Plasticization of Glassy Polymeric Membranes in Gas Separations: Assessment and Prediction

The permeability behavior of glassy membranes as upstream pressure increases shows different trends, including the monotonous decline, a monotonous increase, as well as an initial decline followed by a subsequent increase after a minimum permeability value is reached. The minimum value, whenever present, occurs at a pressure conventionally indicated as the plasticization pressure. The plasticization phenomenon has not been inspected in detail thus far, in terms of material property variations; current physical interpretation associates to the plasticization pressure the onset of a generic softening of the polymer matrix, which is considered responsible of the observed increase in permeability and decrease in selectivity.

On the other hand, by considering several different glassy membranes we have observed that the different behaviors observed for gas permeability can be described by considering only a solution-diffusion model in which molecular mobility varies with penetrant concentration through an exponential law, with two adjustable parameters only. Diffusivity is taken as the product of molecular mobility and a thermodynamic factor, which is calculated by using the NELF model for thermodynamic properties of the glassy phase.

It is observed that by fitting the only two adjustable parameters of the model to the initial branch of the permeability-pressure curve it is possible to predict the plasticization pressure, if existing, by using only the solubility diffusivity model without introducing any additional physical phenomenon.

The agreement observed between model calculations and experimental data of CO₂ permeability in various glassy polymers is very satisfactory. That allow us to offer a deeper insight on the so-called plasticization phenomenon. By considering at the same time the permeability isotherm, the solubility isotherm and the concentration and swelling profiles in the membrane, we can show that in some cases the plasticization pressure takes place after part of the membrane has turned into rubbery phase; while in other cases, e.g. for Matrimid polyimide, plasticization pressure takes place when the entire membrane is still glassy but with a polymer swelling sufficient for a permeability increase.

Finally, it is observed that all parameters used have a defined physical meaning which lead to good general correlations with both polymer and penetrant properties, based on which permeability predictions can be obtained.