An atomistic insight on CO₂ plasticization resistance of thermally rearranged 6FDA-bisAPAF

An emerging class of thermally rearranged (TR) polymer has been of great interest for its extraordinary transport properties. More importantly, harnessing the full potential of TR polymer as gas separation membrane for CO₂/CH₄ separation is through its ability to resist plasticization at high pressures. Accordingly, we report here on the effect of CO₂-induced plasticization on polyimide precursor (6FDA-bis-APAF: 4,4-hexafluoro isopropylidene- diphthalic anhydride – 2,2′-bis(3-amino-4-hydroxyphenyl) – hexafluoropropane) and on the resulting thermally rearranged polybenzoxazole (TR-PBO) polymer membranes as investigated through the radial distribution function and accessible free volume analyses. Using molecular simulation techniques, structural properties such as d-spacing, glass transition temperature, fractional free volume, etc. were estimated in agreement with wide range of experimental observations, which are published within the last decade. Results showed that, TR polymer displayed restricted % FFV increase up to 40 bar due to its limited chain mobility as indicated by the dihedral distribution, and sorption sites on its backbone with lower affinity to CO₂ as shown by the RDF analyses. Additionally, analysis of free volume elements suggests that the ability of TR polymers to maintain their interconnected microstructure and resistance to CO₂-induced plasticization at high pressures leads also to higher diffusion and hence permeation performances and as a result, make them promising materials in gas separation applications.