Facilitating anion transport in polyolefin-based anion exchange membranes via bulky side chains

Highly anion-conductive polymer electrolyte membranes with excellent alkaline stabilities for fuel cell applications were prepared. Thus, a series of polyolefin copolymers with poly(4-methyl-1-pentene) (PMP) moieties containing bulky side chains and side-chain quaternary ammonium (QA) groups were prepared through copolymerization with a Ziegler-Natta catalyst and subsequent quaternization. The separation of hydrophilic microphase and hydrophobic microphase was induced by PMP bulky side chains, and then well-connected ionic domains were formed. This result was confirmed by AFM (atomic force microscopy) and SAXS (small-angle X-ray scattering) analyses. It was discovered that well-defined ionic domains of the PMP-TMA-x (TMA, trimethylamine) membranes depended on the content of PMP moieties. The well-defined ionic domains enhanced the hydroxide conductivity of the PMP-TMA-x membranes despite their lower water uptake (WU) as compared to polypropylene (PP)-containing membranes (PP-TMA-x). The PMP-TMA-41 membrane showed the highest ionic conductivity value (43 mS/cm) while maintaining low WU (29.2 wt %) at room temperature. The membranes mostly preserved (>93.0%) their initial hydroxide conductivity after alkaline treatment (10 M aqueous NaOH, 80 °C, 700 h), thereby revealing desirable alkali stability characteristics. Presumably, the nucleophilic attack from hydroxide or water in the cationic center is inhibited by long alkyl spacers (-CH₂-)_n (n = 9) which are located between the cation groups and the polymer backbone.