Effect of pre-orientation on formation of microstructure of lamella crystal and the stress response of semicrystalline polymers: Molecular dynamics simulations
1.Computational Physics Key Laboratory of Sichuan Province, Yibin University, Yibin 644000, China 2.Weifang University of Science and Technology, Shouguang 262700, China 3.School Material Science and Engineering, Qingdao University, Qiangdao 266003, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 11704329), the Scientific Research Fund of Sichuan Provincial Education Department, China (Grant No. 15ZB293), and the Open Research Fund of Computational Physics Key Laboratory of Sichuan province, Yibin University, China (JSWL2014KF02)
Received Date:04 August 2019
Accepted Date:28 November 2019
Published Online:05 February 2020
Abstract:Molecular dynamics simulations have been used to study the effect of the pre-orientation on the microstructure of lamella crystal and the stress response of polyvinyl alcohol (PVA) semicrystalline polymer under stretching. For the different pre-oriented systems, nucleation is demonstrated to be a two-step process, however, in a different intermediate order. For the isotropic PVA polymer melt, the segment needs more time to adjust its inter-chain structure, therefore, the nucleation is assisted by local order structures, while the nucleation of the oriented PVA melt is promoted by density fluctuation. The nucleation process is the result of coupling effect of conformational and orientational ordering. The transformation from flexible chains into conformational ordered segments circumvents the entropic penalty under the shear flow, which is the most peculiar and rate-limited step in polymer crystallization. Therefore, the current work suggests that the acceleration of the nucleation rate by shear deformation is mainly attributed to the different kinetic pathway via conformational/orientational ordering-density fluctuation-nucleation. From the different pre-oriented PVA semicrystalline polymers, we know that the higher oriented degree corresponds to a higher number of Tie chains and lower Loop chains, and the higher number of Tie chains corresponds to a stronger stress-strain response. And the detailed molecular structural evolution of semicrystalline polymer under stretching is also given in this work. Keywords:pre-oriented/ crystallization/ polyvinyl alcohol melts/ amorphous/ stress-strain
高分子在熔体阶段预取向的不同会导致结晶后力学性能和加工性能存在差异, 然而, 这些加工特性和物理特性的区别主要归因于产生的半晶态高分子晶区与无定型区的结构在预剪切过程中对于高分子成核的影响. 具体发生了那些改变?实验很难准确地检测到晶体和无定型结构的变化, 动力学结果的统计与分析恰好可以弥补实验上在这方面的缺陷和不足. 为了研究无定型结构的区别, 根据实验理解与可能存在的结构, 我们定义了三种类型的无定型结构(图4(a)). Tie 链是连接两个片晶之间的无定型链段, Loop链是折叠链结构, Tail链是链端结构. 从图4(c)可以看出, 本文判定方法可以准确有效地判定出晶体和无定型链的结构分布, 为进一步的研究提供了可靠的理论依据. 图 4 (a)半晶态高分子结构中无定型对应的链结构分类模型; (b), (c) PVA半晶态高分子结构中晶体和无定型的原子结构 Figure4. (a) Schematic diagram of crystalline and amorphous chain structure for the semicrystalline polymers; (b) and (c) atomic structure of crystal and melt of PVA semicrystalline polymers.
根据图4所示的结构定义, 图5(a)给出了不同剪切应变下半晶态高分子结构晶区与无定型结构晶体取向和无定型取向的变化. 从图5(a)中不难发现, 应变越大的熔体形成的半晶态高分子结构对应的结晶度越大, 同时晶体和无定型的取向也越大. 分析不同的无定型链结构在半晶态高分子结构中的数量发现, 应变越大的熔体的Tie链数目也越多, 而Loop链的数目会随着应变的增加而减小. 在小应变下, Tie链数目越小, Loop链数目越多表明, 没有预取向的半晶态高分子晶区主要是以链内的折叠为主; 在高应变下, 主要是以链间的有序为主, 通过Tie链连接片晶与片晶. Tie链数目的改变及Loop链的数目进而会影响到力学性能的改变. 图 5 (a)不同剪切应下PVA半晶态高分子对应的${\phi _{\rm{c}}}$、晶体取向参数Pc和无定型结构取向参数Pa; (b)不同剪切应变下PVA半晶态高分子无定型链结构数目的演化 Figure5. (a) ${\phi _{\rm{c}}}$, crystalline order parameter Pc, and amorphous order parameter Pa for PVA semicrystalline polymers with different shear strains; (b) the evolution of the numbers of amorphous chains for PVA semicrystalline polymers with different shear strains.