Poly(p-terphenyl alkylene)s grafted with highly acidic sulfonated polypentafluorostyrene side chains for proton exchange membranes

Molecularly well-designed proton exchange membranes (PEMs) with a high local concentration of strongly acidic groups have the potential to fulfill the strict requirements for fuel cell operation under high temperature and low humidity. Here, we have prepared a series of well-defined and tunable PEMs based on poly(p-phenylene alkylene) backbones functionalized with sulfonated polypentafluorostyrene grafts having different ionic content, degree of grafting and molar mass. First, backbone copolymers were prepared by superacid-mediated polyhydroxyalkylations of p-terphenyl, 2,2,2-trifluoroacetophenone and 3-bromo-1,1,1-trifluoroacetone. Next, the bromomethyl groups of these copolymers were utilized as initiator sites for atom transfer radical polymerization (ATRP) of pentafluorostyrene. Finally, the polypentafluorostyrene grafts were quantitatively and selectively sulfonated to introduce highly acidic perfluorophenylsulfonic acid groups. Solvent cast PEMs displayed a microphase separated morphology with domains on the nanoscale, which gave a controlled water uptake that increased only very little between 20 and 80 °C. Under fully hydrated conditions, the PEMs reached a maximum proton conductivity of 154 mS cm−1, exceeding that of Nafion NR212. Under reduced humidity, the conductivity was just slightly below that of NR212. In conclusion, the combination of ether-free stiff polymer backbones and the strongly acidic side chains gave rise to nanostructured PEMs with restricted water uptake, high proton conductivity, stability and robust mechanical properties, which merit further investigations of their performance in fuel cells.