Mesoscale Photomechanical Coupling in Photoactive Liquid Crystal Elastomers

Some molecules can absorb light of a certain wavelength and change their shape, dissociate, or combine to form new molecules. Embedding these molecules into a rubbery liquid crystalline polymer network enables photoactive liquid crystal elastomers (LCEs) that generate large deformation and decent amount of mechanical work upon light illumination. Photomechanical actuation of photoactive LCEs has attracted increasing attention in recent years due to its wireless and fast energy transmission, rich tunability, capability of micromachines, and delivery of high energy density. New applications include photo-responsive robots, metamaterials, motors, optical waveguides, fibers, and other light-modulated devices. However, compared to an individual photoactive molecule, most existing macroscopic photoactive LCEs perform much poorer in their actuation performance (e.g., work output, thermodynamic efficiency, deformation) by orders of magnitude, severely limiting their application in real working scenarios. This contrast highlights an urgent research need for mechanistic understanding of photomechanics in these materials at the mesoscale (e.g., micrometer) that bridges a nanoscale molecule and a macroscale material. This talk will present our recent progress in such fundamental understanding of mesoscale photomechanical coupling. Incorporating the single molecule photoreaction and the long-range directional molecular interaction into a continuum theoretical framework, we investigate multiple interesting phenomena including tunable molecular alignment, photomechanical phase transformation and material instability, formation of microstructure, temperature-modulated photo-actuation, and their consequences in macroscopic applications.