Decoupling Mechanical and Conductive Dynamics of Polymeric Ionic Liquids via a Trivalent Anion Additive

The mechanical and conductive properties of a polymeric ionic liquid (PIL) are decoupled through the addition of a fraction of trivalent anions to a chloride single-ion conductor. Trivalent phosphate ions strongly coordinate with polymer-bound imidazoliums, producing an increase in both the ionic conductivity and the polymer viscosity. Both the viscosity and the ionic conductivity increase with phosphate content, and the conductivity is superior to that of the neat PIL at larger trivalent anion concentrations. The interaggregate spacing (determined by X-ray scattering), glass transition temperature (measured by calorimetry), and free volume (estimated by rheology) are each sensitive to the presence of trivalent ions but not to changes in the phosphate concentration. Thus, the presence of a fraction of trivalent anions qualitatively changes the structure and interaction of ions, resulting in modified macroscopic properties of the PIL. We hypothesize that this step change in properties upon introducing phosphate ions is due to a densification of ion aggregates by the trivalent ion, which strongly binds to imidazolium ions. This provides a new mechanism for creating PILs with tailored conductive and rheological behavior.