Ionomer dynamic properties

Structural Organization and Dynamic Properties of Ionomer Membranes... 

Next, an attempt was made to clarify the effect of crystalline phase on the mechanical properties of ethylene ionomer. Dynamic mechanical measurements... 

Several groups, including ours, followed the latter path, studying the structural and dynamical properties of water/Nafion mixtures with atomistic simulations. Let us first briefly consider the key chemical features of the ionomer and the nature of the phase segregation. The chemical composition of Nafion is given by ... 

Rollet, A.L., Blachot, J.F., DelviUe, A., Diat, O., GuiUermo, A., Porion, P., Rubatat, L., Gebel, G. (2003) Characterization of porous structure through the dynamical properties of ions confined in sulfonated polyimide ionomers films. The European Physical Journal E, 12, 131-134. 

The physical properties of the acid- and ion-containing polymers are quite interesting. The storage moduli vs. temperature behavior (Figure 8) was determined by dynamic mechanical thermal analysis (DMTA) for the PS-PIBMA diblock precursor, the polystyrene diblock ionomer and the poly(styrene)-b-poly(isobutyl methacrylate-co-methacrylic acid) diblock. The last two samples were obtained by the KC>2 hydrolysis approach. It is important to note that these three curves are offset for clarity, i.e. the modulus of the precursor is not necessarily higher than the ionomer. In particular, one should note the same Tg of the polystyrene block before and after ionomer formation, and the extension of the rubbery plateau past 200°C. In contrast, flow occurred in... 

Ma, X., Sauer, J. A., and Hara, M. (1995). Poly(methyl methacrylate) based ionomers. 1. dynamic mechanical properties and morphology. Macromolecules 28, 3953-3962. 

Despite the relative simplicity of most ionomers, questions about them remain. One of the key questions is how structure and dynamics on different length scales connect. Specifically, how does the metal coordination to the neutralized acid groups (which is on an angstrom level) correlate with the size, shape, and distribution of the ion-iich aggregates in the hydrophobic matrix (which is on a nanometer length scale) Furthermore, how does the microscopic structure control the macroscopic properties like melt viscosity or elastic modulus ... 

The results of a study of the relation between the oriented structure and drawn poly(e-caprolactone) specimens including CaCO particles, their dynamic mechanical properties and line shape analyses of CP MAS NMR spectra are presented. Solid state studies of C NMR study of poly(methyl acrylate) ionomer have been reported. ... 

Viscoelastic measurements of ionomers have been used to indirectly characterize the microstructure and to establish property structure relationships. Forming an ionomer results in three important changes in the viscoelastic properties of a polymer. First, T usually increases with increasing ionization. This is a conseqi nce of the reduced mobility of the polymer backbone as a result of the formation of physical, ionic crosslinks. Second, an extended rubber plateau is observed in the modulus above T, again as a result of the ionic network. Third, a high temperaturi mechanical loss is observed above T, which is due to motion in the ion-rich phase. The dynamic mechan cal curves for SPS ionomers shown in Fig. 9 clearly demonstrate these three characteristics. 

Various techniques have been used to study the solution properties of ionomers. These include viscosity (4, 1 ), static and dynamic light scattering (12.13.15-18), small-angle neutron scattering (11.14). and spectroscopy (10). Here, we use (static and... 

Dynamic Mechanicetl (DM) Analysis. One of the most straightforward methods of investigating the effects of plasticizers on ionomer properties is through measurements of dynamic mechanical properties. Such studies allow observation of how plasticizers influence the Tg of the ionomer as well as the higher temperature transition (hereafter referred to as the ionic transition or Ti) that is associated with the ionic aggregates in microphase-separated ionomers. A number of such studies, as well as DSC or... 

Of the microphase-structure dependent physical properties of ionomers, perhaps the most widely studied are glass transition temperatures, (Tg), and dynamic mechanical response. The contribution of the Coulombic forces acting at the ionic sites to the cohesive forces of a number of ionomeric materials has been treated by Eisenberg and coworkers (7). In cases in which the interionic cohesive force must be overcome in order for the cooperative relaxation to occur at Tg, this temperature varies with the magnitude of the force. For materials in which other relaxations are forced to occur at Tg, the correlation is less direct. 

