The influence of temperature on the sorption and permeability of CO₂ in poly(fluoroalkoxyphosphazene) membranes

This paper reports the transport and sorption properties of poly(fluoroalkoxyphosphazene) (PFAP) membranes for carbon dioxide and nitrogen in both pure and mixed gas experiments. The CO₂ permeability decreased from 336 to 142 Barrers with an increase in the CO₂/N₂ ideal separation factor from 12 to 21 as the membrane temperature was decreased from 303 K to 258 K at feed pressure of 2.9 bars. At lower feed pressure (1.5 bars) the CO₂ permeability decreased from 327 to 140 Barrers, while the CO₂/N₂ ideal separation factor increased from 13 to 22 over the same temperature range. CO₂ sorption isotherms were measured using the pressure decay equilibrium method. Solubility of CO₂ was determined using the sorption isotherms and the diffusion coefficients were calculated from CO₂ permeabilities and solubilities. Sorption isotherms were linear at each temperature for the pressure range studied and the enthalpy of sorption was -5.8 kcal/mol. The solubility coefficient values for CO₂ increased from 0.95 to 5.43 cm³ CO₂(STP)/cm³ polymer atm whereas the diffusion coefficient decreased from 2.71 × 10^-6 to 0.19 × 10^-6 cm²/s as the temperature decreased from 303 K to 258 K.