Entanglement, segmental relaxation, and polymer crystallization

Entanglement and segmental relaxation can impact crystallization kinetics and semicrystalline morphologies in polymer samples. Using atomistic simulations of cyclic and linear polyethylene (PE), we demonstrate the effect of entanglement on polymer crystal nucleation. Unlinked rings do not exhibit any conventional entanglement topology, and randomly concatenated rings are "permanently" entangled and cannot fully relax. By isotropically compressing individual polymer chains, we reduce the inter-molecular contacts and the entanglement in linear PE melts. We show the isothermal crystal nucleation rate of PE increases mildly with decreasing entanglement density. However, at sufficient supercooling, when the critical nucleus is smaller than the entanglement strand, the formation of a critical nucleus is only weakly impacted by entanglement. When the crystallite size is comparable to the entanglement strand, entanglement impacts crystal growth and lamellar thickness. To demonstrate the role of entanglement in long-time polymer crystal growth, we crystallize PE melts onto post-critical crystalline seeds. Together with topological analyses, MD simulations allow us to track the formation of entangled tie chains and loops during crystal growth. We show trapped entanglement strands formed by loops and ties accumulate over time around polymer crystals, hindering the propagation of crystalline order in the samples. Finally, we show segmental relaxation can significantly affect the polymer crystallization kinetics and semicrystalline morphologies. Free-surface-enhanced segmental mobility can significantly enhance homogeneous nucleation near polymer-air interfaces in free-standing thin films. The resulting film-thickness-dependent nucleation rate correlates linearly with the average segmental orientational relaxation rate.