Ionic liquids as stable solvents for ionic polymer transducers

Ionic liquids as stable solvents for ionic polymer transducers

Sensors and Actuators A 115 (2004) 79–90 Ionic liquids as stable solvents for ionic polymer transducers Matthew D. Bennett∗ , Donald J. Leo Departmen...

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Sensors and Actuators A 115 (2004) 79–90

Ionic liquids as stable solvents for ionic polymer transducers Matthew D. Bennett∗ , Donald J. Leo Department of Mechanical Engineering, Center for Intelligent Material Systems and Structures, Virginia Polytechnic Institute and State University, Blacksburg VA 24061, USA Received 15 October 2003; received in revised form 30 March 2004; accepted 31 March 2004 Available online 14 May 2004

Abstract NafionTM membranes are known to operate as electromechanical actuators and sensors. The transduction in the material is caused by redistribution of the mobile cations in the material, which is made possible because the material is saturated with a solvent. Typically, the solvent used is water, although its use limits the performance of these materials. This is due to the chemical breakdown of the water at relatively low operating voltages and the loss of the water to evaporation when these devices are operated in air, causing a corresponding loss of performance. In the current work, the use of highly stable ionic liquids to replace water is explored. Ionic liquids have the advantage of greater electrochemical stability than water, thus offering the possibility of higher actuation voltages for these materials. Also, ionic liquids are known to be non-volatile and therefore will not evaporate out of the polymer as water will. In this work, the use of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ionic liquid is demonstrated as a viable solvent for NafionTM polymer actuators and sensors. This ionic liquid melts at −9 ◦ C and has an electrochemical stability Window of 4.1 V [Inorg. Chem. 35 (1996) 1168], making it a promising candidate to replace water in ionic polymer transducers. Experimental results indicate that NafionTM transducers solvated with this ionic liquid have improved staility when operated in air as compared to the same materials solvated with water, although the magnitude of the response is decreased as compared to the water samples at high frequencies. The main drawback associated with the use of ionic liquids is a reduction in the speed of the response as compared to water, although the initial results are promising and demonstrate the potential for this approach. © 2004 Elsevier B.V. All rights reserved. Keywords: Ionic liquid; Ionic polymer; NafionTM ; Artificial muscle

1. Introduction Ionic polymer membranes are materials that behave as solid-state electrolytes, making them useful in fuel cells and water electrolyzers. Another application for these membranes is as distributed actuators and sensors. Ionic polymer membrane actuators have existed in their current state since the early 1990s [2–4]. These devices are typically based on NafionTM , which is a polymer consisting of a TeflonTM backbone with pendant sulfonic acid side groups. The addition of these sulfonic acid groups allows a NafionTM polymer membrane to conduct cations and thus serve as a polymer electrolyte. This property also allows the polymer membrane to behave as a distributed actuator or sensor, as the cations within the polymer will redistribute when an electric field or imposed stress is applied to the membrane (see Fig. 1) . ∗ Corresponding author. Tel.: +1-540-2312910; fax: +1-540-2312903. E-mail address: [email protected] (M.D. Bennett).

0924-4247/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2004.03.043

In order for this phenomenon to occur, two conditions must be met. First, the cations must be free to move about within the polymer matrix. This is done by saturating the polymer with a solvent. The solvent dissolves the cations associated with the pendant acidic groups and allows them to move within the polymer. Second, the surfaces of the membrane must be made to conduct electricity. This is accomplished by depositing a thin metal electrode on both surfaces of the membrane. Typically, the ion exchange properties of the polymer are used to facilitate the deposition of the metal. The polymer is pretreated by sandblasting, hydrating, and cleaning in a strong acid. This acid wash also serves to ensure that the polymer is fully saturated with protons. The membrane is then placed into an aqueous solution containing ions of the metal to be plated. These ions are allowed to exchange with the protons in the polymer for a predetermined amount of time and are then reduced to their neutral state at the surface of the polymer by a reducing agent (typically NaBH4 or LiBH4 ) in outer solution. For more information about how this electroding process is

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Fig. 1. An ionic polymer membrane as (a) an actuator (b) a sensor.

typically done, the reader is referred to work by Bennett and Leo [5,6], Takenaka and Torikai [7], Millet et al. [8–12], Cook et al. [13], and Dewulf and Bard [14]. In this solvated and electroded form, a NafionTM membrane can be made to bend towards the anode side when a small voltage (1–5 V) is applied across its thickness, thus making it a soft, distributed actuator. These membranes in this form can also be used as distributed sensors. Several researchers have shown that the transient voltage generated across the membrane is correlated to the quasi-static displacement of the membrane [2,15–17]. More recently, Newbury and Leo [18,19] have shown that the charge generated at the surfaces of the membrane is proportional to the strain in the material.

