Synthesis Of Novel Polybenzimidazoles For Proton Exchange Membranes

Polybenzimidazole (PBI) is one of the classes of aromatic heterocyclic polymers, which have high mechanical properties, excellent thermal stability and chemical resistance. Because of these outstanding comprehensive properties, they have been widely used in automotive industry, aeronautical industry, microelectronics industry, fuel cells components, etc. However, the major drawback of PBI is their extreme processing conditions due to the poor solubility and infusibility. Phthalazinone moiety is a heterocyclic, crank and twisted, non-coplanar and asymmetrical structure, which can reduce the regularity of the main chain, cumber the close packing of chains and increase the free space between the molecular chains, and thus enhance the solubility of polymers.In this study, novel serieses of polybenzimidazoles containing phthalazinone moiety in the polymer main chain have been synthesized. Their applications in proton exchange membrane fuel cells have been studied.A novel polybenzimidazole containing phthalazinone structure (PPBI) was synthesized from4-(4-(4-(4-carboxyphenoxy)phenyl)-1-oxophthalazin-2(1H)-yl)benzoic acid (CPPBC) and3,3’-diaminobenzidine (DAB) by solution polycondensation in polyphosphoric acid (PPA). Furthermore, the model compound of PPBI was synthesized. The chemical structure of the model compound was established by FT-IR and NMR. To provide further insight into the fundamentals of molecular architecture, molecular simulation was carried out for the model compound containing phthalazinone moiety. The PPBI polymer was found to be soluble in aprotic polar solvents such as NMP, DMAc, and DMSO. The PPBI had inherent viscosity of1.78dL/g, and showed amorphous nature as evaluated by WAXD. PPBI had high glass transition temperatures of402℃and excellent thermal stability with the temperatures for5%polymer weight loss of550℃and char yields at800℃of58.3%.A novel series of polybenzimidazoles containing4-phenyl phthalazinone moieties and p-phenylene linkages (p-PPBI) were synthesized from CPPBC, terephthalic acid (TPA) and DAB in PPA. The chemical structures of p-PPBI polymers were characterized by FT-IR and NMR. The p-PPBI exhibited amorphous nature as evaluated by WAXD. The resultant polymers had inherent viscosity up to3.43dL/g and showed good solubility in NMP, DMAc and DMSO. The p-PPBI exhibited high glass transition temperatures of426℃and excellent thermal stability with the5%weight loss temperatures of564℃. The highest char yield of p-PPBI at800℃was75.7%. Moreover, the tensile strength of p-PPBI was up to98.8MPa. A novel series of sulfonated4-phenyl phthalazinone-based polybenzimidazoles (SPPBI) were synthesized by solution polycondensation of DAB, CPPBC and5-sulfoisophthalic acid monosodium salt (SIPN) in PPA. Their chemical structures were characterized by NMR, FTIR and WAXD. The SPPBI membranes were prepared by solution casting technique, displaying low water uptake and swelling ratio. The SPPBI membranes present IEC value up to0.76meq/g. The SPPBI membranes had unexpected low methanol permeability (5.46×10-10cm2/s, almost4order of magnitudes lower than Nafion(?)117), and their proton conductivities were comparable with Nafion(?)117at the temperature below100℃(～1.3×10-2S/cm at80℃in water). The sulfonation degree (SD) as well as the properties of SPPBI membranes can be controlled. The water uptake, swelling ratio, IEC value and proton conductivity increased with the increasing of SD, while the methanol permeability, oxidative stability and hydrolysis resistance were enhanced with the increasing phthalazinone content.To prepare PBI membranes with high proton condcutivity, the amorphous PPBI and p-PPBI polymers were cast into membranes, doped with phosphoric acid and further investigated as proton exchange membranes. To modify the doping conditions, the PPBI membranes were doped in PA with different concentration, at different doping temperature and for different time. Membranes with high acid doping level (-15.2mol H3PO4) and high proton conductivity (～0.13S/cm in anhydrous condition) were obtained after doping with14.7mol/L phosphoric acid at180℃for30h. These obtained membranes present breaking time up to81h in Feton regent and tensile strength up to26.1MPa.To prepare PA doped PBI membranes with higher conductivity as well as better oxdative stability, a series of novel polybenzimidazoles containing4-phenyl phthalazinone moieties and hydroxyl groups (PPBIOH) were synthesized from CPPBC,2,5-dyhydroxyterephthalic acid (TPA-OH) and DAB in PPA. The resultant polymers showed good solubility in H2SO4and NMP. The inherent viscosity of the polymers was up to2.10dL/g. The corresponding acid doped membranes were prepared directly from the polymerization solution by the PPA process. These membranes showed high proton conductivities up to0.22S/cm at high temperature under anhydrous condition. The breaking time in Feton regent was up to155h and the tensile strength was enouth for PEM use.