Reversible Actuation of Polyelectrolyte Films: Expansion-Induced Mechanical Force Enables <i>cis–trans</i> Isomerization of Azobenzenes

Abstract

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) (PAH) produces azobenzene-containing PAZO–PAA/PAH films. UV irradiation induces <i>trans–cis</i> 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 <i>cis–trans</i> 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 <i>cis–trans</i> back isomerization of azobenzenes and restore the bent actuators to their original configuration. The capability of the force produced by film expansion for <i>cis–trans</i> azobenzene isomerization can be helpful for designing novel polymeric actuators