Study of the rheological properties of Nafion in controlled humidity environment for using in artificial muscles and deformation sensors

Abstract

In the last decades, polymer-metal ionic composites (IPMCs) have been widely studied as new materials for actuators and sensors for using in robotics. These actuators are composed of a polymer membrane between fine metal surfaces, these metal surfaces having to be conductive and the polymer must have ionic conductivity at an appropriate level. A good example of this type of polymer is Nafion®.The main function of the polymers in the IPMCs is to store the ions and maintain the mobility required for counterions through the membrane. In addition, they must provide the mechanical tension and force required for the actuators. A large number of combinations of structures and polymer chains satisfying these requirements have already been synthesized, however the Nafion® holds a prominent place for this application due to its high proton conductivity and good mechanical properties. However, it is known that Nafion® changes its properties with the present moisture content. It is also known that Nafion® hydrate undergoes creep. Depending on the water activity and temperature, the mechanical deformation causes irreversible deformations in the Nafion that decreases its ionic conductivity. Thus, knowledge about the influence of water content on the final properties of Nafion and the knowledge of its rheological properties is of the utmost importance. In the modeling area, the first to incorporate mechanical properties in a Nafion model for fuel cell applications were Weber and Newman in 2004. Its one-dimensional model included conductivity, water transport, swelling, and mechanical hydrostatic behavior of Nafion®. Lai et al. in 2009 linearly modeled Nafion viscoelastic behavior with expansions in temperature and water content using master relaxation curves to account for changes due to temperature and hydration. However, there is still no model describing Nafion®'s complete viscoelastic behavior (with fixed temperature and varying frequency). Therefore, the present project has as objective the collection of rheological data for future use in a viscoelastic model to describe the behavior of IPMCs for applications in artificial muscles. For this, studies of static traction, oscillatory traction, oscillatory torsion and flexion, with frequency variation and under controlled humidity, in DMTA will be carried out. (AU)