A practical density functional for polydisperse polymers

The Flory-Huggins equation of state for monodisperse polymers can be turned into density functional by adding square gradient term, with coefficient fixed by appeal to RPA (random phase approximation). We present instead model nonlocal functional in which each polymer is replaced by deterministic, penetrable particle of known shape. This reproduces the RPA and square gradient theories in the small deviation and/or weak gradient limits, and can readily be extended to polydisperse chains. The utility of the new functional is shown for the case of polydisperse polymer solution at coexistence in poor solvent.