Thermodynamic Analysis of CO2 Permeability in Glassy Polymer With and Without Plasticization

The study of the permeability and average diffusivity of gases and vapors in glassy polymeric membranes has been carried out on a variety of different systems, using a simple fundamental approach alternative to the known and established model based on Dual Mode Sorption framework.

The procedure developed to describe permeability and diffusivity in glassy polymers, is based on a notorious expression of the diffusive flux as the product of a kinetic factor, mobility, and a thermodynamic factor, associated to the concentration dependence of the chemical potential of the diffusing species. The latter factor can be obtained from solubility isotherm data, or evaluated by means of the well-known predictive procedures, provided by the Non Equilibrium Thermodynamics for Glassy Polymers models, as for instance NELF or NE-SAFT. On the other hand, the mobility factor considered is endowed with an exponential dependence on penetrant concentration, following the usual trend commonly found experimentally; only two adjustable parameters are thus present. As a result, a rather simple and general expression has been obtained, which contains at most two adjustable parameters: the gas mobility at infinite dilution and the coefficient characterizing the concentration dependence of mobility itself (plasticization factor).

The resulting expressions for diffusivity and for permeability describe rather carefully the pressure dependence of CO₂ permeability observed in a variety of different glassy polymers, both in steady state permeation and in transient mass uptake, in all the cases inspected, even in the presence of the so-called plasticization effects.

The present model is also applied to describe CO₂ permeability behavior in complex heterogeneous systems, such as blends and random copolymers, and it is also suitable to represent the effects produced on the transport properties of glassy polymer matrices by crosslinking, as well as by the addition of low molecular weight species.

Finally, it is also observed that in all cases CO₂ mobility at infinite dilution correlates very well to the fractional free volume of the glassy matrix, and in turn the plasticization factor correlates very well with the penetrant-induced polymer swelling.