The upper bound relationship (both theory and empirical correlation) indicates that the permselectivity as a function of permeability is a consequence of the diffusivity based on gas diameter differences.  The solubility selectivity is assumed to be invariant with permeability (and free volume).  Several literature references note the solubility constant for specific polymeric classes exhibits a free volume variation.  A large database of permeability, diffusivity and solubility data for glassy polymers was compiled to assess the above conclusion.  A critical analysis of the data clearly demonstrates there is a modest solubility selectivity contribution to permselectivity as a function of permeability and free volume.  The solubility selectivity (Si/Sj) generally decreases with increasing permeability (and free volume) where the j gas diameter is larger than the i gas diameter.  This is a consequence of the solubility of the larger gas being less accessible to polymer sorption sites as the polymer packing density increases and free volume decreases.  The diffusion data allows for the determination of a diffusivity upper bound with modest differences versus expectations based on the permeability based upper bound relationship.  The analysis of diffusion data allows for a determination of a new set of gas diameters more appropriate to gas diffusion in polymers than prior correlations.The introduction to this presentation will discuss the upper bound analysis and permeability and permselectivity analysis previously published providing the background for this presentation.  A brief review of relevance to the important technology involving membrane separation of gases will also be presented.

Critical analysis of diffusion selectivity and solubility selectivity: Contribution to permeability selectivity

246th National Meeting (2013)

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