Alvo Aabloo

Molecular Dynamics Modeling of Proton Transport in Nafion and Hyflon Nanostructures

Classical molecular dynamics modeling studies at 363 K are reported of the local atomic-level and macroscopic nanostructures of two well-known perfluorosulfonic acid proton exchange polymer membrane materials: Nafion and Hyflon. The influence of the different side-chain lengths in the two polymers on local structure is relatively small: Hyflon exhibits slightly greater sulfonate-group clustering, while Nafion has more isolated side chains with a higher degree of hydration around the SO(3)(-) side-chain ends. This results in shorter mean residence times for water molecules around the end groups in Nafion. Hyflon also displays a lower degree of phase separation than Nafion. The velocities of the water molecules and hydronium ions are seen to increase steadily from the polymer backbone/water interface toward the center of the water channels. Because of its shorter side chains, the number of hydronium ions is approximately 50% higher at the center of the water channels in Hyflon, and their velocities are approximately 10% higher. The water and H(3)O(+) diffusion coefficients are therefore higher in the shorter side-chain Hyflon system: 6.5 x 10(-6) cm(2)/s and 25.2 x 10(-6) cm(2)/s, respectively; the corresponding values for Nafion are 6.1 x 10(-6) cm(2)/s and 21.3 x 10(-6) cm(2)/s, respectively. These calculated values compare well with experiment: 4 x 10(-6) cm(2)/s for vehicular H(3)O(+) diffusion.

Miniature crystal models of cellulose polymorphs and other carbohydrates

Miniature crystal models of cellulose and other carbohydrates were evaluated with the molecular mechanics program MM3. The models consisted of groups of 24 to 32 monosaccharide residues, with the models of mono- and disaccharides based on well-established, single-crystal work. Structures of the cellulose forms and cellotetraose were based on published work using fibre diffraction methods. A structure for the single-chain I alpha cellulose unit cell was also tested. A dielectric constant of about 4 was best for this type of work. Calculated intra- and intermolecular energy for glucose agreed with literature values for the heat of combustion. Cellulose II had the lowest calculated energy for a cellulose form, followed by I alpha, cellulose III(I), ramie I, IV(II) and IV(I). Optimization of cellulose IV caused larger mean atomic movements from the original crystallographic positions than the other cellulose forms, and cellotetraose had larger movements than any of the other structures. Lattice energies for the cellulose forms were about 20 kcal/mol of glucose residues, with a dominant van der Waals component.

Nanoporous carbon-based electrodes for high strain ionomeric bending actuators

Ionic polymer metal composites (IPMCs) are electroactive material devices that bend at low applied voltage (1–4 V). Inversely, a voltage is generated when the materials are deformed, which makes them useful both as sensors and actuators. In this paper, we propose two new highly porous carbon materials as electrodes for IPMC actuators, generating a high specific area, and compare their electromechanical performance with recently reported RuO2 electrodes and conventional IPMCs. Using a direct assembly process (DAP), we synthesize ionic liquid (Emi-Tf) actuators with either carbide-derived carbon (CDC) or coconut-shell-based activated carbon-based electrodes. The carbon electrodes were applied onto ionic liquid-swollen Nafion membranes using a direct assembly process. The study demonstrates that actuators based on carbon electrodes derived from TiC have the greatest peak-to-peak strain output, reaching up to 20.4 me (equivalent to>2%) at a 2 V actuation signal, exceeding that of the RuO2 electrodes by more than 100%. The electrodes synthesized from TiC-derived carbon also exhibit significantly higher maximum strain rate. The differences between the materials are discussed in terms of molecular interactions and mechanisms upon actuation in the different electrodes.

Nanocarbon based ionic actuators—a review

Nanocarbons represented especially by carbon nanotubes (CNTs) and graphene have been of great interest during the last two decades, both from a fundamental point of view and for future applications. The most eye-catching features of carbon nanostructures (CNSs) are their electronic, mechanical, optical and chemical characteristics, which open a way for versatile applications. Among those future prospects, actuators are one of the promising technologies. Since 1999 when the first macroscopic actuator containing CNTs was reported, the interest of utilizing these materials as well as other CNSs in active systems has been triggered all over the world. This paper gives a thorough review as well as in-depth descriptions of the many aspects of nanocarbon-based actuators. The review covers aspects of worldwide research and development of nanocarbon ionic actuators up to 2012. Materials which are covered by this review include CNTs and their composites, carbon nanofibres (CNFs), graphene and its derivatives, microporous carbon materials (for example carbide derived carbons (CDCs) and carbon aerogels) as well as the possible combinations of these materials. The considered aspects cover the following fields: synthesis and characterization of the investigated materials, the actuation mechanism as well as modelling and simulation. Applications comprising system integration and device development are also reviewed within this paper. (Some figures may appear in colour only in the online journal)

