  Star polymers can exhibit a heterogeneous dynamical behavior due to their
internal structure. In this work we employ atomistic molecular dynamics
simulations to study translational motion in non-entangled polystyrene and
poly(ethylene oxide) star-shaped melts. We focus on the local heterogeneous
dynamics originating from the multi-arm star-like architecture and quantify the
intramolecular dynamical gradient. By examining the translational motion at
length scales of the order of the Kuhn length, we aim to find common features
for both studied chemistries and to provide a critical and direct comparison
with theoretical models of polymer dynamics. We discuss the observed tendencies
with respect to the continuous Rouse model adjusted for the star-like
architectures. Two versions of the Rouse model are examined: one assuming
uniform friction on every Rouse bead and another one considering larger branch
point friction. Apart from chain connectivity between neighboring beads, both
versions disregard interactions between the chains. Despite the tolerable
description of the simulation data, neither model appears to reflect the
mobility gradient accurately. The detailed quantitative atomistic models
employed here bridge the gap between the theoretical and general,
coarse-granined models of star-like polymers which lack the indispensable
chemical details.
