Theoretical Analysis of Multiple Phase Coexistence in Polyelectrolyte Blends

Advanced materials for energy storage and process technology are globally in high demand. Promising components for such materials are polymer blends and copolymers, because of their ability to self-assemble into nanostructured materials that combine desirable material properties, such as high mechanical stability with ion-selective conductivity. It remains a challenge to predict the nanoscale structure in charged polymers, because of intricate ionic correlations that can influence the structure at many length scales. Here we present a free energy analysis of charged polymer blends with electrolytes of the primitive model, by combining a Flory-Huggins-type theory with liquid-state methods, relying on the Ornstein-Zernike equation. We find that different mechanisms, driven by entropy and electrostatics, can stimulate or repress phase separation in blends. These mechanisms can enable the existence of a triple point, where three phases can coexist in an effective two-component blend. We analyze the influence of chain length and electrostatic coupling strength, set by the ion type and dielectric properties of the polymer, on the coexistence lines and nanoscale structure within the blend. Our predictions may add resolution to the perspective on multiphase coexistence in blends of charged polymers, and the formation of nanoscale structure in ionic gels and ionomers such as Nafion. (Graph Presented).