Light-induced mechanical response in crosslinked liquid-crystalline polymers with photoswitchable glass transition temperatures

Youfeng Yue,Yasuo Norikane,Reiko Azumi,Emiko Koyama

doi:10.1038/s41467-018-05744-x

Abstract

Energy conversion of light into mechanical work is of fundamental interest in applications. In particular, diligent molecular design on nanoscale, in order to achieve efficient photomechanical effects on macroscopic scale, has become one of the most interesting study topics. Here, by incorporating a “photomelting” azobenzene monomer crosslinked into liquid crystalline (LC) networks, we generate photoresponsive polymer films that exhibit reversible photoswitchable glass transition temperatures (Tg) at room temperature (~20 °C) and photomechanical actuations under the stimulus of UV/visible light. The trans-to-cis isomerization of azo chromophores results in a change in Tg of the crosslinked LC polymers. The Tg of the polymer network is higher than room temperature in the trans-form and lower than room temperature in the cis-form. We demonstrate the photoswitchable Tg contribute to the photomechanical bending and a new mechanism for photomechanical bending that attributes the process to an inhomogeneous change in Tg of the film is proposed.

Highlights

Energy conversion of light into mechanical work is of fundamental interest in applications
We develop a photoresponsive film based on a liquid crystalline (LC) network using a polymerizable meta-methylazobenzene (M-azo) as the photoresponsive moiety that was covalently crosslinked into a dodecyl glyceryl itaconate (DGI) polymer network (Fig. 1)
We found that the Tg of the LC polymer network decreases after UV absorption; as soon as the UV irradiation is removed, the polymer network recovers its Tg at room temperature upon exposure to visible light or in the dark

Summary

Introduction

Energy conversion of light into mechanical work is of fundamental interest in applications. The free-standing LC film shows a change in its glass transition temperature and reversible photomechanical motions upon exposure to UV/visible light. The trans-to-cis photoisomerization of azo that crosslinked in the polymer network results in a photoswitchable Tg. We demonstrate the reversible change in the Tg of the polymer network contribute to the photomechanical response (e.g. bending) in LC film.

Results

Conclusion