Magnetic ion-exchange nanoparticles and their application in proton exchange membranes

Nanoparticles with dual properties of magnetic susceptibility and ionic conductivity were synthesized for application in proton exchange membranes (PEM). The particles were composite in nature, consisting of γ-Fe2O3 and sulfonated crosslinked polystyrene. The synthesis was carried out using emulsion polymerization with various feed compositions ranging from 0 to 23 wt% ionic monomer and 8–11 wt% crosslinking monomer. The synthesized particles were in the size range of 230–340 nm and had polymer content of about 75–80%. A very unusual morphology of iron oxide localization was observed in the composite particles, and a model is proposed to explain such a structure. The particles had considerable magnetic susceptibility, and aligned easily in a sulfonated poly (ether ketone ketone) (SPEKK) matrix under magnetic field of 0.1 Tesla. The membrane with composite particles had lower ion-exchange capacity (IEC) compared to plain SPEKK (2.4 meq/g), due to the lower IEC (0.35 meq/g) of the particles used. However, a proton conductivity of about 0.0043 ± 0.0028 S/cm was observed by aligning the particles in the SPEKK matrix. By randomly dispersing the particles, the conductivity was about 0.0020 ± 0.0018 S/cm. The water and methanol uptakes were similar for both kinds of membranes and were about 23% and 18%, respectively. The use of magnetic fields for aligning the proton conducting domains was successfully demonstrated.