Rouse Mode Analysis of Chain Relaxation in HomopolymerMelts.

We use molecular dynamics simulationsof the Kremer–Grest(KG) bead–spring model of polymer chains of length between10 and 500, and a closely related analogue that allows for chain crossing,to clearly delineate the effects of entanglements on the length-scale-dependentchain relaxation in polymer melts. We analyze the resulting trajectoriesusing the Rouse modes of the chains and find that entanglements stronglyaffect these modes. The relaxation rates of the chains show two limitingeffective monomeric frictions, with the local modes experiencing muchlower effective friction than the longer modes. The monomeric relaxationrates of longer modes vary approximately inversely with chain lengthdue to kinetic confinement effects. The time-dependent relaxationof Rouse modes has a stretched exponential character with a minimumof stretching exponent in the vicinity of the entanglement chain length.None of these trends are found in models that allow for chain crossing.These facts, in combination, argue for the confined motion of chainsfor time scales between the entanglement time and their ultimate freediffusion. [ABSTRACT FROM AUTHOR]