2. Limitations of ionic polymer transducers Ionic polymer membrane actuators have several advantages over other types of actuation technology. For example, because they are soft and saturated with water, they are amenable to implantation in the human body and therefore have significant potential in biomedical applications. Also, as compared to other many types of smart materials, these actuators generate larger strains and strain rates at lower applied potentials. There are some key limitations of this technology that have prevented it from experiencing widespread use, however. One of the most problematic limitations is the dependence of ionic polymer transducers on a solvent in order to function. Typically water is used as the solvent, but because the water will quickly evaporate in air, the applications for these devices are limited by its use. Also, the water will decompose into hydrogen and oxygen gas once the electrolysis voltage limit is reached (around 1 V). This decomposition will also contribute to rapid loss of the water and a corresponding drop in the performance of the polymer transducer. One way around this problem would be to use a barrier coating to contain the water inside the membrane. Bar-Cohen et al. [20,21] reported that with the aid of a barrier coating

they were able to operate a sample in air for 4 months. However, such a barrier coating will add passive stiffness to the actuator device and reduce the amount of strain that the device can generate [22]. Furthermore, if the electrolysis limit of the aqueous solvent is exceeded, the formation of hydrogen and oxygen gases at the membrane surfaces will create blisters beneath the barrier coating and lead to delamination of this coating from the polymer transducer.

3. Motivation In light of the limitations imposed on ionic polymer transduction technology by its dependence on water as a solvent, this goal of this research has been to identify other solvent systems that will facilitate transduction in these materials and reduce or eliminate the problems associated with water. In this regard, suitable solvents will have a low vapor pressure and a large electrochemical stability window. Additional benefits may be realized from solvents that have a high ionic conductivity. Hydrophobicity will also be of importance, as ionic polymers will readily absorb moisture from the air, which will eventually lead to the same problems associated with using water as a solvent. Although most of the work in the area of ionic polymer transducers has focused on water as the solvent, there have been a limited number of studies on the use of alternative solvents. For example, Nemat-Nasser [23,24] has demonstrated the use of ethylene glycol and glycerol as suitable solvents for these materials. These materials do not suffer from the dehydration problem associated with water, but the speed of the actuation mechanism in the transducers is reduced significantly. Also, Shahinpoor and Kim [25,26] have demonstrated that composites of poly(ethylene oxide) and poly(ethylene glycol) will exhibit electromechanical behavior under an applied electric field with no aqueous solvent necessary. In these materials, the very low-molecular weight poly(ethylene glycol) essentially serves as the solvent, creating soft amorphous phases in the composite polymer that facilitate motion of the cations.

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In the current work, ionic liquids are investigated as possible new solvents for these materials. Ionic liquids (sometimes known as molten salts) are ionic compounds that exist in a liquid form at room temperature, without the need to be dissolved in a solvent. These compounds may be used as electrolytes and have the added benefit of very high stability. This means that they can withstand higher voltages than most common solvents before catastrophic breakdown. Ionic liquids also have a very low vapor pressure, meaning that they do not evaporate easily, as some common solvents (e.g. water) will. Therefore, if an ionic liquid could be used as the solvent for an ionic polymer membrane in the place of water, the resulting device would have the benefit of a larger voltage range in which it could be driven as well as a much reduced tendancy towards loss of performance due to dehydration. The use of an ionic liquid in such a way could in fact overcome the need for a suitable barrier coating against solvent loss, and if an ionic liquid could be used as the solvent in an ionic polymer membrane transducer instead of water, the applications for these devices could potentially be increased dramatically.

