Ionomers, 2-phase systems

Dieterich et al. (1A6) and Taft and Mohar (157) have prepared excellent reviews of urethane ionomers. One of the most Important characteristics of urethane ionomers is the ease with which they form stable water dispersions without the use of emulsifiers (158-16A). These dispersions consist of colloidal two-phase systems that can be readily prepared by adding a solution of the urethane ionomer in polar solvents, such as methyl ethyl ketone or tetrahydrofuran,... 

More recently Dieterich and Reiff (166) have described the formation of aqueous urethane dispersions by the dispersion of ionomer melts with subsequent polycondensation in two-phase systems. The principle of this procedure consists of reacting molten ionic modified polyester or polyether prepolymers containing NCO groups with urea to yield bis(biuret), followed by methylolation by means of aqueous formaldehyde in a homogeneous phase, and the resulting plasticized melt of methylolated ionic urethane bis(biurets) dispersed in water at 50-130 °C. These steps can be represented schematically as follows ... 

Neutralizing the acid groups in the dispersion leads to formation of an ionomer membrane in the latex film [63]. As shown in Figure 14.24, this ionomer phase has a profound effect on slowing down the interdifiusion and on broadening the distribution of diffusion rates. In spite of the complexity of the system, there is a quite simple explanation for these effects. The shell polymer has a higher Tg than that of the core. The Tg of the shell phase depends upon composition the... 

In comparison with the usual hydrophobic isocyanate polyaddition products, polyurethane ionomers are structurally much more suitable for the preparation of aqueous two-phase systems. These polymers, which show hydrophilic ionic sites between predominantly hydrophobic chain segments, are self-dispersing under favourable conditions. These products spontaneously form stable dispersions in water without the influence of shear forces and in the absence of dispersants. There are several techniques available ... 

The proton resistance of the catalyst layer can be reduced by adding more ionomer phase or low EW ionomer to the system, but this is outweighed by reduced oxygen transport. As gas phase diffusion is several orders of magnitude faster than diffusion through liquid water, it is essential to create a system that does not completely fill with liquid water, that is, contains fairly hydrophobic materials and not too small pore sizes. The state-of-the-art carbon blacks do not seem to meet this criterion. CNT or other more graphitic structures seem better suited. Also alternative supports should be selected with their potential for improved mass transport. Oxides may be less suitable in that respect but this certainly requires... 

This section presents a review of atomistic simulations and of a recently introduced meso-scale computational method to evaluate key factors affecting the morphology of CLs. Most of the effort in molecular dynamics simulations for PEFCs has concentrated on dynamic motion of proton and water through the hydrated membrane [96-104], Little attempt has been made to employ MD techniques for elucidating the structure and transport of CLs, particularly in three-phase systems of carbon/Pt, ionomer, and gas phase. In the following subsections, we discuss various MD simulations to study the transport and dynamic behavior of CLs in terms of water and proton diffusivity, Pt-supported electrocatalyst, and microstructure formation. 

Water in the pore is in equilibrium with the surrounding phase that is kept at controlled temperature, T, vapor pressure, and gas pressure, P. Corresponding to these thermodynamic variables, equilibrium of water in the pore entails three independent microscopic conditions, the first two of which are obvious (Bellac et al., 2004) (i) thermal equilibrium of the ionomer-water system implying zero heat flux and uniform T (ii) chemical equilibrium implying zero water diffusion and uniform chemical potential of water in the PEM, which is in equilibrium with the... 

These blends with ionomer were two-phase systems which showed synergistic increase of tensile yield strength and ionomer increased the rebound of EPR [206]. 

This block copolymer melt flow was improved by adding ethylene ionomer [79]. vs blend ratio was S-shaped, indicating a two-phase system and tensile modulus and yield showed evidence of synergism [206]. 

Polystyrene-co-butylacrylate, 7 608t Poly (styrene-co-divinylbenzene), 73 113 Poly(styrene-co-vinylpyridinium methyl iodide) ionomer system, 74 480 Poly(styrenedivinylbenzene) (PSDVB) liquid chromatography stationary phase, 4 623... 

Jang et a. have used an all-atom approach in their MD simulation of phase segregation and transport in Nation at A = 16. It was shown that blocky Nation ionomers with highly nonuniform distributions of side chains on fhe polymer backbones form larger phase-segregated domains compared to systems with uniform distributions of side chains on the backbones. Water diffusion coefficients at 300 and 353K were found to agree well with experimental values. 

Although immiscible polymer blends and ionomers share a common feature in that both exhibit more than a single phase, a major difference between the two systems involves the dispersed phase size. For blends, this is generally of the order of micrometers and may be detected optically. Ionomers, however, are microphase-separated with domain sizes of the order of nanometers. Thus, blends and ionomers represent two extremes of the subject of multiphase polymers. In this book, the reader will observe similarities as well as differences in the problems... 

The characteristics discussed above are mainly related to clustering in the ionic phase, but the role of the hydrophobic phase also is quite important. In some cases it controls the gas transport properties of the material (e.g. 02 through PFSA) (4). And, it makes it possible to keep hydrophobic reactions in the neighborhood of the ionic domain species (5). Moreover, metal complexes with bulky hydrophobic ligands can be supported in the ionomers because of synergystic interaction of both polymer phases (6). Interesting electrocatalytic or photocatalytic systems take advantage of these unique properties of ionomers (7-8). Moreover, support of the reactants in ionomers may be useful for reactant/product separations. 

Erdi and Morawetz (8) attempted to overcome this problem by incorporating ionic groups into PS in the form of metal carboxylate-containing comonomers. When they plasticized the PS phase in these systems, they observed only a very weak physically bonded network of no substantial strength. However, the availability of these metal-sulfonated ionomers coupled with the inherently stronger associations of the metal sulfonates offer a more attractive system to test this hypothesis. Therefore, plasticized S-PS samples of varying sulfonate level were blended with DOP... 

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