Fluorinated PBI

Several authors have determined methanol permeability for PBI [12, 192, 197, 280,413], methyl and ethyl-PBI [197], and fluorinated PBI [198,414], for different acid doping degree. The reported permeability coefficients are summarized in Fig. 6.33, where the A values are indicated. 

A variety of high-temperature-resistant PBIs were synthesized and reported in the Uterature. The most widely used PBI is poly[2,2 -(l,3-phenylene)-5,5-benzimidazole] (known as m-PBI) [22]. The other variations are poly[2,2 -( 1,4- phenylene)-5,5 -benzimidazole] (known as p-PBI) [13], poly(4,4 -diphenylether-5,5 -bibenzimidazole) (OPBI) [23], poly(2,5-benzimidazole) (AB-PBI) [24], pyridine-based PBI [25], sulfonated PBI [26], hyperbranched polybenzimidazole (HPBI) [27], naphthalene-based PBI [28], fluorinated PBI [29], PBI block copolymer [30], sulfonated PBI copolymer with sulfone or sulfonic acid groups in the backbone [31,32], PBFnano-composite [33], cross-linked PBI [34], and many others. Different approaches have been taken by the researchers to modify the structure and properties of the fluorinated PBI. A detailed summary of these works is discussed below. 

Then they used these fluorinated PBIs for the preparation of the hybrid nanocomposite by mixing it with a silica precursor, tetraethoxysilane (TEOS), in a DMAc solution followed by hydrolysis and solution casting (Figure 5.19). The compatibility of the fluorine-containing PBI copolymer with the inorganic silica phase was good, and the bonding site enhanced the interfacial interaction between PBI chains and silica nanoparticles [33]. 

The high-temperature stability of the PBI attracted the attention of the scientists to develop suitable membrane material for high-temperature industrial applications. Asymmetric hollow fiber membranes were successfully prepared by Kumbharkar et al. using the fluorinated PBI synthesized by a solution polycondensation method using PPA (Figure 5.47). To have defect-free asymmetric PBI membranes for gas... 

Researchers over the world tried to use fluorinated PBI for PEM. Hsu et al. studied and compared the proton conductivity of the PBI membranes (Figure 5.3(a)) doped with different amounts of PA, with Nation 117 membrane at different temperatures shown in Figure 5.62. Above 80 °C, conductivity of the Nation 117 membrane unexpectedly decreased due to dehydration. With an increase of the temperature and the doping level, conductivity of the PBI membranes increased. At 160 °C, the proton conductivity of PBI-3.OH3PO4 was 1.7 X10 " (S/cm), much higher than that of the Nation 117 membrane. The proton conductivity of PBI membranes decreased after 180 °C, due to dehydration of PA by forming H4P2O7 [29],... 

Multitudes of other organically modified PBI membranes exist that include, but are not limited to, fluorinated PBI, ionicaUy and covalently cross-linked PBI, PBI blends, and a wide variety of PBI copolymers. Because there are far too many to describe, this subsection wiU highlight select PBI membranes that have not been included in the prior subsections. 

From the abovementioned results, it can be concluded that the combination of the partially lluorinated sulfonated ionomer M with the partially fluorinated PBI B2 is of advantage in terms of chemical stability and compatibility, which can be explained with chemical similarity, since both blend components consist of electron-deficient aromatic ring systems. Good PBI blend membrane stabilities can be achieved by good compatibility between the blend components, electron-deficiency of the aromatic ring systems, or both. To get hints of which is the... 

