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Molecular Dynamics Modeling of PPTA Crystals in Aramid Fibers

机译：芳纶纤维中PPTA晶体的分子动力学建模

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摘要

In this work, molecular dynamics modeling is used to study the mechanical properties of PPTA crystallites, which are the fundamental microstructural building blocks of polymer aramid fibers such as Kevlar. Particular focus is given to constant strain rate axial loading simulations of PPTA crystallites, which is motivated by the rate-dependent mechanical properties observed in some experiments with aramid fibers. In order to accommodate the covalent bond rupture that occurs in loading a crystallite to failure, the reactive bond order force field ReaxFF is employed to conduct the simulations.;Two major topics are addressed: The first is the general behavior of PPTA crystallites under strain rate loading. Constant strain rate loading simulations of crystalline PPTA reveal that the crystal failure strain increases with increasing strain rate, while the modulus is not affected by the strain rate. Increasing temperature lowers both the modulus and the failure strain. The simulations also identify the C--N bond connecting the aromatic rings as weakest primary bond along the backbone of the PPTA chain. The effect of chain-end defects on PPTA micromechanics is explored, and it is found that the presence of a chain-end defect transfers load to the adjacent chains in the hydrogen-bonded sheet in which the defect resides, but does not influence the behavior of any other chains in the crystal. Chain-end defects are found to lower the strength of the crystal when clustered together, inducing bond failure via stress concentrations arising from the load transfer to bonds in adjacent chains near the defect site. The second topic addressed is the nature of primary and secondary bond failure in crystalline PPTA. Failure of both types of bonds is found to be stochastic in nature and driven by thermal fluctuations of the bonds within the crystal. A model is proposed which uses reliability theory to model bonds under constant strain rate loading as components with time-dependent failure rate functions. The model is shown to work well for predicting the onset of primary backbone bond failure, as well as the onset of secondary bond failure via chain slippage for the case of isolated non-interacting chain-end defects.
机译：在这项工作中，使用分子动力学模型研究PPTA晶体的机械性能，这些晶体是聚合物芳纶纤维（如凯夫拉尔）的基本微观结构基础。特别关注PPTA微晶的恒定应变率轴向载荷模拟，这是由在某些使用芳纶纤维进行的实验中观察到的与速率相关的机械性能引起的。为了适应在将微晶加载至破坏过程中发生的共价键断裂，采用了反应性键序力场ReaxFF进行仿真。提出了两个主要主题：第一个是应变速率下PPTA微晶的一般行为。加载中。晶体PPTA的恒定应变速率加载模拟显示，晶体破坏应变随应变速率的增加而增加，而模量不受应变速率的影响。温度升高会降低模量和破坏应变。模拟还确定了连接芳环的C–N键是沿PPTA链主链的最弱的一级键。探索了链端缺陷对PPTA微力学的影响，发现链端缺陷的存在将载荷转移到缺陷所在的氢键合板材中的相邻链上，但不影响其行为。晶体中任何其他链的数量发现链端缺陷会在聚集在一起时降低晶体的强度，并通过应力集中引起键合失效，应力集中是由于载荷转移到缺陷部位附近相邻链中的键上而引起的。讨论的第二个主题是结晶PPTA中一级键和二级键失效的性质。发现这两种类型的键的失效本质上都是随机的，并且是由晶体内键的热波动引起的。提出了一种模型，该模型使用可靠性理论对恒定应变速率载荷下的粘结作为具有时间相关故障率函数的构件进行建模。对于孤立的非相互作用链端缺陷，该模型可以很好地预测主骨架键失效的发生，以及通过链滑移引起的次级键失效的发生。

著录项

  • 作者

    Mercer, Brian Scott.;

  • 作者单位

    University of California, Berkeley.;

  • 授予单位 University of California, Berkeley.;
  • 学科 Mechanical engineering.
  • 学位 Ph.D.
  • 年度 2016
  • 页码 131 p.
  • 总页数 131
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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