Films used for membrane-based separation of gases and for barrier packaging applications rely upon the same fundamental principles. Thermodynamically-controlled partitioning of a penetrant such as oxygen into a membrane or barrier packaging layer is the first step in the permeation process. Cooperative motions of gas molecules and the matrix enable diffusive motion of the penetrant in the direction of decreasing dissolved concentration. Our group is a leader in developing advanced materials for both membrane and barrier applications. We focus on optimization of the composition of materials to either promote or retard the permeation of specific components. This approach allows us to produce a barrier, for example to oxygen, or a selective membrane that promotes oxygen passage while rejecting nitrogen to separate air into its main components. Besides the chemical nature of the substrates, the detailed morphology can be tailored.

An obvious example of such morphological engineering is shown in Figure 1, where a fine asymmetric hollow fiber with an external skin is shown. The ultrathin dense skin and porous support layer in this fiber allow rapid permeation in membrane applications. Nevertheless, the molecular scale processes controlling permeation across the dense skin in Figure 2 are identical to those in thicker common films mentioned above. Our group has contributed significantly to the development of processes for formation of these complex asymmetric structures. The work relies upon an integrated knowledge of thermodynamics, phase separation kinetics, and interphase mass transfer to capture the desired asymmetry in the morphology. The ability to control highly asymmetric morphologies in hollow fibers and the ability to select suitable materials based on specific separation requirements make revolutionary separation processes possible.

Work is also underway in our group to form "mixed matrix composite" membrane comprised of blends of molecular sieving entities within the matrix of a conventional polymer. This project seeks to achieve ultra high permeation membrane properties without sacrificing the ease of membrane formation associated with conventional polymers.

Fascinating effects due to non-equilibrium thermodynamic and non-Fickian transport phenomena are additional topics our group studies. Long-lived conditioning effects due to exposure of membranes and barriers to elevated concentrations of certain penetrants are typical of such non-equilibrium phenomena. Protracted aging of glassy polymers, carbons and inorganic membranes after formation or conditioning treatments also are of interest to us. In many cases, these effects seem to defy logic – until one realizes that an expanded set of rules governs these out-of-equilibrium materials.

• Membrane and barrier materials
• Formation and application of polymeric, ceramic, and carbon membranes
• Penetrant history and temperature-dependent phenomena

• "Occlusion of pores of polymer membranes with colloidal silica", J. Membr. Sci., 136, p 273 (1997) (with Moaddeb, M.)
• "Accelerated physical aging of thin glassy polymer films: evidence from gas transport measurements", Polymer, 36, p 2379 (1995) (with Pfromm, P.)
• "A free volume distribution model of gas sorption and dilation in glassy polymers", Macromolecules, 28, p. 2228 (1995) (with Jordan, S. M.)
• "Tailoring mixed matrix composite membranes for gas separation", J. Membr. Sci., 137, p 145 (1997) (with Zimmerman, C. M. and Singh, A.)
• "The significance of entropic selectivity for advanced gas separation membranes", Ind. & Engr. Chem. Res., 35, p 1231 (1996) (with Singh, A.)