Molecular Dynamics Simulations of Short-Chain Branched Bimodal Polyethylene: Topological Characteristics and Mechanical Behavior
Molecular Dynamics Simulations of Short-Chain Branched Bimodal Polyethylene: Topological Characteristics and Mechanical Behavior
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DOI：
10.1021/acs.macromol.8b01874
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发表时间：
2019-02-12
期刊：
影响因子：
5.5
通讯作者：
Gedde, Ulf W.
中科院分区：
文献类型：
--
作者：
Moyassari, Ali;Gkourmpis, Thomas;Gedde, Ulf W.
先前已经表明，具有双峰摩尔质量分布的聚乙烯（PE）具有高断裂韧性。我们的方法一直是使用粗粒度（CG）分子动力学（MD）模拟研究的影响，包括在双峰PE的高摩尔质量分数的拓扑结构和材料的力学行为的短链分支。CG电位的推导，验证，并用于模拟熔体平衡，冷却，结晶和机械变形。连续监测结晶度、连接链和缠结浓度。在结晶过程中，分支双峰系统解缠结的程度较小，并最终以较高的缠结密度比线性双峰系统在我们以前的研究中模拟。缠结浓度的增加与支化高摩尔质量分数的含量成正比。一个显着更高的连接链的浓度，得到在短链支化双峰系统比在线性系统。领带数量的增加比纠缠数量的增加更明显。连接链浓度与高摩尔质量分数的含量不成正比。尽管较低的晶体厚度和含量，弹性模量和屈服应力值较高的支化双峰系统。一个更明显的应变硬化区中观察到的分支系统。结果表明，较高的连接链和缠结的浓度在变形前，更广泛的解缠结在变形过程中，和失效点之前形成的空隙消失的原因是观察到的更高的韧性的短链支化双峰聚乙烯相比，线性双峰系统。最强硬的系统，其中分别含有25和75重量%的低摩尔质量和支化的高摩尔质量的馏分，具有最高的连接链浓度和第二高的缠结浓度的模拟系统。
It has previously been shown that polyethylene (PE) with a bimodal molar mass distribution has a high fracture toughness. Our approach has been to use coarse-grained (CG) molecular dynamics (MD) simulations to investigate the effects of including short-chain branches in the high molar mass fraction of bimodal PE on topological features and mechanical behavior of the material. The CG potentials were derived, validated, and utilized to simulate melt equilibration, cooling, crystallization, and mechanical deformation. Crystallinity, tie chain, and entanglement concentrations were continuously monitored. During crystallization, the branched bimodal systems disentangled to a lesser degree and ended up with a higher entanglement density than the linear bimodal systems simulated in our previous study. The increase in entanglement concentration was proportional to the content of the branched high molar mass fraction. A significantly higher tie chain concentration was obtained in the short-chain branched bimodal systems than in the linear systems. The increase in the number of ties was more pronounced than the increase in the number of entanglements. The tie chain concentration was not proportional to the content of the high molar mass fraction. Despite a lower crystal thickness and content, the elastic modulus and yield stress values were higher in the branched bimodal systems. A more pronounced strain hardening region was observed in the branched systems. It was suggested that the higher tie chain and entanglement concentration prior to the deformation, the more extensive disentanglement during the deformation, and the disappearance of formed voids prior to failure point were the reasons for the observed higher toughness of the short-chain branched bimodal PE compared with that of the linear bimodal systems. The toughest system, which contained respectively 25 and 75 wt % low molar mass and branched high molar mass fractions, had the highest tie chain concentration and the second highest entanglement concentration of the simulated systems.