Tailoring tetracationic linkers to improve the physical and CO2 gas separation properties of doubly segmented PEEK-ionene + ionic liquid composites

Abstract

A series of doubly segmented (DS) poly(ether ether ketone)-ionenes (PEEK-ionenes) was synthesized through polycondensation via the 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 their 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-ionene + IL composites have moderate CO₂ permeability up to 73 barrer, and the separation performance approaches the 1991 and 2008 upper bounds for O₂/N₂ and CO₂/H₂, respectively, with moderate selectivities for CO₂/N₂ and CO₂/CH₄. The elongated charged moieties per segment and the introduction of C(2)–Me are key factors for finely tuning and maximizing the separation performance of designed materials.