Matthew D. Bennett

Ionic liquids as novel solvents for ionic polymer transducers

The use of ionic liquids as solvents for ionic polymer (specifically, Nafion) transducers is demonstrated. Ionic liquids are attractive for this application because of their high inherent stability. Ionic liquids are salts that exist as liquids at room temperature and have no measureable vapor pressure. Therefore, the use of ionic liquids as solvents for ionic polymer transducers can eliminate the traditional problem of water evaporation in these devices. Another benefit of the use of ionic liquids in this way is the reduction or elimination of the characteristic back-relaxation common in water-solvated ionic polymer actuators. The results demonstrate that the viscosity of the ionic liquid and the degree to which the ionic liquid swells the membrane are the important physical parameters to consider. Five ionic liquids were studied, based on substituted pyrrolidinium, phosphonium, or imidazolium cations and fluoroanions. Of these five ionic liquids, transduction is demonstrated in three of them and the best results are obtained with 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ionic liquid. This substance has an electrochemical stability window of 4.1 V, a melting point of -10 °C, and a viscosity of 35-45 cP [19]. Results demonstrate that platinum-plated Nafion transducers solvated with this ionic liquid exhibit sensing and actuation responses and that these transducers are stable in air. Endurance testing of this sample reveals a decrease in the free strain of only 25 % after 250,000 actuation cycles in air.

Directly Copolymerized Poly(arylene sulfide sulfone) and Poly(arylene ether sulfone) Disulfonated Copolymers for Use in Ionic Polymer Transducers

Polymeric electromechanical transducers were identified and based on various novel ion-exchange membranes bonded between two conductive metal layer electrodes. Imposed deformations and small electric fields allowed both sensing and actuation applications. Soft actuator materials produced large bending displacements when only a small voltage was applied across the membrane electrode assembly. Charge motion from one pole to the other pole produced electromechanical coupling effects in the ionic materials through the electric double layer. By increasing the surface area of the electrodes, thereby increasing the capacitance, it was shown that the motion of charges and actuator performance increases, thus indicating a strong correlation between the capacitance and charge motion/performance. Manipulation of the morphology of the electrodes by enhancing the capacitance and effective interfacial area of the conductive electrodes produced major effects on performance and transduction. Transducer actuation performance at lower frequencies was enhanced by employing a novel electrode fabrication technique which could utilize RuO 2 instead of platinum. At higher frequencies, mass transport and interfacial resistance appeared to play pivotal roles in actuator performance.

Morphological and electromechanical characterization of ionic liquid/Nafion polymer composites

Ionic liquids have shown promise as replacements for water in ionic polymer transducers. Ionic liquids are non-volatile and have a larger electrochemical stability window than water. Therefore, transducers employing ionic liquids can be operated for long periods of time in air and can be actuated with higher voltages. Furthermore, transducers based on ionic liquids do not exhibit the characteristic back relaxation that is common with water-swollen materials. However, the physics of transduction in the ionic liquid-swollen materials is not well understood. In this paper, the morphology of Nafion/ionic liquid composites is characterized using small-angle X-ray scattering (SAXS). The electromechanical transduction behavior of the composites is also investigated. For this testing, five different counterions and two ionic liquids are used. The results reveal that both the morphology and transduction performance of the composites is affected by the identity of the ionic liquid, the cation, and the swelling level of ionic liquid within the membrane. Specifically, speed of response is found to be lower for the membranes that were exchanged with the smaller lithium and potassium ions. The response speed is also found to increase with increased content of ionic liquid. Furthermore, for the two ionic liquids studied, the actuators swollen with the less viscous ionic liquid exhibited a slower response. The slower speed of response corresponds to less contrast between the ionically conductive phase and the inert phase of the polymer. This suggests that disruption of the clustered morphology in the ionic liquid-swollen membranes as compared to water-swollen membranes attenuates ion mobility within the polymer. This attenuation is attributed to swelling of the non-conductive phase by the ionic liquids.

