Ionic polymers are a recently discovered class of active materials that exhibit bidirectional electromechanical coupling. They are `soft' transducers that perform best when the mechanical deformation involves bending of the transducer. Ionic polymers are low voltage actuators -- they only require inputs on the order of 1V and cannot tolerate voltages above approximately 10V. The mechanisms responsible for the electromechanical coupling are not yet fully understood, and reports of the capabilities and limitations of ionic polymer transducers vary widely. In addition, suitable engineering models have not been developed. This document presents a dynamic model for ionic polymer transducers that is based on a pair of symmetric, linearly coupled equations with frequency dependent coefficients. The model is presented in the form of an equivalent circuit, employing an ideal transformer with a frequency dependent turns ratio to represent the electromechanical coupling. The circuit elements have clear physical interpretations, and expressions relating them to transducer dimensions and material properties are derived herein. The material parameters required for the model: modulus, density, electrical properties, and electromechanical coupling term are determined experimentally. The model is then validated by comparing simulated and experimental responses, and the agreement is good. Further validation is presented in the form of extensive experiments that confirm the predicted changes in transducer performance as transducer dimensions are varied. In addition, reciprocity between mechanical and electrical domains is demonstrated. This reciprocity is predicted by the model, and is a direct result of the symmetry in the equations on which the model is based. The capabilities of ionic polymer sensors and actuators, when used in the cantilevered bender configuration, are discussed and compared to piezoceramic and piezo polymer cantilevered benders. The energy density of all three actuators are within an order of magnitude of one another, with peak values of approximately 10J/m^3 and 4mJ/kg for ionic polymer actuators actuated with a 1.2V signal. Ionic polymer sensors compare favorably to piezoelectric sensors. Their charge sensitivity is approximately 320E-6C/m for a 0.2 x 5 x 17mm cantilevered bender, two orders of magnitude greater than a piezo polymer sensor with identical dimensions. This work is concluded with a demonstration of feedback control of a device powered by ionic polymer actuators. An ionic polymer sensor was used to provide the displacement feedback signal. This experiment is the first demonstration of feedback control using an ionic polymer sensor. Compensator design was performed using the model developed in the first chapter of this document, and experiments confirmed that implementation of the control scheme improved, in a narrow frequency range, the system's ability to track sinusoidal inputs. / Ph. D.

Ionic polymer transducers are a class of electroactive polymer materials that exhibit coupling between the electrical, chemical, and mechanical domains. With the ability for use as both sensors and actuators, these compliant, light weight, low voltage materials have the potential to benefit diverse application areas. Since the transduction properties of these materials were recently discovered, full understanding of their dynamic characteristics has not yet been achieved. This research has the goal of better understanding the actuation response of ionic polymers. A specific emphasis has been placed on investigating the observed nonlinear behavior because the existing proposed models do not account for these characteristics. Employing the Volterra representation, harmonic ratio analysis, and multisine excitations, characterization results for cantilever samples showed that the nonlinearity is dynamic and input-dependent, dominant at low frequencies, and that its influence varies depending on the solvent. It was determined that lower viscosity solvents trigger the nonlinear mechanisms at higher frequencies. Additionally, the primary components of the harmonic distortion appear to result from quadratic and cubic nonlinearities. Using knowledge gained from the characterization study, the utility of different candidate system structures was explored to model these nonlinear response characteristics. The ideal structure for modeling the current-controlled voltage and tip velocity was shown to consist of an underlying linear system with a dynamic input nonlinearity. The input nonlinearity is composed of a parallel connection of linear and nonlinear terms, where each nonlinear element has the form of a Hammerstein system. This system structure was validated against data from measured time and frequency responses. As a potential application, and consequently further validation of the chosen model structure, a square-plate polymer actuator was considered. In this study, the plate was clamped at the four corners where a uniform input was applied, measuring the center-point displacement. Characterization and modeling were performed on this system, with results similar to the cantilever sample. Applying output feedback control, in the form of proportional-integral compensation, showed that accurate tracking performance could be achieved in the presence of nonlinear distortions. Special attention was extended here to the potential application in deformable mirror systems. / Ph. D.

