Stretch-Induced Coil-Helix Transition in Isotactic Polypropylene: A Molecular Dynamics Simulation
Stretch-Induced Coil-Helix Transition in Isotactic Polypropylene: A Molecular Dynamics Simulation
复制标题
等规聚丙烯中拉伸诱导的螺旋-螺旋转变：分子动力学模拟
DOI：
10.1021/acs.macromol.8b00325
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发表时间：
2018
期刊：
影响因子：
5.5
通讯作者：
Li Liangbin
中科院分区：
文献类型：
--
作者：
Xie Chun;Tang Xiaoliang;Yang Junsheng;Xu Tingyu;Tian Fucheng;Li Liangbin
用全原子分子动力学(MD)方法研究了等规聚丙烯(IPP)单轴拉伸过程中的拉伸诱导螺旋-螺旋转变(CHT)过程。结果表明，施加拉伸会引起CHT，增加螺旋的含量和平均长度。当应变超过一定值时，最初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-延伸螺旋的转变。通过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，单链模型足以解释拉伸诱导的CHT形成短螺旋，但缝隙减小主要是由链内能量而不是熵贡献的，这与目前拉伸诱导CHT的理论不同。而大应变下长螺旋的形成需要链间的协同作用，这伴随着富含螺旋的团簇的形成。此外，我们还发现主链中含有奇数原子的螺旋的含量高于其偶数原子的含量，这是由于其相应的螺旋长度导致的奇偶效应。
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.