Comparative effect of phthalazinone units in sulfonated poly(arylene ether ether ketone ketone) copolymers as proton exchange membrane materials

A new series of aromatic poly(arylene ether ether ketone ketone) copolymers containing pendant sulfonic acid groups (SPAEEKK‐D) were synthesized from commercially available monomers 1,3‐bis(4‐fluorobenzoyl)‐benzene, sodium 6,7‐dihydroxy‐2‐naphthalenesulfonate, and 4‐(4‐hydroxyphenyl)‐2,3‐phthalazin‐1‐one (DHPZ). Structure–property relationships of the phthalazinone SPAEEKK‐D series poly(arylene ether ether ketone ketone) copolymer were compared with copolymers SPAEEKK‐B and SPAEEKK‐H containing different diols such as 4,4′‐biphenol and hydroquinone, respectively, prepared in our earlier work. Ion exchange capacity (IEC_w, weight‐based; IEC_v, volume‐based), thermal stabilities, swelling, proton and methanol transport properties of the membranes were investigated in relation to their structures and compared with those of perfluorinated ionomer (Nafion 117). The SPAEEKK‐D membrane incorporating the phthalazinone monomer DHPZ showed relatively lower water uptake and methanol permeability compared with earlier SPAEEKK‐B and SPAEEKK‐H membranes incorporating biphenol and hydroquinone monomers, respectively. Inclusion of phthalazinone in the SPAEEKK‐D copolymers led to lower water absorption, enabling increased proton exchange concentrations in the hydrated polymer matrix that resulted in more desirable membrane properties for future direct methanol fuel cell applications. The SPAEEKK‐D membranes also showed improved mechanical and thermal properties and oxidative stability compared with the earlier SPAEEKK‐B and ‐H membranes. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 989–1002, 2008