Ionic polymer transducers (IPT) are a class of electroactive polymer devices that exhibit electromechanical coupling through charge transport in ionomeric membranes that contain a charge mobilizing diluent and are interfaced with conducting electrodes. Applications of these active materials have been broadly developed in the field of actuators and sensors. Advances in fundamental understanding of IPT performance mechanisms and tuning of the device components has primarily focused on transducers constructed with the commercial ionomer Nafion® due to its overall stability, high ionic conductivity, and availability. The much smaller number of studies conducted with non-perfluorosulfonated ionomers concentrated on changes in chemical composition to address processability, price, ionic conductivity, and hydrated modulus of the final IPT. Also, nearly all ionic polymer transducers operated with water as the diluent until the recent successful development of IPTs with ionic liquids. The objective of this research is to increase the understanding of electromechanical transduction in ionic polymer transducers through the synthesis and characterization of novel branched ionomers. Controlled branching is achieved in sulfonated polysulfones (sBPS) through employment of an oligomeric A₂ + B₃ step-growth polymerization. Structure – property relationships are established for a series of linear and branched sulfonated polysulfones to resolve the effects of polymer topology and charge content on ionomer properties such as hydrated modulus and ionic conductivity. Furthermore, the variation of these parameters is investigated in the presence of ionic liquids as a function of ionic liquid uptake using two methods for introduction of the diluent. One of those methods, based on casting of IPT components in the presence of the ionic liquid, was applied to the Direct Application Process to produce a controlled set of IPT electrodes and transducers to investigate percolation effects of RuO₂ on the device's electrical properties and actuation characteristics. Equivalent circuit modeling of the component and transducer electrical impedance accurately modeled variations in contributing processes and material interfaces to estimate the evolution of effective capacitance based on the electrode composition. Combination of optimized electrode composition, ionic liquid uptake, and the series of linear and branched sulfonated polysulfones allowed for fabrication of a tailored set of novel ionic polymer transducers. Effects of the fabrication process on the ionic conductivity of the membranes and transducers are evaluated using electrical impedance spectroscopy, which also allowed for equivalent circuit modeling to calculate effective capacitance for the series of IPTs that varied in composition, topology, and uptake for both types of fabrication processes. The transducers described in this dissertation are the first IPTs to be designed and actuated with novel ionomers, specifically linear and branched sulfonated polysulfones, in the presence of ionic liquids. Use of sulfonated polysulfones allowed for realization of transducers with high uptakes of the ionic liquid diluent that retained significant hydrated modulus on the order of 2 GPa. Characterization of electromechanical transduction for the series of sBPS – IPTs was demonstrated in cantilever bending through frequency response analysis and step responses in the time domain to low input voltages. Both the ion content and polymer topology of the sBPS ionomeric matrix demonstrated a significant effect on the final actuation performance in relation to variations in charge transport. Also, IPTs constructed with a co-diluent swelling method which emphasized the formation and stability of the ionomer's charge transport pathway demonstrated the greatest actuation responses, up to a peak-to-peak strain of ~0.45 % and strain rates on the order of 0.1 % / s while producing significant blocked force (180 N/Vm). Combination of these actuation performance metrics resulted in maximum energy densities of 1150 mJ/kg and 2.23 mJ/mm³ for the corresponding IPT. / Ph. D.

Ionomeric polymer transducers exhibit electromechanical coupling capabilities. The transport of charge due to electric stimulus is the primary mechanism of actuation for a class of polymeric active materials known as ionomeric polymer transducers (IPTs). The research presented in this dissertation focuses on modeling the cation transport and cation steady state distribution due to the actuation of an IPT. Ion transport in the IPT depends on the morphology of the hydrated Nafion membrane and the morphology of the metal electrodes. Recent experimental findings show that adding conducting powders at the polymer-conductor interface increases the displacement output. However, it is difficult for a traditional continuum model based on transport theory to include morphology in the model. In this dissertation, a two-dimensional Monte Carlo simulation of ion hopping has been developed to describe ion transport in materials that have fixed and mobile charge similar to the structure of the ionic polymer transducer. In the simulation, cations can hop around in a square lattice. A step voltage is applied between the electrodes of the IPT, causing the thermally-activated hopping between multiwell energy structures. By sampling the ion transition time interval as a random variable, the system evolution is obtained. Conducting powder spheres have been incorporated into the Monte Carlo simulation. Simulation results demonstrate that conducting powders increase the ion conductivity. Successful implementation of parallel computation makes it possible for the simulation to include more powder spheres to find out the saturation percentage of conducting powders for the ion conductivity. To compare simulation results with experimental data, a multiscale model has been developed to increase the scale of Monte Carlo simulation. Both transient responses and steady state responses show good agreement with experimental measurements. / Ph. D.

Ionic polymer transducers (IPTs) are soft sensors and actuators which operate through a coupling of micro-scale chemical, electrical, and mechanical mechanisms. The use of ionic liquid as solvent for an IPT has been shown to dramatically increase transducer lifetime in free-air use, while also allowing for higher applied voltages without electrolysis. This work aims to further the understanding of the dominant mechanisms of IPT actuation and how these are affected when an ionic liquid is used as solvent. A micromechanical model of IPT actuation is developed following a previous approach given by Nemat-Nasser, and the dominant relationships in actuation are demonstrated through an analysis of electrostatic cluster interactions. The elastic modulus of Nafion as a function of ionic liquid uptake is measured using uniaxial tension tests and modeled in a micromechanical framework, showing an excellent fit to the data. Charge transport is modeled by considering both the cation and anion of the ionic liquid as mobile charge carriers, a phenomenon which is unique to ionic liquid IPTs as compared to their water-based counterparts. Numerical simulations are performed using the finite element method, and a modified theory of ion transport is discussed which can be extended to accurately describe electrochemical migration of ionic liquid ions at higher applied voltages. The results presented here demonstrate the dominant mechanisms of IPT actuation and identify those unique to ionic liquid IPTs, giving directions for future research and transducer development. / Master of Science