  By applying local primitive path and Rouse modes analysis we study the chains
conformations, local dynamics and viscosity of a model polymer melt in a
polymer-wall interface. We establish that the presence of a repulsive wall
leads to acceleration of the dynamics both for unentangled and weakly entangled
melts and to a depletion in the entanglement density in the wall vicinity. When
the surface bears some grafted chains, we show that the melt chains are
accelerated in the unentangled regime and slowed down in the entangled regime.
By analyzing the primitive paths we attribute the observed slowdown to an
increase in the entanglement density in the interfacial layer. The presence of
a relatively small density of grafting sites thus leads to improved mechanical
properties (reinforcement) and decreases locally the entanglement length even
if the surface is repulsive.
