Poly(ether ether ketone) Ionenes: Ultrahigh-Performance Polymers Meet Ionic Liquids

This work presents the first example of an imidazolium ionene containing aromatic ether-ketone-ether linkages inspired by poly(ether ether ketone) (PEEK), a well-known ultrahigh-performance (UHP) engineering polymer. The requisite starting materials for this “PEEK ionene” were efficiently synthesized in good yields and then polymerized through condensation (Menshutkin reaction), followed by anion metathesis to form the final polymer product, which had a number-average molecular weight (M_n) of ∼90 kDa. The properties of the PEEK-ionene were thoroughly characterized, and its potential utility was demonstrated by analyzing this material as a gas separation membrane and 3D-printing this ionic UHP polymer. Thin films of this material and composites containing “free” ionic liquids (ILs) were also tested as membranes, to evaluate the gas/permeation behaviors. While the addition of [C₄mim][Tf₂N] IL to the PEEK-ionene enhanced membrane performance, the ether-functionalized [PEG₁mim][Tf₂N] IL exhibited notably different interactions with the PEEK-ionene, resulting in diminished selectivity and irregular ordering due to partial exfoliation and dissolution of the polymer. To better understand the range of processability and morphological effects with ILs, the PEEK-ionene was extruded into fibers and was 3D-printed through fused deposition modeling (FDM) techniques, to demonstrate how the PEEK-ionene is more readily processable than conventional PEEK. Compared to neutral PEEK, the PEEK-ionene with bistriflimide (Tf₂N) counterions exhibited reduced glass-transition temperature (T_g) and melting point (T_m) and increased permeability to gases, as well as the ability to hold ILs within their structures. This approach to the design of ionenes and PEEK-containing materials can provide great opportunities to create tailored materials for a variety of applications.