Novel Actuating System Based on a Composite of Single-Walled Carbon Nanotubes and an Ionomeric Polymer

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Augustus Lawrence
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1 Mat. Res. Soc. Symp. Proc. Vol Materials Research Society D9.1.1 Novel Actuating System Based on a Composite of Single-Walled Carbon Nanotubes and an Ionomeric Polymer Igor A. Levitsky, 1 Peter T. Kanelos 2, and William B. Euler 2 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 gold- Nafion) 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, respectively [6]. However, at high charge density the electrostatic effect dominates and material expansion occurs regardless of the charge sign. Therefore, such an actuating system, at high charge density, principally cannot work in the bimorph cantilever geometry when asymmetrical

Since ionomeric polymers represent a solid electrolyte, their coupling with SWNTs, as electrodes, could result in an efficient novel actuating system utilizing the advantages of IPCM and SWNT

2 D9.1.2 electromechanical properties of both sides are required for cantilever bending. Also, SWNT actuation needs an electrolytic solution to provide a high charge concentration in the SWNT/electrolyte interface (double layer). Since ionomeric polymers represent a solid electrolyte, their coupling with SWNTs, as electrodes, could result in an efficient novel actuating system utilizing the advantages of IPCM and SWNT materials. Such a composite structure allows not only the investigation of electrochemical processes at the Nafion-SWNT interface, but also amplifies the actuation with respect to pure SWNT mats or IPMCs. Indeed, the SWNT cathode should exhibit its own stretching at low bias voltage (quantum effect) which coincides with the Nafion stretching due to hydraulic and electrostatic effects. In addition, as we will demonstrate, this composite exhibits an efficient actuation in the open air as distinct from SWNT mats and most IPMCs, which require a liquid environment. EXPERIMENTAL DETAILS SWNTs were synthesized by the arc discharge method and purified (85%) using air oxidation, acid treatment and thermal annealing, as purchased from BuckyUSA, Inc. The average diameter of the nanotubes was in the range of nm according to NIR absorption/raman spectroscopy and TEM observation. Nafion 117 membrane (180 µm thickness, H + -exchanged form) was purchased from Aldrich. The SWNT/Nafion/SWNT (SNS) composite was prepared by airbrush spraying of a SWNT suspension in chloroform (~ 0.8 mg/ml, 30 min sonication) onto both sides of Nafion at ºC. The deposited SWNTs form uniform and dense films (thickness about µm) with a high adhesion to the polymer surface (Fig. 1). Neither spincast nor coating methods could provide the same film quality. The reference samples Graphite/Nafion/Graphite (GrNGr) and Gold/Nafion/Gold (Au/N/Au) were prepared by the airbrush technique at the same conditions and the Gold sputtering (100 nm thickness), respectively. Additional sets of reference samples, for testing of possible actuation of SWNTs 200µm Nafion CNT film Figure 1. Optical (left) and SEM (right) image of the cross section of SWNT/Nafion/SWNT composite. SWNT film thickness is about 15 µm.

3 D9.1.3 without Nafion in the open air, were prepared by spraying nanotubes onto the following substrates: glass, paper, polyethylene (non-ionomeric polymer), and nanoporous alumina oxide soaked in the NaCl (1M) solution. Finally, a bimorph cantilever was fabricated from the composite to form a strip (~ 3 mm x 20 mm), and was clamped between two glass slides using platinum foil to maximize electrical contact with the voltage source. I-V characteristics and current time scans in response to step voltage were carried out with a Keithley-236 source-measurement unit. Cantilever displacement was measured by a CCD video camera coupled with an optical microscope and connected to a computer video capture card. RESULTS AND DISCUSSION The mechanical response of the SNS bimorph cantilever under DC step voltage is shown in Figure 2 (left). The mechanism of actuation presumably can be associated with both SWNTs and Nafion s electromechanical properties, as it was mentioned before. To clarify the SWNT role in the composite s actuation the reference samples, sputtered Gold/Nafion/Gold (AuNAu) and sprayed Graphite/Nafion/Graphite (GrNGr) were tested at the same condition. Another set of the reference samples, SWNT /X/SWNT where X is the glass, paper, polyethylene (nonionomeric polymer), and nanoporous alumina oxide soaked in the NaCl (1 M) solution, were used to elucidate Nafion's contribution to the mechanical response. All these samples exhibited no actuation under DC applied bias in the range of 1-5 V. Thus, we can conclude that actuation occurs only for Nafion-SWNT composites with a high interface area resulting in efficient current flow. A high ionic current through the composite due to proton mobility should induce the swelling of the cathode side and consequently cantilever bending toward the anode side. Apparently, only SWNTs with a huge surface to volume ratio [6], with respect to other tested materials, can provide the current values necessary for the cantilever bending. Similar situations occurred for IPMC composites, where a high interface area is the result of the fractal-like microstructure between Nafion and electrochemically plated platinum [2] c 1 r 1 Tip Displacement, mm r 0 c 2 r 2 ε Figure 2. Left: Displacement of SNS cantilever (dots) driven by 3.5 V rectangular DC voltage; Right: Electrical circuit simulating the current response in the SNS composite.

