Thermally Generated Isometric Stress Performances of Polydomain and Monodomain Nematic Elastomers

Nematic liquid-crystalline elastomers (NLCEs) have potential applications as soft actuators because of their ability to undergo large deformations or generate stress by phase transitions. However, the mechanical properties of the materials required to design their actual applications remain limited. In this study, isometric stresses generated via temperature changes on polydomain (PD) and monodomain (MD) NLCEs composed of the same components were evaluated at different strains, and their performances were compared. The results confirmed that the generated stress increased with the applied uniaxial strain. This implicates that the slope in the stress–strain response curve, i.e., the differential coefficient with respect to strain, increases with temperature at fixed strain. In terms of the true stress and strain units, the increasing trend of the PD and MD liquid-crystal elastomers (LCEs) did not differ significantly, which can be ascribed to similar thermodynamic state of both LCEs with similar effective cross-link densities and degree of liquid crystalline order in their stretched state. In MD, the deformation during the phase transition is expected to be higher owing to the initial alignment; therefore, the generated stress is also higher, at first glance. However, this study revealed that the creation method (genesis) did not make a significant qualitative difference when using the stresses generated in the stretched state. On the other hand, in the case of PD and MD materials of exactly the same dimensions, the ranges of useable strain, stress and generated stress are apparently very different, and this variety is useful as application choices. In addition, the evaluation of strain dependence on generated stress and the data analysis method for true stress employed in this study may be beneficial for understanding and enhancing the performance of polymer actuators in general, beyond just NLCEs.