4D-Printing of Photoswitchable Actuators*

Liquid crystal elastomers (LCEs) modified with a photoisomerizable chromophore can be designed to undergo reversible shape change in response to light. The return of the original shape is dictated by the chromophore isomerization kinetics. To decouple isomerization kinetics in the LCE from macroscopic shape change, a photoresponsive LCE has been synthesized with dynamic covalent crosslinking (DA), supramolecular crosslinking (UPy), and azobenzene. Dynamic covalent crosslinking of the LCE enables 3D printing to be used to process the material, which enables the patterning of shape change. During light-driven actuation, UPy crosslinks break and undergo rearrangement to lock the shape change. Locking decouples the spontaneous cis-trans isomerization of azobenzene from the shape change of the material. This material can be reversibly switched from one state to another and used in either state for indefinite periods. After UV irradiation, a LCE with 30% of the repeat units functionalized with UPy and 20% of the repeat units having a DA crosslink is able to fix between 80-95% of the shape change for more than 3 days. The initial shape can be recovered by heating to break UPy crosslinks. Moreover, this shape switching behavior can be repeatedly observed over at least 10 cycles.