CO₂ diffusion in bulk polymerized ionic liquids

Ionic liquids (ILs) are potential candidates for CO₂ capture due to their high solubility and selectivity for CO₂. Polymerized ionic liquids (PILs) have shown promising CO₂ permeability and CO₂/N₂ selectivity, but the mechanism of CO₂ transport within the bulk PILs remains unclear. In this work, we investigate the contributions of segmental and ion motion to the diffusion of CO₂ in two different PILs. We synthesized two different cationic PILs that have the same TFSI anion but different cations (imidazolium or trimethylammonium) on the pendant groups. Differential scanning calorimetry and temperature-dependent rheology reveal that these PILs have distinct T_g values (approx. 20 C and 50 C for the imidazolium and trimethylammonium variants, respectively). Rheology and broadband dielectric spectroscopy (BDS) reveal that ion transport and structural relaxation are decoupled in these materials, as has been observed in other PILs. Finally, diffusion of CO₂ was measured by FTIR at temperatures above and below T_g. Comparison of the diffusion constants to the timescales of segmental and ionic relaxations enables the contributions of these processes to CO₂ diffusion to be determined. Understanding the contributions of segmental and ion dynamics to CO₂ transport will guide the future design of PILs for CO₂ capture applications.