  We perform molecular dynamics simulations of an idealized polymer melt
surrounding a nanoscopic filler particle to probe the effects of a filler on
the local melt structure and dynamics. We show that the glass transition
temperature $T_g$ of the melt can be shifted to either higher or lower
temperatures by appropriately tuning the interactions between polymer and
filler. A gradual change of the polymer dynamics approaching the filler surface
causes the change in the glass transition. We also find that while the bulk
structure of the polymers changes little, the polymers close to the surface
tend to be elongated and flattened, independent of the type of interaction we
study. Consequently, the dynamics appear strongly influenced by the
interactions, while the melt structure is only altered by the geometric
constraints imposed by the presence of the filler. Our findings show a strong
similarity to those obtained for ultra-thin polymer films (thickness $\lesssim
100$ nm) suggesting that both ultra-thin films and filled-polymer systems might
be understood in the same context.
