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.  This may occur by over-twisting a monofilament such that the material torsionally buckles and coils.  In this configuration, these actuators are called a Twisted Coiled Polymer Actuators (TCPAs) or artificial muscles. These same drawn polymers can be used to create torsional actuation when the precursor monofilament is twisted but still remains straight. In this twisted configuration, the torsional actuators are known as Straight Twisted Polymer Actuators (STPAs). The untwist of STPAs is claimed to be a result of the anisotropic thermal properties of the material (axial thermal contraction and radial thermal expansion) and, in turn, the contraction produced in TCPAs is driven by the untwist of STPAs. In this work we report, for the first time, that STPAs can be actuated by moisture content variations. We have tested the response of STPAs under no thermal or mechanical loads and found that moisture absorption can cause a similar untwist response as seen when a thermal load is applied. This work presents hygroscopic actuation results as a function of moisture absorption for STPAs with various initial twist (specifically we consider STPAs with initial twists of 15 degrees and 36 degrees relative to the vertical axis). We believe that swelling and the anisotropic nature of the precursor monofilament creates a radial expansion but little change in the axial direction, in a similar manner as thermal loading, thereby hygroscopically untwisting the STPAs. It has been assumed that water molecules are stored in polymers by taking occupied volume (a.k.a. Van der Waal volume) where water molecules are bounded with the polymeric matrix and free volume (voids or imperfections found in the polymeric monofilament). Occupied volume typically takes about 10% of the total absorbed moisture in the sample, while free volume usually takes about 90%. The occupied volume is the only portion of moisture absorption that contributes to the swelling of the material, and thus is thought to be responsible for the hygroscopic actuation. Additionally, this work studies how moisture content affects the mechanical and thermal properties of the precursor (straight, untwisted) monofilament. The properties of precursor monofilaments are often used in closed-form models to predict torsional thermal actuation.  Thus, characterizing the effect of moisture content on these properties can highlight how absorbed water molecules affect the thermally driven actuation mechanism.  In this work, we show that moisture content in the precursor monofilament can highly affect the thermo-mechanical behavior of monofilaments. Future work will aim to fully characterize the mechanical and thermal properties of the precursor monofilament as a function of both temperature and moisture content. This will allow actuation models to predict STPAs thermal actuation responses under real operating environmental condition.


Moisture Driven Actuation in Twisted Polymers Actuators and Moisture Dependence of Thermal and Mechanical Properties Used to Predict Thermal Driven Actuation of Twisted Polymer Actuators

Division / Sponsors: 
Aerospace Division

Paper Proceedings: https://asmedigitalcollection.asme.org/SMASIS/SMASIS2020/volume/84027

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