From plasticization to anti-plasticization: the critical role of residual solvent and thermal history on CO2 transport in 6FDA polyimide membranes

This study investigates the profound impact of residual solvents and thermal history on the CO 2 /CH 4 and CO 2 /N 2 separation performance of 6FDA-based polyimide membranes by providing a comprehensive understanding of how solvent-polymer-penetrant interactions dictate macroscopic separation efficiency. Dense 6FDA-DAM membranes were cast using three solvent molecules with varying boiling points (NMP, DMF, and THF) and subjected to distinct annealing protocols (sub-boiling, supra-boiling, and near the glass transition temperature, T g ). Experimental evaluations underscore the persistent nature of solvent entrapment; achieving a completely solvent-free matrix is practically unattainable, as solvent molecules become structurally locked within the polymer chains depending on the evaporation kinetics. These trapped solvents act as plasticizers, expanding fractional free volume and increasing permeability at the expense of selectivity. Conversely, precise thermal treatment near T g significantly tightens polymer chain packing, effectively doubling CO 2 /CH 4 selectivity, with a considerable drop in CO 2 permeability, as the minute amount of remaining trapped solvent begins to act as an anti-plasticizer. To elucidate these macroscopic findings, molecular simulations were performed to capture the coupled effects of the residual solvent and CO 2 on rigid (6FDA-DAM) and flexible (6FDA-ODA) polyimide matrices. The computational models emphasize that structural alterations are driven by the combined presence of both CO 2 and the trapped solvent, rather than the isolated effect of the solvent. This reveals a direct competition between CO 2 and the solvent for preferential sorption sites, primarily nitrogen in the imide group and ether linkages. Furthermore, fractional free volume (FFV) analyses identified a critical transition threshold (1–2 wt% solvent content) where this combined interaction reverses the matrix behavior from synergistic plasticization to anti-plasticization with the decrease of residual solvent content. By detailing the dependence of gas transport properties on both specific solvent selection and thermal treatment, this dual-methodological work establishes a new fundamental perspective for optimizing the fabrication of high-performance polyimide membranes.