Ionomers aggregates

Wang S, Sun G, Wu Z, Xin Q. Effect of Nafion ionomer aggregation on the stmcture of the cathode catalyst layer of a DMFC. J Power Sources 2007 165(l) 128-33. 

How does the primary chemical architecture of ionomer molecules determine ionomer aggregation, formation of water containing pathways in the PEM, and water sorption properties of the PEM ... 

The self-organized structure and mechanical properties of ionomer aggregates determine the water sorption properties and the stability in PEMs. These properties in turn are the key to PEM operation in PEFCs they govern water distribution and transport, proton density and conductivity, and the membrane response to mechanical stressors. 

All the major trends discussed in this section are in agreement with experimental findings on the impact of sidechain properties and sidechain grafting density on ionomer aggregation. 

Figure 2.13 shows snapshots of equiUbrated microstructures of the ionomer-water system at different water content A, as indicated in the plot. Figure 2.14 shows corresponding RDFs. These structural correlation functions can be analyzed in view of (i) ionomer aggregation into hydrophobic domains, (ii) formation of ionic domains, (iii) distribution of water and hydronium ions with respect to sidechains and backbones, and (iv) connectivity of water and ionomer networks. 

Theoretical studies of ionomer aggregation in solution can rationalize and predict stable configurations of ionomer bundles as a function of basic ionomer properties. Theoretical results can be highly insightful to narrow down the configuration space for molecular simulations. 

The morphology of ionomer aggregates in CL is clearly different from that in the PEM, even if the base ionomer is the same. A simple extrapolation of ionomer properties in CLs, from properties of the bulk PEM, using percolation theory is generally insufficient. Ionomer forms a thin adhesive skin layer at agglomerate surfaces with undetectable internal porosity. The Pt loading could play a significant role in the transition of pore surface properties from hydrophobic to hydrophilic. 

Melchy, A. M. and Eikerling, M. 2014. Physical theory of ionomer aggregation in water. 

Dynamic light scattering has also been used to study ionomer aggregates in solution. It was shown that hydrodynamic radius increases with increasing ion content and ionomer concentration (up to a certain point, above which the hydrodynamic radius becomes almost constant in the case of the THF solution [8]). While Lantman et al. only obtained hydrodynamic radii from scattering data at an angle of 90°, Pedley et al. used the cumulant method to obtain information on the distribution of aggregates and found that the quality factor. 

In addition, the quality factor that can be a measure of polydispersity for ionomer aggregates is plotted against polymer concentration in Fig. 20.4, By considering the rather large errors involved in quality factors, the difference between ionomer samples with different ion contents may be small. However it is clear that the width of the distribution of particles in the ionomer solution is... 

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