Nanophase-Separated Block-co-Polymers Based on Phosphonated Pentafluorostyrene and Octylstyrene for Proton-Exchange Membranes

Nanophase separationinto hydrophobic and hydrophilicdomains incommercial perfluorosulfonic acid polymers promotes high conductivityby forming proton-conductive channels within a matrix. To transferthis beneficial phase separation to phosphonic acid functionalizedionomers, we combine phosphonated polypentafluoro-styrene andflexible polyoctylstyrene in a di-block-co-polymer. We introduce astepwise approach, including mesophase simulations, synthesis, andspectroscopic imaging. After the required block lengths were calculated,controlled radical polymerization led to a narrowly distributed block-co-polymer.The respective block-co-polymer membrane outperforms a phosphonatedpentafluorostyrene blend concerning conductivity and water uptake.Stained membrane cross-sections revealed bicontinuous nanophase separationin the 13 to 25 nm range in transmission electron microscopy. Theion-conducting phosphonated polymer block assembled into an isotropic,three-dimensional gyroidal network across the membrane. Our stepwiseapproach is transferable toward other block-co-polymer systems featuringdifferent monomers or functional groups. Applying the proposed principlesallows for the prediction of structure-related phase separation whilereducing the amount of synthesis work.

Automated summary

Commercial perfluorosulfonic acid polymers undergo nanophase separation, forming proton-conductive channels that result in high conductivity. Researchers have successfully transferred this phase separation behavior to phosphonic acid functionalized ionomers by synthesizing a di-block-co-polymer containing phosphonated polypentafluorostyrene and flexible polyoctylstyrene. The resulting membrane outperforms a phosphonated pentafluorostyrene blend in terms of conductivity and water uptake.