Reversible Actuation of Polyelectrolyte Films: Expansion-Induced Mechanical Force Enables cis-trans Isomerization of Azobenzenes

Fabrication of light-driven actuators that can prolong their deformation without constant irradiation poses a challenge. This study shows the preparation of polymeric actuators that are capable of reversible bending/unbending movements and prolonging their bending deformation without UV irradiation by releasing thermally cross-linked azobenzene-containing polyelectrolyte films with a limited free volume from substrates. Layer-by-layer assembly of poly{1-4[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl sodium salt} (PAZO)-poly(acrylic acid) (PAA) complexes (noted as PAZO=PAA) with poly(allylamine hydrochloride) (PAR) produces azobenzene-containing PAZO-PAA/PAH films. UV irradiation induces trans-cis isomerization of azobenzenes and allows large-scale bending deformation of the actuators. The actuators prolong the bending deformation even under visible light irradiation because the cis-trans back isomerization of azobenzenes is inhibited by the limited free volume in the actuators. Unbending of actuators is attained by exposing the actuators to a humid environment at room temperature. Film expansion in a humid environment produces a mechanical force that is sufficiently strong to enable the cis-trans back isomerization of azobenzenes and restore the bent actuators to their original configuration. The capability of the force produced by film expansion for cis-trans azobenzene isomerization can be helpful for designing novel polymeric actuators.