Photomechanics of blanket and patterned liquid crystal elastomer films

Recent studies have demonstrated the coupling of optical and mechanical energy in liquid crystal elastomers that contain light-sensitive molecules. The molecular structure of these materials consists of stiff. ordered, rod-like molecules that are connected to long, crosslinked molecular chains, resulting in a coupling between the orientational order of the rods and mechanical deformation of the network. Irradiation with light at a specific wavelength causes the stiff rod-like molecules to bend which dilutes the orientational order and thus causes deformation. Irradiation at a different specific wavelength then reverses the process by straightening the rod-like molecules which enhances orientational order and causes deformation. When unconstrained this deformation can be considered a stress-free eigen-strain and we call it photostrain; it can vary from a few to hundreds of percent. When the material exists as a plate-like film with nematic order, it will undergo an incompressible expansion or contraction in the film plane if it is uniformly irradiated through its thickness. However, if the light intensity varies with position, either through the thickness or within the plane of the film, it will undergo more complex deformation due to the spatial variation of the photostrain. Here we study the deformation of thin-film plates with photostrains and build on existing work by incorporating the effects of geometric nonlinearity, while retaining the assumptions of isotropic linear elastic material behavior. We develop fairly simple analytical estimates of the overall deformation of photo-actuated liquid crystal elastomer plates. and also carry out more accurate nonlinear finite element simulations. As shown experimentally in the literature, large shape changes can occur, and geometric nonlinearity effects can be significant. Finally, we use a recently-developed topology optimization approach to design irradiation patterns in the plane of the film that will lead to specified target shapes by taking advantage of geometric nonlinearity and photostrain anisotropy. Spatial patterning of light intensity, which can potentially be realized using various existing optical techniques, opens the door to the realization of a rich set of photomechanical materials and structures. (C) 2009 Elsevier Ltd. All rights reserved.