4. Ionic liquids Ionic liquids are ionic compounds that exist as liquids at low temperatures. They consist of organic cations and (mostly) inorganic anions. There is no formal definition of the minimum melting point for a compound to be described as an ionic liquid; many researchers use 80 ◦ C, although some compounds with melting points as high as 100 ◦ C are referred to as ionic liquids [27,28]. The low melting point of these compounds is due primarily to the bulky and cumbersome structure of the corresponding ions, which inhibits the formation of a crystalline solid. One of the advantages of ionic liquids are that they are non-volatile, and in fact have no measurable vapor pressure, meaning that they will not be lost to evaporation. Also, they are able to dissolve a wide range of organic and inorganic compounds, including some polymers and minerals, are non-flammable, have a high thermal stability, and have a high electrochemical stability. Moreover, Adam [28] reports that “compared to the 300 organic solvents commonly used in the chemical industry, there are over a trillion possible ionic liquids”. These compounds can also be tailored to give properties desirable for specific applications. Many researchers believe that the use of ionic liquids could drastically reduce the chemical industry’s reliance on flammable, volatile, environmentally damaging organic solvents and lead to new processes that allow for recycling of solvents and catalysts and are more friendly to the environment. One of the most common classes of ionic liquids are those based on substituted imidazolium cations. These imidazolium-based ionic liquids are currently being studied as electrolytes for battery [29] and electrochemical capacitor [30] applications. They have also been explored for use as

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replacement solvents in many common reactions [31], and in photoelectrochemical solar cells [32]. The advantages that they can offer these systems are high ionic conductivity, high electrochemical stability, non-flammability, and non-volatility [33,34]. Ionic liquids have also been used as electrolytes for conducting polymer actuator systems. Lu et al. [35] have found that polypyrrole and polyaniline conducting polymer actuators operated in 1-butyl3-methlyimidazolium tetrafluoroborate (BMI-BF4) and 1-butyl-3-methlyimidazolium hexafluorophosphate (BMIPF6) ionic liquids exhibited larger induced strains, less polymer degradation, and less electrolyte degradation than the same polymers operated in aqueous or organic (propylene carbonate) electrolytes. The use of ionic liquids as electrolytes for conducting polymer actuators also allowed the researchers to apply higher actuation voltages due to the increased electrochemical stability of these materials. Wallace et al. [36,37] have found similar improvements to the performance and stability of their conducting polymer actuators when using ionic liquids as the solvent/electrolyte. Vidal et al. have also utilized ionic liquids as electrolytes for conducting polymer actuators and have demonstrated improved stability of these devices as compared to propylene carbonate/lithium perchlorate electrolyte systems [38]. For their work the ionic liquid used was 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI-Im) and they report that the actuators solvated with ionic liquid can be cycled 7(10)6 times in air with no degradation of performance. Although many applications of ionic liquids for electrochemical devices have been demonstrated, this paper represents the first published work documenting the use of ionic liquids as solvents for ionic polymer transducers.

5. Experimental methods The transducer samples studied in this work were plated with either gold or platinum using an impregnation/reduction process. A critical feature of electrodes made in this way is interpenetration of the metal into the polymer membrane. As compared to a purely surface electrode this interpenetration accomplishes two things: first, the adhesion of the electrode to the TeflonTM like polymer is enhanced and second, the effective capacitance of the membrane is increased by the larger polymer/metal interfacial area. This is important because the transduction performance of these devices has been shown to be strongly linked to the capacitance of the membrane [23,39,40]. In the case of platinum, the electroding process is carried out as follows. The NafionTM 117membrane (product of DuPont, nominal thickness 183 microns) is first roughened using emery paper. It is then cleaned in an ultrasonic bath and subsequently boiled in 1 M HCl to become fully saturated with water and protons. The membrane is then placed into a solution of [Pt(NH3 )4 ]Cl2 overnight to allow the protons in the membrane to exchange with Pt(NH3 )+2 ions. These platinum ions are then reduced

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to metallic platinum at the surface of the membrane by treatment with NaBH4 . Several researchers have utilized this process for plating platinum onto NafionTM membranes [41,9,10]. The membrane may also be plated with gold using a similar process, as described by Fujiwara et al. [42] and Onishi et al. [43]. Following the deposition of this interpenetrating metal layer, a thin coating of gold is deposited on the outer surface of the membrane by electroplating in order to enhance the surface conductivity of the electrodes. The plated membrane is then boiled in HCl and soaked overnight in LiOH in order to exchange it into the lithium cation form. Small strips of the plated membrane are then cut from the parent sample and characterized for their strain generation and sensing performance. In order to impregnate these materials with ionic liquid, the first step after the plating process is to completely remove the water from the membranes. This is done by baking the samples in a vacuum oven, at a temperature of 75–85 ◦ C and a pressure of 25 Torr (absolute) for at least 1 h. After dehydrating the polymer samples, they are placed into a mixture of methanol (30–40% by weight) and ionic liquid and sonicated at 65–80 ◦ C for 1–3 h. This technique has been proven effective in imbibing the ionic liquid into the NafionTM membranes. Upon removal from the ionic liquid/methanol mixture the samples are blotted with a clean filter paper to remove residual solvent on the surface. One of the metrics that is used to compare the transducer materials employing different solvents is the free strain generated by the transducers when driven by a small voltage. Because NafionTM membranes bend when stimulated by an applied voltage, they are tested in a cantilevered configuration. One end is fixed in a spring-loaded clamp fitted with gold foil electrodes that contact the conductive metal surfaces of the sample and the deflection of the free end is measured with a laser vibrometer (see Fig. 2(a)). This test is performed in two different ways. In order to obtain the frequency response between the generated strain and the applied voltage, a random voltage signal is used to drive the

