The Role of Multivalent Ions on the Mechanics and Ionic Conductivity of Metal-Ligand Coordinating Polymers

Decoupling mechanical properties and ionic conductivity in conventional ion conducting polymers is challenging due to the highly correlated nature of ion motion and segmental chain dynamics. Polymeric ionic liquids (PILs) formed via metal-ligand coordination interactions present a promising pathway towards this goal. This work explores the effect of the nature and concentration of metal cations and ligands in PILs on both mechanical properties and ionic conductivity. Rheological analysis of the addition of metal salts into a ligand-containing polymer suggests the formation of temporary networks. Impedance spectroscopy reveals comparable conductivities for monovalent, divalent and trivalent salts, suggesting conductivity is governed by the ratio of cations to ligands rather than valency, total ion concentration, or strength of the metal-ligand interaction. Pulse-field-gradient NMR diffusion measurements enable a comparison of measured conductivity to conductivity calculated from the Nernst-Einstein equation assuming full salt dissociation. These results suggest a contribution to the conductivity from cation species in the multivalent salt systems.