Comparison of transport properties of rubbery and glassy polymers and the relevance to the upper bound relationship

Abstract

A database of permeability, diffusivity and solubility for He, H2, O2, N2, CO2 and CH4 was compiled from the literature for rubbery polymers. These data were compared with results for a similar study conducted for glassy polymers published in 2013. Based on this comparison, glassy polymers have higher solubility coefficients than rubbery polymers due to the excess volume (or free volume) stemming from the non-equilibrium nature of polymers below the glass transition temperature, Tg, which is well described by the dual-mode sorption model. When compared at equal permeability, rubbery polymers have higher diffusion coefficients and lower solubility coefficients. Polymers having permselectivity values at or near the upper bound are virtually all glassy polymers. This phenomenon is widely ascribed to the better diffusivity selectivity of glassy versus rubbery polymers. Comparison of the upper-bound plot of log Pi/Pj versus log Pi (where Pi represents the permeability of the more permeable gas) shows that glassy polymers dominate the upper bounds for all 15 gas pairs possible. However, when log Pi/Pj is plotted versus log Di, many of the gas pairs have overlapping data for glassy and rubbery polymers. Thus, glassy polymers dominate the upper bound due, in part, to their higher solubility coefficients.