459986 How to Describe and Predict Plasticization of Glassy Polymeric Membranes in Gas Separations
<span">Francesco M. Benedetti, Matteo Minelli and Giulio C. Sarti<span">
<span">Department of Civil, Chemical, Environmental and Materials Engineering (DICAM)
<span">Alma Mater Studiorum - University of Bologna, via Terracini 28, 40131, Bologna, Italy.
<span">Plasticization of glassy membranes due to gases or vapors is typically recognized through an increase in gas permeability as the upstream pressure increases. Qualitatively that is associated to an enhancement of polymer chain mobility with consequent increase in penetrant diffusivity, starting from a certain relatively high upstream pressure below which such effects are not present. The phenomenon is normally associated to the sorption of large quantities of low molecular weight species into the polymer matrix, leading to important volume dilation, reduced mechanical rigidity, depression of the glass transition. The increased mobility affects the transport properties of the plasticizing agent as well as of the other gases possibly present in the feed, thus reducing the membrane mixed gas selectivity. The quantitative description of gas permeability and, possibly, the prediction of its dependence on operating pressure is thus relevant for a better understanding of the phenomenon and for the selection of suitable operating conditions.
By considering several different glassy membranes, we have observed that the various different behaviors shown by gas permeability, including plasticization, can be described by considering only a solution-diffusion model in which penetrant molecular mobility varies with its concentration through an exponential law, with two adjustable parameters only. Diffusivity is taken as the product of molecular mobility and a thermodynamic factor, calculated by using the NELF model for thermodynamic properties of the glassy phase.
In particular, the fitting of the only two adjustable parameters of the model to the initially declining branch of the permeability-pressure curve allows the prediction of the plasticization pressure, if existing, by using only the solubility diffusivity model without invoking the onset of any additional physical phenomenon.
Interestingly, we have also inspected the permeability dependence on upstream pressure for CO2in a commercial polyimide (Matrimid), at different downstream pressure values. The resulting behaviour is indeed unexpected and not fully consistent with the qualitative description recalled above. The model analysis has been thus focused on the experimental data obtained at different downstream pressures, offering a deeper insight on the so-called plasticization phenomenon.
The simultaneous consideration of permeability and solubility isotherms, together with the concentration and swelling profiles in the membrane, reveals 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, 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 properties of both polymer and penetrant, based on which permeability predictions can be obtained.