Ionic liquids are organic salts that are liquid at room temperature. Polymerizing the charged species in these materials dramatically restricts their mobility. While most applications of these materials rely on polymerization of either the cation or the anion, resulting in single-ion conductors with good mechanical properties, polymerizing both of the charged species can slow down the ion motion so much that the ions are effectively “locked” in place.
Working with collaborators in chemical engineering, we demonstrated that this “ion locking” effect can be used to prepare functional 2D electronic devices. By depositing the polymerizable ionic liquid onto a 2D semiconductor in its monomeric form, applying a voltage to drive ions to the interface, and then triggering polymerization before removing the voltage, we generated a persistent electric double layer at the interface that effectively doped the underlying semiconductor and created a p-i-n junction. We are now investigating how long the “locked” double layer persists for, and how this time can be tuned, in an effort to optimize performance of such devices.
For some applications, it may also be useful to be able to release the locked ion distribution on command. To this end, we have also developed polymerizable ionic liquid monomers containing thermally-labile Diels-Alder linkages. Like the materials described above, polymerizing these materials results in a dramatic drop in ion mobility that effectively locks the ions in place. Unlike our other materials, however, ion mobility can then be restored by cycling the polymer above the retro Diels-Alder temperature, which breaks the bonds between the polymer and the charged groups and releases the ions into the material.
As a physical chemistry group, however, we are interested not only in the device applications of these materials, but also in their fundamental physics. As such, we have begun characterizing the dynamics of ion motion in “ion-locked” polymerizable ionic liquids using broadband dielectric spectroscopy. Using this approach, we showed that the timescales for relaxation are more than four orders of magnitude slower in the materials in which both charged species are polymerized than when only one is, and that polymerizing both ionic species decouples global charge transport processes from local ionic rearrangements.
These experiments have allowed to begin to understand how the ion locking process in polymerized ionic liquids enables interesting device responses. Going forward, we look forward both to continuing to develop our fundamental understanding of these materials and to further development them for use in electronic devices and a wide range of other applications.
Our work on polymerized ionic liquids has been supported by the Air Force Office of Scientific Research under awards FA9550-19-1-0196 and FA9550-20-1-0403 (DURIP award funding acquisition of a broadband dielectric spectrometer).