Experimental Studies of the Mechanisms of Photomechanical Effects in a Nematic Liquid Crystal Elastomer in a Photomechanical Optical Device Geometry

TL;DR

This study investigates the photomechanical mechanisms in azo-dye-doped liquid crystal elastomers, revealing that photothermal heating primarily drives their length changes, which is crucial for designing smart actuating devices.

Contribution

The paper provides experimental evidence that photothermal heating is the main mechanism behind photomechanical effects in surface-constrained LCEs, advancing understanding of their microscopic behavior.

Findings

Photothermal heating causes length change in LCEs.

Absorption occurs mainly near the surface at high dye densities.

Thermal diffusion contributes to nonlocal strain.

Abstract

Azo-dye-doped liquid crystal elastomers (LCEs) are known to show a strong photomechanical response. We report on experiments that suggest that photothermal heating is the underlying mechanism in surface-constrained geometry. In particular, we use optical interferometry to probe the length change of the material and direct temperature measurements to determine heating. LCEs with various dopants and optical density were used to study the individual mechanisms. In the high dye-doped limit, most of the light is absorbed near the entry surface, which causes a local strain from photothermal heating and a nonlocal strain from thermal diffusion. The results of our research on the microscopic mechanisms of the photomechanical response can be applied to designing photomechanical materials for actuating/sensing devices, the potential basis of smart structures.