Ion Correlation-Induced Phase Separation in Polyelectrolyte Blends

Inhomogeneous polyelectrolyte materials have been of both longstanding and recent interest; polymer blends exhibit technologically advantageous properties for adhesives and fuel cell membranes and serve as an ideal model system to study more complicated behaviors in polyelectrolyte materials. However, the physics governing the phase behavior of polyelectrolyte blends remains poorly understood. Traditional self-consistent field theory (SCFT) can include Coulombic interactions that arise in polyelectrolytes but can only reproduce Poisson-Boltzmann behavior or perturbations thereof due to the mean-field nature of the SCFT calculation. Recently, tools have been developed to couple SCFT with liquid state (LS) integral equation theory, which can calculate ion correlations in a quantitative fashion. This permits the articulation of ion effects in very low dielectric epsilon(r) constant regimes that are relevant to polymer blends in nonaqueous conditions. We show that the inclusion of local ion correlations can give rise to marked enhancement of phase separation, contrary to theories invoking the Poisson-Boltzmann approximation, even to the extent of driving phase separation when two polymers are fully miscible (chi N = 0). We provide both a demonstration of this effect as well as a conceptual explanation.