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1. Modeling Actuation of Ionomer Cilia in Salt Solution Under an External Electric Field

   https://doi.org/10.1115/DSCC2019-9060  

   Boldini, Alain; Rosen, Maxwell; Cha, Youngsu; Porfiri, Maurizio (October 2019, Proceedings of the ASME 2019 Dynamic Systems and Control Conference DSCC 2019)

   Abstract A recent experiment by Kim’s group from the University of Nevada, Las Vegas has demonstrated the possibility of actuating ionomer cilia in salt solution. When these actuators are placed between two external electrodes, across which a small voltage is applied, they move toward the cathode. This is in stark contrast with the case of ionic polymer metal composites, where these ionomers are plated by metal electrodes and bending occurs towards the anode. Here, we seek to unravel the factors underlying the motion of ionomer cilia in salt solution through a physically-based model of actuation. In our model, electrochemistry is described through the Poisson-Nernst-Planck system in terms of concentrations of cations and anions and voltage, which is solved through the finite element method. Based on computer simulations, we establish that Maxwell stress is the main driving force for the motion of the cilia. 

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2. On Structural Theories for Ionic Polymer Metal Composites: Balancing Between Accuracy and Simplicity

   https://doi.org/10.1007/s10659-020-09779-4  

   Boldini, Alain; Bardella, Lorenzo; Porfiri, Maurizio (June 2020, Journal of Elasticity)

   Ionic polymer metal composites (IPMCs) are soft electroactive materials that are finding increasing use as actuators in several engineering domains, where there is a need of large compliance and low activation voltage. Similar to traditional sandwich structures, an IPMC comprises a hydrated ionomer core that is sandwiched by two stiffer electrodes. The application of a voltage across the electrodes drives charge migration within the ionomer, which, in turn, contributes to the development of an eigenstress, associated with osmotic pressure and Maxwell stress. Critical to IPMC actuation is the variation of the eigenstress through the thickness of the ionomer, which is responsible for strain localization at the ionomer-electrode interfaces. Despite considerable progress in the development of reliable continuum theories and finite element tools, accurate structural theories that could beget physical insight into the inner workings of IPMC actuation are lacking. Here, we seek to bridge this gap by contributing a principled methodology to structural modeling of IPMC actuation. Our approach begins with the study of the IPMC electrochemistry through the method of matched asymptotic expansions, which yields a semi-analytical expression for the eigenstress as a function of the applied voltage. Hence, we establish a total potential energy that accounts for the strain energy of the ionomer, the strain energy of the electrodes, and the work performed by the eigenstress. By projecting the IPMC kinematics on select beam-like representations and imposing the stationarity of the total potential energy, we formulate rigorous structural theories for IPMC actuation. Not only do we examine classical low-order and higher-order beam theories, but we also propose enriched theories that account for strain localization near the electrodes. The accuracy of these theories is assessed through comparison with finite element simulations on a plane-strain problem of non-uniform bending. Our results indicate that an enriched Euler-Bernoulli beam theory, with three independent field variables, is successful in capturing the main features of IPMC actuation at a limited computational cost. 

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3. On structural models for ionic polymer metal composites (SPIE Best Student Paper Finalist)

   https://doi.org/10.1117/12.2558302  

   Boldini, Alain; Bardella, Lorenzo; Porfiri, Maurizio (April 2020, SPIE Smart Structures + Nondestructive Evaluation, Electroactive Polymer Actuators and Devices (EAPAD) XXII)

