Polybenzimidazoles with Pendant Quaternary Ammonium Groups as Anion Exchange Membranes: Synthesis, Characterization and
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Polybenzimidazoles with Pendant Quaternary Ammonium Groups as Anion Exchange Membranes: Synthesis, Characterization and Alkaline Stability Mahesh P. Kulkarni, Timothy J. Peckham, Owen D. Thomas, and Steven Holdcroft* Department of Chemistry, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia, Canada V5A 1S6. ABSTRACT A novel method for introducing quaternary ammonium-bearing side chains has been developed and applied to a polybenzimidazole backbone to generate new anionic-exchange polymers, TMHA-m-FPBI where m = the degree of substitution of available sites (values ranging from 74 to 100%). These polymers have been shown to exhibit hydroxide ion conductivities up to 34 mS cm-1 in hydrated membranes at ambient temperature and 13 mS cm-1 at 60 ºC, RH = 95%. Immersion of these polymers in 2 M KOH at 60 ºC has shown they remain stable with no sign of chemical structure degradation to a period of at least 15 days. INTRODUCTION Research on alkali anion exchange membranes has recently garnered increased interest due to potential applications for these materials as battery separators,1 electrodialysis membranes for water purification and salt production,2-5 sensors6 and solid polymer electrolytes in alkali anion exchange membranes fuel cells (AAEMFCs).7-16 Most AAEMs are based on quaternary, tetra-alkyl (primarily tetramethyl) nitrogen centres tethered to a polymer backbone via short alkyl chains.10, 12, 16-21 These functional groups, however, have generally have been found to be unstable under strongly alkaline conditions22-26 and thus alternative functionalities have been explored. The tethered cationic centres have included imidazolium,27, 28 phosphonium,29, 30 guanidinium14 and transition-metal-containing groups.15 Recently, the use of methylated polybenzidimidazole (PBI), where both nitrogen centres in the benzimidazole rings are quaternized, has been explored as a potential AAEM;31-33 however, the material was found to be decompose over time when immersed in a strong base. Very recently, Thomas et al. reported that increasing the steric hindrance around the C2 carbon in methylated PBI results in a polymer that shows no signs of structural degradation under strongly alkaline conditions (2 M KOH, 60 ºC, 10 d) and, when blended with a non-methylated derivative of PBI, produces water-insoluble membranes that possess high levels of hydroxide conductivity (up to 13.2 mS cm-1).34 Although considerable research in the area of AAEMs has been directed towards alternative functional groups, there remains an interest in tetraalkylammonium functional groups due to their relatively facile synthesis, particularly in the case of polymer postfunctionalization. A report by Tomoi et al. demonstrated the effect of different spacer groups upon the thermal stability of tetraalkylammonium groups, with hydroxide counter-ions, attached to a polystyrene (PS) backbone.35 It was found that if the length of the alkyl chain between the phenyl ring of the backbone and the ammonium centre was at least four carbons long, at lea
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