Abstract
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 ( T g) of the PAIs are calculated to investigate the microstructure of the PAIs and the structural dependence of sorption and diffusion properties of CO 2/CH 4 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.
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