Gas sorption and diffusion in perfluoro(butenyl vinyl ether) based perfluoropolymeric membranes
Moataz Ali El-Okazy, Liang Liu, Mohamed H. Abdellah, Eirini Goudeli, Sandra E. Kentish
Abstract
Amorphous glassy perfluoropolymers are characterized by high fractional free volume and hydrocarbon phobicity which means that they perform at the upper bound for several gas pairs. In this study, the sorption of a range of pure gases in Cytop® and a CyclAFlorTM random copolymer of perfluoro(butenyl vinyl ether) (PBVE) and perfluoro(2,2-dimethyl-1,3-dioxole) (PDD) are studied for the first time at different temperatures. The Non-Equilibrium Lattice Fluid (NELF) model is used to model the sorption isotherms in Cytop®, after the determination of the Sanchez-Lacombe model parameters from literature data of the polymer volumetric behavior. The Dual Mode Sorption model is used to model the sorption isotherms in Poly(50%PBVE-co-50%PDD). These results and previously published permeability data are then used within a transport model to determine the diffusivity and activation energy of diffusion for a range of gases within these polymers. The CyclAFlor™ copolymer is shown to have a higher CO2 diffusivity than Cytop®, while maintaining comparable CO2/CH4 diffusivity selectivity. Molecular Dynamics (MD) simulations are performed as a comparison, to calculate the Fickian diffusion coefficients at 35 °C. The diffusion coefficients obtained from MD are consistent with those calculated by the transport model for He and H2 but deviate for larger penetrants (i.e. CO2 and CH4). The results confirm the superior performance of the CyclAFlor™ copolymer for gas separation applications, with greater CO2 solubility and diffusivity than comparable polymers.