Nonlinear photomechanics of nematic networks: upscaling microscopic behaviour to macroscopic deformation

Abstract

A liquid crystal network whose chromophores are functionalized by photochromic dye exhibits light-induced mechanical behaviour. As a result, the micro-scaled thermotropic traits of the network and the macroscopic phase behaviour are both influenced as light alternates the shape of the dyes. In this paper, we present an analysis of this photomechanical behaviour based on the proposed multiscale framework, which incorporates the molecular details of microstate evolution into a continuum-based understanding. The effects of trans-to-cis photoisomerization driven by actinic light irradiation are first examined using molecular dynamics simulations, and are compared against the predictions of the classical dilution model; this reveals certain characteristics of mesogenic interaction upon isomerization, followed by changes in the polymeric structure. We then upscale the thermotropic phase-related information with the aid of a nonlinear finite element analysis; macroscopic deflection with respect to the wide ranges of temperature and actinic light intensity are thereby examined, which reveals that the classical model underestimates the true deformation. This work therefore provides measures for analysing photomechanics in general by bridging the gap between the micro- and macro-scales.

Multiresponse Shape-Memory Nanocomposite with a Reversible Cycle for Powerful Artificial Muscles

“…This trans-to-cis isomerization of azobenzene was modeled with molecular dynamics (MD) simulation . Then, by utilizing the micromechanics-based homogenization method, the effective elastic properties of elastomers were numerically derived with respect to the isomerization ratio, and the microscopic behavior was transduced to the macroscopic deformation . Finally, the macroscale photomechanical response of the composites was simulated by finite element analysis, where the effective mechanical properties were usually identified numerically by matching the FE model with the MD model.…”

Numerical Methods in Studies of Liquid Crystal Elastomers

“…Workflow of the multiscale model. At an instance in time (t), a photoisomerization ratio is calculated for given values of light intensity (I o ) and temperature (T); and then is used for carrying out MD simulations, which in turn will provide microscopic information to the nonlinear FEA [103] (Reprinted with permission from [103]).…”

“…(1) predict the way the mechanical response of LCEs with side-chain architectures depends on their chemical structure [60] (2) studying how phase transition from monodomain to polydomain occurs in LCEs on a molecular scale [8] (3) developed a multiscale framework for studying the photomechanical behavior of photo-responsive polymer networks [103] (4) developed a coarse-grained model consisting of mesogenic molecules and smeared charges for the study of Liquid crystal polymers (LCPs) [107] (5) studying which force field among the three, Deriding, PCFF, and SciPCFF potentials should be applied to LCE modeling system [110] Other numerical methods…”

“…Chung et al [103] presented a newly developed multiscale framework for studying the photomechanical behavior of photo-responsive polymer networks (PRPNs). Regarding the photomechanical behavior from the viewpoint of trans-to-cis photoisomerization, azobenzene-based acrylate side-chain PRPNs were the focus of this study [104].…”

Cyclic Photoisomerization of Azobenzene in Atomistic Simulations: Modeling the Effect of Light on Columnar Aggregates of Azo Stars

“…Out of multiple reported MD implementations of the photoisomerization reaction [39,42,43,[66][67][68][69][70][71][72][73], we employ the model of Heinz et al [42]. It has been used in various other simulation works [39,42,[73][74][75][76][77] and is consistent with the most important features of the isomerization of azobenzene. Furthermore, the model is directly compatible with a range of force fields, among them the DREIDING force field employed in this study.…”