4 D9.1.4 I-V characteristics for SNS, GrNGr, and AuNAu are consistent with our assumption about the high current flow through SWNT-Nafion interface. Beginning from 2.5 V bias, the current density of the SNS sample is significantly higher than that of references samples. At the same bias, the SNS cantilever begins to exhibit a sizable actuation (~ 0.5 mm). More detailed information about the actuation mechanisms can be obtained from the temporal current response on the step voltage. Figure 3 shows the current decay of SNS cantilever at different applied step-voltages. At low bias (0.8 V) the current decay can be fitted satisfactory by monoexponential decay, but not for a bias that is above of an actuation threshold (2.5 V). In this case only a bi-exponential function is fitted well (Fig. 3). Hence, bi-exponential decay with a long time component can be considered as a sign of the electromechanical effect (Fig. 3a). To explain the above features we propose a model based on the electrical circuit, simulating the current time response in the actuating system (Fig. 2, right). Here r 0 is the internal polymer resistance between SWNT electrodes; r 1, c 1 and r 2, c 2 are leakage resistances and capacitances of both SWNT-Nafion interfaces. The actuating process leads to the asymmetry in electro-chemical properties of the cathodic and anodic interfaces, and consequently to the differences between resistances and capacitances at both cantilever sides. The solution of the current time response, i(t) on the step voltage, ε, for such a circuit is follows: i ( t) = B1 exp( z1t) + B2 exp( z2t) + B o (1) a b c d Figure 3. Current time scans of SWNT/Nafion/SWNT cantilever (dots) at applied step-voltage of 3.5 V (a), 2.5 V(b), 1.5 V(c), and 0.8 V(d) and corresponding mono- (dash) and bi- (solid) exponential fit curves

5 D9.1.5 where kinetic parameters z 1, z 2, B 1, B 2, B 0 are functions of r 0, r 1, r 2, c 1, c 2 and ε (not presented here). The current decay of the SNS sample at step voltages below and above the actuating threshold (Fig. 3) exhibit a significant difference in the i(t) function (Table 1). At low voltage (0.8 V) the decay is almost monoexponential (B 1 /B 2 ~ 10, B 1 /B 0 ~ 40) and the major contribution is defined by the short-time component (0.5 s). At voltages of 3.5 V and greater (above threshold) the contribution of the long-time component is increased dramatically with the increasing of the background value (B 1 /B 2 ~ 1, B 1 /B 0 < 0.1). Also, the magnitude of τ 2= z -1 2 is increased by a factor of 3 with respect to the low voltage case. The i(t) decay can be described by the mono-exponential function according to the above model (Fig. 2, right) if r 1 = r 2 = r and c 1 = c 2 = c. Then, ε 2r t r i( t) 1 exp( ), r0 2r + 0r = r where R = (2) + 0 Rc r0 + 2r This is consistent with the low voltage case when no actuation is observed and consequently there is no asymmetry in the properties of both cantilever sides (equal interface capacitances and resistors). The mono-exponential fit of the experimental decay (0.8 V) by eq.(6) gives the r 0 = 20 kω, r = 400 kω, and c = 60 µf. For i(t) dependence at 3.5 V (bi-exponential decay) we can make quantitative conclusions about resistances and capacitances according to expressions for kinetic parameters of eq.(1) (not presented here). First, the resistance sum, r 0 + r 1 + r 2, is 1.4 kω, which indicates considerable reduction of the resistances compared with the low voltage case. Second, an estimation for interface capacitances under actuation are c 1, c 2 1 mf. A decrease of the resistances and increase of the capacitances can be understood in terms of ionic conductivity, change of the water dielectric constant and SWNTs electrostatic expansion under change injection. When an applied bias is enough to induce the proton flux from the anode to cathode, internal and interfacial resistances are reduced due to imbalanced charge carriers. According to the model [4,5], such a redistribution of mobile cations results in the polymer chains contraction between Nafion hydrophilic clusters at the anodic side and chain expansion at the cathode. It is known that free water has a dielectric constant of 78 at room temperature and this value can be reduced on the order of six for hydrated water bound to ions in solvents [7]. When an electric field moves protons, an excess of free water near the anodic side can strongly increase the dielectric constant at the CNT-Nafion interface. Moreover, at the cathode side the protons bind to the surface oxides of SWNTs and cannot participate in the water hydration, which Table 1. Parameters of the best fitting of i(t) function by bi-exponential decay at the different values of the step voltage. Volts τ 1, s z 1, s -1 τ 2, s z 2, s -1 B 1 /B 2 B 1 /B

