Poly(fluorene)-Based Anion Exchange Membranes, Part 1: Effect of Pendant Alkyl Chain Length on Membrane Properties

A series of poly(fluorene)-based anion exchange membranes (AEMs) featuring alkylene-spacer-separated piperidinium cations were synthesized with the pendant chain length in the hydrophobic component as a key variable. Membrane properties, including morphology, water absorbability, ion conduction, and alkaline stability, and their relationship with the polymer structures were thoroughly investigated. For membranes with a target IEC of 2.60 mequiv g^–1, as the pendant alkyl side chain length was extended from C₂ to C₁₀, the size of the hydrophilic and hydrophobic domains increased while water uptake and swelling ratio decreased. The highest hydroxide ion conductivity of 154 mS cm^–1 was obtained at 80 °C for the C₁₀-PFDMP-2.60 membrane. In the accelerated alkaline stability test (in 8 M KOH at 80 °C), all C_R-PFDMP-x membranes retained over 64% of their initial conductivity after 1000 h, with C₁₀-PFDMP-2.60 showing the highest stability, retaining 87%, highlighting the combined effect of alkyl chain length and IEC on the alkaline stability. Density functional theory (DFT) calculations suggested that increasing the pendant chain length enhanced the electrostatic shielding of the piperidinium cations and led to higher resistance to nucleophilic attack by hydroxide ions. These findings provide new insights into how the pendant chain length modulates the physical and chemical properties of AEMs, offering a strategy for the design of more durable and efficient AEMs for electrochemical applications.