Clustering in ionomers

Such ionic clusters have been studied in ionomers such as Naflon poly(perfluoropropylene oxide sulfonic acid) membranes clusters in ionomers have been reviewed recently by Mauritz and Hopfinger ( 7). These reviewers also quote a model for hydrated Naflon proposed by T. D. Gierke at the October, 1977, meeting of the Electrochemical Society, Atlanta, Georgia, in which aqueous spher-... 

In summary, I have discussed a semi-phenomenological elastic theory for ion clustering in ionomers. The theory is consistent with observed trends in perfluorinated ionomers. I have also demonstrated the percolatlve nature of ion transport in these ionomers and computed quantitatively their tensile modulus. Finally, I have discussed the Influence of morphology on ion selectivity in perfluorinated ionomer blends. In particular, I have pointed out that an universally preferred morphology beneficial to all blends does not exist the ideal morphology must be individually determined based on component properties. Most of the theories and conclusions here are very general and applicable to other composite polymer systems. 

Tadano K, Hirasawa E, Yamamoto H, Yano S (1989) Order-disorder transition of ionic clusters in ionomers. Macromolecules 22 226—233... 

Neppel A, Butler IS, Eisenberg A (1979) Vibrational spectra of polymers. 2. Variable-temperature raman spectroscopy as a probe for ion clustering in ionomers. Macromolecules 12 948-952... 

Presently, some hybrid polyblends, such as the thermoplastic apparent interpenetrating polymer networks (AIPNs), call for a broader view, hi contrast to traditional IPNs, in thermoplastic AIPNs the components are cross-linked by means of physical, instead of chemical, bonds. These physical bonds are glassy domains of block copolymers, ionic clusters in ionomers, or crystalline domains in semicrystalline polymers. The components of thermoplastic AIPNs are capable of forming physical networks and are characterized by mutual penetration of phases. Thermoplastic AIPNs are intermediate between mixtures of linear polymers and true IPNs because they behave like chemically cross-Unked polymers at relatively low temperatures, but as thermoplastics at elevated temperature [208]. The blends based on combinations of physically cross-Unked polymer and Unear polymer, or physicaUy cross-Unked polymer and chemically cross-Unked (thermoset) polymer, where the physically cross-Unked polymer network constitutes the continuous phase and the other component disperses into domains, will also exhibit the properties of thermoplastic compositions. 

The original cluster-network model proposed by Gierke et al. (also referred to as the cluster-channel model) has been the most widely referenced model in the history of perfluorosulfonate ionomers. Despite the very large number of papers and reports that have strictly relied on this model to explain a wide variety of physical properties and other characteristics of Nafion, this model was never meant to be a definitive description of the actual morphology of Nafion, and the authors recognized that further experimental work would be required to completely define the nature of ionic clustering in these iono-mers. For example, the paracrystalline, cubic lattice... 

The ionic clusters act as sites of cross-linking at low temperatures. The interchain forces resulting from this ionic bond produces properties normally associated with a cross-linked thermoset polymer. The association in ionomers can be partially overcome through application of heat and pressure allowing processability while truly" cross-linked network polymers cannot be remelted, dissolved or reshaped. Thus, ionomers are often referred to as processable thermosets. 

IR spectroscopy can be used to characterise the formation of ionomers by studying the environment of the anions [85, 86]. Risen and co-workers [87, 88] used far-IR spectra (150 to 400 cm"1) to demonstrate the sensitivity of low frequency vibrations to the anions and cations and the degree of cluster formation in ionomers. For example, styrene sulfonic acid ionomers with Na+ cation shows absorption bands at 220 cm 1, whereas the Cs+ cation shows bands at 100 cm 1. 

Recent proton NMR relaxation studies have shown that the method can be used to study the formation of ionic clusters and the structure of the physical network in ionomers [153]. 

Figure 10. Cumulative number of molecules in clusters in hydrated SSC PFSA ionomer and Nafion for Rc = 3.5 A as a function of molecular weight - 15mer (gray) and trimer (black).

