Molecular Dynamics (MD) techniques have been used to study the structure and dynamics of a model system of an
interpenetrating polymer (IPN) network for actuator devices. The systems simulated were generated using a Monte
Carlo-approach, and consisted of poly(ethylene oxide) (PEO) and poly(butadiene) (PB) in a 80-20 percent weight ratio
immersed into propylene carbonate (PC) solutions of LiClO<sub>4</sub>. The total polymer content was 32%, in order to model
experimental conditions. The dependence of LiClO<sub>4</sub> concentration in PC has been studied by studying five different
concentrations: 0.25, 0.5, 0.75, 1.0 and 1.25 M. After equilibration, local structural properties and dynamical features
such as phase separation, coordination, cluster stability and ion conductivity were studied. In an effort to study the
conduction processes more carefully, external electric fields of 1×10<sup>6</sup> V/m and 5×10<sup>6</sup> V/m has been applied to the
simulation boxes. A clear relationship between the degree of local phase separation and ion mobility is established. It is
also shown that although the ion pairing increases with concentration, there are still significantly more potential charge
carriers in the higher concentrated systems, while concentrations around 0.5-0.75 M of LiClO<sub>4</sub> in PC seem to be
favorable in terms of ion mobility. Furthermore, the anions exhibit higher conductivity than the cations, and there are
tendencies to solvent drag from the PC molecules.
This paper presents an electro-mechanical Finite Element Model of an ionic polymer-metal composite (IPMC) material. Mobile counter ions inside the polymer are drifted by an applied electric field, causing mass imbalance inside the material. This is the main cause of the bending motion of this kind of materials. All foregoing physical effects have been considered as time dependent and modeled with FEM. Time dependent mechanics is modeled with continuum mechanics equations. The model also considers the fact that there is a surface of platinum on both sides of the polymer backbone. The described basic model has been under developement for a while and has been improved over the time. Simulation comparisons with experimental data have shown good harmony. Our previous paper described most of the basic model. Additionally, the model was coupled with equations, which described self-oscillatory behavior of the IPMC material. It included describing electrochemical processes with additional four differential equations. The Finite Element Method turned out to be very reasonable for coupling together and solving all equations as a single package. We were able to achieve reasonably precise model to describe this complicated phenomenon. Our most recent goal has been improving the basic model. Studies have shown that some electrical parameters of an IPMC, such as surface resistance and voltage drop are dependent on the curvature of the IPMC. Therefore the new model takes surface resistance into account to some extent. It has added an extra level of complexity to the model, because now all simulations are done in three dimensional domain. However, the result is advanced visual and numerical behavior of an IPMC with different surface characteristics.
Nafion is widely known as one of the most popular membrane materials for low temperature fuel cell applications.
However, the particular exchange membrane material properties make it also valuable for other applications. One of the
electroactive polymer (EAP) subclasses, ionic polymer metal composites (IPMC) commonly exploits Nafion as the ion
exchange polymer membrane. The ion conducting properties of Nafion are extremely important for IPMCs. Although,
ion conductivity depends strongly on the structural properties of the polymer matrix, there has been very little insight at
the atomistic level. Molecular dynamics simulations are one of the possibilities to study the ion conduction mechanism
at atomistic level. So far, the simulation results have been rather contradictory and very much dependent from the force
fields and polymer matrix setup used. In the present work, new force field parameters for Li<sup>+</sup> and Na<sup>+</sup> - nafion based on
DFT calculations are presented. The developed potentials and the force field were tested by molecular dynamics
simulations. It can be concluded that Li<sup>+</sup> and Na<sup>+</sup> ions are coordinated to different Nafion side-chain terminal group
(SO<sub>3</sub><sup>-</sup>) oxygens and to very few water molecules. One cation is coordinated to three different side-chains. Oxygens of
SO<sub>3</sub> groups and cations form complicated multi-header systems. In the equilibrium state, no cations dissociated from
side chains were found.
This paper presents a electro-mechanical model of an IPMC sheet. The model is developed using Finite Element method. The physical bending of an IPMC sheet due to the drift of counter-ions (e.g Na+) and water in applied electric field are simulated. Our model establishes a cause-effect relationship between the charge imbalance of the counter-ions and the mechanical bending of the IPMC sheet. The model takes into account the mechanical properties of the Nafion polymer as well as the platinum coating. As the simulations are time dependent, a transient model is used and some additional parameters, such as damping coefficients, are included. In addition to electro-mechanical model, electrochemical reactions are introduced. Equations describing periodic adsorption and desorption of CO and OH on a platinum electrode of an IPMC muscle immersed into formaldehyde solution are coupled to mechanical properties of the proposed model. This permits us to simulate self-oscillatory behavious of an IPMC sheet. The simulation results are compared to experimental data.
A program called <i>mcgen</i> was written for creating initial models for Molecular Dynamics simulations with capability
to arrange at least the following into simulation cell: branched and non-branched polymers, copolymers,
nanoparticles, dissolved salts (ions), liquids. The program was tested with non-branched poly(ethylene oxide)
molecules and the optimal values were found for the control parameters the Monte Carlo algorithm depends on,
such that the program works steady and fast enough. Generation features of mcgen allow to generate one or
several chains of the same or different types; add side-chains with fixed or random spacing along the main chain;
insert atoms and ions into the simulation cell before generating the polymers; mark given atoms as "invisible" so
that those atoms are not checked against any geometric constraints and will be removed from the simulation cell,
if they happen to be on the way of the growing polymer chain; establish geometric constraints (sphere, upper
and/or lower limit on one, two or all three axes) and generate polymer chains either inside or outside them.