two modelling approaches.