Synthesis and properties of sulfonated and unsulfonated poly(arylene ether triazine)s with pendant diphenylamine groups for fuel cell applications

Abstract

A series of poly(arylene ether triazine) homopolymers were synthesized that contain pendant diphenylamine groups. The polymers had inherent viscosities from 0.66 to 1.01 dL/g in DMAc at 25 °C, thermal stabilities >500 °C in air, glass transition temperatures from 156 °C to 309 °C, and solubilities that depended upon the bis(4-fluorophenyl) monomer that was used for polymerization. Polymers could be sulfonated with chlorosulfonic acid exclusively at the para position of the diphenylamine rings, with ion exchange capacities from 1.88 to 2.12 meq/g. Transmission electron microscopy images show that the morphology of the sulfonated polymer films depends on the functional group in the polymer backbone. Polymers containing sulfone groups exhibited small ionic clusters within a uniform ion-containing background, while polymers with ketone and isophthaloyl groups exhibited phase separation with different sizes of spherical hydrophilic clusters. The uniform distribution of ionic groups within polymers that contain sulfone groups resulted in higher proton conductivity, 0.11 S/cm at 90 °C and 100% relative humidity, in spite of having a lower degree of sulfonation and water uptake. Small angle neutron scattering data also shows this film has a robust morphology that does not change as a function of temperature or by counterion exchange. Films cast from unsulfonated polymers that contained phosphine oxide groups in the polymer backbone, as well polymer blends with polybenzimidazole soaked in 85% phosphoric acid at 75 °C, had phosphoric acid uptakes above 350 wt%, and as high as 830 wt%. However, these films lost dimensional stability at elevated temperatures.