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

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. [Macromolecules, 2014, 47, 4738–4749]. Motivated by this recent
discovery, the crystallization behavior 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
crystallization 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.