Ionic electroactive hybrid transducers

Ionic electroactive actuators have received considerable attention in the past ten years. Ionic electroactive polymers, sometimes referred to as artificial muscles, have the ability to generate large bending strain and moderate stress at low applied voltages. Typical types of ionic electroactive polymer transducers include ionic polymers, conducting polymers, and carbon nanotubes. Preliminary research combining multiple types of materials proved to enhance certain transduction properties such as speed of response, maximum strain, or quasi-static actuation. Recently it was demonstrated that ionomer-ionic liquid transducers can operate in air for long periods of time (>250,000 cycles) and showed potential to reduce or eliminate the back-relaxation issue associated with ionomeric polymers. In addition, ionic liquids have higher electrical stability window than those operated with water as the solvent thereby increasing the maximum strain that the actuator can produce. In this work, a new technique developed for plating metal particulates on the surface of ionomeric materials is applied to the development of hybrid transducers that incorporate carbon nanotubes and conducting polymers as electrode materials. The new plating technique, named the direct assembly process, consists of mixing a conducting powder with an ionomer solution. This technique has demonstrated improved response time and strain output as compared to previous methods. Furthermore, the direct assembly process is less costly to implement than traditional impregnation-reduction methods due to less dependence on reducing agents, it requires less time, and is easier to implement than other processes. Electrodes applied using this new technique of mixing RuO2 (surface area 45~65m2/g) particles and Nafion dispersion provided 5x the displacement and 10x the force compared to a transducer made with conventional methods. Furthermore, the study illustrated that the response speed of the transducer is optimized by varying the vol% of metal in the electrode. For RuO2, the optimal loading was approximately 45%. This study shows that carbon nanotubes electrodes have an optimal performance at loadings around 30 vol%, while PANI electrodes are optimized at 95 vol%. Due to low percolation threshold, carbon nanotubes actuators perform better at lower loading than other conducting powders. The addition of nanotubes to the electrode tends to increase both the strain rate and the maximum strain of the hybrid actuator. SWNT/RuO2 hybrid transducer has a strain rate of 2.5%/sec, and a maximum attainable peak-to-peak strain of 9.38% (+/- 2V). SWNT/PANI hybrid also increased both strain and strain rate but not as significant as with RuO2. PANI/RuO2 actuator had an overwhelming back relaxation.

Physics of transduction in ionic liquid-swollen Nafion membranes

Ionic polymer transducers are a class of electroactive polymers that are able to generate large strains (1-5%) in response to low voltage inputs (1-5 V). Additionally, these materials generate electrical charge in response to mechanical strain and are therefore able to operate as soft, distributed sensors. Traditionally, ionic polymer transducers have been limited in their application by their hydration dependence. This work seeks to overcome this limitation by replacing the water with an ionic liquid. Ionic liquids are molten salts that exhibit very high thermal and electrochemical stability while also possessing high ionic conductivity. Results have shown that an ionic liquid-swollen ionic polymer transducer can operate for more than 250,000 cycles in air as compared to about 2,000 cycles for a water-swollen transducer. The current work examines the mechanisms of transduction in ionic liquid-swollen transducers based on Nafion polymer membranes. Specifically, the morphology and relevant ion associations within these membranes are investigated by the use of small-angle X-ray scattering (SAXS), Fourier transform infrared spectroscopy (FTIR), and nuclear magnetic resonance spectroscopy (NMR). These results reveal that the ionic liquid interacts with the membrane in much the same way that water does, and that the counterions of the Nafion polymer are the primary charge carriers. The results of these analyses are compared to the macroscopic transduction behavior in order to develop a model of the charge transport mechanism responsible for electromechanical coupling in these membranes.

Optimization of active electrodes for novel ionomer-based ionic polymer transducers

This study expands the number of novel synthetic ionomers specifically designed for performance as ionic polymer transducers (IPT) membranes, specifically employing a highly branched sulfonated polysulfone. Control of the synthetic design, characterization, and application of the novel ionomer is intended to allow fundamental study of the effect of polymer branching on electromechanical transduction in IPTs. Fabrication methods were developed based upon the direct application process (DAP) to construct a series of stand-alone electrodes as well as full IPTs with corresponding electrode compositions. Specifically, the volumetric ratio of RuO2 conducting particles to the novel ionomeric matrix was varied from 0 - 45 vol % in the electrodes. Electrical impedance spectroscopy was employed to determine the electrical properties and their variation with electrode composition separate from and in the IPT. A percolation threshold was detected for increased ionic conductivity of the stand-alone electrodes and the full IPTs based on increased loading of conducting particles in the electrodes. An equivalent electrical circuit model was applied to fit the impedance data and implicated interfacial and bulk effects contributing differently to the electrical properties of the electrodes and IPT as a whole. The fabricated IPT series was further tested for bending actuation in response to applied step voltages and represents the first demonstration of IPTs constructed with the DAP process using 100 % novel ionomer in all components. The percolation behavior extended to the bending actuation responses for strain and voltage-normalized strain rate and is useful in optimizing IPT components for maximum performance regardless of the ionomer employed.

