Influence of Relative Humidity On the Gas Transport Properties of Polyimides

Gas separation membranes are constantly gaining importance in chemical engineering operations, thanks to low costs and reduced environmental impact.¹ The main applications are related to the energy production, concerning both pre and post-combustion CO₂ capture, but also purification of streams coming from bioreactors. In real separation processes, the presence of minor compounds is frequent and for a reliable characterization its influence must be taken into account.^2,3 In particular, water vapor can affect significantly the membrane performance, depending on the hydrophobic or hydrophilic nature of the material and the affinity of the different penetrants.^4,5 Different phenomena can take place in the polymer matrix, such as plasticization or clustering, which can modify the trans-membrane flux of gas molecules.

Aromatic polyimides are high T_g polymers, which present good mechanical and chemical stability, as well as excellent film-forming properties. The rigidity of the molecular bond between the carbonyl and the nitrogen atoms confers them a remarkable selectivity, which makes them suitable for several separations.⁶

In the present work, the effect of the presence of water vapor in the feed stream has been investigated for two different polyimides, Matrimid and 6FDA-6FpDA. Four different penetrants have been studied (CH₄, N₂, CO₂ and He) through a manometric technique purposely developed to analyze  gas permeation in humid conditions.⁷ Experiments were performed at three different temperatures (25, 35 and 45°C) and allowed the evaluation of the gas permeability and also of the activation energy for permeation at different relative humidity of the feed stream.

Polyimides are known to be hydrophobic materials, according to the low water uptake.^8,9 Because of the low affinity between polymer and water vapor, no significant swelling is induced by the vapor, even at high activity. On the other hand, when the concentration of the solvent in the polymer matrix reaches a certain value, the water molecules tend to interact with each other due to their polar nature, forming clusters which reduce water diffusivity in the polymer matrix.⁴

Despite their hydrophobic nature, for both polyimides the presence of the water vapor at high activities lead to permeability losses larger than 50% of the dry value. No significant difference in the permeability reduction has been observed for the different penetrants. Due to the higher condensability of water compared to the other gases, a strong competitive sorption effect seems to take place during the permeation process, affecting significantly the membrane performance. The monotonic decrease of permeability with increasing relative humidity points out also that the competitive sorption effect dominates any possible plasticization of polymer chains, possibly related to the water vapor uptake. Remarkably, the permeability drop due to relative humidity remained practically the same at all temperatures, for all penetrants. This result is consistent with the fact that for Matrimid polyimide⁸ no significant differences have been observed for the water vapor sorption isotherms at different temperatures, i.e. water uptake seems to be a T independent process for a given water activity. Moreover, the activation energy of permeation increased for the more permeable gases (CO₂ and He) and decreased for the less permeable ones (CH₄ and N₂).   

Finally, a simple but effective empirical model based on the free volume theory¹⁰ is proposed to analyze the results obtained, . The model considers that water molecules, more condensable, are absorbed in the polymeric matrix preferentially than the other penetrants, partially filling the free volume. From the solubility isotherm it is possible to calculate the amount of volume occupied by water at any activity, hence the residual free volume available for the other penetrants to diffuse. Then, using the free volume theory, the permeability values are obtained foe each penetrant. In spite of its simplicity, the model proposed shows a good agreement with the experimental data.     

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