G. Xu. High-resolution x-ray-scattering of ionic clusters in perfiuorinated ionomers. Polymer Journal 26, 840-844 1994. 

It should be acknowledged that Risen utilized the concept of the ionic domains in ionomers (Nafion sulfonates, sulfonated linear polystyrene) as microreactors within which transition metal partides can be grown and utilized as catalysts (23-25). Transition metal (e.g. Rh, Ru, Pt, Ag) cations were sorbed by these ionomers from aqueous solutions and preferentially aggregated within the pre-existing clusters of fixed anions. Then, the ionomers were dehydrated, heated and reduced to the metallic state with Hg. Risen discussed the idea of utilizing ionomeric heterophasic morphology to tailor the size and size distributions of the incorporated metal particles. The affected particle sizes in Nafion were observed, by electron microscopy, to be in the range of 25-40 A, which indeed is of the established order of cluster sizes in the pre-modified ionomer. 

Even less is known about ionomer/plasticizer interactions on a molecular level. A variety of scattering and spectroscopic techniques that can probe this level have been mentioned, but they have been applied primarily to the specific case of water in ionomers, and in particular to hjdrated perfluorinated ionomers. At the least, these studies demonstrate the powerful potential of the techniques to contribute to a more complete understanding of structure-property relationships in plasticizer/ionomer systems. For e.xample, to return to the question of the effect of nonpolar plasticizers on the ionic domains how can the decrease in the ionic transition temperature be reconciled with the apparently minimal effect on the SAXS ionomer peaks and with the ESR studies that indicate (not surprisingly) tiiat these plasticizers have essentially no influence on the local structure of the ions Is it due to their association with the hydrocai bon component of the large aggregates or clusters Or if these entities do not exist, as some researchers postulate, what is the interaction between the nonpolar plasticizer, the hydrocarbon component and the ionic domains These questions are, of course, intimately related to the understanding of ionomer microstructure even in the absence of plasticizer. The interpretation of SAXS data in particular is subject to the choice of model used. 

The existence of ion clustering in perfluorinated sulfonate ionomers was first reported by Yeo and Eisenberg in 1975. This phenomenon has been subsequently studied for perfluorinated sulfonate and carboxylate ionomers by many Experimental evidence to support the conclusion that ion clustering occurs in these materials includes thermorheological behavior/ X-ray diffraction results/" " " IR data/ " NMR data,"" " ESR data/ Mossbauer spectroscopic fluores-... 

Reference 7 reviews a number of electron microscopy studies of ionomer morphology in the period up to 1979. None of these studies makes a convincing case for the direct imaging of ionic clusters. This is because of the small size of the clusters (less than 5 nm based on scattering studies) and difficulties encountered in sample preparation. The entire problem was reexamined in 1980(21). In this study ionomers based on ethylene-methacrylic acid copolymers, sulfonated polypentenamer, sulfonated polystyrene and sulfonated ethylene-propylene-diene rubber (EPDM) were examined. The transfer theory of imaging was used to interpret the results. Solvent casting was found to produce no useful information about ionic clusters, and microtomed sections showed no distinct domain structure even in ionomers neutralized with cesium. Microtomed sections of sulfonated EPDM, however,... 

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. 

Electrostatic and Entropic Terms. Just as charges coalesce into clusters in the dry state, upon hydration, the charges and water molecules are gathered into vesicles, which we assume to be spherical (radius R). Experiments show that the location of the ionomer peak varies continuously with the water content (v) Thus we assume that the sample is in thermodynamical equilibrium, and we have to look for its free energy. 

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-... 

The effect of temperature on a SAXS scan is shown in Figure 11. Note that the reflection at 4 nm persists above the melting point of the polymer, 265°C. Similar behavior was observed in ethylene/methacrylic acid ionomer (21, 22) and is strong support for the existence of ionic clustering in the dry state of the polymer. Indeed, we will demonstrate shortly that this reflection is observed in dry polymers at room temperature neutralized with heavier metal ions. 

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