Preparation And Characterization Of Soluble Polybenzimidazole Based Sulfonic Acid Membranes

Polybenzimidazoles (PBI) based proton exchange membranes (PEM) with excellent properties attracted great attention, including four sorts such as the doped PBI membranes, the grafted PBI membranes, the blending PBI membranes and the direct polycondensation PBI membranes. Solubility of sulfonated polybenzimidazoles (sPBI) is the key challenge in the fuel cell applications. The direct polycondensation method shows greatly flexible in the molecular design of soluble sPBI according to the requirements of PEM. However, there are only few kinds of sPBI prepared by the direct polycondensation in the previous studies.In this dissertation, 2-sulfonate p-terephthalic acid (STA) was synthesized, the structure of which was confirmed by 1H NMR, 13C NMR, Dept135 and IR spectroscopy. sPBI were prepared by direct polycondensation of 3,3'-diaminobenzidine (DAB) with STA and 4,4'-oxybis(benzoic acid) (OBBA) or 2,2'-bis(4-carboxylphenyl) -hexafluoropropane (BCHFP), the sulfonate groups of which were introduced onto the molecular chain. Meanwhile, the ether groups and hexafluoroisopropylidene units were incorporated into the backbone of the sPBI in order to improve the flexibility, solubility as well as processability. However, the sPBI derived from OBBA showed poor solubility, and their membrane samples could not be obtained by casting from solution. On the contrary, the sPBI containing hexafluoroisopropylidene units have good solubility, sPBI-20 ~ sPBI-60 could be cast into membranes by solution. The microscopic phase separation of sPBI membranes became more and more notable with increasing the sulfonation degree, promoting the water retention and proton conduction. The sPBI indicated high thermal stability, the 5% weight loss temperature (T5%) of which is higher than 520℃, decreasing with the increase of sulfonation degrees. There were no endothermal peak, exothermal peak and glass transition in the DSC curves of sPBI at 120~380℃, designating that sPBI are amorphous. Besides, sPBI showed low swelling and excellent resistance to oxidation. Thus the sPBI indicated potential prospect in the fuel cell application.Soluble sPBI containing hydroxyl groups (PBI-OH) were also prepared by direct polycondensation of 3,3'-diaminobenzidine (DAB) with 5-hydroxyisophthalic acid (HIPA) and 2,2'-bis(4-carboxylphenyl) -hexafluoropropane (BCHFP). After modifying the hydroxyl groups of PBI-OH with K2CO3, soluble sPBI were prepared by grafting with 1,3-propanesultone or p-fluorobenzene sulfonate. This route of grafting required much more moderate conditions than that of grafting originating from the -NH of the benzimidazole groups of sPBI, showing good controllability and practicability. The structure of sPBI was confirmed by IR and NMR spectroscopy. PBI-OH showed high thermal properties and no microscopic phase separation, while the corresponding grafted products have moderate thermal properties but notable microscopic phase separation.