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 f ixed by appeal to RPA (random phase approximation). We present instead mode l nonlocal functional in which each polymer is replaced by deterministic, p enetrable particle of known shape. This reproduces the RPA and square gradi ent theories in the small deviation and/or weak gradient limits, and can re adily be extended to polydisperse chains. The utility of the new functional is shown for the case of polydisperse polymer solution at coexistence in p oor solvent.