First steps in modeling thermal actuation of twisted polymer actuators using virgin material properties

Artificial muscle systems have the potential to impact many technologies ranging from advanced prosthesis to miniature robotics. Recently, it has been shown that twisting drawn polymer fibers such as nylon can result in torsional or tensile actuators depending on the final fiber configuration. The actuation phenomenon relies on the anisotropic nature of the fibers moduli and thermal expansion. They have high axial stiffness, low shear stiffness, and expand more radially when heated than axially. If a polymer fiber is twisted but not coiled, these characteristics result in a torsional actuator that will untwist when heated. During the fabrication process, these twisted polymers can be configured helically before annealing. In this configuration, the untwisting that occurs in a straight twisted fiber results in a contraction or extension depending on relative directions of twist and coiling. In these ways, these materials can be used to create both torsional or axial actuators with extremely high specific work capabilities. To date, the focus of research on twisted polymer actuators (TPAs) and twisted-coiled polymer actuators (TCPAs) has been actuator characterization that demonstrates the technologies capabilities. Our work focuses here on applying a 2D analysis of individual layers of the TPAs to predict thermally induced twisting angle and fiber length based on virgin (untwisted) material properties and actuator parameters like fiber length and inserted twist. A multi-axis rheometer with a controlled thermal environmental chamber was used to twist, anneal, and test thermally induced actuation. Experimentally measured angle of untwist and axial contraction after heating are compare the the model. In comparing the experimental results with the two dimensional model, it appears that the difference between the 2D model and experimental results can be explained by the longitudinal stresses that develop inside the material. Future work will aim to include these effects in the model in order to be able to use this model in the design of TPAs.