Local dynamics and primitive path analysis for a model polymer melt near a surface

Using molecular dynamics simulations, we apply primitive path and local Rouse modes analysis to study the chain conformations and the local dynamics and viscosity of a model polymer melt near a flat wall and a wall presenting some bonding sites. The presence of the flat wall leads to acceleration of the dynamics both for unentangled and weakly entangled melts and to a depletion in the entanglement density in the vicinity of the wall. When the surface bears 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 show that the observed slowing down in presence of grafted chains is due to an increase in the entanglement density in the interfacial layer. In contrast, for a bare surface we observe a depletion of the entanglements at the interface. The presence of a relatively small density of grafting sites leads to a reinforcement of the entanglement network locally increasing the entanglement density.