Bond-orientational Order Assisted Crystal Nucleation in Polyethylene

Xiaoliang Tang, Junsheng Yang, Tingyu Xu, Fucheng Tian, Chun Xie, Liangbin Li

National Synchrotron Radiation Lab and CAS Key Laboratory of Soft Matter Chemistry, University of Science and Technology of China, Hefei, China

Flexibility and connectivity are the most prominent characteristics of polymer, how does flexible chain transform into rigid conformational ordered segments may be the key step for crystallization [1]. We investigate the nucleation process of polyethylene with full-atom molecular dynamic simulation, during which the structural evolution is analyzed with three order parameters, including conformational order, modified bond-orientational order [2] and density. Coupling between conformational and bond-orientational orderings results in the formation of hexagonal clusters first, which is dynamic in nature and absence of density order. Whilst nucleation of orthorhombic clusters occurs inside hexagonal clusters later, which involves all three order parameters and proceeds via the coalescence of neighboring hexagonal clusters rather than standard stepwise growth process. This demonstrates that nucleation of PE crystal is a two-step process with the assistance of bond-orientational order, which is different from early models for polymer crystallization but in line with that proposed for spherical “atoms” like colloid and metal.

[1] Su F, Ji Y, Meng L, et al. Coupling of Multiscale Orderings during Flow- Induced Crystallization of Isotactic Polypropylene. Macromolecules, 2017, 50(5): 1991-1997. (link)
[2] Yang J, Tang X, et al. Coupling between intra-and inter-chain orderings in flow-induced crystallization of polyethylene: A non-equilibrium molecular dynamics simulation study. The Journal of Chemical Physics, 2017, 146(1): 014901. (link)