Base stable poly(diallylpiperidinium hydroxide) multiblock copolymers for anion exchange membranes

Abstract

The development of base-stable cationic groups for anion exchange membranes is important for application in alkaline fuel cells. Spirocyclic ammonium groups have been shown to be particularly base stable, although few examples exist that incorporate them into polymer structures. A telechelic polymer with spirocyclic ammonium repeat units was designed by the cyclopolymerization of diallylpiperidinium chloride with a photoiniferter. A series of hydrophobic–hydrophilic multiblock copolymers with varying ion exchange capacities were produced by copolymerizing the end-functionalized polydiallylpiperidinium oligomers with polysulfone monomers. The multiblock copolymers were solution cast in the hexafluorophosphate form from DMAc resulting in mechanically robust, colorless, transparent membranes. Tapping mode atomic force microscopy and differential scanning calorimetry demonstrated microphase separation of the blocks. The multiblock copolymers were determined to be highly conductive with hydroxide conductivities as high as 102 mS cm⁻¹ at 80 °C with 45% water uptake. Thermogravimetric analysis of the polydiallylpiperidinium oligomers and the multiblock copolymers demonstrated the materials to be highly thermally stable with the multiblock copolymers in the hydroxide form showing 5% weight loss at 360 °C. No degradation of the polydiallylpiperidinium was observed by proton NMR after 1000 hours at 80 °C in a 1 M KOH/methanol-d₄ solution. Membranes were found to maintain at least 92% of their hydroxide conductivity after being treated in 1 M KOH at 80 °C for 5 days.