Stretch-Induced Coil-Helix Transition in Isotactic Polypropylene: A Molecular Dynamics Simulation
C Xie and XL Tang and JS Yang and TY Xu and FC Tian and LB Li, MACROMOLECULES, 51, 3994-4002 (2018).
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.
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