Photo-induced changes in azobenzene-containing soft materials

Photocontrol of molecular alignment is an exceptionally-intelligent and useful strategy. It enables us to control optical coefficients, peripheral molecular alignments, surface relief structure, and actuation of substances by means of photoirradiation. Epoxy linear polymers with azobenzene and alkyl side groups were synthesized and studied. They showed interesting and variable photo-response upon visible-light irradiation due to trans-cis-trans photoisomerization of the azobenzene units. The influence of physical crosslinks between alkyl chains on optical and mechanical properties was also investigated. Mechanical characterization of the polymers was done by nanoindentation. Light induced reversible or irreversible changes in the polymer after being irradiated with visible light, depend on the polarization states of light used. When using circularly polarized light, reversible mechanical response is observed. Permanent structural changes in films are related to anisotropic alignment of azobenzene molecules (after being illuminated with linear polarized light) and/or physical crosslinks of alkyl chains. This study demonstrated the importance of the polymer architecture and the presence of domains of alkyl chains on the potential photo responsiveness.