Bond-orientational Order Assisted Crystal Nucleation in Polyethylene

Homogeneous nucleation process of polyethylene is studied with full-atom molecular dynamic simulation,during which the structural evolution is analyzed with three order parameters, including conformational order, modified bond-orientational order 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, 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 fundamentally different from early models for polymer crystallization but in line with that proposed for spherical "atoms" like colloid and metal.