Polymeric membrane gas separation performance improvements through supercritical CO2 treatment

Supercritical carbon dioxide (sc-CO2) will plasticize and partially solubilise polymeric membranes, resulting in alteration to the polymer morphology, impacting the gas separation properties. Here, cellulose triacetate (CTA) and polyimides, Matrimid and 6FDA-TMPDA membranes were exposed to supercritical CO2 for 2 and 8 h, followed by two depressurization protocols; a rapid depressurization of 12 MPa/min and a slow depressurization of 0.17 MPa/min. The resulting impact on He, N2, CH4 and CO2 permeability as well as the corresponding selectivities were then quantified. Matrimid membranes undergo substantial plasticization in the presence of sc-CO2 resulting in significant increases in permeability and loss of selectivity, irrespective of the sc-CO2 exposure protocol. CTA and 6FDA-TMPDA membranes experience competing phenomenon under supercritical conditions, both demonstrate limited CO2 plasticization which is offset by sc-CO2 partly solubilising the polymers, enabling rearrangement to a more compact morphology. The depressurization protocol strongly impacted the underlying morphology for these two membranes. Rapid depressurization resulted in a higher fractional free volume and greater gas permeability, which is attributed to the sudden expansion of CO2 sorbed in the polymer opening up the morphology. Slower depressurization resulted in a lower fractional free volume and decreased gas permeabilities, because the CO2 gradually desorbs, leaving behind a more dense morphology. The resulting decrease in permeability also corresponded with a significant increase in selectivity. For both CTA and 6FDA-TMPDA membranes the sc-CO2 treatment improved the gas permselectivity relative to the original state. Therefore, exposure to sc-CO2 presents an advantageous process to improve the performance of common polymeric membranes for gas separation.