Bulk and inhomogeneous mixtures of hard rods and excluded-volume polymer: a density functional approach

We present a density functional theory for a mixture of hard rods and polymer modeled as chains of hard tangent spheres which refines the theory proposed in the paper by Phys. Rev. E 68, 062501 (2003)]. The improvement involves a semiempirical formula for the contact value of the sphere-sphere radial distribution function of the sphere and needle reference system, which includes the important depletion effect induced by the needles. The new functional yields slightly broader phase coexistence envelopes but the changes affect mainly the polymer-rich binodal branches. After analyzing the bulk phase behavior the structure of hard-rod-polymer mixture close to a hard wall is examined. An increase of the chain length leads to an increase of the average polymer segment contact value. This behavior may lead to a qualitative difference of the polymer segment profiles: from an effective repulsion of the polymer segments to an effective attraction, which can be observed by a change of sign of the excess adsorption. By analyzing the orientational order parameter profiles we have found that the polymer coils decrease the tendency of needles to adopt anisotropic configurations.