Asymmetric Block Copolyelectrolyte Morphology Mapping Induced by Strong Electrostatics

A central goal in the design of polymer electrolyte membranes is to improve their integrity while maintaining high selective ion conduction. Here we develop a new class of copolymers with one block containing a cyclopropenium (CP) ion per monomer and show that they form either disordered structures or cylindrical morphologies with an ion-conducting matrix even when the charge block fraction is as low as 10 vol%. We attribute these results to the CP polyelectrolyte having a remarkably low static dielectric constant, ε_r = 2.2 D, resulting in very strong electrostatic interactions that dominate the microphase segregation behavior. We define a packing parameter that balances the range of ionic interactions against the size of the charge-neutral block. This treatment demonstrates the CP block has a positive spontaneous interfacial curvature for all investigated charge neutral macroinitiators and rationalizes the observed asymmetric morphology mapping. We stress that the morphology of materials in this limit of strong electrostatics are unusually beneficial for selective charge transport applications.