Multifunctional flexible sensors based on ionogel composed entirely of ionic liquid with long alkyl chains for enhancing mechanical properties

Abstract

Ionogel, with high transparency, excellent mechanical properties, and notable recoverability is an up-and-coming material for making skin-like iontronic devices. Herein, a pure ionic liquid design strategy is proposed for preparing tough ionogel material by one-pot copolymerization of two polymerizable ionic liquids, 1-vinyl-3-butylimidazolium tetrafluoroborate ([VBIm][BF4]) and 1-dodecyl-3-vinylimidazolium tetrafluoroborate ([C12VIm][BF4]), in a non-polymerizable ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF4]). Experiments and theoretical simulations confirm the important role of an ionic liquid-phobic “microdomain” made-up of the long alkyl chains of [C12VIm]+ in the improvement of mechanical properties. Due to a synergistic effect of the ionic liquid-phobic microdomains, ionic bond interactions, and molecular entanglement, the resultant ionic-liquid-based ionogels (IL-PILs-IG) show good mechanical properties (tensile stress ca. 4.5 MPa, tensile modulus ca. 3.2 MPa), excellent transparency (91%), high thermal stability (>300 °C), and good recoverability. Several IL-PILs-IG-based sensors are further developed. IL-PILs-IG-based resistive-type sensors show diverse sensory capabilities towards temperature, strain, and humidity. Capacitive-type sensors based on IL-PILs-IG exhibited a low detection limit as low as 25 Pa, and showed no deviation over 1,000 loading/unloading cycles. This study might provide a new path for designing and fabricating ionogels with good mechanical properties, transparency, and excellent fatigue resilience for skin-inspired ionic conductive sensors in detecting complex human motions.