Substituted poly(p-phenylene) oligomer as a physical crosslinker in Nafion® membrane

Oligomeric poly(p-phenylene-2,5, diacetoxy) (POAc), a rigid rod bearing acetyl side-chain groups, was incorporated into a Nafion matrix through solution mixing and casting. The resulting composite matrix is characterized by the interactions between the sulfonic acid groups of Nafion and POAc through hydrogen bonding as well as the proton-π system affinity. Hence, these interactions serve as physical crosslinking to strengthen the Nafion matrix. The variation of glass transition temperatures of Nafion with an increase in POAc content exhibits a special pattern, which supports the occurrence of physical crosslinking. Alternatively, the UV–vis spectroscopic study endows the evidence for the adsorption of Nafion molecules to POAc particles in a substantially dilute environment. In addition, an examination on the morphology of composite matrix and on the fusion behaviour of matrix-bound water also supports the association of Nafion with POAc. The optimal loading of POAc in the Nafion matrix was 1 wt% through balancing the enhancements on storage modulus and proton conductivity by POAc. In contrast with pristine Nafion membrane, the composite membrane, Nafion-POAc (1%) in particular, revealed greater proton conductivity at either elevated temperatures or under low relative humidity conditions. The single H2-fuel cell evaluation showed that the composite membrane sustains a higher power density than the pristine Nafion membrane and such difference increases with rise of cell operation temperature.