Photo-isomerization effect of the azobenzene chain on the opto-mechanical behavior of nematic polymer: A molecular dynamics study

Abstract

The opto-mechanical properties of a photo-responsive nematic polymer network (PRPN) are investigated using molecular dynamics simulation. For the implementation of the trans-to-cis isomerization of azo compounds, a switchable potential formalism for the N = N bond is applied to the crosslinked PRPN unit cell model. During the light switch-on and heating-up simulations at a wide range of temperatures, the scalar orientational order parameter for the mesogenic side group molecules, the effective photo-induced strain of the bulk polymer network, and the opto-mechanical properties are characterized. The correlation between the microstate which belongs to the molecular location and the macroscopically observed photostrain is identified according to the isomerization ratio of the diazene groups.

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

“…The multiscale framework presented in this work, which consists of micro-scale simulations and a scale-bridging method, incorporates information on microstate evolution and macro-scale deformation. Below we present a detailed description of the framework, although we omit certain equations and simulation conditions that can be found in the references 23 31 .…”

“…We model the molecular unit cell of the acrylate-based PRPN, a polymeric structure with many mesogenic constituents, following the cell construction scheme suggested by Choi et al 23 , which incorporates energetic relaxation and multi-step crosslinking. The molar crosslinking ratio is set to 7:1 45 , and energetic interactions and equilibrated polymeric conformations are computed via a photoactive potential 23 24 46 . The dihedral parameters of the azodye, as well as the potential coefficients of peripheral atoms, surrogate those of the conventional polymer consistent force field (PCFF).…”

“…the non-convexity of the energy landscape 12 13 14 , the dynamics of stress evolution and diverse stress-accommodating shape change (such as bent elastica 15 16 17 18 19 20 21 ), etc. 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.…”

“…These thermotropic parameters reflect detailed molecular effects that are not captured by experimental observations; the parameters are extracted by averaging across wide windows of both time and temperature. The MD simulations used herein are largely indebted to recent work 23 regarding how an increase in the cis- population is correlated with photo-induced cell shrinkage and order collapse in general; however, this macroscopic viewpoint fails to grasp key microscopic information, as was evident in the case of the experimental approach. We therefore focus on extracting the modulated thermomechanical phase behaviours, which reflect changes in the long-range mesogenic interactions in the presence of short-range atomic interactions.…”

Enhanced Deformation of Azobenzene-Modified Liquid Crystal Polymers under Dual Wavelength Exposure: A Photophysical Model

“…This actuation can be reversed by exposing the material to visible (VIS) light or heat, which accelerate the cis azobenzenes to fall back to the trans state. The current consensus in experimental [5,[7][8][9] and theoretical studies [10][11][12][13] is that the amplitude of the photoinduced deformation is related to the statistical building up of molecules in the cis state. However, a recent experimental study [14] has revealed that this is only one side of the story.…”