Molecular Simulations\nof Quantized Lamellar Thickening\nin Polyethylenes with Regularly Spaced Brominated Groups

Polyethylene (PE)\nchains, with CH<sub>2</sub> groups\nreplaced by\nCBr<sub>2</sub> at regular intervals (“precision PE”),\nhave been observed to exhibit competing polymorphs driven by a preference\nfor quantized fold lengths by Tasaki et al. [<i>Macromolecules</i>, <b>2014,</b> 47, 4738–4749]. Motivated by this recent\ndiscovery, the crystallization behavior of such precision PE chains,\n400 carbons long with CBr<sub>2</sub> groups placed regularly at every\n21st carbon, is investigated using molecular dynamics simulations.\nThe united-monomer model of PE is extended to include dibromo groups,\nwith steric clashes at the bromines reflected in a triple-well bending\npotential, demonstrating its function as a preferred fold site. Different\ncrystallization protocols, continuous-cooling and self-seeding, reveal\nremarkably different crystals. Using self-seeding, the crystalline\nlamellar thickness increases monotonically with temperature, in quantized\nmultiples of the distance between dibromo units. Polymer chains are\nobserved to fold preferentially at the dibromo groups, and such groups\nappear to be tolerated within the crystal lamellae. On quenching the\nbromos assemble to form registered\nlayers, not unlike Smectic phases observed in liquid crystals, which\nconfirms the experimental observation of competing Form I and Form\nI′ polymorphs.