Ion transport and softening in a polymerized ionic liquid

Abstract

Polymerized ionic liquids retain mechanical properties of regular polymers and exhibit conductive properties of the room temperature ionic liquids. As such, they are an important class of materials for practical applications in various solid state electrical devices. Additional these materials are excellent model systems for understanding intricate coupling between electrostatics and crowding effects. In this study, we examine the local charge transport and structural changes in films of a polymerized ionic liquid (PolyIL) are studied using an integrated experiment-theory based approach. Experimental data for the kinetics of charging and steady state current-voltage relations can be explained by taking into account the dissociation of ions under an applied electric field (known as the Wien effect). Onsager’s theory of the Wien effect coupled with the Poisson-Nernst-Planck formalism for the charge transport is found to be in excellent agreement with the experimental results. The agreement between the theory and experiments allows us to predict structural properties of the PolyIL films. For example, we have observed significant softening of the PolyIL films beyond cer- tain threshold voltages and formation of holes under a scanning probe microscopy (SPM) tip through which electric field was applied. The observed softening is explained by theory of melting point depression resulting from enhanced dissociation of ions with an increase in applied electric field.