The perm-selectivity for species A and B, α A/B = k A k B , can be then divided into two contributing factors: sorption selectivity ( S A S B ) and diffusion selectivity ( D A D B ). Depending on which contribution dominates the process, we * tianmu.yuan@manchester.ac.uk FIG. . CO2/N2 perm-selectivity as a function of (a) CO2 permeability and (b) sorption selectivity for various glassy polymer membranes . In panel (a), the red, blue, and yellow lines are the upper bounds from Robeson , Comesaña-Gándara et al. , and Freeman , respectively. The black line is a hypothetical upper bound based on the highest selectivity available where the purple line is the selectivity calculated from Eq. 4. The black triangles in panel (a) highlight membranes based on PIMs. In panel (b), results from experiments and simulations are shown as red triangles and blue circles, respectively.