Reprocessable and healable room temperature photoactuators based on a main-chain azobenzene liquid crystalline poly(ester-urea)

The development of reprocessable and healable room temperature photoactuators from a main-chain azobenzene (azo) liquid crystalline poly(ester-urea) (PEU) is described for the first time. The azo PEU was prepared via Michael addition polymerization of an ester and urea unit-containing azo monomer with both acrylate and methacrylate end-groups and 1,2-ethanedithiol. It showed high thermal stability, low glass transition temperature, a broad range of a smectic liquid crystalline phase, reversible photoresponsivity, and dynamic hydrogen bonding interactions among urea groups. Its well-oriented fibers and films were prepared via the melt spinning method and special melt pressing approach, respectively, whose azo mesogenic alignment was along the fiber axes and homeotropic to the film surfaces, respectively. The fibers showed highly reversible UV light-induced bending (toward the light source) and visible light-induced unbending, whereas the films exhibited rapid and reversible bending (away from the light source) under UV irradiation alone together with the spontaneous quick unbending upon turning off the UV light (both at room temperature). The reasons for the special photomobile behaviors of the azo PEU films and the decisive role of the urea unit-induced physical crosslinking in their photomobility were disclosed. The successful fabrication of a remotely controlled electric circuit by using a photodeformable azo PEU film (with attached filter paper slips soaked with aqueous NaCl solution) as a photoswitch as well as the high reprocessability and healability of the azo PEU actuators was also demonstrated. Such a multifunctional main-chain azo PEU with good film-forming ability, room temperature photomobility, high reprocessability and healability holds much promise in various photoactuating applications.