From local jumps to entangled chain dynamics in polyethylene melts

Well equilibrated strongly entangled (C1000) polyethylene melts at 450 K are studied by molecular dynamics on the basis of an atomistic potential over a time window going from a few fs up to 20 ns. This wide time range allows to extract from a single MD trajectory a large class of experimentally relevant time correlation functions, in particular the incoherent intermediate scattering function Fs(Q,t) on which we focus in the present contribution. The free Rouse relaxation regime, associated to the time window where the mean square displacement is expected to grow as 〈Δr2(t)〉≈At0.5, can be explored in the Q range going from Q=1nm−1 to Q=5nm−1. In this monomer diffusive regime where monomers feel the chain connectivity but not yet the tube confinement effects, the Fs(Q,t) decay is characterized by a relaxation time τ∝Q−3.7 which is not far from the Q−4 Rouse theory prediction. Respect to the gaussian approximation of Fs(Q,t) based solely on the time evolution of the hydrogen mean square displacement, the nongaussian effects are responsible for a 25% increase of the relaxation times without affecting their Q-dependence.