Thermal Torsion Effect of Twisted Polymer Actuators

Twisted polymer artificial muscles activated by thermal heating represent a new class of soft actuators capable of generating torsional actuation. The thermal torsion effect, characterized by the reversible untwisting of twisted fibers as temperature increases due to greater radial than axial thermal expansion, is crucial to the actuation performance of these artificial muscles. This study explores the thermal torsion effect of polymer muscles made of twisted Nylon 6 fibers in experimental and theoretical aspects, focusing on the interplay between material properties and temperature. It is revealed that the thermal torsion effect enhances the actuation performance of the twisted polymer actuator while the thermal softening effect diminishes it. A thermal–mechanical model incorporating both the thermal torsion effect and thermal softening effect is used to predict the recovered torque of the twisted polymer actuators. An optimal bias angle and operating temperature are identified to maximize the recovered torque. Analysis of strain and stress distributions in the cross-section of the twisted polymer fiber shows that the outer layers of the fiber predominantly contribute to the torsional actuation. This work aids in the precise control and structural optimization of the thermally-activated twisted polymer actuators.