Effect of cross-linking density on photoinduced bending behavior of oriented liquid-crystalline network films containing azobenzene

Oriented liquid-crystalline network films with various cross-linking densities were prepared by polymerization of mixtures of mono- and diacrylates, both of which contain azobenzene chromophores. The optical anisotropy in the films was evaluated by polarizing optical microscopy and polarized IR spectroscopy. The dichroic ratios and the order parameters of the films were estimated from polarized UV absorbance. The free-standing films were found to show photoinduced bending and unbending behavior upon alternate exposure to unpolarized UV light at 366 nm and unpolarized visible light at > 540 nm. It was observed that the films with different cross-linking densities exhibited different bending extents and speeds.