Molecular Simulations of Quantized Lamellar Thickening in Polyethylenes with Regularly Spaced Brominated Groups

TL;DR

This study uses molecular dynamics simulations to explore how regular brominated groups in polyethylene influence lamellar thickening and polymorph formation, revealing quantized fold lengths and layered structures similar to smectic phases.

Contribution

It introduces a new molecular model for brominated polyethylene and demonstrates how bromine groups induce quantized lamellar thickening and layered polymorphs during crystallization.

Findings

Lamellar thickness increases in quantized steps with temperature.

Brominated groups serve as preferred fold sites within crystals.

Formation of layered structures akin to smectic phases observed.

Abstract

Polyethylene (PE) chains, with CH2 groups replaced by CBr2 at regular intervals ("precision PE"), have been observed to exhibit competing polymorphs driven by a preference for quantized fold lengths by Tasaki et al. Motivated by this recent discovery, the crystallisation behaviour of such precision PE chains, 400 carbons long with CBr2 groups placed regularly at every 21st carbon, is investigated using molecular dynamics simulations. The united-monomer model of PE is extended to include dibromo groups, with steric clashes at the bromines reflected in a triple-well bending potential, demonstrating its function as a preferred fold site. Different crystallisation protocols, continuous-cooling and self-seeding, reveal remarkably different crystals. Using self-seeding, the crystalline lamellar thickness increases monotonically with temperature, in quantized multiples of the distance between dibromo units. Polymer chains are observed to fold preferentially at the dibromo groups and such groups appear to be tolerated within the crystal lamellae. On quenching the bromos assemble to form registered layers, not unlike Smectic phases observed in liquid crystals, which confirms the experimental observation of competing Form I and Form I' polymorphs.