S4E1

Abstract

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 solid enables various photoactive materials ranging from amorphous polymers to densely packed crystals, which generate large deformation or mechanical work upon light illumination. Photomechanical actuation 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 materials perform much poorer in their actuation efficiency and work output by orders of magnitude, severely limiting their application in real working scenarios. This contrast highlights an urgent research need for mechanistic understanding of photomechanics 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. Using statistical and continuum theoretical frameworks, we investigate various photoactive materials including liquid crystal elastomers, semicrystalline polymers, and molecular crystals. When embedded in a solid, photoactive molecules do not react independently, but behave collectively through the long-range interaction between themselves and with the solid matrix. As a result, the actuation involves a three-way photomechanical coupling between light propagation, photoreaction, and mechanics, where one process greatly affects the others. Such collective behaviors give rise to multiple interesting phenomena including tunable molecular alignment, kinetics and equilibrium of photomechanical phase transformation, formation of microstructure, instability, and their consequences in the macroscopic material actuation.