Improved properties of sulfonated octaphenyl polyhedral silsequioxane cross-link with highly sulfonated polyphenylsulfone as proton exchange membrane

This study explored the concept of improving the properties of the cross-linked membrane using a 1.5-nm closed cage octaphenyl polyhedral silsesquioxane (POSS) form of nanosilica carrying the sulfonic acid group. POSS functioned with SO3H groups (SPOSS) at 0, 1, 2, and 5 wt% were cross-linked with water-soluble sulfonated polyphenylsulfone (SPPSU) polymer. The cross-linking between SPPSU and SPOSS was accomplished through the interchain condensation of sulfonic acid functionalities initiated by thermal curing treatment. In this study, a covalently cross-linked membrane was obtained under stepwise thermal curing from 80 to 180 degrees C. Upon curing at 180 degrees C, the SPPSU-SPOSS showed considerable improvement on the membrane proton conductivity under low and high RH (%) conditions compared with the pristine SPPSU membrane. The membrane proton conductivity shows similar patterns with the membrane water uptake as the presence of water greatly influences the cross-linked membrane. The proton conductivity of the SPPSU cross-linked with 1 wt% SPOSS that was conducted under low RH (%) and at elevated temperature exhibited about six times higher proton conductivity as compared with pristine SPPSU membrane. However, increasing the loading of SPOSS beyond 1 wt% significantly dropped the membrane water uptake and proton conductivity due to SPOSS aggregation, blocking the hydrophilic domains in the polymer matrix. The results indicated that the incorporation of SPOSS in the SPPSU membrane by curing at 180 degrees C exhibit improvement on membrane water management and proton conductivity as compared with the pristine SPPSU membrane.