The progress in the field of gas separation by membranes has been very fast. It has grown from early diffusion experiments, through the basic concepts of diffusion and permeation, to industrially accepted products. Since the membrane is the most decisive part of the gas separation technology, it has attracted maximum attention in terms of research and development. Efforts to correlate the basic structure with permeability and selectivity have resulted in the synthesis of newer polymers. Concurrent with these studies, newer theories to explain the phenomena of diffusion, solution and permeation have also emerged. The theoretical description of small molecule diffusion in polymers falls into several categories, depending on the state of polymer. These have been supplemented by computer simulations and improved models. Because of these instrumental aids, as well as continual efforts over the years, the relationships involving the structures/permeabilities/selectivity of polymeric membranes have become the subject of systematic studies. This review gives a brief outline of the field that has emerged on the basis of theoretical models on porous as well as non-porous membranes, and discusses mainly the solution cast polymeric membranes. The effects of casting conditions on membrane properties have also been discussed. Established and emerging technologies in ceramic and in other inorganic membranes such as slip casting, electrodeless plating, sputtering and chemical, and electrochemical vapour deposition techniques are being successfully adopted on a laboratory scale to produce membranes with improved separation factors and high fluxes. In addition, new materials are being developed and new preparation techniques developed to produce thinner membranes and/or smaller pore-sized, defect-free membranes. The emerging field of inorganic membranes for specific gas separations has also been briefly reviewed herein. (C) 2001 Elsevier Science Ltd. All tights reserved.
