Novel anion exchange membranes based on quaternized diblock copolystyrene containing a fluorinated hydrophobic block

Abstract

With the aim to enhance the hydrophilic–hydrophobic separation and thus improve the anion conductivity of diblock copolymer anion exchange membranes (AEMs), a fluorinated hydrophobic block was introduced into quaternized diblock copolystyrene. Two fluoro-containing monomers (i.e., 4-fluorostyrene (4FS) and 2, 3, 4, 5, 6-pentafluorostyrene (PFS)) were polymerized into a poly(4-vinyl benzyl chloride)-based macroinitiator (Macro-PVBC263) via reversible addition–fragmentation chain transfer (RAFT) polymerization. This route led to the corresponding diblock copolymers PVBC263-b-P4FSy and PVBC263-b-PPFSy, which served to prepare AEMs (i.e., PVBC263-b-PPFSy-OH and PVBC263-b-P4FSy-OH, respectively) via quaternization with trimethyl amine (TMA) and subsequent ion-exchange. As expected, the as-obtained AEMs showed well-defined lamellar morphologies, as revealed by small-angle X-ray scattering (SAXS) and atomic force microscopy (AFM). Consequently, all the fluorinated diblock copolymer-based AEMs showed high anion conductivities. PVBC140-b-P4FS38-OH showed the highest hydroxide conductivity (86.1 mS/cm) with an ion exchange capacity (IEC) of 4.27 meq/g. Interestingly, despite its high IEC, PVBC263-b-P4FS38-OH showed a reasonable water uptake (WU) of 37.5% as a result of the presence of hydrophobic fluorinated block segments. Thus, proper ion conducting channels are believed to be built by facilitating hydrophilic–hydrophobic phase separation processes driven by the fluoro-substituted functionalities. Despite their high IEC values in terms of hydrophobicity, the WU and swelling ratio of the AEMs prepared herein did not increase significantly, especially at elevated temperatures. All the AEMs preserved over 86% of their initial hydroxide conductivity after 20 d of alkaline stability testing (1 M aqueous NaOH) at 80 °C.