Local structure order assisted two-step crystal nucleation in polyethylene
The homogeneous nucleation process of polyethylene (PE) is studied with full-atom molecular dynamic simulation. The related work has been published in Physical Review Materials (2017, 1, 073401).
To account for the complex shape with low symmetry and the peculiar intrachain conformational order of polymer, we introduce a shape descriptor OCB coupling conformational order and interchain rotational symmetry, which is able to differentiate hexagonal and orthorhombic clusters from melt. With the shape descriptor OCB, we find that coupling between conformational and interchain rotational orderings results in the formation of hexagonal clusters first, which is dynamic in nature. After the formation of hexagonal clusters, the nucleation of orthorhombic structure occurs inside of them, which proceeds via the coalescence of neighboring hexagonal clusters rather than a standard stepwise growth process. This demonstrates that nucleation of PE crystal is an OCB order assisted two-step process, which is different from previous models for polymer crystallization but similar with that proposed for spherical “atoms” such as colloid and metal.
(a, b) Snapshots of the nucleation process of PE with different types of atoms colored differently. (c) The XRD of the orthorhombic clusters. (d, e, f) The evolutions of the number of OCB structures at 330, 350, and 375 K, respectively.
