Viscoelastic properties and ion dynamics in star-shaped polymerized ionic liquids

The viscoelastic and dielectric response of linear and star-shaped polymeric ionic liquids (PolyILs) based on imidazolium/bis(trifluoromethane)sulfonimide are probed over a broad range of frequencies and temperatures and are evaluated in connection to the polymer architecture and morphology. At longer timescales, the arm dynamics of star PolyILs are shown to be more sluggish and elastic relative to those of linear chains of comparable size. Arm retraction delays terminal relaxation in the melt, resulting in an enhanced zero-shear viscosity that increases exponentially with the arm length. Yet at shorter timescales, long-chain branching is shown to have only secondary effects on the segmental and ion motions. That is, star PolyIL melts are found to have a lower viscoelastic fragility, more frustrated secondary β fluctuations, and—for stars with the shortest arms—higher activation energies for ion hopping at low temperatures. Even so, all PolyILs ultimately exhibit nearly similar rates of segmental relaxation, ion disassociation, and dc conductivity over the investigated temperature range, regardless of polymer architecture including arm length. This study thus demonstrates how the disparity between chain, segmental, and ion dynamics in branched PolyILs can be exploited in the assembly of functional polymer electrolyte materials with divergent morphologies and controlled viscoelasticity properties, both can be varied independently of ion transport.