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

Ionic-conducive 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 similar to 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.