  The Rouse model can be regarded as the standard model to describe the
dynamics of a short polymer chain under melt conditions. In this contribution,
we explicitly check one of the fundamental assumptions of this model, namely
that of a uniform friction coefficient for all monomers, on the basis of MD
simulation data of a poly(ethylene oxide) (PEO) melt. This question immediately
arises from the fact that in a real polymer melt the terminal monomers have on
average more intermolecular neighbors than the central monomers, and one would
expect that exactly these details affect the precise value of the friction
coefficient. The mobilities are determined by our recently developed
statistical method, which provides detailed insights about the local polymer
dynamics. Moreover, it yields complementary information to that obtained from
the mean square displacement (MSD) or the Rouse mode analysis. It turns out
that the Rouse assumption of a uniform mobility is fulfilled to a good
approximation for the PEO melt. However, a more detailed analysis reveals that
the underlying microscopic dynamics is highly affected by different
contributions from intra- and intermolecular excluded volume interactions,
which cannot be taken into account by a modified friction coefficient. Minor
deviations occur only for the terminal monomers on larger time scales, which
can be attributed to the presence of two different escape mechanisms from their
first coordination sphere. These effects remain elusive when studying the
dynamics with the MSD only.
