  The shear rate dependence of the slip length in thin polymer films confined
between atomically flat surfaces is investigated by molecular dynamics
simulations. The polymer melt is described by the bead-spring model of linear
flexible chains. We found that at low shear rates the velocity profiles acquire
a pronounced curvature near the wall and the absolute value of the negative
slip length is approximately equal to thickness of the viscous interfacial
layer. At higher shear rates, the velocity profiles become linear and the slip
length increases rapidly as a function of shear rate. The gradual transition
from no-slip to steady-state slip flow is associated with faster relaxation of
the polymer chains near the wall evaluated from decay of the time
autocorrelation function of the first normal mode. We also show that at high
melt densities the friction coefficient at the interface between the polymer
melt and the solid wall follows power law decay as a function of the slip
velocity. At large slip velocities the friction coefficient is determined by
the product of the surface induced peak in the structure factor, temperature
and the contact density of the first fluid layer near the solid wall.
