Multiscale Study of the Relationship between Photoisomerization and Mechanical Behavior of Azo-Polymer Based on the Coarse-Grained Molecular Dynamics Simulation

Cross-linked liquid crystalline polymers (CLCPs) incorporated with photochromic azobenzene moieties can exhibit large and reversible deformations under the exposure of actinic lights. To practically utilize CLCPs in applications such as flexible photoresponsive actuators and soft robots, the user-defined diverse deformation modes have been demonstrated by controlling the liquid crystalline phases and polymer architectures. However, conventional all-atom molecular dynamics (AA MD) simulation is unsuitable for modeling complicated phases and conducting a parametric study of the photomechanical effect in terms of the polymer morphology due to its excessive computational cost. Therefore, a multiscale analysis framework based on coarse-grained molecular dynamics (CG MD) simulations is developed in this study. The mesoscale CLCP network model is constructed to satisfy the structural and thermodynamic properties of the full-atomistic reference. Also, the photoisomerization process is operated by integrating multiscale phenomena as follows. The AA MD reference describes the quantum events by implementing reactive rotational potential for trans-to-cis conversion, and corresponding CG potential sets are derived based on the associated structural changes. Because the mesoscale photoswitching potential is transferable to a wide range of cis-concentrations, the light-induced sequential-smectic-nematic-isotropic phase transition is realized. The results show that corruptions of the smectic layer and nematic orientational order reversely influence the overall shape of the network, which causes different photodeformation modes under each transition. Furthermore, the light-induced disordering behaviors are combined with the mechanical deformations of the polymer according to various light and temperature conditions. We expect the presented work contributes to the precise prediction and design of the optomechanical behaviors of photoresponsive devices.