I wound like to take part in the session of "Active Materials".Abstract:&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Photoactive nematic elastomers are soft rubbery solids that undergo deformation when illuminated. They are made by incorporating photoactive molecules like azobenzene into nematic liquid crystal elastomers. Since its initial demonstration, it has received increasing interest with many recent studies of periodic and buckling behavior. Theoretical models have been developed for photochemistry induced deformation in liquid crystal elastomers (LCEs). However, most of them have been focused on describing specific actuation modes (e.g., beam bending and uniaxial contraction). The light actuation is subsequently treated using a 1D thermomechanical model, where the effective temperature increase depends on the intensity of light. Many more continuum theories were developed based on this analogy of temperature effect, essentially treating the photomechanical deformation as an effective thermal expansion. Another group of theoretical models focus on bending, due to the limited penetration depth of light into a LCE in most applications. While these models predict the material response under specified loading conditions of interest and largely study free deformation with no applied mechanical loads, they do not describe detailed multiphysical interaction in the material during actuation. This lack of microscopic details hinders the opportunity of discovering new actuation modes, or exploring material behaviors under more complex conditions. Besides these models of specific actuation modes, theories combining statistical mechanics and continuum mechanics were also developed. Despite this effort, an important question has yet been answered: how does mechanical stress affect the light-induced deformation in a LCE? A full coupling between light illumination and mechanical stress, as well as the resulting photomechanical response of the material, is still largely unexplored.&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; In this talk, we will explore photomechanical coupling in a photoactive LCE under both light illumination and mechanical stress. We begin with a continuum framework built on free energy. We focus on the photo-stationary state where the system reaches equilibrium. Through energy minimization with homogeneous deformation, we study ground states and show that the nematic director tends to align perpendicular to the polarization of a linearly polarized light. We also study the competition between illumination and stress since the director aligns parallel to any applied uniaxial stress. We demonstrate such photo-alignment and mechano-alignment, and the induced large deformation in a sheet of nematic LCE under both light illumination and mechanical stress. The transition between these two modes induces a nonzero shear strain, giving rise to formation of stripe domains in the material. Finally, we show the first-order nematic-isotropic phase transformation as the light intensity increases, and a corresponding snap-through instability in the deformation of the material. The phase diagram under various light intensity and mechanical stress is plotted, and a critical point is identified.

Photomechanical Coupling in Photoactive Nematic Elastomers

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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