Supramolecular photoresponsive polyurethane with movable crosslinks based on photoisomerization of azobenzene

Light-driven actuators are widely used for smart devices such as soft robots. One of the main challenges for actuators is achieving rapid responsiveness, in addition to ensuring favorable mechanical properties. Herein, we focused on photoresponsive polyurethane (CD-Azo-PU) based on controlling the crystallization of the hard segments in polyurethane (PU) by complexation between azobenzene (Azo) and cyclodextrins (CDs). CD-Azo-PU incorporated polyurethane as the main chain and a 1:2 inclusion complex between Azo and gamma CD as a movable crosslink point. Upon ultraviolet light (UV, lambda = 365 nm) irradiation, the photoresponsiveness of CD-Azo-PU bent toward the light source (defined as positive), while that of the linear Azo polyurethane (Azo-LPU) without peracetylated gamma-cyclodextrin diol (TAc gamma CD-diOH) as a movable crosslinker bent in the direction opposite the light source. The bending rates were determined to be 0.25 degrees/s for CD-Azo-PU and 0.083 degrees/s for Azo-LPU, indicating that the bending rate for CD-Azo-PU was faster than that for Azo-LPU. By incorporating movable crosslinks into CD-Azo-PU, we successfully achieved specific photoresponsive actuation with an enhanced rate. Supramolecular photoresponsive polyurethane (CD-Azo-PU) with movable crosslinks between azobenzene (Azo) and gamma-cyclodextrins (CD) exhibits rapid photoresponsive actuation irradiated by UV light (lambda = 365 nm). The films of CD-Azo-PU bend towards the light source. Photostimuli increase crystallinity via the formation of hydrogen-bond driven by the dissociation of the CD/Azo complexes.image