Molecular Design of Precise Network Polymerized Ionic Liquids for Improved Understanding of Soft Actuators

Crosslinked network polymerized ionic liquids (n-PILs), with tethered imidazolium cations and mobile bis(trifluoromethane sulfonimide) (TFSI) anions, were investigated as dopant-free (no water or free ionic liquid) soft actuators by sandwiching between flexible electrodes and applying 3V, either AC or DC. These networks are made via a modular synthetic route such that they have tunable polarity and modulus through the choice of either hydrocarbon (HC) or ethylene oxide (EO) monomers and crosslinking density. The T_g is not substantially affected, and the role of conductivity, crosslinking, and modulus can be probed to understand how they systematically affect actuation. Under DC conditions, where conductivity is not a factor, the lower modulus (E) allows for greater bending strain with a non-linear 1/E relation. Under 0.1 Hz AC potentials, the conductivity is important and couples with modulus to affect beam displacement. The choice of EO monomers and low crosslink density lead to the best performance due to high conductivity and low modulus. Because the n-PILs are hydrophobic (< 1 wt% water uptake under ambient conditions), they can be operated over a thousand cycles at 3 V with minimal degradation of performance, as leaching of ionic liquids or water splitting are not an issue.