Heavily Crosslinked Polymerized Ionic Liquid (PIL) Networks for High-Performance Ionic Diodes and Low-Voltage Electroadhesives

Heterojunctions of oppositely charged, crosslinked polymerized ionic liquids (PILs) have attracted recent attention as solvent-free ionic diodes and electroadhesives. However, the mechanism of operation for such devices has been poorly understood, limiting the rational design of materials for PIL heterojunction-based electronics. Recently, we found that speeding up bulk PIL dynamics reduced ionic diode performance by limiting the ability of the interface to block charges under reverse bias. As such, we reasoned that reducing bulk dynamics by heavily crosslinking PILs would improve diode rectification performance. As such, we developed dianionic and dicationic ‘gemini’-style diacrylate crosslinkers that can form polycationic and polyanionic PIL networks with crosslink densities ranging from 2 mol% to 100 mol% crosslinker. This enables decreased bulk dynamics and increased moduli by simply increasing crosslinker content; all without significant changes in charge density. As anticipated, diodes made with high crosslink density PILs outperformed those consisting of loosely crosslinked material; with up to a four order of magnitude decrease in interfacial resistance between small reverse (-0.15 V) and forward (+0.25 V) biases. Moreover, highly crosslinked PIL networks exhibited enhanced electroadhesive switchability (i.e. the ratio of ‘on’ to ‘off’ –state adhesive force), indicating the applicability of this approach to different types of devices. In addition to the advancements in PIL network synthesis, these findings highlight the important role of crosslink density on ionic diode and low-voltage electroadhesive performance through the lens of polymer dynamics.