An explicit physics-based model of ionic polymer-metal composite actuators

The Poisson-Nernst-Planck system of equations is used to simulate the charge dynamics due to ionic current and resulting time-dependent displacement of ionic polymer-metal composite (IPMC) materials. Measured data show that currents through the polymer of IPMC cause potential gradients on the electrodes. Existing physics based models of IPMC do not explicitly consider how this affects the charge formation near the electrodes and resulting actuation of IPMC. We have developed an explicit physics based model that couples the currents in the polymer to the electric current in the continuous electrodes of IPMC. The coupling is based on the Ramo-Shockley theorem. The circular dependency concept is used to explain how the dependency between the ionic current and the potential drop in the electrodes is calculated and how they affect each other. Simulations were carried out using the finite element method. Calculated potential gradients, electric currents, and tip displacement of IPMC were validated against exper...

A self-oscillating ionic polymer-metal composite bending actuator

This paper presents an electromechanical model of an ionic polymer-metal composite (IPMC) material. The modeling technique is a finite element method (FEM). An applied electric field causes the drift of counterions (e.g., Na+), which, in turn, drags water molecules. The mass and charge imbalance inside the polymer is the main cause of the bending motion of the IPMC. The studied physical effects have been considered as time dependent and modeled with FEM. The model takes into account the mechanical properties of the Nafion polymer as well as the thin coating of the platinum electrodes and the platinum diffusion layer. The modeling of the electrochemical reactions, in connection with the self-oscillating behavior of an IPMC, is also considered. Reactions occurring on the surface of the platinum electrode, which is immersed into formaldehyde (HCHO) solution during the testing, are described using partial differential equations and also modeled using FEM. By coupling the equations with the rest of the model, ...

In search of better electroactive polymer actuator materials: PPy versus PEDOT versus PEDOT–PPy composites

A comparative study of metal-free air-operated polypyrrole and PEDOT based trilayer actuators is presented. Actuators made of both pure and combined conducting polymers are considered. Trilayer bending actuators, synthesized in similar conditions, are characterized in terms of the structure, electrochemical and electro-chemo-mechanical properties. The characterization was carried out using two popular electrolytes: LiTFSI in propylene carbonate and a room-temperature ionic liquid EMIm TFSI. The results reveal that structure and actuation properties of the synthesized actuators depend on both the polymer chosen for the chemically synthesized electrode layer as well as the electrochemically synthesized working layer.

A Distributed Model of Ionomeric Polymer Metal Composite

This article presents a novel model of an ionomeric polymer metal composite (IPMC) material. An IPMC is modeled as a lossy RC distributed line. Unlike other electro-mechanical models of an IPMC, the distributed nature of our model permits modeling the non-uniform bending of the material. Instead of modeling the tip deflection or uniform deformation of the material, we model the changing curvature. The transient behavior of the electrical signal as well as the transient bending of the IPMC are described by partial differential equations. By implementing the proper initial and boundary conditions we develop the analytical description of the possibly non-uniform transient behavior of an IPMC consistent with the experimental results.

Ionic electroactive polymer artificial muscles in space applications

A large-scale effort was carried out to test the performance of seven types of ionic electroactive polymer (IEAP) actuators in space-hazardous environmental factors in laboratory conditions. The results substantiate that the IEAP materials are tolerant to long-term freezing and vacuum environments as well as ionizing Gamma-, X-ray, and UV radiation at the levels corresponding to low Earth orbit (LEO) conditions. The main aim of this material behaviour investigation is to understand and predict device service time for prolonged exposure to space environment.

Molecular dynamics simulation of the LiBF4-PEO system containing Al2O3 nanoparticles

Abstract The amorphous LiBF4(PEO)20 system has been simulated alone and containing a ca. 14-A diameter Al2O3 nanoparticle and in juxtaposition with a ca. 65-A thick α-Al2O3 slab at a nominal temperature of 293 K by Molecular Dynamics (MD) methods. Li-ion mobility in the poly(ethylene oxide) (PEO) host is found to increase on the addition of the nanoparticle; the effect is also noticeable for the alumina slab. This can be seen as theoretical confirmation of the positive influence of nanoparticles on ion mobility in a PEO–salt system, as observed earlier experimentally. Other effects observed are related to this Li-ion mobility enhancement: PEO forms an immobilised coordination sphere around the particle and an immobilised layer at the surface of the α-alumina slab. No Li ions are found near the particle or at the slab surface. Instead, two to three unpaired BF4− anions are found attached to the particle within the region of immobilised PEO and at least one is found immobilised on the slab surface, leaving free Li ions in the regions away from the particle and slab surfaces. No more than 60% of the Li ions form ion pairs and ion clusters in the regions away from the particle surface and up to 87% of the Li ions form ion pairs and ion clusters in the regions away from the slab surface.