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 DD and DH, the average diffusion coefficient Davg, the local diffusion coefficient (or the effective diffusion coefficient) Deff as well as the apparent diffusion coefficient Dapp, based on the permeation and sorption measurements. The Henry mode diffusion coefficients DD,t obtained from the time-lag method are in fair agreement with that DD calculated from the permeation and sorption isotherms. Except CO2, the magnitude of DD, DH, Davg and Dapp of the three non-interacting gases increases in the order of CH4Deff>Davg>Dapp>DH. However, the deviations among Deff, Davg and Dapp apparently diminish if the upstream pressure is extremely low or at an infinite diluted situation. While the values of Deff and Davg are close to the values of DD or DD,t under a sufficiently high upstream pressure, as the Langmuir sites are saturated and the Henry mode primarily takes charge of gas transportation.