polymer and the driving voltage and tip displacement are measured by a Tektronics Fourier analyzer. The tip displacement is then converted into strain using the sample geometry and Eq. (1), where δ/V is the frequency response between tip displacement (zero-to-peak) and input voltage, t is the thickness of the sample, and Lf is the free length of the sample. This equation assumes that the actuator deforms with a constant curvature. In order to obtain very low frequency measurements of strain, the sample is driven with a harmonic (single frequency) voltage signal supplied by a function generator and amplifier. As before, the tip displacement is measured using the laser vibrometer. The time history of the tip displacement and input voltage are measured using a DSpace digital signal processor and Eq. (1) is used to convert the tip displacement into strain. In this case, δ is half of the measured peak-to-peak tip displacement. The step response of the samples is also measured using the DSpace board.  (δ/V)t = (1) V L2f A second experimental setup is used to measure the Young’s modulus of the polymer transducers (see Fig. 2(b)). In this setup, the polymer sample is again fixed between two gold foil electrodes in a spring-loaded clamp. However, in this case the clamp is linearly displaced by an electromagnetic shaker. The tip of the sample is in contact with the load application point of a 10-gm load cell and is constrained as such (a small prebend in the sample ensures that it remains in contact with the load cell). A random voltage signal is used to drive the shaker; the motion of the clamp is measured with a linear potentiometer and the resulting reaction force at the tip of the sample is measured by the load cell. The displacement and force signals are collected by a Tektronics Fourier analyzer and a frequency response is computed. This frequency response is the bending stiffness of the sample, which asymptotically approaches a constant value at low frequency. This low-frequency stiffness can be used with the sample’s geometry and Eq. (2) to compute the Young’s modulus of the

Fig. 2. A schematic of the experimental setup used to measure (a) the free strain generated by the transducer as an actuator, and (b) the modulus and sensing response of the transducer.

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Fig. 3. The circuit used to measure the charge output of a NafionTM sensor.

material. In this equation F/δ is the bending stiffness of the sample, w is the sample width, t is the thickness of the sample, and Lf is the free length of the sample. This computation assumes that that the sample is homogeneous and isotropic. Y=

(F/δ)L3f 3(wt3 /12)

(2)

NafionTM transducers can also function as electromechanical sensors. In order to measure the sensing response of these materials, the setup shown in Fig. 2(b) is used. As with the modulus test, the tip of the sample is constrained and the base of the sample is displaced by an electromagnetic shaker; the displacement is measured with a linear potentiometer. This situation generates an induced strain in the membrane that gives rise to a charge buildup at the electrodes. This charge can be measured using the circuit shown in Fig. 3. This circuit behaves as a filter and its gain is therefore frequency dependent, but at frequencies sufficiently larger than the corner frequency its gain can be approximated as (V/q) = Cf−1 , where V is voltage in Volts, q is charge in Coulombs, and Cf is the value of the feedback capacitor in Farads. The output of this circuit and the displacement of the polymer clamp are measured by a Tektronics Fourier analyzer and a transfer function between charge and deflection is generated. This transfer function can be used with

