Binary star-polymer solutions: bulk and interfacial properties

Abstract

Using an effective logarithmic–Gaussian pair potential that models theinteraction between star polymers, we compare the hypernetted chain(HNC) and random phase approximations (RPA) for calculating the bulkstructure (including the Fisher–Widom and Lifshitz lines), thermodynamicfunctions and phase diagram of a phase-separating binary fluid of starpolymers, of two-arm length ratio 2:1. Thereby, the stars considered hereare equivalent to linear chains in the mid-point representation of theireffective interaction. We find that at densities where the star coronasoverlap, the quasi-exact HNC and RPA give very similar results. Using adensity functional approach, with a functional which generates the RPA, wecalculate properties of the inhomogeneous binary fluid. We determinethe surface tension and one-body density profiles at the free fluid–fluidinterface. For states well removed from the critical point the profiles exhibitpronounced oscillations. For a purely repulsive planar wall potential thatmodels the effective potential between a star polymer and a hard wall, wefind a first-order wetting transition with the associated pre-wetting line.