Polyelectrolyte Blends and Nontrivial Behavior in Effective Flory–Huggins Parameters

The Flory χ-parameter is a ubiquitous description of the extent of immiscibility that is apparent between two or more polymeric species. While the formalism is a powerful one in most systems of technological interest, experimentally obtaining this parameter requires the assumption of an underlying theoretical model. For charged systems, mapping to analogous uncharged systems is often assumed by introducing an “effective χ”, χ eff . Random phase approximation (RPA) analysis based on the Hamiltonian used for recent self-consistent field theory–liquid-state theories (SCFT–LS) demonstrates that χ eff incorporates molecular-level details such as charge ordering. Even for simple polyelectrolyte blends where the bare χ is kept constant, the observed χ eff will drastically change as a function of composition; prediction of heterogeneous polyelectrolyte material phase behavior using χ eff is thus highly nontrivial since an understanding of local charge structure is required.