Improving Physical Properties of Perfluorinated Ionomers by Crosslinking Using SuFEx Reaction

Abstract

Perfluorosulfonic acid (PFSA) ionomers are widely used in proton exchange membrane fuel cells; however, improving their physicochemical robustness without substantially compromising ion-exchange-related properties remains a major challenge. In this study, sulfur(VI) fluoride exchange (SuFEx) click chemistry was employed as a post-polymerization crosslinking strategy for poly(TFE-co-PSEPVE), a fully fluorinated PFSA precursor copolymer. Two nucleophilic crosslinkers, p-phenylenediamine (PPD) and (perfluoro-1,4-phenylene)bis(oxy)bis(tert-butyldimethylsilane) (F-HQ), were compared. FT-IR and rheological analyses showed that PPD was substantially more effective than F-HQ in promoting crosslink formation under the conditions used in this study. PPD crosslinking improved the thermal stability, tensile strength, toughness, and solid-like viscoelastic behavior of the polymer, whereas the glass transition temperature changed only slightly. In contrast, F-HQ did not provide clear evidence of effective crosslinking. The equivalent weight exhibited a non-monotonic dependence on the PPD content, with a marked increase in the apparent EW at 20 wt% PPD, which is likely related to the heterogeneous nature of both the crosslinking reaction and titration process. These results demonstrate that SuFEx-based crosslinking using PPD is an effective approach for enhancing the physicochemical robustness of poly(TFE-co-PSEPVE).