Nanoindentation study of light-induced softening of supramolecular and covalently functionalized azo polymers

Abstract

Nanoindentation studies on thin films of the widely used azo polymer pDR1A and a supramolecular polymer–azobenzene complex p4VP(DY7)0.5 demonstrate significant light-induced softening upon visible-light irradiation due to trans–cis–trans photoisomerization of the azobenzene units. More specifically, the strain-rate sensitivities of pDR1A and p4VP(DY7)0.5 upon 532 nm irradiation increase by 80% and 120%, respectively. These results imply a photosoftening contribution to the mechanisms of light-induced surface patterning of azo polymers and the photomechanical effect. The finding that under the experimental conditions used photosoftening is more significant in the supramolecular complex than in the covalently functionalized polymer highlights the potential of noncovalent functionalization strategies in designing materials with efficient photomechanical response, and nanoindentation provides a powerful technique to quantify the connection between the photoinduced changes in mechanical properties and photoinduced macroscopic movement of azo polymer films.