Dependence of mechanical properties on crystal orientation of semi-crystalline polyethylene structures
Dependence of mechanical properties on crystal orientation of semi-crystalline polyethylene structures
复制标题
DOI：
10.1016/j.polymer.2014.03.045
复制
发表时间：
2014-08-05
期刊：
影响因子：
4.6
通讯作者：
Jacob, Karl I.
中科院分区：
文献类型：
--
作者：
Dong, Xin;McDowell, David L.;Jacob, Karl I.
采用基于统一原子法的分子动力学模拟方法，研究了具有单向和双向分子取向的半晶聚乙烯结构的分子响应。预取向熔体在375 K的等温结晶过程中产生了初始结构。在恒体积（NVT）和恒侧应力（NLz sigma(x)sigma T-y）条件下，以恒定应变速率1.24 nm/ns进行变形模拟。为了将不同长度尺度的表征结果联系起来，基于非晶区和晶区的密度和取向差异，引入两点统计来捕捉变形过程中半晶结构的形态变化。在这种新方法中，分子的构象变化在两点自相关和相互相关光谱中被很好地捕获，从而在统计水平上理解由大分子链组成的不同结构域的形态演变。在应变水平为0.5时，两种结构均保留了伪层流半晶结构，且晶区链的旋转和取向均面向单轴变形方向。在较高的应变水平下，观察到双向定向半晶结构的层状向纤维状转变。恢复试验证实了层状和纤维状结构在变形的某些阶段的稳定性。(C) 2014 Elsevier Ltd.版权所有。
Molecular dynamics simulations based on a united atom method are employed to study the molecular response of semi-crystalline polyethylene structures with unidirectional and bidirectional molecular orientation. The initial structures are generated from isothermal crystallization at 375 K of pre-oriented melts. Deformation simulations are conducted at a constant strain rate of 1.24 nm/ns at both constant volume (NVT) and constant lateral stress (NLz sigma(x)sigma T-y) conditions. In order to connect characterization results at various length scales two-point statistics are introduced to capture the morphological changes in semi-crystalline structures during the deformation, based on the density and orientational differences of the amorphous and crystalline regions. In this new approach, molecular conformational changes are well captured in the two-point auto- and cross-correlation spectra, to understand the morphological evolution of different domains consisting of macromolecular chains on a statistical level. The pseudo-laminar semi-crystalline structures are preserved for both structures up to a strain level of 0.5, with the rotation and orientation of chains in the crystalline region facing towards the uniaxial deformation direction. A laminar to fibrillar transition is observed for the bidirectional oriented semi-crystalline structure at higher strain levels. Recovery tests confirm the stability of the laminar and fibrillar structures at certain stages of the deformation. (C) 2014 Elsevier Ltd. All rights reserved.