Optimal Conditions for Efficient Photomechanical Response in Nematic Elastomers

Photoresponsive nematic liquid crystal elastomers (LCEs)containingazobenzene units have potential applications in light-controlled softactuators. Photo-isomerization of the trans to bent cis azobenzeneform disturbs the nematic order, modulating the mechanical propertiesbetween soft and entropic elasticity. Controlling and optimizing themodulation range are critical for the application of LCEs. In thisstudy, by investigating a series of photo-LCEs with various phasetransition temperatures, which affect the correlation between thescalar nematic local order parameter Q and the mechanicalproperties, we explore how the thermodynamic properties and operatingconditions affect the efficiency of conversion of light energy tomechanical changes. The generation of photostress or photostrain underconstant strain or stress, respectively, is maximized when the operatingtemperature is close to the nematic-isotopic transition temperatureof the cis-rich state. Under these conditions, the light-induced changein Q is also maximized, which is further supportedby a simple model that considers the light-induced shift of the nonlinearcurve of the temperature-dependent Q along the temperatureaxis. The present results provide useful guidelines for optimizingthe materials and operating conditions for the efficient conversionof light energy into mechanical work in photo-LCEs.