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Eq. (1) and the geometry of the sample to compute the frequency response between charge and induced strain, which is referred to as the sensitivity of the sample. The sensitivity can also be converted to a function between charge and stress by dividing it by the Young’s modulus of the sample. The impedance of these materials was also measured in the course of this work and used to compute the capacitance of the membranes. In order to determine the electrical impedance, the current drawn by the membrane for a given voltage input is measured using the circuit shown in Fig. 4. As can be seen, this circuit operates by amplifying the voltage drop across a 0.1  resistor placed in series with the polymer sample; the gain of the circuit is (V/ i) = 41.5 mA/V. The value of this resistor is chosen to be small enough that it’s effect on the current through the polymer can be neglected. The polymer is driven with a random voltage signal and the voltage and current are measured with a Tektronics Fourier analyzer. From this the voltageto-current frequency response is generated, which is directly the complex impedance of the polymer sample. If this complex impedance is represented as Z(ω) = R(ω) + jI(ω), where ω is the frequency in rad/s and j is the imaginary unit, then the capacitance can be computed from Eq. (5). This result is arrived at by considering the equation for the complex impedance of a pure capacitor (3). If the complex part of the impedance (I) is assumed to be the impedance of a pure capacitor, then Eq. (3) can be re-arranged to yield Eq. (4). The capacitance of the sample is then found by taking the magnitude of this value (Eq. (5)). This exercise is only valid at low frequencies, where the impedance of the NafionTM transducers is dominated by the capacitance and therefore the capacitance values quoted are those obtained at 1 Hz. Typically these capacitance values are normalized by the sample area to facilitate comparisons. ZC = C=

1 jωC

1 jωI

C = (ωI)−1

Fig. 4. The circuit used to measure the current through a NafionTM transducer when driven with a voltage input.

(3) (4) (5)

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Fig. 5. Free strain response of an ionic polymer transducer in three states: saturated with water, dry, and saturated with ionic liquid. The ionic liquid was EMI-Br.

6. Environmental stability characterization In order to evaluate the effectiveness of the ionic liquid as a solvent for these materials, the free strain response of a sample in both the water and ionic liquid-solvated forms has been measured (see Fig. 5). For this test the ionic liquid used was 1-ethyl-3-methylimidazolium bromide (EMI-Br). As can be seen in the figure, the free strain generated in the actuator is decreased by about four orders of magnitude after dehydration, whereas after incorporation of the ionic

liquid the free strain response recovers to about half of the response with the water solvent. Note that the very small response of the sample in the dehydrated state is likely due to a small amount of residual water inside the polymer; complete dehydration of the material is difficult to achieve. Both of these samples were tested in their as-prepared form and were then tested after being allowed to sit in a free air environment for 1, 2, and 3 days in order to evaluate the evaporative stability of these new solvents as compared to water (see Fig. 6). Two identical samples, one in the

Fig. 6. Free strain responses of two samples (water and ionic liquid-solvated) over three consecutive days. The ionic liquid was EMI-Br.

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ionic liquid form and one in the water form, were tested on consecutive days to determine the effect of leaving the samples exposed to ambient conditions, with no effort made to protect them from dehydration or other degradation. As can be seen from Fig. 6, a dramatic difference in the responses from day 1 to day 2 is evident in the water-solvated sample. This is due to the evaporative loss of the water between the two tests. Further change in the response of the water-solvated sample is witnessed from day 2 to day 3. This change is likely due to changes in the relative humidity of the testing environment. Also, it should be noted that although this sample had lost most of the water solvent by the second day of testing, it was not completely dry. Rather, the water content inside the sample becomes equilibrated with the water content of the surrounding air, hence there is still a quantifiable amount of water inside the polymer and some electromechanical effect is still evident. By contrast, the sample that was dried in the vacuum oven is much drier and generates almost an unmeasurably small amount of strain (see Fig. 5). The sample that was solvated with ionic liquid did not display the same changes in its free strain response, however. As can be seen from the figure, its response at 1 Hz changes by less than 10% over the course of 3 days, whereas the strain response of the water-solvated sample drops by 96% (at 1 Hz) after drying under standard room conditions for 1 day. Thus, although in the present form there is some loss of strain caused by using ionic liquids as solvents instead of water, the advantage of long-term stability in air is demonstrated. Following this initial testing, a more rigorous test of the air stability of NafionTM actuators solvated with ionic liquids was performed. For this testing, the ionic liquid used was 1-ethyl-3-methylimidazolium trifluoromethanesulfonate

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(EMI-Tf). This ionic liquid will be the focus of the remainder of this paper and has a melting point of 263–264 K, an ionic conductivity of 8.6–9.3 mS/cm, and an electrochemical stability windown of 4.1 V [1,44]. In this test stability was quantified by cycling the actuators (one solvated with the ionic liquid and one solvated with water) continuosly with a 1.5 V (peak), 2 Hz sine voltage input and monitoring the change in their free tip displacement; this result is shown in Fig. 7. As can be seen, the strain response of the sample in the ionic liquid form is far more stable than the water sample when these actuators are operated in an air environment. Two possible explanations for the attentuation of the displacement response in the ionic liquid sample are decomposition of impurities in the ionic liquid and absorption of water from the air by the hygroscopic NafionTM polymer or the ionic liquid, which is reported to be miscible with water [1].

