Moisture’s significant impact on twisted polymer actuation

Abstract

It was recently shown that inexpensive drawn polymer monofilaments, such as nylon fishing line, can be used to create thermally driven actuators. These actuators are called twisted polymer actuators (TPAs). TPAs can produce linear actuation when they are both twisted and coiled. In this configuration, these actuators are called twisted coiled polymer actuators (TCPAs). These same drawn polymers can be used to create torsional actuation when the precursor monofilament is twisted but still remains straight, known as straight twisted polymer actuators (STPAs), which is also thought to be the elemental unit of TCPAs. The torsional thermal actuation of STPAs is primarily a result of the anisotropic thermal properties of the virgin material (axial thermal contraction and radial thermal expansion), which manifests as linear actuation in the coiled configuration (TCPA). This paper presents two moisture related matters: moisture content impact on the thermal actuation of TPAs and the capability of TPAs to actuate as a function of moisture absorption at room temperature. For the former, we first present moisture dependencies of the axial thermal contraction and axial modulus of the precursor (straight, untwisted) monofilament. This study is conducted because closed-form and finite element models often use the physical properties of the precursor monofilament as inputs to predict the thermal actuation of TPAs. The results show that, both, axial thermal contraction and axial modulus, are strongly dependent on moisture content. Second, we present the experimental thermal actuation for STPAs and TCPAs at different moisture content percentages. We present torsional actuation responses for three different pitch angles STPAs (36, 25, and 15°) at two percentages added moisture by weight (0% and 4%). Similarly, we study the linear thermal actuation of TCPAs under an isotonic tensile load at the above moisture percentages. The results show an increase in actuation for those samples at 4% moisture content of approximately 100% for STPAs at 75 °C and a 50% for TCPAs samples at 100 °C. Finally, we report for the first time, that TPAs can be hygroscopically actuated. Here, we present torsional actuation responses under free torsion conditions for a 36° pitch angle STPA as well as axial contraction of a TCPA under an isotonic tensile load as a function of moisture absorption and show that moisture absorption can cause a similar actuation responses as seen when a thermal load is applied. Like the thermal actuation, we expect this hygroscopic actuation of TPAs is produced by a combination between the swelling that occurs on the precursor monofilament during moisture absorption and its anisotropic nature.