6 D9.1.6 results in an increase of the free water at the cathode interface and consequently and an increase ε of the dielectric constant. Since interface capacitance is given as c ~ S, where ε is the d dielectric constant, S is the SWNT-Nafion contact surface, and d is the thickness of the interface double layer, the ε value change should increase the interface capacitance at both sides of SNS cantilever. Besides, charge injection in the SWNTs should enlarge the SWNT Nafion contact surface, S, due to electrostatic repulsion between SWNTs bundles and individual nanotubes inside bundles [8]. This effect can also contribute to increase of the interface capacitances. CONCLUSION This study demonstrates a novel, pure organic electro-mechanical actuator based on an ionomeric polymer and singled-wall carbon nanotubes. SWNTs sprayed onto both sides of the polymer membrane work as electrodes providing a high current density due to enormous SWNT- Nafion interface area. As a result, cantilever actuation is occurred in open air at relatively low driven DC bias as distinct from the references samples. With an applied AC signal in the range of Hz, the cantilever exhibits resonant frequencies depending on its size and displays excellent robustness (more than one million cycles with 10% of amplitude reduction). Such an unique system and actuation effect has a high potential for various MEMS/NEMS applications. ACKNOWLEDGETMENTS This work is supported by the Missile Defense Agency grant F C Authors are grateful to M. Platek for his help in optical/sem imaging. REFERENCES 1. "Electroactive Polymer Actuators as Artificial Muscles", Y. Bar-Cohen ed., SPIE press, (a) Shahinpoor, Y Bar-Cohen, J. O. Simpson, and J. Smith, A review, Smart Mater. Struct., 7(1998) R15; (b) K. Sadeghipour, R. Salomon, and S. Neogi, Smart Mater. Struct., 1(1992)172; (c) K. Onishi, S. Sewa, K. Asaka, N. Fujiwara, and K. Oguro, Electrochim. Acta 46(2000) K. Asaka, K. Oguro, J. Electroanal. Chem. 480(2000) S. Nemat-Nasser, and J. Y. Li, J. Appl. Phys. 87(2000) S. Nemat-Nasser, and C. W. Thomas, in "Electroactive Polymer Actuators as Artificial Muscles", Y. Bar-Cohen ed., SPIE press, 2001, pp (a)r. H. Baughman, C. Cui, A. A. Zakhidov, Z. Iqbal, J. N. Barisci, G. M. Spinks, G. G. Wallace, A. Mazzoldi, D. De Rossi, A. G. Ronzler, O. Jaschinski, S. Roth, and M. Kertesz, Science 284, 1340 (1999); (b) G. M. Spinks, G. G. Wallace, R. H. Baughman, and L. Dai in "Electroactive Polymer Actuators as Artificial Muscles", Y. Bar-Cohen ed., SPIE press, 2001, pp ; 7. O'M. J. Bokris, and A. K. N. Reddy, "Modern Electrochemistry, v.1 New York, Plenum Press, Y. Zang, and S. Iijima, Phys. Rev. Lett. 17(1999)3472.

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