Andrew J. Duncan

Ionomer design for augmented charge transport in novel ionic polymer transducers

Ionic polymer transducers are devices that display electromechanical transduction and are projected to have extensive applications as actuators and sensors. This study employs novel, highly branched sulfonated polysulfones (sBPS) as part of an investigation into the contribution of polymer topology to electromechanical transduction. Specifically, the ionomers are combined with an ionic liquid to determine the optimal ratio and method for maximizing ionic conductivity, where charge transport is essential to device performance. Two uptake methods are assessed for introduction of ionic liquid into the central ionomeric membrane. The effects of casting membranes in the presence of ionic liquid and swelling preformed membranes in ionic liquid on film stability and ionic conductivity are examined. Membranes cast from a solution of the ionomer and ionic liquid allow for direct targeting of the component ratio and a single-step process for membrane formation. Swelling conditions for preformed neat membranes combine time, temperature, and the presence of organic co-diluents to achieve the maximum stable uptake of ionic liquid. Comparison of optimal conditions for the various methods reveals that swelling with co-diluents achieves ionic conductivity of the imbibed membrane per uptake higher than the levels achieved with the casting process for highly sulfonated sBPS. However, for less sulfonated sBPS the casting process successfully produced membranes with ionic conductivities unreachable with the co-diluent process. Both methods will enable the production of high performance ionic polymer transducers constructed from novel sBPS ionomers and ionic liquids.

Forced and free displacement characterization of ionic polymer transducers

Ionic polymer transducers (IPT), sometimes referred to as artificial muscles, are known to generate a large bending strain and a moderate stress at low applied voltages (<5V). Recently Akle and Leo[1] reported extensional actuation in ionic polymer transducers. In this study, extensional IPTs are characterized under forced and free displacement boundary condition as a function of transducer architecture. The electrode thickness is varied from 10 μm up to 40 μm while three extensional actuators with Lithium, Cesium, and tetraethylammonium (TEA) mobile cations are characterized. Three fixtures are built in order to characterize the extensional actuation response. The first fixture measures the free displacement of an IPT sample sandwiched between two aluminum plates glued using the electrically conductive silver paste. In the second fixture a spring is compressed against the test sample with variable amounts to generate different levels of pre-stress and prevents the bending of the IPT. In the third fixture dead weights are placed on top of the sample in order to prevent bending. In the spring loaded fixture a thermocouple is placed in the proximity of the actuator and temperature is measured. The different transducers are characterized using a step voltage input and an alternating current (AC) sine wave input. The step input resulted in a logarithmic rise like displacement curve, while the low frequency (<0.1 Hz) AC excitation generated a sine wave displacement response with a strong first harmonic. The high frequency AC excitation generated a response similar to that of the step input. Comparing the measured temperature for step and AC response demonstrated that the sample is heating up when exited with a high frequency signal; which is leading to the expansion of the sample. Initial experimental results demonstrate a strong correlation between electrode architecture and the peak strain response. Strains on the order of 2% are observed with air stable ionic liquid based transducers. A correlation between the strain and charge buildup in the polymer is also characterized. Cesium (Cs) mobile cation outperformed all other tested mobile charges, while Potassium displaced the least. Keywords: Ionic Polymers, Transducer, Actuator, Electroactive Polymer, Extensional Actuator.

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.

Electromechanical performance and membrane stability of novel ionic polymer transducers constructed in the presence of ionic liquids

