Photomechanics of mono- and polydomain liquid crystal elastomer films

The coupling of optical and mechanical energy has recently been demonstrated in a class of liquid crystal elastomers that contain light-sensitive molecules. Their molecular structure consists of stiff, ordered, rod-like molecules (the liquid crystal feature) connected to long cross-linked molecular chains (the elastomer feature). This connection leads to a coupling between the orientational order of the rods and mechanical deformation of the network. Irradiation with light at appropriate wavelengths leads to isomerization of the stiff rod-like molecules, resulting in bending (straightening) of the molecules which dilutes (enhances) the orientational order, thus causing macroscopic deformation. This deformation, photostrain, can be as much as hundreds of percent. These materials are typically fabricated as thin monodomain or polydomain films. If uniformly irradiated through the thickness, they undergo anisotropic incompressible deformation in the film plane. If the light intensity varies through the thickness, then the photostrain also varies and the film bends. The nature of the deformation depends on, among other things, whether the film is a monodomain or polydomain, whether the light is polarized or not, and if so, in what direction. In all cases, molecules with axes aligned with the polarization component preferentially isomerize; the relationship between the molecular alignment and polarization determines the nature of the resulting strain. In this study, we analyze the deformation of such photoactuated thin films with anisotropic photostrains that are uniform or vary through the thickness. Building on existing work we incorporate the effects of geometric nonlinearity, while retaining the assumptions of isotropic linear elastic material behavior, to develop fairly simple analytical estimates of the overall deformation. By virtue of their simplicity these provide considerable insight into the deformation phenomena and at the same time are in good agreement with more accurate nonlinear finite element calculations. Consistent with reports in the literature, we find that large shape changes can occur, and that the effects of geometric nonlinearity can be significant. Our analysis explains the recently reported phenomena of photoinduced anisotropic saddle-shaped films [M. Camacho-Lopez, H. Finkelmann, P. Palffy-Muhoray, and M. Shelley, Nat. Mater. 3, 307 (2004)] and directionally-dependent bending of polydomain films [T. Ikeda, M. Nakano, Y. L. Yu, O. Tsutsumi, and A. Kanazawa, Adv. Mater. 15, 201 (2003)]. These results have somewhat broad applicability, including technological applications such as actuators, artificial muscles, and energy harvesting.