Partially Fluorinated Aromatic Polymers

This section seeks to report the preparation and characterization of PEMs based on partially fluorinated aromatic polymers. In the past few decades, attention has focused on the preparation of new fluorinated monomers and aromatic fluoropolymers. This topic was recently reviewed [70], summarizing the characteristic effects of the aromatic group on the physicochemical properties (e.g., Tg and the thermostability of the obtained polymers). 

As early as the 1940s Emeleus and Haszeldine [17] discovered that perfluoroalkyl iodides are not only cleaved into perfluoroalkyl radicals by light but also that they add readily to a variety of olefins to yield telomers and 1 1 adducts [18]. This kind of radical chain reaction can also be initiated by high temperatures (Scheme 2.100). The addition of perfluoroalkyl iodides to olefins is a very important method for synthesis of partially fluorinated alkanes, polymers, oligomers, and their derivatives [19]. The synthesis of some perfluoroalkyl aromatic compounds can also be achieved [20]. 

Wholly aromatic polymers are thought to be one of the more promising routes to high performance PEMs because of their availability, processability, wide variety of chemical compositions, and anticipated stability in the fuel cell environment. Specifically, poly(arylene ether) materials such as poly-(arylene ether ether ketone) (PEEK), poly(arylene ether sulfone), and their derivatives are the focus of many investigations, and the synthesis of these materials has been widely reported.This family of copolymers is attractive for use in PEMs because of their well-known oxidative and hydrolytic stability under harsh conditions and because many different chemical structures, including partially fluorinated materials, are possible, as shown in Figure 8. Introduction of active proton exchange sites to poly-(arylene ether) s has been accomplished by both a polymer postmodification approach and direct co-... 

Current photoresists cannot be used for 157 nm technology, mainly because their transmittance at 157 nm is too low. Although materials with aromatic substructures are quite useful for the 248-nm process, only purely aliphatic polymers are employed in the current 193 nm technology. For an illuminating wavelength of 157 nm, even the absorptivity of most aliphatic compounds is too high. Therefore, only partially fluorinated polymers with absorption characteristics carefully optimized by experiment [10] and molecular modeling [11] can be used. The solubility switch after illumination is usually achieved by addition of a photo-activatable super-acid (e.g. a diaryl iodonium hexafluoroantimonate) [12], which typically cleaves an add-labile tert-butyl ester in the polymer (Scheme 4.9). 

Therefore, in a comparative study, different nonfluorinated and partially fluorinated sulfonated aromatic polymers were synthesized and characterized in terms of chemical and thermal stabilities. It was found that SO3H groups introduced in the electron-deficient sections of the aromatic polymer main chains were much more stable against splitting-off than SO3H groups pendent to electron-rich sections of the polymer backbones [29]. One of the most stable polymers of this series was a polymer prepared by polycondensation of decafluorobiphenyl and bisphenol (AF), followed by sulfonation with... 

The mentioned disadvantages of PFI membranes induced many efforts to synthesize PEM based on hydrocarbon-type polymers and brought about the emergence of partially fluorinated and fluorine-free ionomer membranes as alternatives to Nafion membranes. Among them the membranes based on aromatic PEEK were shown to be promising for fuel cell application, as they possess good mechanical... 

Table III lists the properties of perfluorinated polyimides, along with those of partially fluorinated and unfluorinated polyimides. The structure of PMDA/ODA is the same as that of Dupont s Kapton. Because of the direct introduction of fluorines into the aromatic rings and the flexible structure of the lOFEDA component, the polymer decomposition temperatures and Tg s of perfluorinated polyimides are slightly lower than those of conventional poly imides. The thermal stability of these films is nonetheless high enough to withstand the manufacturing process for IC s and muitichip modules.

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