where b 0 is a constant usually taken as 0.017 K -1 . This brings the selectivity to an approximate value of 20.6 for CO 2 /N 2 separation and this boundary is also indicated in Fig. by the purple line. However, the Eq. 3 is generally not valid. Fig. compiles data for polymers for which data for α CO2/N2 and deviations from the parity plot, clearly indicating the importance of transport effects.