On the Effects of Different trans and cis Populations in Azobenzene Liquid Crystal Elastomers: A Monte Carlo Investigation

We investigate main-chainliquid crystal elastomers (LCEs) formedby photoresponsive azobenzene units with different populations of trans and cis conformers (from fully trans to fully cis). We study their macroscopicproperties as well as their molecular organization using extensiveMonte Carlo simulations of a simple coarse-grained model where the trans and cis conformers are representedby soft-core biaxial Gay-Berne particles with size and interactionenergy parameters obtained by fitting a bare bone azobenzene moietyrepresented at atomistic level. We find that increasing the fractionof cis conformers, as could be obtained by near-UVirradiation, shifts the nematic-isotropic transition to a lowertemperature, consistently with experiment, while generating internalstress in a clamped sample. An analysis of pair distributions showsthat the immediate surroundings of a bent cis moleculeare slightly less dense and more orientationally disordered in comparisonwith that of a trans conformer. Comparing nematicand smectic LCEs, actuation in the smectic phase proved less effective,disrupting the smectic layers to some extent but preserving orientationalorder of the azobenzene moieties.