Molecular simulation of CO2/CH4 permeabilities in polyamide–imide isomers

Molecular simulation techniques have been adopted for the first time for understanding the structure–property relationships of three highly fluorinated polyamide–imide (PAI) isomers (6FDA/8p, 6FDA/8m and 6FDA/12p). The PAIs being considered in this work are based on 2,2-bis(3,4-decarboxy-phenyl) hexafluoropropane dianhydride (6FDA) and N,N-bis(amino-phenyl)-perfluoroalkane-α,ω-dicarboxamide diamines. The density, fractional free volume (FFV) and glass transition temperature (Tg) of the PAIs are calculated to investigate the microstructure of the PAIs and the structural dependence of sorption and diffusion properties of CO2/CH4 pair. The 6FDA/8p and 6FDA/12p give both higher permeability and permselectivity than the 6FDA/8m because the rigid para-isomer functions as a molecular sieve. With a longer length of oligo(tetrafluoroethene) segment, the 6FDA/12p has permeability and selectivity greater than 6FDA/8p. Hence, this study may provide a guideline for designing PAI membranes to have desirable properties of permeability and permselectivity.