Dynamic hydrogen bonding induced hygroscopicity of ionogel enabling lubricating regulation

Stimuli-responsive hydrogels with tunable lubrication have aroused attention for the development intelligent lubricating materials. However, the limited bearing capacity and the decaying of lubrication induced by water evaporation have hindered the broad application of hydrogels in the practical fields. Here, we present an ionogel composite that combines the hygroscopic ionogel with 3D printed polymer substrates to achieve a humidity-responsive friction behavior enabled by dynamic hydrogen bonding (HB). The ionogel composed of the ionic liquid (IL) 1-Ethyl-3-methylimidazolium acetate ([EMIM][OAc]) and poly(N-isopropylacrylamide) (PNIPAM), exhibited the moisture absorption induced stiffening and swelling, which is attributed to the reconfiguration of the polymer network through dynamic HB. The material shows a large change in tension strength (as much as 3.8 times) and Young's modulus (as much as 83 times). Moreover, the hygroscopic ionogel composites demonstrate a stimuli-responsive friction behavior, with the coefficient of the friction (COF varying from 0.07 to 0.70) depending on relative humidity, hygroscopic time, and 3D printing patterns. This behavior can be attributed to the combined effects of modulus and roughness changes. Our strategy introduces the hygroscopic IL for the stimuli-responsive polymer for the realizing of water-induced stiffening and provides a design to access the smart material with lubrication regulation.