Acid doping degree

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. 

The proton conductivity of PBI and modified PBI membranes has been studied under different conditirms of membrane preparation, acid doping degree, temperature, and water activity. The results for PBI membranes prepared by casting from dimethyl acetamide (DMA) and other solvents (NMP N-Methyl pyrrolydOTie, DMSO dimethyl sulfoxide, TFA, trifluoroacetic acid) are summarized in Table 6.6. 

The thermodynamic data from the adsorption isotherms can be combined successfully with spectroscopic data from Raman investigations. From the changes of characteristic Raman bands as a function of doping degree 6, two distinguishable stages of protic electrolyte (phosphoric acid) uptake can be identified ... 

Acid-doped sulfonated PBIs (sPBI) have been synthesized by multiple research groups [12,15,40-42,50-54]. Typical approaches to sulfonation include direct sulfonation of the PBl backbone, chemical grafting of functionalized monomers onto the chain, or copolycondensation of sulfonated monomers. The last approach is highly favored, because side reactions can be avoided and degree of sulfonation easily controlled. A recent review by Rikukawa and Sanui [54] thoroughly describes the preparation of sulfonated hydrocarbon polymers and the properties of the sulfonated membranes. 

Jeong et al. [69] synthesized acid-doped sulfonated poly(aryl ether benzimidazole) (s-PAEBI) copolymers for use in high-temperature fuel cells. The polymer was made in a direct polymerization (structure confirmed with NMR) and doped with phosphoric acid to levels of 0.7-5.7. The degree of sulfonation was varied from 0-60%. The copolymer s physicochemical properties were studied using AFM, TGA, and conductivity measurements. TGA runs showed good stability of the nonsulfonated, sulfonated, and acid-doped sulfonated membranes up to 450 °C, with a slow decline above this temperature. The conductivity depended on the doping level of the polymer. At 130 °C with no hiunidification, a polymer with a doping level of 5.7 had a conductivity of 7.3 X 10 S cm ... 

Emeraldine salt films can be prepared directly from solutions when particular counterions are used to impart solubility. The most extensively studied system is the camphorsulfonic acid-doped ES as cast from a m-cresol solution. Lunzy and co-workers have considered the crystal structure of camphorsulfonic acid-doped polyaniline (PAn-CSA) as cast from m-cresol. Although presenting a clear crystal structure with a degree of crystallinity of 25%, the unit cell structure has no resemblance to the structures formed by the emeraldine base. 

A sulfonated polybenzimidazole that has all the desired properties of conductivity and thermal stability to be used at 150-200 °C is sulfonated ABPBI prepared by direct sulfonation of a previously cast membrane, followed by phosphoric acid doping as detailed in Fig. 3.34. Sulfonation of ABPBI reduces the phosphoric acid uptake at low doping bath concentrations, but at high bath concentrations the sulfonated ABPBI absorbs more acid than the nonsulfonated ABPBI (Fig. 3.35), and the phosphoric acid uptake in these concentrated baths increases with the sulfonation degree (Fig. 3.36). As it could be expected, this higher doping level leads to an increase in conductivity as shown in Fig. 3.37 [382,396]. 

For a given polybenezimidazole, both DL and Sp are useful for characterization of the degree of acid doping. However, in some cases, Sp is more useful for comparison of the sorption capacities between different PBIs. For example, the commercial PBI possesses two imidazole rings per repeat unit, whereas the AB-PBI possesses only one imidazole ring per repeat unit. As a result, when the DL is the same for both polymers, the Sp of the commercial PBI is only less than half of that of the AB-PBI. 

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