Stretch-Induced Coil–Helix Transition in Isotactic Polypropylene: A Molecular Dynamics Simulation,Macromolecules

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Stretch-Induced Coil–Helix Transition in Isotactic Polypropylene: A Molecular Dynamics Simulation
Macromolecules ( IF 5.1 ) Pub Date : 2018-05-15 00:00:00 , DOI: 10.1021/acs.macromol.8b00325
Chun Xie ₁ , Xiaoliang Tang ₁ , Junsheng Yang _{1,

2} , Tingyu Xu ₁ , Fucheng Tian ₁ , Liangbin Li ₁

Affiliation

The stretch-induced coil–helix transition (CHT) of isotactic polypropylene (iPP) was studied with full-atom molecular dynamics (MD) simulations during the uniaxial stretch process. The results show that imposing stretch induces CHT, which increases both the content and the average length of helices. As strain exceeding a certain value, long helices initially not presented in melt start to emerge, which mainly follow a kinetic pathway of merging adjacent short helices, while overstretch at large strain leads to the helix-extended coil transition. Based on statistics on the distribution of helical length and theoretical calculation, stretch is found to reduce free energy gap for CHT. At small strain, the single-chain model is sufficient to account stretch-induced CHT for the formation of short helices, but the gap reduction is mainly contributed by intrachain energy rather than entropy, which is different from current theories for stretch-induced CHT. While the formation of long helices at large strain requires interchain cooperative interactions, which is accompanied by the formation of helix-rich clusters. Additionally, we found that the content of helices with odd atoms in backbone is higher than their even counterparts, which exhibits an odd–even effect due to their corresponding helical lengths.

中文翻译：

全同立构聚丙烯中拉伸诱导的螺旋-螺旋转变：分子动力学模拟

在单轴拉伸过程中，通过全原子分子动力学（MD）模拟研究了等规聚丙烯（iPP）的拉伸诱导的螺旋-螺旋转变（CHT）。结果表明强加的拉伸诱导了CHT，这增加了螺旋的含量和平均长度。当应变超过某个值时，开始没有出现在熔体中的长螺旋开始出现，主要遵循合并相邻短螺旋的动力学路径，而在大应变下的过度拉伸导致螺旋延伸的螺旋转变。基于对螺旋长度分布的统计和理论计算，发现拉伸可以减小CHT的自由能隙。在小应变下，单链模型足以说明拉伸诱导的CHT形成短螺旋，但是缺口的减少主要是由链内能量而不是熵引起的，这与目前的拉伸诱导CHT理论不同。虽然在大应变下长螺旋的形成需要链间协作相互作用，但伴随有富含螺旋的簇的形成。另外，我们发现在主链中带有奇数原子的螺旋的含量高于其偶数螺旋，这由于其相应的螺旋长度而表现出奇偶效应。

更新日期：2018-05-15

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