npj: 多尺度计算框架—聚合物纤维的构型调控和声子输运增强

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无定形高分子聚合物的导热性能通常很差，但当其受热拉伸后，分子链有序排布、结晶度和导热性能可以显著提高。然而热拉伸工艺条件参数，如拉伸温度和应变速率，和所产生的链取向度、结晶度以及导热性能之间的关系还有待研究，尤其是这些工艺参数对聚合物构型和性能的复杂影响尚不清楚。

来自上海交通大学的林尚超副教授领导的团队，应用大尺度分子动力学模拟和仔细训练的粗粒化力场，系统性地研究了热拉伸条件对块体聚乙烯链取向度、结晶度和热导率的影响，并完整地建立了聚合物纤维的加工-结构-导热性能的复杂非线性关系。研究发现一个适中的拉伸温度和应变速率的优化组合可实现最高程度的链取向度、结晶度和最终的热导率。这种组合可由粘弹性弛豫中的竞争效应合理地解释，并凝聚为无量纲的德博拉数，反映了应力局域化和分子链扩散效应之间的巧妙平衡。一个串-并联复合热导等效介质模型被建立来预测链取向度和结晶度对热导率的影响。热导率的提高主要归因于本征声子平均自由程和纵波群速度的增加。这项工作为聚合物热拉伸工艺提供了根本性认识，并为全有机电子设备中的声子输运和聚合物散热器效率的增强建立了完整的加工-结构-属性关系。

该文近期发表于npj Computational Materials 5: 126 (2019)，英文标题与摘要如下，点击左下角“阅读原文”可以自由获取论文PDF。

Tailored Morphology and Highly Enhanced Phonon Transport in Polymer Fibers: a Multiscale Computational Framework

Shangchao Lin, Zhuangli Cai, Yang Wang, Lingling Zhao, Chenxi Zhai

Although tremendous efforts have been devoted to enhance thermal conductivity in polymer fibers, correlation between the thermal-drawing conditions and the resulting chain alignment, crystallinity, and phonon transport properties have remained obscure. Using a carefully trained coarse-grained force field, we systematically interrogate the thermal-drawing conditions of bulk polyethylene samples using large-scale molecular dynamics simulations. An optimal combination of moderate drawing temperature and strain rate is found to achieve highest degrees of chain alignment, crystallinity, and the resulting thermal conductivity. Such combination is rationalized by competing effects in viscoelastic relaxation and condensed to the Deborah number, a predictive metric for the thermal-drawing protocols, showing a delicate balance between stress localizations and chain diffusions. Upon tensile deformation, the thermal conductivity of amorphous polyethylene is enhanced to 80% of the theoretical limit, that is, its pure crystalline counterpart. An effective-medium-theory model, based on the serial-parallel heat conducting nature of semicrystalline polymers, is developed here to predict the impacts from both chain alignment and crystallinity on thermal conductivity. The enhancement in thermal conductivity is mainly attributed to the increases in the intrinsic phonon mean free path and the longitudinal group velocity. This work provides fundamental insights into the polymer thermal-drawing process and establishes a complete process–structure–property relationship for enhanced phonon transport in all-organic electronic devices and efficiency of polymeric heat dissipaters.

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