Modification of PBIs

1 N-Substitut0d PBI The NH groups in the imidazole rings are chemically reactive. For some applications, the chemical reactivity can be reduced by, for example, replacement of the hydrogen of the imidazole ring with less reactive substituents such as hydroxyethyl [179], sulfoalkyl [180,181], cyanoethyl [182], and phenyl [183], as well as alkyl, alkenyl, or aryl [184] groups. The methods developed by Sansone et al. [180-184] use a PBI solution in DMAc or 7V-methyl-pyrrilidone. The unsubstituted PBI is first reacted with an alkah hydride to produce a polybenzimidazole polyanion, which is then reacted with a substituted or unsubstituted alkyl, aryl, or alkenyl methyl halide to produce an iV-substituted PBI, as shown in Fig. 4.13. 

ANHYDROUS PROTON-CONDUCTING POLYMERS FOR HIGH-TEMPERATVJRE PEMFCs 

15 Partially fluoriiiated svdfoDated polymers used for piepaiation of acid-base blend membranes with PBI [148,204]. 

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Another prorrtising modification of PBI is the introduction of immobilized polyvirtylphosphonic acid (PVPA) in the PBI matrix. This approach is especially interesting since the acid is not likely to be washed out dttring operation, thus maintaining steady fuel cell performance. PEMEAS-BASF Fuel Cells has developed an MEA product, Celtec -V, mainly for liqttid feed DMFCs, based on this concept Single-cell performance tests of CeltecV and Na... 

In liquid separation, hollow fiber membranes based on PBI have shown excellent performance for pervaporation dehydration of organic liquids. For example, a dual layer PEI-PBI hollow fiber membrane with an outer selective layer of PBI showed better performance than most other polymeric membranes in pervaporation dehydration of ethylene glycol. Sulfonation modifications of PBI membranes have demonstrated excellent separation efficacies in the dehydration of acetic acid. Studies have shown that PBI hollow fiber membranes were effective in separating chromates from solutions. Also, PBI nanofiltration hollow fiber membranes are promising candidates as forward osmosis membranes. In gas separation, recent studies sponsored by the Department of Energy at Los Alamos National Laboratories and SRI International demonstrated potential applications of PBI membranes in carbon capture and Hj purification from synthesis gas streams at elevated temperatures. H2/CO2 selectivity > 40 has been achieved at H2 permeability of 200 GPU at 250°C. ... 

Ga( Zn), Sn, Te( I) Mossbauer spectroscopy, no modifications of the local symmetry of lattice sites, electronic structure of atoms and intensity of electron-phonon interaction are revealed for Pbi Sn Te solid solutions in the gapless state at 80 and 295 K... 

Because of good thermal and hydrolytic stability, excellent mechanical and chemical stability, low cost, and commercial availability of sulfonated aromatic hydrocarbon polymers, recent research has focused on the synthesis and development of sulfonated aromatic hydrocarbon polymers specifically for high-temperature PEMFCs. Typical examples include sulfonated poly(ether ether ketone) (SPEEK) or poly(ether ketone ketone) (SPEKK) [1,2], sulfonated poly(ether sulfone) (SPSE) [3], alkyl sulfonated polybenzimidazole (PBI), sulfonated naphthalenic polyinrides (sNPl) [4-6], sulfonated polyCphenylene sulfide) [7,8]. Both post- and pre-sulfona-tion methods have been used in the past. Other than the post-sulfonation modification of aromatic polymers, recently, efforts have been dedicated to direct polycondensation from sulfonic acid containing monomers to synthesize sulfonated polymers [9]. The latter approach, namely pre-sulfonation, is widely applied because of the ease of controlling sulfonation degree and deactivated sites in the arylene backbones, which further avoid side reactions such as decomposition and hydrolysis of polymers resulted from the post-sulfonation method. 

More efforts have been made to modify the polymer stmctures for fuel cell uses. One motivation of these efforts is to improve the properties such as high molecular weight (MW) and good solubility and proeessability, which are of significance for mechanieal stability and functionalization processing of PBI membranes. Another motivation is to tailor the basicity of the polymers for improving the acid-base membranes. These modifications are accomplished in two ways either by syntheti-eaUy modifying the monomers prior to polymerization or by the postpolymerization substitution of the polymer at the reactive benzimidazole N-H sites. 

The good miscibility of PBI-PVPy blends can be useful for property modifications in the plastics industry, particularly in the areas of processability, where intermediate Tg values are sufficient and economical, and with retention of good mechanical properties at elevated temperatures. Blending can be achieved by solution mixing, and hydrogen bonding promotes compatibility. 

In subsequent years (1988), the MAGIC system was commerciahzed, first by Hewlett-Packard (nowadays Agilent Technologies), and subsequently by other instrument manufacturers. Four commercial versions of the system have been available (1) the particle-beam interface, featuring an adjustable concentric pneumatic nebulizer, (2) the thermabeam interface with a combined pneumatic-TSP nebulizer, (3) the universal interface, in which TSP nebulization and an additional gas diffusion membrane is applied, and (4) the capillary-EI interface, which resulted from systematic modifications to existing PBI systems by Cappiello [83]. The first system was most widely used, and is discussed in more detail below. For some years, PBI was widely used for environmental analysis, especially in the US. 

Functionalisation of MWNTs via a microwave-induced polymerisation modification route to produce polybenzimidazole (PBI) nanocomposite films has been developed by Zhang et al These PBI-MWNT demonstrated an increase in the Young s modulus of approximately 43.9% at 2 wt% CNT loading, and further modulus growth was observed at higher filler loading. " ... 

Other type of inorganic modifications used for PBI membranes was the formation of composites with sohd acids such as heteropolyacids or derived salts and pyrophosphates. The strategy of these inorganic materials was to increase the conductivity of the membrane at lower acid contents, keeping or even improving other properties such as the mechanical and chemical stability [39]. 

FIGURE 8.6 Synthesis of various PBIs via condensation polymerization in Eaton s reagent. (Reprinted with modification from Ueda, M. et al.. Macromolecules, 18, 2723, 1985. With Permission.)... 

FIGURE 8.37 Synthesis of poly(arylene ether sulfone)-f>-PBI (BPS-PBI) multiblock copolymers (a) synthesis of the PBI oligomer, (b) synthesis of the carboxyl-terminated polyfethyl sulfone) oligomer (BPS), and (c) block copolymerization. (Reprinted with modification from Lee, H.-S. et al.. Polymer, 49, 5387, 2008. With permission.)... 

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