Testing the Photo/Piezo Actuation of Azo-Containing Polymers with Supramolecular Structures for Applications as Wireless Actuators

A polyimide-based supramolecular system including an azo component, namely, 4-[(4-cyanophenyl)diazenyl]phenol, was tested regarding its photomechanical and piezoelectrical behavior. According to theoretical calculations, the polymer system exhibits a significant internal free volume (15.7%), suggesting a molecular rearrangement capacity that can sustain the trans – cis isomerization. Photoactuation tests performed under moderate intensities of an unpolarized light from XeCl excimer laser irradiation revealed a peak bending angle of 24° after 7 min at 30 mJ/cm 2 . When the laser fluence was doubled, the bending angle increased to 34° within 30 s and eventually exceeded 60° after 7 min. They induced only physical changes in the molecular structure of the polyimide, with no chemical modifications being detected. Porous, ordered structures with depths of ∼700 nm and 1.2 μm, similar to “honeycomb” formations, appeared on the surface of the photoactuated films, compared to the initial relatively uniform surface, increasing their roughness. The evolution of the texture parameters was an indicator of the formation of 3D spatial structures with high complexity due to physical phenomena that occurred during laser irradiation, such as photoinduced structuring, localized thermal expansion and internal stress, or Marangoni currents. The piezoelectric constant d 33 of the pristine sample attributed to the presence of highly polarizable −CN groups within both the main chain and the azo component was improved after the photoactuation tests, facilitated by the orientation of the azo group dipoles and a slight relaxation of internal stresses. These properties recommend this multifunctional material for applications in smart devices as wireless actuators.