A critical review on diffusivity and the characterization of diffusivity of 6FDA–6FpDA polyimide membranes for gas separation

Abstract

We have used 2,2′-bis(3,4-carboxylphenyl) hexafluoropropane dianhydride (6FDA)–4,4′-(hexafluoroisopropylidene) dianiline (6FpDA) polyimide synthesized from 6FDA moiety and 6FpDA moiety as a model polymer to analyze and compare different diffusion coefficients of gases to provide a deeper insight into the characteristics of gas penetration through dense membranes. The gas diffusions of N 2, O 2, CH 4 and CO 2 in 6FDA–6FpDA polyimide dense films were characterized by means of the Henry and Langmuir mode diffusion coefficients D D and D H, the average diffusion coefficient D avg, the local diffusion coefficient (or the effective diffusion coefficient) D eff as well as the apparent diffusion coefficient D app, based on the permeation and sorption measurements. The Henry mode diffusion coefficients D D, t obtained from the time-lag method are in fair agreement with that D D calculated from the permeation and sorption isotherms. Except CO 2, the magnitude of D D, D H, D avg and D app of the three non-interacting gases increases in the order of CH 4<N 2<O 2, which is consistent with the sequence of decreasing kinetic diameter. Diffusivity of CO 2 has the strongest pressure dependence. The magnitude of diffusion coefficients follows the sequence of D D ( D D, t )> D eff> D avg> D app> D H. However, the deviations among D eff, D avg and D app apparently diminish if the upstream pressure is extremely low or at an infinite diluted situation. While the values of D eff and D avg are close to the values of D D or D D, t under a sufficiently high upstream pressure, as the Langmuir sites are saturated and the Henry mode primarily takes charge of gas transportation.