Local Structure Order Assisted Two-step Crystal Nucleation in Polyethylene

TL;DR

This study uses molecular dynamics simulations and a novel shape descriptor to reveal that polyethylene crystallization occurs via a two-step process involving initial hexagonal cluster formation followed by orthorhombic nucleation, differing from traditional models.

Contribution

Introduces the OCB shape descriptor to differentiate polymer crystal structures and uncovers a two-step nucleation mechanism in polyethylene using molecular dynamics.

Findings

Hexagonal clusters form first and are dynamic.

Orthorhombic nucleation occurs inside hexagonal clusters.

Polymer crystallization proceeds via a two-step process, similar to colloids and metals.

Abstract

Homogeneous nucleation process of polyethylene (PE) is studied with full-atom molecular dynamic simulation. To account the complex shape with low symmetry and the peculiar intra-chain conformational order of polymer, we introduce a shape descriptor OCB coupling conformational order and inter-chain 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 inter-chain rotational orderings results in the formation of hexagonal clusters first, which is dynamic in nature. Whilst nucleation of orthorhombic structure occurs inside of hexagonal clusters later, which 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 OCB order, which is different from early models for polymer crystallization but similar with that proposed for spherical 'atoms' like colloid and metal.