The acid-base Nafion composite membranes include blends of Nafion with polypyrrole (PPy) [98-104], polybenzimidazole (PBI) [105-107], poly (propyleneoxide) (PPO) [108, 109], polyfurfuryl alcohol (PFA) [110], poly(vinyl alcohol) (PVA) [111-115], sulfonated phenol-formaldehyde (sPF) [116], polyvinylidene fluoride (PVdF) [117-122], poly(p-phenylene vinylene) (PPV) [123], poly(vinyl pyrrolidone) (PVP) [124] polyanifine (PANI) [125-128], polyethylene (PE) [129], poly(ethylene-terephtalate) [130], sulfated p-cyclodextrin (sCD) [131], sulfonated poly(ether ether ketone) (sPEEK) [132-135], sulfonated poly(aryl ether ketone) (sPAEK) [136], poly(arylene ether sulfone) (PAES) [137], poly(vinylimidazole) (PVl) [138], poly(vinyl pyridine) (PVPy) [139], poly (tetrafluoroethylene) (PTFE) [140-142], poly(fluorinated ethylene-propylene) [143], sulfonated polyhedral oligomeric silsesquioxane (sPOSS) [144], poly (3,4-ethylenedioxythiophene) (PEDT) [145, 146], polyrotaxanes (PR) [147], purple membrane [148], sulfonated polystyrene (PSSA) [149, 150], polystyrene-b-poly(ethylene-ran-butylene)-bpolystyrene (SEES) [151], poly(2-acrylamido-2-methyl-l-propanesulphonic acid-co-l,6-hexanediol propoxylate diacrylate-co-ethyl methacrylate) (AMPS) [152], and chitosan [31]. A binary PVA/chitosan [153] and a ternary Nafion composite with PVA, polyimide (PI) and 8-trimethoxy silylpropyl glycerin ether-1,3,6-pyrenetrisulfonic acid (TSPS) has also been reported [154]. 

Pu et al. [197] have prepared and tested N-methyl (PNMBI) and N-ethyl (PNEBI) derivates of PBI, and Chuan and Hsu [198] reported fluorine-containing PBI. A number of PBI membranes modified by ionic or covalent cross-linking and composite PBI membranes have been reviewed by Li et al. [199]. 

Suegusa et al. first synthesized two organosoluble fluorine-containing aromatic PBIs by direct polycondensation of 4,4 -(hexafluoroisopropylidene) dibenzoic acid (HFA) with 3,3 -DAB and 1,2,4,5-benzenetetra-mine tetrahydrochloride in PPMA or PPA (Figure 5.3) [3]. PPMA was used as a solvent and reaction of a 1 mmol scale, 5 mL of PPMA was found to be appropriate. An optimized reaction solution concentration was required because the polycondensation reaction did not proceed homogeneously at high concentration. 

Jana et al. reported that HFA containing PBI polymer (Figure 5.15) absorbed less water and also that the swelling ratio was low due to the presence of hydrophobic fluorine atoms in the polymer chain. This polymer was chemically stable, with only -10% weight loss after 120h in Fenton s reagent. The Py-PBI membrane was doped with a different percent of... 

Polyazoles, a class of high-strength heterocyclic polymers, are classified into three broad classes depending on the hetero atoms, like PBO, PBI, and PBT. However, their inherent insolubility in common organic solvents restricts their growth in practical applications. Different methods have been applied to enhance their solubility and processability. Incorporation of the fluorinated moiety in the polymer backbone is one of the important ways to improve the solubility and processability. The fluorinated poly azoles obtained... 

X. Li, G. Qian, X. Chen, B.C. Benicewicz, Synthesis and characterization of a new fluorine-containing polybenzimidazole (PBI) for proton conducting membranes in fuel cells. Fuel Cells 13 (5) (2013) 832-842. 

Li Q, Jensen JO, Pan C et al (2008) Partially fluorinated arylene polyethers and their ternary blends with PBI and H3PO4 Part II. Characterisatirai and fuel cell tests of the ternary membranes. Fuel Cells 8 188-199... 

From Fig. 4.16 can be read that both covalently cross-linked blend membranes expose nearly similar thermal stabilities, while the thermal stability of pure partially fluorinated B2 is slightly better than that of the pure fluorine-free B4 only above 450 °C which is however not relevant for the intended intermediate-T fuel cell application of these membranes. Therefore, covalent cross-linking can be regarded as a good method to improve thermal (and chemical, see below) stabilities of PBI-type membranes. 

From the reviewed work, it can be concluded that obviously the use of partially fluorinated aromatic cationomers as ionical cross-linkers leads in most cases to better chemical and thermal stabilities of the blend membranes than if nonfluorinated cationomers would be applied as acidic blend components. Among aU acidic cross-linkers, the sulfonated and partially fluorinated ionomer S9 (Fig. 4.5) leads to the best chemical stability of the referring base-excess PBI blend membranes. 

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