Studies On The Solid Electrolyte Materials Of Polybenzimidazole

Polybenzimidazole(PBI) is a class of aromatic heterocyclic polymers,which has excellent thermal and chemical resistance,fire retarding capacity.Duing to its excellent performance,PBI has been used as textile fibers,high temperature matrix resins,adhesives,foams and so on.Recently,acid doped PBI membranes were found to be the most promising materials as polymer electrolyte membranes in low-cost, high-temperature polymer electrolyte membrane fuel cell(PEMFC).However,the lack of either solubility or mechanical stability of these materials is the main factors of impeding its industrial processing and application in fuel cell.We want to synthesize the PBI with higher molecular weight and try to improve its forming process using new solvent system to relatively overcome the problem mentioned above.In this work,poly[2,2-(m-phenylene)-5,5-bibenzimidazole](mPBI) and poly(2, 5-benzimidazole)(ABPBI) with higher molecular weight were obtained by optimizing reaction conditions.Then,quaternary ammonium groups were successfully introduced into the chain of ABPBI.FT-IR,¹H-NMR,TGA and SEM were used to characterize the structure and properties of the obtained polymers. Furthermore,the solubility of ABPBI and mPBI in various ionic liquids was investigated,and the dissolution mechanism of mPBI in ionic liquids was also disscussed.The main conclusions of our work are summarized as follows:(1) mPBI with the intrinsic viscosity of 1.12 dL/g was synthesized by polycondensation of 3,3',4,4'-tetraminobiphenyl(TAB) and isophthalic acid(IPA) in polyphosphoric acid(PPA).(3 h pre-reaction at 140℃,and then 20 h reaction at 200℃).ABPBI with the intrinsic viscosity of 2.3 dL/g was prepared by polycondensation of 3,4-diaminobenzoic acid(DABA) in PPA at 210℃for 10h.(2) A novel quaternary ammonium salt,ABPBI containing quaternary ammonium groups,and functionalized ABPBI with tertiary amino side groups were designed and synthesized,which could be used as the electrolyte membrane for alkaline fuel cell.(3) Ionic liquids can be used as nonderivatizing solvents for mPBI.Ionic liquids incorporating anions which are strong hydrogen bond acceptors,such as 1-butyl-3-methylimidazolium chloride(BMIMCl) and 1-butyl-3-methylimidazole hydroxide(BMIMOH),were most effective,especially when combined with microwave heating.The highest solubility of mPBI in BMIMCl reached 12%by heating in an oil bath at 140℃and 15%by microwave heating.The solubility of mPBI in BMIMOH was 8%when heated in an oil bath at 140℃and 10%by microwave heating.However,ionic liquids containing 'noncoordinating' anions, like 1-butyl-3-methyl tetrafluoroborate(BMIMBF₄) were nonsolvents for mPBI.(4) mPBI fibers were prepared by extruding mPBI/BMIMCl solution into water, stretching,cleaning,and drying.The surface of the obtained fibers was very smooth,which indicated that ionic liquids could be used to replace the traditional organic solvent for wet-spinning of mPBI fibers.The stability and chemical structure of the mPBI weren't shown obvious changes after regenerating.(5) BMIMOH doped mPBI membranes were prepared by one step method.The ionic conductivity of the obtained BMIMOH/mPBI membranes with mass ratio of 5 was 4.75×10^-3 S/cm at room temperature.BMIMOH/mPBI membranes may be the potential materials as the electrolyte membranes for alkaline fuel cell.