Bisphenol-A-polycarbonate is by far the most utilized and, therefore, intensively studied variety of polycarbonates. High impact strength, ductility, glass transition and melting temperatures make it a valuable material for industrial applications.
We use coarse-grained models to study confined melts of this polymer. We are interested in how the polydispersity of the melt changes its adhesion behavior and how the melt rearranges itself under shear.
We demonstrate that the adhesion behavior of a bisphenol-A polycarbonate melt to a (111) nickel surface changes significantly if a small amount of short chains is present, i.~e. in polydispersed melts or self-blends. Attractive interaction of the chain ends with the surface results in an adsorbed layer, made of single- and two-end attached chains. Shorter chains, however, diffuse from the bulk and occupy the adsorption sites much faster than the long ones. Interplay between the surface concentration of short chains, their molecular conformation and excluded volume results in a non-monotonic coverage of the surface by long chains. The smallest coverage is obtained for diphenyl carbonate, due to its high mobility and relatively large excluded volume. We propose that self-blending can be used to modify, in a controlled fashion, the friction coefficient of a melt sheared past a nickel surface.
We propose that several mechanisms contribute to friction in a polymer melt adsorbed at a structured surface. The first one is the well known disentanglement of bulk polymer chains from the surface layer. However, if the surface is ideal at the atomic scale, the adsorbed parts of polymer can move along the equipotential lines of the surface potential. This gives rise to a strong slippage of polymer melt. For high shear rates, the chains partially desorb; however, the friction force on adsorbed chains increases, resulting in quasi-stick boundary conditions. We propose that the surface-adsorbed layers can be efficiently used to adjust the friction force between the polymer melt and the surface.
Snapshots show a 240 chain melt of 20 repeat units for several shear rates. The chain ends adsorb on the surface, while the rest of the beads repell from it. Coloring illustrates division on the single-end adsorbed chains (red), two-end adsorbed chains (green), and chains with no adsorbed ends.