As a result, this class of materials has recently redefined the Robeson upper bound for various gas pairs, including CO 2 /N 2 , highlighting their potential in advanced gas separation applications. To systematically design new materials with desirable characteristics for membrane separations, it is crucial to be able to predict their properties accurately. Over the years, a wide range of theories and phenomenological models have been developed for this purpose. They aim to predict diffusion and solubility in bulk polymer materials based on structure and composition. Typically, the solution-diffusion theory is then invoked to estimate permeability as the product of diffusivity and solubility, while ideal perm-selectivity is obtained as the ratio of permeabilities for two gas species.