Membrane technologies for gas separations are considered to be an energy efficient alternative to the traditional absorption processes. The separation performance of a membrane process is governed by two key characteristics of the polymer material: permeability and permselectivity. These characteristics are, however, in competition with each other: polymers with high permeability do not exhibit high selectivity and vice versa. Robeson clearly demonstrated this trade-off by compiling a large amount of data for the performance of various polymer materials and different gas couples and by plotting perm-selectivity as a function of permeability on log-log scale . Moreover, generalizing these trends, Robeson noted that the trade-off between perm-selectivity and permeability must lead to a practical limitation (now known as the Robeson bound) to the performance of polymer materials in separation of a particular gas pair. This empirical observation played an important role in assessing the economic potential of the membrane separation processes and in guiding various strategies to modify polymer materials. For example, a strategy where properties of two materials (polymer and filler) are combined in order to overcome the Robeson limit led to the development of the mixed matrix membranes .