A photo-mechanical-nematic order coupling viscoelastic model for photoactive nematic liquid crystal elastomers

Accurate modeling of photo-induced deformation in photoactive liquid crystal elastomers (LCEs) requires capturing both their intrinsic time-dependent viscoelastic properties and the nematic order coupling between the liquid crystal (LC) director and order parameters. To address these requirements, a comprehensive photo-mechanical-nematic order coupling viscoelastic model is established. Within the framework of continuum mechanics, specific forms of the free energy functions that encompass the elastic potential energy of the crosslinked network and the free energy of the nematic mixture were formulated based on the neo-classical model. The validity and predictive accuracy of the model were verified by experiments. Distinct from existing models, which are typically limited to describing either LC mesogens reorientation or photo-isomerization in isolation, our model uniquely enables the simultaneous characterization of these two processes and their intricate coupling interactions, filling a key gap in the current literature. Additionally, by applying external mechanical loads and accounting for LC director rotation, our model successfully reproduces the semisoft elasticity of LCEs observed in tensile experiments. Furthermore, numerical examples are presented to investigate the time-dependent deformation of LCEs with different viscoelastic parameters under varying light intensities, offering a comprehensive understanding of how these factors regulate the time-dependent deformation of LCEs. This work lays a theoretical foundation for the development of novel actuation modes and advanced applications of photoactive LCEs.