The recently reported opto-mechanical effect of some photochromic liquid crystal elastomers (LCEs) is studied. It is found that in such LCEs, material parameters such as the Young's modulus and the stress-free strains will become nonhomogeneous under light irradiations. One may call them the light-induced functionally gradient materials. Analytical expressions for the dependence of the material parameters on the space variable and possibly on the time variable are obtained. Exponential dependence can be derived under certain approximations. As an example, the light-induced bending of a beam is studied. Two neutral planes are found in the beam. Thus, along the thickness of the beam, there are extensions in the upper and lower parts and contractions in the middle.Keywords: opto-mechanical effect, material nonhomogeneity, functionally gradient materials, light-induced bending.Liquid crystal elastomers are cross-linked polymeric liquid crystalline (LC) solid materials. They combine rubber elasticity and liquid crystal properties [1] . Upon cooling from the isotropic phase to a certain critical temperature, the LC molecules will transform from a randomly oriented state to a more ordered state such as the nematic phase in which the LC molecules align themselves in a certain average direction, called the director n. The critical temperature is then called the nematic-isotropic (NI) transition temperature T ni [2] . During the transition, the cross-linked polymer network formed by prolate LC polymers will extend along the direction n. The extension can be well fitted as ni niwhere α and ξ are positive constants. From this formula, it can be found that the deformation of LCEs can be induced not only in the heating-cooling process but also by manipulating the NI transition temperature T ni . The latter can be indeed achieved by photoisomerization of certain photochromic LCEs [3] . Light-induced contraction of LCE by 20%
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