Effect of polymer molecular weight on the physical properties and CO2/N2 separation of pyrrolidinium-based poly(ionic liquid) membranes

Abstract

Aiming at investigating the effect of the polymer molecular weight (Mw) on the physical and gas permeation properties of poly(ionic liquid)-ionic liquid (PIL–IL) composites, this work focuses on membranes based on variable Mw pyrrolidinium-PILs having [C(CN)3]– as counter-anion and different amounts (20, 40 and 60wt%) of free [C2mim][C(CN)3] IL. Although all the prepared composite materials have high thermal stability (Tonset > 556K) for post-combustion CO2 separation, the evaluation of the film forming ability shows that it is not possible to obtain free standing PIL–IL membranes using the Low Mw PIL (average < 100kDa). The formed Medium Mw (average 200 – 350kDa) and High Mw (average 400 – 500kDa) PIL–IL membranes present similar mechanical properties in terms of Young´s modulus, tensile strength and elongation at break. The gas permeabilities and diffusivities are dependent on the Mw of the PIL used. The Medium Mw PIL–IL membranes display higher CO2 permeabilities (14.6 – 542 Barrer) than those (8.0–439 Barrer) observed for High Mw PIL–IL composites. Despite the Mw of the PIL used, the incorporation of high free IL contents increases both CO2 permeability and CO2/N2 permselectivity. Consequently, the finest CO2/N2 separation performances, overcoming the 2008 upper bound in the Robeson plot, were obtained for the High and Medium Mw PIL–60 IL composites, respectively, with CO2 permeabilities of 439 and 542 Barrer and CO2/N2 permselectivities of 64.4 and 54.0.