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.
Using proprietary technology, Discover Technologies has developed ionomeric polymer transducers that are capable of
long-term operation in air. These "Plastic MuscleTM" transducers are useful as soft distributed actuators and sensors and
have a wide range of applications in the aerospace, robotics, automotive, electronics, and biomedical industries.
Discover Technologies is developing novel fabrication methods that allow the Plastic MusclesTM to be manufactured on
a commercial scale.
The Plastic MuscleTM transducers are capable of generating more than 0.5% bending strain at a peak strain rate of over
0.1 %/s with a 3 V input. Because the Plastic MusclesTM use an ionic liquid as a replacement solvent for water, they are
able to operate in air for long periods of time. Also, the Plastic MusclesTM do not exhibit the characteristic "back
relaxation" phenomenon that is common in water-swollen devices.
The elastic modulus of the Plastic MuscleTM transducers is estimated to be 200 MPa and the maximum generated stress
is estimated to be 1 MPa. Based on these values, the maximum blocked force at the tip of a 6 mm wide, 35 mm long
actuator is estimated to be 19 mN. Modeling of the step response with an exponential series reveals nonlinearity in the
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.
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.
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.
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 . 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.
Triton Systems Inc. has teamed with Virginia Tech to develop smart active materials for mirror shape control and stabilization systems utilized in thin film, space-based, polymeric mirrors. The development of novel lightweight space-qualified optics and support structures is of vital importance to science, industry, and national defense. Primary mirrors are one of the main drivers of the mass of space based optical systems. Therefore, lightweight optics is an essential component to reducing launch costs while increasing payload utility. Electroactive polymers represent a special class of “smart materials” whose electronic and physical properties such as conductivity, charge distribution, and shape can be changed in response to the environment (voltage and stress). The ability of electroactive polymers to change structure within a matrix in response to electrical stimulation has several applications for large ultra-lightweight optics. The Triton/VT team has begun the development of castable electroactive materials that do not depend on aqueous systems. These novel non-aqueous materials allow both an increase in voltage limits, and the ability to be used in environments where water would rapidly evaporate. These materials will allow the adjustment of the shape, to remove aberrations, as well as solve issues such as damping vibrations after re-pointing of large space telescopes.