Ionomers for proton exchange membrane fuel cells with sulfonic acid groups on the end groups: Novel branched poly(ether-ketone)s

The first examples of branched wholely aromatic polyether ketones (PEK) bearing clusters of sulfonic acid groups only on the end groups are presented. The novel trifunctional branching agent, 1,3,5-tris(4-(4-fluorophenylsulfonyl) phenyl)benzene was synthesized by Friedel-Crafts reaction of 1,3,5-triphenylbenzene with 4-fluorobenzenesulfonyl chloride in the presence of iron(III) chloride. Branched polymers PEK 5 with relatively short average block lengths of 1-(4-Hydroxyphenyl)-2,3,4,5,6-pentaphenylbenzene was employed as the end group bearing a number of pendant phenyl rings to provide a number of post-sulfonation sites. Stability of the main chain of 5 under sulfonation conditions was examined using unend-capped linear 5 prepared by the same procedure. Transmission electron microscopy (TEM) analysis showed significantly phase-separated and worm like, highly connected morphology in the membranes.