Entanglement Reduction and Anisotropic Chain and Primitive Path Conformations in Polymer Melts under Thin Film and Cylindrical Confinement

We simulate and theoretically analyze the properties of entangled polymer melts confined in thin film and cylindrical geometries. Macromolecular-scale conformational changes are observed in our simulations: the average end-to-end vector is reduced normal to the confining surfaces and slightly extended parallel to them, and we find that the orientational distribution of the chain end-to-end vectors is transmitted to the primitive path entanglement strand level. Treating the chains as ideal random walks and the surfaces via a reflecting boundary condition we are able to accurately theoretically predict the anisotropic global and primitive-path level conformational changes. Combining this result with a recently developed microscopic theory for the dependence of the tube diameter on orientational order allows a priori predictions of how the number of entanglements decreases with confinement in a geometry-dependent manner. The theoretical results are in excellent agreement with our simulations.