Single-Material Solvent-Sensitive Actuator From Poly(Ionic Liquid) Inverse Opals Based On Gradient Wetting/Dewetting

In nature, many organisms respond to water in the environment or solvent for actuation. Typically, the seed dispersal units of pinecones, wheat awns and seedpods remain in their closed state when on the tree or under a wet environment, and they are able to open and release the seeds as they dry out. Inspired by these smart organisms, various stimulus actuators have been fabricated from a passive-active bilayer structure, with responsive materials as the active layer combined with the passive material as the substrate. These smart materials are able to generate mechanical work under external stimuli, such as light, electricity, moisture, solvent, magnetism and heat, with potential applications in microrobotic, artificial muscles, and energy generators. Recently, a type of simple actuator derived from gradient or single materials has stimulated great research interest based on the inhomogeneous swelling/shrinking within the films by efficiently overcoming the stress restriction of the bilayer-actuator. It is therefore essential to develop simple, economy sensing material/devices.In this study, we demonstrated a single-material solvent-sensitive actuator based on a gradient wetting/dewetting process. The sample was fabricated from a single-material of poly(ionic liquid)(PIL) inverse opals, a type of excellent candidate for actuator materials, since they possess bulk organic counter anions and can interact with a variety of organic solvents. Furthermore, the porous structure of inverse opals gives rise to a fast responsiveness by accelerating the internal mass transport and large-scale actuation arising from their high compressibility. The as-prepared PIL actuator demonstrates a bending angle of 14°in 4 s. The actuator magnitude/rate of the PIL inverse opals can be modulated by the solvent type and PIL composition, which can be attributed to the gradient wetting/ dewetting behavior of the solvents on the PILs combined with the strong hydrogen bonding interactions between PIL and the solvent. The fabrication of a single-material bending actuator reported herein will give an important insight for the design and manufacture of novel and advanced solvent-actuators.