Ionic polymer transducers (IPT) are a class of devices that leverage electroactive polymers (EAP), specifically electrolyte-swollen ionomeric membranes, to perform energy conversions. Energy transformation from input to output is referred to as transduction and occurs between the electrical and mechanical domains. The present study expands on IPT investigations with a novel series of sulfonated polysulfones (sBPS), with specific interest in the effect of polymer topology on actuator performance. A hydrophilic ionic liquid was combined with a series of sBPS through a casting method to create hydrated membranes that contained target uptakes (f) of the diluent. The ionic liquids hydrophilic, yet organic nature raised the issue of its degree of compatibility and miscibility with the microphase separated domains of the host ionomeric membrane. Initial studies of the ionomer - ionic liquid morphology were performed with synchrotron small angle X-ray scattering (SAXS). The effective plasticization of the membranes was identified with dynamic mechanical analysis (DMA) in terms of varied storage modulus and thermal transitions with ionic liquid uptake. Electrical impedance spectroscopy (EIS) was employed to quantify the changes in ionic conductivity for each sBPS ionomer across a range of uptake. Combined results from these techniques implied that the presence of large amounts of ionic liquid swelled the hydrophilic domains of the ionomer and greatly increased the ionic conductivity. Decreases in storage modulus and the glass transition temperature were proportional to one another but of a lesser magnitude than changes in conductivity. The present range of ionic liquid uptake for sBPS was sufficient to identify the critical uptake (fc) for three of the four ionomers in the series. Future work to construct IPTs with these components will use the critical uptake as a minimum allowable content of ionic liquid to optimize the balance of electrical and mechanical properties for the device components.

Design for Optimized Electromechanical Transduction in Ionic Polymer Transducers Fabricated With Architecturally Controlled Ionomers

Ionic polymer transducers (IPT) are devices composed of ionomeric membranes, high surface area electrodes, and ion-conducting electrolytes that are capable of electromechanical transduction. This study aims to optimize the interactions between all three of these components to design a high performance IPT with novel ionomers. Equivalent circuit modeling of impedance data allowed for estimations of IPT capacitance due to changes in the compositions of the electrodes. Various methods for control of electrolyte uptake resulted in a range of ionic conductivity when combined with novel ionomers that vary in polymer backbone architecture and charge content. Although the ionic liquid was found to dominate the magnitude of the conductivity, the pathway for uptake was significant in determination of the overall maximum values. Combination of these optimized parameters for capacitance and ionic conductivity identified design criteria for potentially high performance IPTs to investigate the benefits of these novel ionomers in electroactive devices.Copyright

A correlation between extensional displacement and architecture of ionic polymer transducers

Ionic polymer transducers (IPT), sometimes referred to as artificial muscles, are known to generate a large bending strain and a moderate stress at low applied voltages (<5V). Bending actuators have limited engineering applications due to the low forcing capabilities and the need for complicated external devices to convert the bending action into rotating or linear motion desired in most devices. Recently Akle and Leo reported extensional actuation in ionic polymer transducers. In this study, extensional IPTs are characterized as a function of transducer architecture. In this study 2 actuators are built and there extensional displacement response is characterized. The transducers have similar electrodes while the middle membrane in the first is a Nafion / ionic liquid and an aluminum oxide - ionic liquid in the second. The first transducer is characterized for constant current input, voltage step input, and sweep voltage input. The model prediction is in agreement in both shape and magnitude for the constant current experiment. The values of α and β used are within the range of values reported in Akle and Leo. Both experiments and model demonstrate that there is a preferred direction of applying the potential so that the transducer will exhibit large deformations. In step response the model well predicted the negative potential and the early part of the step in the positive potential and failed to predict the displacement after approximately 180s has elapsed. The model well predicted the sweep response, and the observed 1st harmonic in the displacement further confirmed the existence of a quadratic in the charge response. Finally the aluminum oxide based transducer is characterized for a step response and compared to the Nafion based transducer. The second actuator demonstrated electromechanical extensional response faster than that in the Nafion based transducer. The Aluminum oxide based transducer is expected to provide larger forces and hence larger energy density.

Ionomer Design for Augmented Charge Transport in Novel Ionic Polymer Transducers

Ionic polymer transducers (IPT) are devices that display electromechanical transduction and have been applied extensively both as actuators and sensors. This study employs novel, highly-branched sulfonated polysulfones to investigate the contribution of polymer topology to electromechanical transduction. We assess two methods for ionic liquid uptake in the central ionomeric membrane. The effects of casting membranes in the presence of ionic liquid and swelling cast membranes in ionic liquid on film stability and ionic conductivity are examined. Casting in the presence of ionic liquid appears to cause macrophase separation of the ionic liquid from the polymer, causing limited charge transport. Overall, swelling appears to be a more stable method and achieves higher conductivity at lower uptake levels.Copyright