Dynamics of a supercooled polymer melt above the mode-coupling critical temperature: cage versus polymer-specific effects

This paper reports results of molecular dynamics simulations fora glassy polymer melt consisting of shea, non-entangled chains. The temperature region studied covers the supercooled state of the melt above the mode-coupling critical temperature. The analysis focuses on the interplay of simple-liquid and polymer-specific effects. One can clearly distinguish two regimes: a regime of small and one of large monomer displacements. The first regime corresponds to motion of a monomer in its local environment. It is dominated by the cage effect and well described by the idealized mode-coupling theory. The second regime is governed by the late-beta/early-alpha process. In this regime the connectivity of the monomers begins to interfere with the cage dynamics and finally becomes dominant. The monomer displacement is compared with simulation results for a binary Lennard-Jones mixture to highlight the differences which are introduced by the connectivity of the particles.