Temperature Dependence of the Slip Length in Polymer Melts at Attractive Surfaces

Using Couette and Poiseuille flows, we extract the temperature dependence of the slip length, $\delta$, from molecular dynamics simulations of a coarse-grained polymer model in contact with an attractive surface. $\delta$ is dictated by the ratio of bulk viscosity and surface mobility. At weakly attractive surfaces, lubrication layers form; $\delta$ is large and increases upon cooling. Close to the glass transition temperature $T_g$, very large slip lengths are observed. At a more attractive surface, a sticky surface layer is built up, giving rise to small slip lengths. Upon cooling, $\delta$ decreases at high temperatures, passes through a minimum, and grows for $T\to T_g$. At strongly attractive surfaces, the Navier-slip condition fails to describe Couette and Poiseuille flows simultaneously. The simulations are corroborated by a schematic, two-layer model suggesting that the observations do not depend on details of the computational model.