A Smart Self-Healing Material with Reversible Optical, Mechanical, and Electrical Transition Induced by Humidity and Temperature

Abstract

Smart responsive materials that can alter their function in response to environmental changes are attractive for their potential applications in intelligent devices and products. Herein, a smart material that exhibits reversible changes in multiple properties upon variations in humidity or temperature is created. The material spontaneously transits between hydrated and dehydrated states in response to fluctuations in the surrounding humidity or temperature. Consisting of a mixture of poly(propylene glycol) (PPG) with urea linkages (PPGurea) and ionic liquid [EMIM][TFSI], the transition is attributed to a series of synergetic interactions among various chemical components and groups, including ether-cation coordination, water-anion complex, urea-urea bidentate hydrogen bonds, and cation–anion electrostatic interactions. In the hydrated state, with a very small amount (4–5 wt%) of spontaneously absorbed moisture content, the smart material is soft, transparent, and conductive, and possesses rapid self-healing ability. Upon dehydration, the material transits into a phase-separated system with PPG-rich and IL-rich phases, resulting in opacity, severely reduced ionic conductivity, yet significantly enhanced stiffness, strength, and toughness. The drastic change in multiple properties makes it an intelligent material well-suited for various smart applications such as sensors, 3D printed optoelectronics and smart windows, which can automatically alter their functions to adapt to environmental changes.