Other ionomers

At lEC s above the latter value, the position of the scattering angle remains constant. We also observed that the SAXS peak is not influenced by the water content of the sample. This is in contrast to other ionomer systems in which the "ionic" peak disappears upon water saturation (12). 

A new angle on the proximity needed between catalyst and the adjacent layer is offered by an alternative thin-film catalyst process requiring neither carbon fibre backup nor channels of Nafion or other ionomer proton conductors (Debe, 2003). Pt-Ru catalysts are deposited by sputtering on oriented crystalline organic whiskers (length about 1 pm), and this assembly is then... 

Arguably the most important amorphous ionomer is sulfonated polystyrene (SPS). Other ionomers include poly(styrene-rfln-methacrylic acid) (SMAA), polyurethanes, siloxanes, butadiene-based elastomers, ethylene-propylene-diene terpolymers, acrylates and methacrylates, polyphosphoesters, polyimides, and many others. ... 

By using Equation 4, the effective ionic diameters, D, can be estimated. The initial slope of each curve in Figure 1 may be obtained by either a simple graphical method or a curve fitting method. The effective diameter is a measure of the distance of closest approach of the centers of the macrolons and reflects the range of interaction of ionomer molecules with other ionomer molecules. 

One of these, which has recently become increasingly important in electrochemical applications, and which is one of the materials discussed extensively in this volume, is the Nafion ionomer family. These materials were developed by the duPont company, and consist of hydrophobic fluorocarbon backbone chains, with hydrophilic per-fluorinated ether side chains terminated by sulfonic acid groups or corresponding alkali salts. The Nafions possess many exceptional properties which are not encountered in other ionomer systems, particularly the high water permeability (26,27), permselectivity with regard to ion transport (28-30), durability in strong alkali (26), thermal stability (26,31), and others. 

There have been Mossbauer studies of Nafion membranes by several groups (J —6), besides some work on other ionomers (7,8) and a body of results on polymers where iron or tin is introduced into the polymeric matrix as a probe impurity (8). This chapter will concentrate on the information that has been extracted from the Mossbauer studies of Nafion membranes, and their variations as a function of parameters such as temperature, applied magnetic field, water content and chemical treatment. The results will be discussed in terms of the information they provide about the structure of the ionic phase in perfluorosulfonate materials. 

As in other ionomers, the ion exchange sites in "Nafion" membranes are observed to aggregate and form clusters. Ionic clustering in "Nafion" membranes has been indicated by a variety of physical studies including dielectric relaxation (1), small angle x-ray scattering (1-4), neutron scattering (4), electron micro-... 

Y Relaxation. Unlike the other dynamic mechanical relaxations observed in this study, the Y relaxation does not have an analog in the dynamic mechanical behavior of polyethylene, hydrogenated PP s, or other ionomer systems. In addition, it displays no definite trends in changing temperature or magnitude as the level of sulfonation and thermal history are altered. Coupled with the fact that these systems are known to contain water as well as nitrogen, it is not possible to assign this relaxation to any specific phase or mechanism. Additional studies are necessary before this task can be approached adequately. 

In this section, the metal-cationic salts of copoly(ethylene-methacrylic acid) are called the ethylene ionomers. This ethylene ionomer is one of the well-known commercial ionomers, marketed under the trade name Surlyn by DuPont. Many ethylene ionomers have crystalline and amorphous phases of ethylene chain units as well as polyethylene. Therefore, there is a three-phase structure, with crystalline, amorphous, and ionic aggregate phases this is a unique characteristic of ethylene ionomers compared with other ionomers. Although the ionic aggregate structure of the ethylene ionomer has not been fully established, its structural model is represented5 as shown in Fig. 1. In ethylene ionomers, therefore, it is necessary that some physical properties should be considered by correlating to not only the ionic aggregates but also the crystalline phases. 

Thus a basic similarity exists between the acid and the salt form in the overall structure, showing that the neutralization does not lead to a large scale reorganization as is observed in other ionomers. 

The in situ construction of the inorganic component within a cast polymer solution is not limited to metal oxides and in practice a range of other inorganic materials can be formed depending on the choice of precursor(s) incorporated in the polymer solution, and the nature of post-treatment following solvent removal. Roziere and Jones and co-workers have developed nano composite membranes in which zirconium phosphate is formed from zirconyl propionate introduced into a DMAc solution of sPEEK, by immersion of the cast film, after solvent removal, into phosphoric acid. This approach provides a robust synthetic route that can be generalised to other ionomers, and allows the amount of ZrP to be readily varied, even up to ca. 40-50 wt. %. 

A wide range of ionomers based on other polymer backbones has also been stndied. Even thongh an extensive discnssion of properties of partly crystalline ionomers as well as the other ionomer famiUes is beyond the scope of the present review, it will be nsefiil to discnss veiy briefly the properties of some of them. In partly crystalline polyethylene-based and perfluorinated ionomers the degree of crystallinity makes the interpretation of experimental results difficult. 

LSV but the potential scan is reversed once potential limits are reached. Thus, CV is a reversal technique and is the potential scan equivalent of double potential step chrOTioamperometry [24]. CV is commonly used to diagnose the electrochemical activity of die catalyst spread on top of the electrodes. Typically, the catalyst for HT-PEMFC consists of platinum (Pt) carbon supported and a binder material such as phosphoric acid-doped PBI or other ionomers [55-58]. The binder serves to facilitate proton conduction between electrolyte membrane and the Pt active sites, whereas the carbon support enhances Pt dispersion through the catalyst layer and the electronic conductivity. Therefore, the electrochemical activity of the electrode catalyst layer strongly depends on the contact between catalyst active sites, reactants, and proton/elec-tron-conducting materials. 

Those critical functions of membrane for DMFC are simple but most important. Required functions are ionic conductivity, electrical insulation, gas and liquid (especially methanol) tightness, and chemical and mechanical stability. As indicated in Fig. 13.2, ohmic polarization is mainly due to the ionic resistance of membranes, but the low open circuit potential of cathode is also mainly coming from the voltage drop by mixed potential made of fuel crossover through the membrane. The low cost of material and process is also another factor in terms of commercialization. Especially for mobile applications, membranes have the additional function for mass balance of liquid fuel and water products circulated out of or through the membrane. In this manner, alternative membranes are under development and researchers are focused on four types perfluorinated and partially fluorinated membranes hydrocarbon and composite and other ionomer modifications inorganic materials. The current state of the art and technical approaches to these materials are discussed in detail elsewhere in this volume. 

In perfluorinated ionomers, a PTFE-based polymeric backbone offers chemical stability from the radical species or acid-base, which causes hydrolytic degradation of the polymer chain. Ionic conductivity is provided by pendant acidic moiety in carboxylate or sulfonate form. There are some reports on perfluorinated carboxylic acid (PFCA) materials, most of which are derived from Nafion [26-29]. However, PFCA is not suitable for fuel cell application due to its low proton conductivity. Perfluorosulfonic acid (PFSA) is the most favored choice among not only perfluorinated membranes but all other ionomers in fuel cell applications. Sulfonic acid form of Nafion is a representative PFSA and thus has been intensively studied since 1960s. Reported chemical structure of Nafion membrane is given in Fig. 13.8. 

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