Dynamics of Ion Locking in Doubly-Polymerized Ionic Liquids

In this work, we investigate the dynamics of ion motion in a "doubly-polymerized" ionic liquid (DPIL) in which both charged species of an ionic liquid are covalently linked to the same polymer chains. Broad-band dielectric spectroscopy is used to characterize these materials over a broad frequency and temperature range, and their behavior is compared to that of conventional "singly-polymerized" ionic liquids (SPILs) in which only one of the charged species is attached to the polymer chains. Polymerization of the DPIL decreases the bulk ionic conductivity by 4 orders of magnitude relative to both SPILs. The time scales for local ionic rearrangement are similarly found to be approximately 4 orders of magnitude slower in the DPIL than in the SPILs, and the DPIL also has a lower static dielectric constant. These results suggest that copolymerization of the ionic monomers affects ion motion on both the bulk and the local scales, with ion pairs serving to form strong physical cross-links between the polymer chains. This study provides quantitative insight into the energetics and time scales of ion motion that drive the phenomenon of "ion locking" currently under investigation for new classes of organic electronics.