Tailoring Tetracationic Linkers to Improve the Physical and CO2 Gas Separation Properties of Doubly Segmented PEEK-Ionene + Ionic Liquid Composites

A series of doubly segmented (DS) poly(ether ether ketone)-ionenes (PEEK-ionenes) was synthesized through polycondensation via Menshutkin reaction, followed by bistriflimide [Tf2N]– anion-exchange. These newly designed tetracationic 2-methylimidazolium (C(2)-Me) linker groups employ a sequence of aromatic (p-xylyl) and aliphatic (hexylene) linkages between cations. The synthesized DS PEEK-ionenes exhibit good solubility in common organic solvents at room temperature, high number average molecular weights ranging from 123 to 159 kDa, and thermal stability up to 410 °C, which are improved compared to counterparts with C(2)-H imidazolium cations. The flexibility of the membranes depends on both the amount of free ionic liquid (IL) added to the PEEK-ionene materials and the characteristic features of the linker groups. The structure-property relationships within the series were established by comprehensively studying the physical properties and gas separation performances. All the newly developed PEEK-ionenes + IL composites have moderate CO2 permeability up to 73 barrer, and the separation performance approaches the 1991 and 2008 upper-bound for O2/N2 and CO2/H2, respectively, and with moderate selectivities for CO2/N2 and CO2/CH4. The elongated charged moieties per segment and introduction of C(2)-Me are key factors for finely tuning and maximizing the separation performance of designed materials.