Nanotube fibers for electromechanical and shape memory actuators

Carbon nanotubes are light, stiff and electroactive materials particularly promising in the field of actuating materials. Indeed, carbon nanotubes can expand and contract upon charge injection and be used for the development of electromechanical actuators. Carbon nanotubes can also be included in polymers to improve their properties and bring specific functionalities. When added to shape memory polymers, carbon nanotubes yield an improved stiffness and the possibility to heat the material through Joule's heating. Nevertheless, spatial ordering of the nanotubes is a critical issue in all these classes of actively moving materials. It is shown in this article that assembling nanotubes under the form of pure or composite fibers is an effective approach to orient carbon nanotubes on a large scale along a well defined direction. Nanotube alignment achieved via fiber drawing allows the optimization of properties of shape memory polymer fibers and electrochemical actuators. In particular, the mechanical response of pure nanotube fibers to electrical stimulations is investigated in liquid electrolytes. It is observed that the fibers can generate a stress one order of magnitude greater than that achieved with unaligned assemblies of nanotubes. We also present the properties of shape memory polymer fibers loaded with carbon nanotubes. These fibers generate a very large stress when they recover their shape after they have been stretched and cooled under tensile load. Composite nanotube polymer fibers also exhibit a temperature memory behavior, which is still raising fundamental questions regarding its microscopic origin.