   Ionic polymer metal composites (IPMCs) are a class of soft electroactive polymers. IPMCs comprise a soft ionic polymer core, on which two stiff metal electrodes are plated. These active materials exhibit large bend- ing upon the application of a small driving voltage across their electrodes, in air or in aqueous environments. In a recent work, we presented compelling theoretical and numerical evidence suggesting that ionic polymer membranes exhibit complex multiaxial deformations neglected by reduced-order structural models. Where most beam theories (including Euler-Bernoulli, Timoshenko, and most higher-order shear deformation models) would suggest vanishing through-the-thickness deformation, we discover the onset of localized deformation that rever- berates into axial stretching. Building upon this effort, here we investigate the role of the electrodes and shear on multiaxial deformations of IPMCs. We establish a novel structural theory for IPMCs, based on the Euler- Bernoulli kinematics enriched with the through-the-thickness deformation in the ionic polymer, computed from a Saint-Venant-like problem for uniform bending. While considering boundary conditions that elicit non-uniform bending, we compare the results of this model against classical Euler-Bernoulli beam theory without enrichment and finite element simulations, encapsulating the nonlinear response of the material. We demonstrate that our theory can predict the macroscopic displacement of the IPMC, along with the localized deformation in the ionic polymer at the interface with the electrodes, which are not captured by the classical Euler-Bernoulli beam theory. This work paves the way to the development of more sophisticated structural theories for IPMCs and analogous active materials, affording an accurate description of deformations at a limited computational cost. 

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4. Advancing Structural Supercapacitors with Hydrated Polymer

   https://doi.org/10.1149/MA2024-0115mtgabs  

   Teymoory, Parya; Chang, Yu-Che; Shen, Caiwei (August 2024, ECS Meeting Abstracts)

   Structural supercapacitors, capable of bearing mechanical loads while storing electrical energy, hold great promise for enhancing mobile system efficiencies. However, developing practical structural supercapacitors often involves a challenging balance between mechanical and electrochemical performance, particularly in their electrolytes. Traditional research has focused on bi-continuous phase electrolytes (BPEs), which typically comprise high liquid content that weakens mechanical strength, and inert solid phases that hinder ion conduction and block electrode surfaces. Our previous work introduces a novel approach with a hydrated polymer electrolyte, demonstrating enhanced multifunctionality. This electrolyte, derived from controlled hydration of PET-LiClO₄, forms a trihydrate (LiClO₄∙3H₂O) structure, where water molecules bond with ions without forming a liquid phase, thereby improving ion mobility while maintaining the base polymer's mechanical properties. This new design also promotes better electrochemical interfaces with electrodes, a significant advancement over traditional BPEs. In this study, we further enhance the performance and processability of such hydrated polymer electrolytes by incorporating polylactic acid (PLA) as the base polymer and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as the salt. The electrolyte, prepared through solution casting and subsequent controlled hydration, consistently remains an amorphous solid solution in both dry and hydrated states, as confirmed by DSC, XRD, and FTIR analyses. Our tests on ionic conductivity and mechanical properties reveal that adding water to the polymer electrolyte substantially increases ionic conductivity while retaining mechanical properties. A specific composition demonstrated a remarkable increase in ionic conductivity coupled with superior toughness surpassing the base polymer. Furthermore, we successfully fabricated and tested structural supercapacitor devices made of composites of carbon fibers and these new electrolytes. The prototypes presented enhanced toughness with significant energy storage performance, demonstrating their vast application potential due to their outstanding multifunctionality. 

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5. Surface roughness effects on ionic polymer-metal composite (IPMC) sensitivity for compression loads

   https://doi.org/10.1117/12.2613127  

   Nagel, William S.; Hussain, Omar A.; Fakharian, Omid; Aureli, Matteo; Leang, Kam K. (January 2022, Electroactive Polymer Actuators and Devices (EAPAD) XXIV)

   The soft and compliant nature of ionic polymer-metal composite (IPMC) sensors has recently been investigated for various applications in soft robotic and mechatronic devices. Recent results of physics-based chemoelectromechanical modeling suggest that IPMC asymmetric surface roughening may enhance the sensitivity under compression. This paper presents initial experimental results on IPMC compression sensors fabricated with varying degrees of asymmetric surface roughness. The roughness is created through a simple mechanical sanding process on the base polymer material, referred to as "polymer abrading technique'", followed by traditional electroless plating to create electrodes. Sample sensors are characterized by measuring the voltage response under different compressive loads. The results show consistently increased sensor sensitivity of the asymmetrically roughened IPMCs versus a control sample. Sensitivity increases non-monotonically with rougher electrode surfaces, where maximum sensitivity of about 0.0433 mV/kPa is achieved with sensor electrodes with 53-74~micrometer abrasions. More variability is also observed through augmented electrode roughness, suggesting greater flexibility for IPMC sensor design. These results align with predictions from the existing physics-based chemoelectromechanical model. 

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