Azobenzene-Containing Liquid Single Crystal Elastomers for Photoresponsive Artificial Muscles

Abstract

Light is an endless, free and environmentally friendly energy source. Photoactive artificial muscle-like actuators are human-made materials capable to convert such luminous energy into motion or force. In fact, these materials try to mimic the abilities of biological muscles, which are responsible for locomotion in living organisms. Indeed, light-triggered artificial muscle-like actuators have experienced a current growing interest within materials science owing to their great potential applicability within actual technology. Specifically, liquid single crystal elastomers (LSCEs), which are macroscopically oriented polymer networks, have been proved to be invaluable materials for this purpose. LSCEs combine uniquely the molecular organization of liquid crystals with the elasticity of conventional rubbers. As a result, if light-sensitive molecules, such as azobenzenes, are somehow incorporated into the elastomeric network, the macroscopic dimensions of the whole material can be easily modified just by irradiating it with light of the appropriate wavelength, owing to the drastic geometrical change the azo chromophore suffers upon isomerisation. Two key parameters should be considered in the overall performance of artificial muscle-like actuators, namely, the maximum opto-mechanical response generated by the actuator and the time it requires to recover its initial dimensions. These two magnitudes are mainly controlled not only by the connectivity between the azo dye and the elastomeric network but also by the thermal isomerisation rate of the azo chromophore used. Hence, the main aim of the present chapter will be to afford the reader with a broad overview of the different strategies that have been developed during recent years to improve these two key parameters and, therefore, to achieve more efficient and fast-responding artificial muscle-like actuators.