Photo-activated dynamic isomerization induced large density changes in liquid crystal polymers: A molecular dynamics study

TL;DR

This study uses molecular dynamics simulations to explore how light-induced trans-cis-trans isomerization cycles in azo-mesogens within liquid crystal polymers cause significant density changes, aligning with experimental findings.

Contribution

It introduces cyclic and probabilistic simulation approaches to elucidate the physics of light-induced density changes in liquid crystal polymers due to dynamic isomerization.

Findings

Dynamic isomerization leads to density changes at specific switch intervals.

Optimal isomerization probabilities maximize density reduction.

Simulations match experimental observations of non-monotonous density responses.

Abstract

We use molecular dynamics simulations to unravel the physics underpinning the light-induced density changes caused by the dynamic trans-cis-trans isomerization cycles of azo-mesogens embedded in a liquid crystal polymer network, an intriguing experimental observation reported in the literature. We employ two approaches, cyclic and probabilistic switching of isomers, to simulate dynamic isomerization. The cyclic switching of isomers confirms that dynamic isomerization can lead to density changes at specific switch-time intervals. The probabilistic switching approach further deciphers the physics behind the non-monotonous relation between density reduction and light intensities observed in experiments. Light intensity variations in experiments are accounted for in simulations by varying the trans-to-cis and cis-to-trans isomerization probabilities. The simulations show that an optimal combination of these two probabilities results in a maximum density reduction, corroborating the experimental observations. At such an optimal combination of probabilities, the dynamic trans-cis-trans isomerization cycles occur at a specific frequency, causing significant distortion in the polymer network, resulting in a maximum density reduction.