A Molecular Dynamics Study of Single-Gas and Mixed-Gas N₂ and CH₄ Transport in Triptycene-Based Polyimide Membranes

Fluorinated polyimides incorporated with triptycene units have gained growing attention over the last decade since they present potentially interesting selectivities and a higher free volume with respect to their triptycene-free counterparts. This work examines the transport of single-gas and mixed-gas N₂ and CH₄ in the triptycene-based 6FDA-BAPT homopolyimide and in a block 15,000 g mol^-1/15,000 g mol^-1 6FDA-mPDA/BAPT copolyimide by using molecular dynamics (MD) simulations. The void-space analyses reveal that, while the free volume consists of small-to-medium holes in the 6FDA-BAPT homopolyimide, there are more medium-to-large holes in the 6FDA-mPDA/BAPT copolyimide. The single-gas sorption isotherms for N₂ and CH₄ over the 0-70 bar range at 338.5 K show that both gases are more soluble in the block copolyimide, with a higher affinity for methane. CH₄ favours sites with the most favourable energetic interactions, while N₂ probes more sites in the matrices. The volume swellings remain limited since neither N₂ nor CH₄ plasticise penetrants. The transport of a binary-gas 2:1 CH₄/N₂ mixture is also examined in both polyimides under operating conditions similar to those used in current natural gas processing, i.e., at 65.5 bar and 338.5 K. In the mixed-gas simulations, the solubility selectivities in favour of CH₄ are enhanced similarly in both matrices. Although diffusion is higher in 6FDA-BAPT/6FDA-mPDA, the diffusion selectivities are also close. Both triptycene-based polyimides under study favour, to a similar extent, the transport of methane over that of nitrogen under the conditions studied.