Mechanics and Ion Transport in Dynamic Polymer Networks based on Metal-Ligand Coordination in Polymeric Ionic Liquids

Polymers that contain ionic liquid constituent retain many of the properties of ionic liquids including ionic conductivity. Further, the polymerized cation (such as imidazolium) forms transient ligand bond interactions with dissolved metal ions allowing them to conduct while also forming a dynamic network. Dynamic polymer networks based on metal-ligand coordination are promising materials to accomplish such decoupling due to the transient nature of the coordination interaction. The general system of interest comprises a metal salt mixed in a polymeric medium with ligands located either along the backbone or on pendant side-chains. The molecular design of these materials allows for precise and independent control over the nature and concentration of ligand and metal, the salt dissolution, and the binding energy between the ligand and metal, all of which shown to be critical for controlling bulk ion conduction and polymer mechanics. Salt dissociation is universally governed by coordination number, equilibrium constant, and initial salt and ligand concentrations. Salt dissociation is enhanced by larger equilibrium constants and higher cation valency. The sensitivity of the ionic conductivity on equilibrium constant, coordination number, and the ratio of cation to anion diffusion coefficients is much higher for monovalent salts compared to divalent or trivalent salts. In a model system composed of poly(ethylene oxide) with tethered imidazole moieties that facilitate salt dissociation of both nickel (II) bis(trifluoromethylsulfonyl)imide (NiTFSI) and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), the nickel-imidazole interactions physically crosslink the polymer, increase the number of elastically active strands, and dramatically enhance the modulus while allowing Li+ to conduct quickly through the matrix.