Opto-mechanical behavior of photochromic liquid crystal polymer film composites

Light induced bending of photochromic liquid crystal polymer (LCP) thin films sandwiched with some transparent polymer substrates is studied. Kirchhoff plate model is analyzed. We found that under the irradiation of light with suitable wavelength, an effective bending moment of the composite film will be produced and has two contributions, the opto moment and the bi-film moment. The opto moment is due to the inhomogeneous distribution of the light induced contraction resulted from the light absorption decay. It acts as a bending moment for single layered LCP film as well and has been studied by many authors. The bi-film moment, on the hand, is resulted from the homogeneous contraction induced by light and will act only to the composite similar to the well-known bi-metal bending. By utilizing the two contributions, the magnitude of the total effective moment and the bending curvature can be enhanced largely. Moreover, the bending direction can be well controlled by suitable design of the multilayered structures. The variational formulation of the bending equation is derived, according to which any commercial finite element code can be used to simulate the light induced bending behavior under various boundary conditions. Some interesting bending patterns of the film composites are shown to be possibly produced by patterned layer structures.