Soft actuators based on the flexible MOF MIL-88B(Fe) with a fast response to various organic solvent vapours

Abstract

Solvent vapor-driven soft actuators are promising devices for human–environment interaction and have gained considerable attention in recent years. To enhance the interaction with the solvents and deliver a fast response and superior sensitivity, polymer/MOF composites are promising candidates for the fabrication of solvent-driven actuators. This paper presents a multiple-solvent-driven intelligent MOF–polymer soft actuator, namely MIL-88B/PVDF asymmetric film, and compares it to analogs based on other MIL-88(Fe) series flexible MOFs. This comparison investigates the deformation–structure relationship and the solvent-driven mechanism. The deformation of the MIL-88(Fe)/PVDF composite films in response to various organic solvents has been studied, and the structure and composition of the MOF fillers and the composites were characterized by SEM, FT-IR, and XRD. Furthermore, the MIL-88B/PVDF composite, which shows ability to respond to multiple solvents and a fast deformation response to several organic solvents through the solvent swelling/adsorption of the MOF skeleton and the appropriate flexibility and group affinity from the terephthalic acid ligand, was used to fabricate an inverted opal gradient actuator composed of MIL-88B, carbon black (CB) and PVDF, namely MIL-88B/CB/PVDF. The MIL-88B/CB/PVDF film not only realizes excellent dual-responsive deformation and discoloration to methanol vapor with fast visible deformation within 1 s with a maximum crimp angle of up to 558° (10 vol% concentration) and a wide color change range covering six viewing-angle-dependent high-saturation colors (red, orange, yellow, green, blue and purple) but also shows an approximate linear relationship between the maximum curl angle and the solvent concentration for three solvents (methanol, toluene and chloroform). In addition, several smart devices have also been fabricated, including methanol-driven bionic petals, a worm robot, an alertor and a LIG sensor. Therefore, such polymer/MOF soft actuators have great potential in bio-robots and this work provides a new design strategy for smart materials.