7. Transducer characterization For the characterization of the transduction properties of NafionTM membranes, 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ionic liquid was used. Fig. 8 is a plot of the free strain response of two samples of the gold-plated polymer in the water form and two samples in the ionic liquid form. As can be seen from the figure, the first resonant peak of the samples in the ionic liquid form occurs at a lower frequency than that of the water-solvated samples. This reduction in the resonant frequency is consistent with a reduction in the stiffness of the device and correlates with a reduction in the measured elastic modulus, from approximately 250 MPa in the water form to approximately 85 MPa in the ionic liquid form.

Fig. 7. Time history of the tip displacement of a NafionTM actuator sample in the water and ionic liquid (EMI-Tf) forms. The input was a 1.5 V (peak), 2 Hz sine wave and the displacement has been normalized by the initial value.

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Fig. 8. Free strain frequency response of two samples each in the ionic liquid (EMI-Tf) and water forms.

Another interesting characteristic of the free strain response of these materials is the large difference in the strain at low frequencies (see Fig. 9) . Note that in this plot the low frequency response of the ionic liquid samples was determined from time response data; the low frequency response of the water samples is a representative fit inferred from the step response but is intended only for reference. The ionic liquid samples seem to display a higher strain response at low frequencies than do the hydrated samples. Typically, NafionTM transducers solvated with water are hindered at low frequencies by a characteristic relaxation in the material. However, the ionic liquids do not exhibit this relaxation and

in fact generate larger strains at low frequencies. Based on this data a reasonable explanation is that the strain generated by the ionic liquid samples at higher frequencies is limited by the speed of the ionic diffusion within the polymer. This difference can also be seen in low frequency square wave responses of the two materials (see Fig. 10). As can be seen, the ionic liquid sample has a much slower rise time than the water sample, but does not exhibit relaxation in the response. Similar phenomena have been observed by Onishi et al. [45] in FlemionTM polymers and by Nemat-Nasser and Wu [46] in NafionTM polymers when using mobile cations of different sizes. This slower behavior of the ionic liquid

Fig. 9. Free strain frequency response of two samples each in the ionic liquid (EMI-Tf) and water forms.

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Fig. 10. Free cantilever time responses of (a) a sample in the ionic liquid (EMI-Tf) form to a 3.5 V square wave input (b) a sample in the water form to a 1.5 V square wave input.

samples is similar to the responses of water-solvated samples in the tetrabutylammonium ion form and is likely due to the large and bulky 1-ethyl-3-methylimidazolium cation in the ionic liquid. Also characterized was the sensing response of these polymers as displacement transducers. Previous work by Newbury and Leo [18,19] has shown that ionic polymer membranes generate a charge in response to an induced stress or strain in the material. Fig. 11 shows a frequency response of the charge output to induced strain in the water and

ionic liquid-solvated samples. As can be seen in the figure, the charge sensitivity of the water-solvated samples is higher than that of the ionic liquid-solvated samples. However, the water samples have a higher elastic modulus than the ionic liquid samples. When the sensing data is adjusted for the difference in the modulus and plotted as charge-per-stress, the responses agree more closely (see Fig. 12). This would seem to indicate that the fundamental sensing mechanism in these polymers is stress induced and not strain induced. Also, the effect of frequency on the sensing data is not as

Fig. 11. Charge-to-strain sensing response of samples in the water and ionic liquid (EMI-Tf) form.

