Molecular Dynamics Simulations of the Effects of Entanglement on Polymer Crystal Nucleation

We investigate the effects of entanglement on polymer crystallization using atomistic molecular dynamics simulations of linear and cyclic polyethylene (PE). While linear chains entangle with their neighbors, unlinked rings adopt compact conformations and do not exhibit conventional entanglements. By isotropically compressing linear chains, we also systematically reduce the entanglement density in PE melts without enhancing any local order that may promote crystallization. After demonstrating that the linear and cyclic chains exhibit similar crystal melting temperatures, we quantitatively show that the isothermal nucleation rate of PE increases with decreasing entanglement density at similar degrees of supercooling. The effect of entanglement density on crystal nucleation, however, is mild. The absence of conventional entanglements in unlinked rings only enhances the nucleation rate by a factor of two. Finally, using randomly linked and permanently entangled rings, which nucleate at a rate similar to that of the linear counterparts, we show that polymer nucleation is local and does not require large-scale relaxation of polymer chains.