Novel Actuating System Based on a Composite of Single-Walled Carbon Nanotubes and an Ionomeric Polymer
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Novel Actuating System Based on a Composite of Single-Walled Carbon Nanotubes and an Ionomeric Polymer Igor A. Levitsky,1 Peter T. Kanelos2, and William B. Euler2 1 Emitech, Inc., Fall River, MA 02720, U.S. A. 2 Department of Chemistry, University of Rhode Island, Kingston, RI 02881, U.S.A.
ABSTRACT We report the fabrication and characterization of a novel composite material based on single walled carbon nanotubes (SWNT)s and the ionomeric polymer Nafion. SWNTs were airbrushed from a chloroform suspension onto both sides of a Nafion membrane (180 µm) and the electromechanical properties of the composite material were explored. The outer layers of carbon nanotubes acted as electrodes in order to pass electrical current through the system while the mechanical response was monitored. Under this design, the mechanical response could be characterized, with respect to the electrical signal, as a function of: voltage, waveform (AC vs. DC), and frequency (AC). Data was also compiled to gauge the effect of size and thickness of each individual layer of the system. The reference samples (graphite-Nafion and sputtered goldNafion) did not exhibit mechanical actuation at the same conditions. An analytical model for current decay was considered that is in agreement with the experimental data. Bi-exponential decay with a long time component was found for bias, which is above the actuating threshold. That was explained in terms of increasing of the water dielectric constant and polymer-SWNT interface area. The possible mechanisms of the actuation in this novel composite are discussed. INTRODUCTION There is an increasing interest in using electroactive polymers that convert electrical energy into mechanical energy for numerous applications in MEMS/NEMS technology [1]. Electrochemical and electromechanical properties of ionomeric polymer-metal composites (IPMCs) [2] have attracted great attention due to their ability to provide effective mechanical actuation under low bias (several volts), high strain with respect to ferroelectric polymers like PVDF, and relatively fast response time compared to ionic gels and conductive polymers . The most studied IPMC material is Nafion, (a perfluorinated ionomer) membrane, coupled with electrochemically plated Pt electrodes on both sides. The actuation mechanism of this composite was described in terms of electro-osmotic water transport driven by solvated cations and charging of the double layer at the interface between Nafion and platinum [2b], or an interfacial stress (Nafion/Pt) inducing the composite motion [3]. Recently, a general model was proposed taking into account both the hydraulic and the electrostatic effects in the IPMC [4, 5]. The discovery of single-walled carbon nanotube (SWNT) electro-mechanical actuation [6] introduced a unique material enabling the conversion of an electrical stimulus to mechanical displacement due to a novel quantum mechanical mechanism. For low charge density, SWNT mats demonstrate expansion and contraction with electron and hole injection, respectiv
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