Light and thermal responses of liquid-crystal-network films: A finite element study

Abstract

As a polymeric system incorporating rigid molecules within its structure, the liquid-crystal network (LCN) has been envisaged as a novel heterogeneous material. Under the influence of external stimuli, the orientational order of the liquid-crystalline phase becomes dilute and overall anisotropy is hence decreased; the actinic light absorbed by photochromic molecules, for example, induces the geometric isomerization and subsequently yields internal stress within the local network. In this study we investigate light- and temperature-induced spontaneous deformations of the LCN structure via a three-dimensional finite element model that incorporates geometric nonlinearity with a photomechanical constitutive model. We first examine the bending behavior and its nonlinearity and then parametrically study the various behaviors that stem from different origins ranging from the microscale to the macroscale: (i) the geometry of the LCN film, (ii) the macroscopic global order, (iii) the distorted mesogenic orientation due to the Fredericks distortion, and (iv) defect-induced instability. These interrelated behaviors demonstrate both the simulation capability and the necessity of the presenting framework. By employing a nonlinear consideration along with a microscopic shape parameter r the present approach facilitates further understanding of photomechanical physics such as the deconvolution of various stimuli and the deformed shape obtained due to snap-through instability. Furthermore, this study may offer insight into the design of light-sensitive actuation systems by deepening our knowledge and providing an efficient measure.

Modeling Defects, Shape Evolution, and Programmed Auto-Origami in Liquid Crystal Elastomers

“…While less computationally efficient than a two-dimensional shell model (Chung et al, 2015), our fully three-dimensional model can describe material systems with director or crosslink density gradients across the sample thickness. With GPU implementation we can achieve speeds of up to 100 time steps per CPU second for a mesh of more than 10 5 elements.…”

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

“…4 . All classical results present hereafter (marked as “Dilute”) are obtained using the previous finite element study 31 with constant thermotropic parameters ( α = 1.4, ζ = 0.33, and through MD simulations under non-irradiation conditions ( n cis = 0). Additionally, the order-clearing temperature is assumed to decrease linearly according to the following formula: , where β = 88.8.…”

“…At the other side of the analysis, small-scale simulations 22 23 24 25 have demonstrated that thermomechanical phase phenomena are related to the light-induced effects, and that they correlate well with the observed light-induced motions. Although a few recent studies have suggested connections between changes in nematic order and large-scale behaviour 26 27 28 29 30 31 , these works still resort to classical modelling by relying on Landau-de Gennes coupling along with the linear dilute model. Accordingly, such approaches fail to take into account crucial molecular details regarding light irradiation.…”

Level-set topology optimization considering nonlinear thermoelasticity

“…In such cases, nonlinear thermoelasticity must be considered, meaning that the infinitesimal strain should be replaced with nonlinear strain; otherwise, displacement and stress states obtained by structural analysis are highly overestimated due to the neglected stress-stiffening effect, and these inaccurate variables potentially lead to non-optimal structures [28]. Such an inaccuracy is exacerbated when temperature change is involved [27,29] as the thermoelastic behavior incorporates the temperature-induced deformation that in effect changes the stress-free state with respect to the undeformed state [30]. Therefore, it has been concluded that incorporation of the nonlinearity in the thermoelastic topology optimization is significant in getting the optimal design when mechanical and thermal loads are applied to the structure [27].…”