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Fig. 12. Charge-to-stress sensing response of samples in the water and ionic liquid (EMI-Tf) form.

pronounced as in the free strain data. This could indicate that the mechanism for sensing in NafionTM involves a shifting or redistribution of the ions, whereas the actuation mechanism involves diffusion of the ions within the polymer membrane. The electrical impedance and capacitance of these materials was also measured and compared. The capacitance of ionic polymer transducers has been shown to be linked to the strain-to-voltage coupling in these devices. The capacitance at 1 Hz was measured to be between 50 and 70 ␮F/cm2 , which is low for ionic polymer transducers. In order to achieve good transduction, capacitance measurements of 1000–5000 ␮F/cm2 should be obtained. Because the low capacitance was also exhibited by the samples in the water-solvated form, this is likely due to issues with the electrodes, however. Of interest is that there did not seem to be any difference in the capacitance of the water and ionic liquid-solvated samples. If the speed of the ion diffusion in the ionic liquid samples is in fact reduced as compared to the water samples, then one would expect to see a sharp increase in the capacitance of the ionic liquid samples at very low frequencies and a decrease at higher frequencies. Improvements to the electroding process need to be made before further study of these devices can be carried out.

pared to water will allow for higher actuation voltages to be applied, which may eventually lead to improvements in the performance of these devices. Although the results presented in this paper are for a group of transducers that exhibited relatively little strain response, this can be attributed to the low capacitance of these devices, which is caused by ineffective metal plating. Regardless, a valid comparison of NafionTM transducers solvated with water and ionic liquid has been made and important differences in the responses of these transducers have been identified. The major limitation associated with the use of ionic liquids as solvents for ionic polymer actuators has been identified as the slow speed of response. However, the use of the ionic liquids to make effective transducers is demonstrated and the motivation for future work in this area is established.

Acknowledgements This work was supported by the U.S. Army Research Laboratory and U.S. Army Research Office under contract/grant number DAAD19-02-1-0275 Macromolecular Architecture for Performance (MAP) MURI. Supplementary funding was provided by the Virginia Space Grant Consortium.

8. Conclusions This work has investigated the performance of NafionTM ionic polymer transducers with a 1-ethyl-3-methylimidazolium trifluoromethanesulfonylimide ionic liquid. The use of this ionic liquid to replace water as the solvent in these devices shows promise for improving the stability of these transducers when operated in air. Furthermore, the larger electrochemical stability window of this ionic liquid as com-

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Biographies Matthew D. Bennett Matthew D. Bennett did his MS in mechanical engineering, from Virginia Tech in 2002; BS mechanical engineering from Virginia Tech in 2000 He graduated with a Bachelor’s degree in mechanical engineering from Virginia Tech in the spring of 2000. In the fall of 2000, Matthew enrolled in graduate school at Virginia Tech. For his Master’s work, he studied the plating process for ionic polymer transducers and developed a new process that utilizes a noble and non-noble metal alloy composite and is therefore less expensive than the traditional process. This work was done under the guidance of Dr. Donald Leo in the center for intelligent material systems and structures in the mechanical engineering department. Following the completion of his Master’s degree in the spring of 2002, Matthew began work on a doctoral degree, also under Dr. Leo in the center for intelligent material systems and structures. For the first semester of this work Matthew travelled to Australia to work with Dr. Gordon Wallace of the intelligent polymer research institute at the university of Wollongong. This work resulted in the development of novel ionic/conducting polymer hybrid actuators. This experience also introduced Matthew to the concept of using ionic liquids as stable electrolytes for these devices. Following the completion of his PhD degree, Matthew would like either to pursue a career in academia or to become an entrepreneur and start a company. During his time at Virginia Tech Matthew has been responsible for fabricating ionic polymer transducers

for his own and his collaborators’ research and has extensive experience with the process. Matthew has co-authored two journal papers and has presented his work at many highly respected conferences in the area of active materials. Donald J. Leo Dr. Donald Leo did his PhD mechanical and aerospace engineering from University of Buffalo in 1995, MS mechanical and aerospace engineering from University of Buffalo in 1992, BS aerospace and astronautics engineering from University of Illinois at Urbana-Champaign in 1990 After completion of his PhD degree at the University of Buffalo in 1995, Dr. Leo served as a project engineer for CSA in Palo Alto, CA for 2 years. Following this work, he was an assistant professor in the department of mechanical, industrial, and manufacturing engineering at the University of Toledo. In June of 1998, Dr. Leo joined the faculty at Virginia Tech as an assistant professor in the mechanical engineering department. In June of 2002, Dr. Leo was promoted to the position of associate professor. He is the assistant director of the center for intelligent material systems and structures at Virginia Tech and the associate editor of the journal of intelligent material systems and structures. Dr. Leo has published 27 journal papers and 23 refereed and 43 unrefereed conference papers. He is the recipient of the 2001 NSF CAREER award and has served as advisor to 16 completed Master’s students and two completed Doctoral students. Dr. Leo is currently advising six PhD students and two Master’s students.