Ion Transport in Dynamic Polymer Networks Based on Metal-Ligand Coordination: Effect of Cross-Linker Concentration

The development of high-performance ion conducting polymers requires a comprehensive multiscale understanding of the connection between ion-polymer associations, ionic conductivity, and polymer mechanics. We present polymer networks based on dynamic metal-ligand coordination as model systems to illustrate this relationship. The molecular design of these materials allows for precise and independent control over the nature and concentration of ligand and metal, which are molecular properties critical for bulk ion conduction and polymer mechanics. The model system investigated, inspired by polymerized ionic liquids, is composed of poly(ethylene oxide) with tethered imidazole moieties that facilitate dissociation upon incorporation of nickel(II) bis(trifluoromethylsulfonyl)imide. Nickel-imidazole interactions physically cross-link the polymer, increase the number of elastically active strands, and dramatically enhance the modulus. In addition, a maximum in ionic conductivity is observed due to the competing effects of increasing ion concentration and decreasing ion mobility upon network formation. The simultaneous enhancement of conducting and mechanical properties within a specific concentration regime demonstrates a promising pathway for the development of mechanically robust ion conducting polymers.