Photoinduced Deformation of Rigid Azobenzene-Containing Polymer Networks

Photoresponsive poly­(amide acid) (PAA) gels containing multiple azobenzene units in a rigid aromatic backbone were synthesized. A centimeter-long cantilever made up of the photoresponsive PAA gel exhibited reversible bending motions upon blue (442 nm) and visible light (>490 nm) irradiation. The network structure in the PAA gels during alternating photoirradiation of blue and visible light was characterized using <i>in situ</i> scanning microscopic dynamic light scattering (SMILS), which revealed reversible mesh-size changes synchronized with the photoisomerization of azobenzene moieties. The photomechanical responses of the PAA gel were likely due to photoinduced contracting and stretching motions of the polymer backbone. A numerical calculation of photon absorptions revealed that photoisomerization in a very thin layer of the surface (∼40 μm) generated large macroscopic motion and large strain in the gel cantilever. The photoresponsive capability is, however, reduced or eliminated when the PAA gels are transformed to the corresponding polyimide (PI) gels, due to the large shrinkage caused by poor solubility of the backbone in the polyimide state