A Dynamic Wall Shear Stress Sensor Based on Ionic Polymers

This paper presents the first implementation of a novel class of dynamic time-resolved direct skin friction measurements sensor based on active ionic polymer transducers. These ionic polymer sensors have the advantage that they contain no moving parts, perform a direct measurement of shear, and can be mounted directly to the surface of an existing vessel with no modification. During the present effort we characterize the accuracy of the sensors and validate their dynamic measurement response. Using an oscillating Stokes layer calibration procedure we demonstrate measurement accuracy in fluctuating shear on the order of 4.92% over a range of stresses of +/- 3 Pa and signal-to-noise-ratio on the order of 60 dB. The frequency response of the sensor is over 10 kHz however due to experimental limitations we were not able to calibrate for frequencies higher than 140 Hz. These sensors have been shown to be insensitive to vibration or pressure. Also, an automatic change of impedance compensation approach is proposed that allows in-situ recalibration of the sensors and accounts for environmental effects such as changes of temperature on the sensors performance. The results demonstrate the potential for using ionic polymer sensors to perform accurate, high frequency measurements of shear in turbulent boundary layers.Copyright

Hybrid Actuation in Coupled Ionic / Conducting Polymer Devices

Ionic polymer membrane actuators represent a relatively new and exciting entry into the field of smart materials. Several key limitations of these transducers have prevented them from experiencing widespread use, however. For example, the bandwidth of these devices is limited at very low frequencies by characteristic relaxation and at high frequencies by the low elastic modulus of the polymer. In this paper, an overview of the initial results of work with hybrid ionic / conducting polymer actuators is presented. These hybrid actuators are devices that combine the electromechanical coupling of ionic polymer actuators and conducting polymer actuators into one coupled device. Initial results show that these hybrid devices have the potential to offer marked advantages over traditional ionic polymer membrane transducers, including increased stress and strain generation and higher actuation bandwidth. Details of the preparation of these devices and performance metrics are presented and comparisons to baseline materials are made.

Investigation of the physics of transduction in nafion / Ionic liquid composite membranes

Ionomeric polymer actuators based on Nafion membranes exhibit large bending motion (1%) under the application of small voltages (1-5 V). Actuation in these materials is believed to arise from the field-induced motion of mobile charges when a voltage is applied. In order for this charge motion to occur, the material must be swollen with a diluent, typically water. However, dehydration of the water limits the lifetime of these actuators in non-aqueous environments. Recently, highly stable ionic liquids have been demonstrated as viable diluents for these actuators. In the current paper, the physics of transduction in these ionic liquid-swollen Nafion membranes are investigated. Small-angle X-ray scattering reveals that the structure and properties of the ionic liquid have a strong influence on the morphology of the composites. Infrared spectroscopy is used to probe the ion associations within the films and shows that the ionic liquids are able to effectively mobilize the counterions of the Nafion membrane. Nuclear magnetic resonance spectroscopy is also used to investigate the composites and reveals that the mobility of the counterions increases as the content of ionic liquid within the membrane is increased. The results of these characterizations are compared to an experimental investigation of transduction in Nafion / ionic liquid composites to form an interpretation of the mechanisms of actuation. This comparison reveals that the counterions of the Nafion membrane are the primary charge carriers and that it is the motion of these mobile charges that gives rise to the actuation behavior of the films.

Ionic Liquids as Hyper-Stable Solvents for Ionic Polymer Transducers

Nafion™ 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 leach out of the polymer by evaporation as water will. This paper will present the results of some initial work with ionic liquids and will compare these materials to the same polymers solvated with water.Copyright