Side-chain-type imidazolium-functionalized anion exchange membranes: The effects of additional hydrophobic side chains and their hydrophobicity

Ionic-conductive polymers with highly ordered, phase-separated structure are the promising materials used as anion exchange membranes (AEMs), thanks to their pronounced ion mobility. In this study, apart from imidazolium-functionalized hydrophilic pendants, additional octyl and perfluorooctyl-containing grafts were tethered to the polymer backbone and the resultant structure-morphology relationships were systemically investigated. The results revealed that the introduction of hydrophobic side chains as well as the corresponding hydrophobicity plays an important role in the formation of a phase-separated morphology in the membrane. AEMs containing perfluorooctyl-containing pendants ( PPO- x -Im y 8F z ) exhibited a pronounced microphase-separated morphology with ion clusters of about 2–5 nm size and a spacing of ∼6 nm, while the membranes with octyl pendants ( PPO- x -Im y 8C z ) or no additional hydrophobic side chains ( PPO- x -Im ) only showed indistinct or no phase separation. In addition, PPO- x -Im y 8F z AEMs showed better ion conductivity and suppressed water absorption, thereby exhibiting a good balance between water uptake and ion conductivity. The results of this study provide an efficient strategy to guide the architectural design of high-performance AEMs.