Nonspherical Uniaxial Azobenzene Polymer Particles and Their Shape Changes under UV- or White-Light Irradiation for Stimuli-Response Applications

In this study, we developed azobenzene polymer particles showing several light-induced shape changes. Conventional azobenzene particles require exposure to polarized white or blue light on a substrate for shape elongation. First, the dispersion polymerization of 6-[4-(4'-methoxyphenylazo)phenoxy]hexyl methacrylate afforded unique oval or short cylindrical shapes with controlled sizes and narrow size distributions. Then, polarized optical microscopic observations and wide-angle X-ray diffraction measurements suggest that the azobenzene moiety formed a uniaxially orientated smectic-layered structure in the particles. Differential scanning calorimetric measurements indicated that the packing structures and particle shapes were closely related to each other. Finally, the particle shape was altered by the trans-cis photoisomerization of azobenzene accompanied by the deformation of the azobenzene packing structure. Interestingly, their shape anisotropy was increased by ultraviolet and white-light irradiation. The particle shape was changed in a dispersed medium and on a substrate by unpolarized light. Consequently, the combination of a light source and site varied the light-induced shape changes of the azobenzene particles.