Dynamic mechanical and melt rheological properties of sulfonated poly(butylene succinate) ionomers. Polymer, 44, 7165. 

Comparison of the dynamic mechanical properties of the p-carboxylated polystyrene ionomers and those of the P(S-co-MANa) ionomers shows a more subtle difference. For the p-carboxylate ionomers the ion pairs are farther from the poljuner backbone than for the P(S-co-MANa), and thus form larger multiplets (102,103). Again, since the size of multiplets is smaller for the P(S-co-MANa), the total number of multiplets is larger, which results in the smaller amount of reduced mobility regions for the carboxylated polystyrene system, at a comparable ion content. As a result, the ionic modulus should be lower in the p-carboxylate system than in the methacrylate system. 

Block Ionomers. The block ionomers to be discussed are of AB or ABA type, in which one of the blocks is ionic (eg, sodium methacrylate) and the other consists of nonionic units (eg, polystyrene). While ionic block copolymers in a micelle form in both aqueous and nonaqueous solutions have been studied extensively (99-101,130,131), the viscoelastic properties of block ionomers in bulk have not received much attention (132-137). If the short ionic blocks formed micelle-like aggregates, which were surrounded by nonionic blocks, the viscoelastic properties of the diblock ionomers would be very similar to those of stars or polymers of low molecular weight (136). Thus, above the Tg of the nonionic blocks, as the temperature increased the modulus dropped rapidly with a very short rubbery plateau. For example, in a dynamic mechanical study, it was found that a homopolymer containing 490 styrene units showed a Tg at ca 115°C, and started to flow at ca 150°C. However, in the case of a diblock ionomer containing 490 styrene units and 40 sodium methacrylate ionic units showed a Tg at ca 116°C, and flow behavior was observed above ca 165°C, which was only 15°C higher than that of nonionic polystyrene (135). 

Several studies have focused on extensive MD simulations of Pt nanoparticles adsorbed on carbon in the presence or absence of ionomers [109-113]. Lamas and Balbuena performed classical molecular dynamics simulations on a simple model for the interface between graphite-supported Pt nanoparticles and hydrated Nation [113]. In MD studies of CLs, the equilibrium shape and structure of Pt clusters are usually simulated using the embedded atom method (EAM). Semi-empirical potentials such as the many-body Sutton-Chen potential (SC) [114] are popular choices for the close-packed metal clusters. Such potential models include the effect of the local electron density to account for many-body terms. The SC potential for Pt-Pt and Pt-C interactions provides a reasonable description of the properties of small Pt clusters. The potential energy in the SC potential is expressed by... 

To improve the structure-dynamics relationships of CLs, the effects of applicable solvents, particle sizes of primary carbon powders, wetting properties of carbon materials, and composition of the catalyst layer ink should be explored. These factors determine the complex interactions between Pt/carbon particles, ionomer molecules, and solvent molecules and, therefore, control the catalyst layer formation process. Mixing the ionomer with dispersed Pt/C catalysts in the ink suspension prior to deposition will increase the interfacial area between ionomer and Pt/C nanoparticles. The choice of a dispersion medium determines whether ionomer is to be found in the solubilized, colloidal, or precipitated forms. 

This coarse-grained molecular dynamics model helped consolidate the main features of microstructure formation in CLs of PEFCs. These showed that the final microstructure depends on carbon particle choices and ionomer-carbon interactions. While ionomer sidechains are buried inside hydrophilic domains with a weak contact to carbon domains, the ionomer backbones are attached to the surface of carbon agglomerates. The evolving structural characteristics of the catalyst layers (CL) are particularly important for further analysis of transport of protons, electrons, reactant molecules (O2) and water as well as the distribution of electrocatalytic activity at Pt/water interfaces. In principle, such meso-scale simulation studies allow relating of these properties to the selection of solvent, carbon (particle sizes and wettability), catalyst loading, and level of membrane hydration in the catalyst layer. There is still a lack of explicit experimental data with which these results could be compared. Versatile experimental techniques have to be employed to study particle-particle interactions, structural characteristics of phases and interfaces, and phase correlations of carbon, ionomer, and water in pores. 

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