Indirect Actuation of Thermally-Triggered Shape-Changing Effect

3 Indirect Actuation of Thermally-Triggered Shape-Changing Effect 

Incorporation of light-sensitive functional groups such as azobenzene or triph-enylmethane leuco derivatives enabled SCP with responsiveness towards light [153-158]. Here, the light-induced changes of shape were based on electrostatic repulsion, on photomechanical effects, or light-stimulated phase transitions. 

In contrast to triphenylmethane leuco derivatives, the backward reaction of azobenzene systems was photoreversible. Irradiation with light having wavelengths between 330 and 380 nm resulted in the trans —s- cis isomerization while a temperature increase or irradiation with light having wavelengths of more than 420 nm induced the reverse cis — trans isomerization reaction. 

The transfer of the geometrical change caused by photoisomerization on the molecular level to macroscopic shape changes was demonstrated in different concepts. While azo-dye loaded nylon filament fabrics showed shrinkage of approximately 0.1% after irradiation under load [162], the incorporation of azobenzene-containing crosslinkers in poly(ethyl acrylate) network films enhanced this photomechanical effect to 0.25% [163]. This is a significant difference to 

Additionally, the bending direction of azobenzene systems could be controlled by the orientation of the photoactive moieties [170]. While in a homogeneous alignment (parallel to the surface) the films bent away from the light source, in a homeotropic alignment (perpendicular to the surface) the films bent towards the irradiation direction [171]. When exposed to UV-light, the surface of the homogeneous film contracted and the load on the films increased while the surface of the homeotropic film expanded and the load on the film decreased. 

---