(130b) Gas Separation Membranes: Issues Beyond the Upper Bound
In 1991 L. M. Robeson proposed that the well-known trade-off between the permeability of gases and the selectivity of gas permeation had an upper bound beyond which no polymers existed at that time. Finding polymers that exceed this upper-bound relationship for certain gas pairs has become a challenge being addressed in many research laboratories around the world. Another research trend has been to develop novel polymer structures that lead to unusually high levels of free volume in order to achieve very high gas permeability, but usually with a compromise in selectivity. Generally these searches for new membrane materials involve testing pure gas permeability in relatively thick films (25-100 microns) that are relatively easy to prepare in the laboratory.
Of course, the development of asymmetric and composite membranes with very thin dense “skins” needed to achieve high fluxes enabled the commercial use of membranes for molecular level gas separations. It has been generally assumed that these thin skins, with thicknesses of the order of 100 nm or less, have the same permeation characteristics as films with thicknesses of 25 microns or more. There is a growing body of evidence that this assumption can be in significant error, and use of thick film data to select membrane materials or predict performance should be done with caution.
This presentation will address issues for some of the new materials being explored for gas separation applications that go beyond where thick film data place them on the trade-off relationship and its upper bound. These include differences in behavior between thick laboratory films and thin films needed for high fluxes; these differences result from a more rapid physical aging and a more exaggerated response to plasticization by highly soluble gases as films become thinner. One focus will be on recent data for some high free volume polymers.