The body of work on polymerized ionic liquids has been growing rapidly in recent years as researchers expand the synthesis space to achieve novel membrane materials with high conductivity, excellent mechanical stability, and high transference number. Despite progress in identifying specific new polymers and useful properties, there has been limited agreement over the mechanism for ion transport in these materials. It is essential that we resolve said mechanism for polymerized-ionic-liquid conduction, with the goal of streamlining future material design. Molecular dynamics is an excellent tool for analyzing local coordination behavior, ion-hopping pathways, and other phenomena of length- and time-scales that are currently inaccessible to direct experimental observation.
Ion transport is seen to proceed via a "climbing the ladder" mechanism involving the formation and breaking of ion-association pairs with, on average, four polymerized ions from two polymer chains. This results in a link between ion-association lifetime and diffusivity for chemically similar polymerized ionic liquids, a feature that distinguishes polymerized ionic liquids from a broad class of polymer electrolytes and low fragility ionomers. This is also shown to be the case for a set of backbone-polymerized ionic liquids, when compared to a chemically similar pendent-polymerized ionic liquid. This is particularly interesting because the pendent architectural motif proves to have significantly higher reversibility of ion-hopping events. The application of design rules inspired by this research has already led to the experimental discovery of highly decoupled polymerized ionic liquids with excellent conductivity at ambient temperature.
Parametric simulation studies of poly(vinylimidazolium) polymerized ionic liquids and counterion variants have revealed a decoupling of ion mobility from polymer segmental dynamics. Small counterions are generally more decoupled, but results show that size is not the sole arbiter. For this set of different chemical components, encompassed by the anionic study, ion-association relaxation time, rather than lifetime, was proven to better correlate with diffusivity. Similar physics is observed between polymerized ionic liquids and salt-doped polymerized zwitterions for the population of mobile ions whose polymerized counter-charge is located on the end of a monomeric pendant. However, the cage-relaxation timescale appears to correlate better with diffusivity for the opposite ion in such materials.Chemical Engineerin
