UV-isomerisation in nematic elastomers as a route to photo-mechanical transducer

TL;DR

This study investigates how UV-induced isomerisation of azobenzene groups in nematic elastomers can be used to control their shape and mechanical response, highlighting temperature-dependent dynamics and the role of nematic order.

Contribution

It demonstrates the large photo-mechanical response of azobenzene-containing nematic elastomers and clarifies that their relaxation is governed by nematic order dynamics rather than polymer network relaxation.

Findings

Response time depends on temperature and is generally long.

Macroscopic relaxation is driven by nematic order dynamics.

The material exhibits repeatable photo-mechanical behavior.

Abstract

The macroscopic shape of liquid crystalline elastomers strongly depends on the order parameter of the mesogenic groups. This order can be manipulated if photoisomerisable groups, e.g. containing N=N bonds, are introduced into the material. We have explored the large photo-mechanical response of such an azobenzene-containing nematic elastomer at different temperatures, using force and optical birefringence measurements, and focusing on fundamental aspects of population dynamics and the related speed and repeatability of the response. The characteristic time of ``on'' and ``off'' regimes strongly depends on temperature, but is generally found to be very long. We were able to verify that the macroscopic relaxation of the elastomer is determined by the nematic order dynamics and not, for instance, by the polymer network relaxation.