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

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 N₂, O₂, CH₄ and CO₂ 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 CO₂, the magnitude of D_D, D_H, D_avg and D_app of the three non-interacting gases increases in the order of CH₄ < N₂ < O2, which is consistent with the sequence of decreasing kinetic diameter. Diffusivity of CO₂ 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.