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A series of poly(organophosphazenes) with the general structure [NPtRhtOCeHsh.,],,, wherex < 2 and R = OCeRUSiMes, OCeH4SiMe2Ph, OCeH4SiMePh2, OCeH4Br, and OCH2CF3, and the ferrocenyl polymer [NsPstOC^CFsMij-CsH^Fe] were synthesized. Molecular structural characterization for these polymers was achieved by and 31P NMR, gel permeation chromatography, elemental microanalysis, and differential scanning calorimetry. Films of these polymers were examined with respect to their permeability to O2, N2, C02, He, and CH4, and selectivity ratios were established. The effect of cross-linking on both the permeation and selectivity values for films of the silyl-bearing polymers was also investigated. The change in permeability and selectivity as a function of side group structure variations, free volume effects, gas pressure, and glass transition temperature (Tt) is discussed. Poly[bis(trifluoroethoxy)phosphazene] was found to have oxygen permeabilities comparable to those of poly(dimethylsiloxane) but with higher permselectivities.

Materials from the polyimide and polycarbonate families with an attractive combination of productivity and permselectivity for important gas pairs have been reported recently. A systematic group of materials from both of these families will be considered in this paper. Material optimization strategies in the absence of strong interactions between the membrane material and the penetrating gas mixture will also be discussed in the context of these materials. In addition, a material from a relatively unexplored family of rigid chain polymers, the polypyrrolones, will be compared to the behavior of the polycarbonates and polyimides. Performance for mixtures such as carbon dioxide and methane that interact relatively strongly will be contrasted those such as oxygen and nitrogen that interact weakly with the membrane materials. KEY WORDS Polycarbonates / Polyimides / Polypyrrolone / Permeation / Gas Separation / Sorption / DiffusionThe permeability, P, of a penetrant through the selective layer of a gas separation membrane can be expressed as the product of an effective solubility of the penetrant in the polymer matrix, S, and an average diffusivity of the penetrant through the polymer matrix, Di_ P = D SFor conditions of negligible downstream pressure such as those considered here, the solubility coefficient, S, equals the secant slope of the gas sorption isotherm, S = C/p, evaluated at the upstream conditions, and D is determined as the ratio of P and S. The solubility coefficient is known to be affected by the inherent condensibility of the penetrant, by polymer-penetrant interactions, and the amount of excess or free volume existing in the glassy polymer 1 . In addition to permeability, the effects on membrane permselectivity must be considered when structural modifications are proposed to optimize performance. When the downstream pressure is negligible, the separation factor for a mixture of gases A and B, aAJB, is equal to the ideal separation factor based on the individual permeabilities of the gases A and B, a* A/B given by Eq.

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