(491a) Sorption in PIM-1 from Cryogenic to Room Temperature: Experimental and Model Analysis

The so-called polymers of intrinsic microporosity, among the class of high free-volume glasses, show a tremendous potential as membranes for gas separations,¹ and their peculiar features provide excellent gas permeability and selectivity so that their performances are often located near the technological limit for polymeric membranes represented by the Robeson upper bound.² For these reasons, PIMs have been extensively studied for a variety of molecular separations,³ ranging from CO₂ capture⁴ to organic solvent nanofiltration.⁵

In this work, the sorption properties of CO₂, N₂ and Ar in glassy polymer of intrinsic microporosity PIM-1 have been thoroughly investigated in a wide temperature range, from cryogenic temperatures (77 K) up near room conditions (275 K). All isotherms show the typical behavior of gas sorption with a pronounced downward curvature on the activity axis; data at different temperatures allowed the determination of the heat of sorption for the three different penetrants inspected.

The experimental results obtained have been analyzed by the nonequilibrium thermodynamics for glassy polymers NET-GP,⁶ suitable to evaluate penetrant solubility in polymer phases, treated as macroscopic, uniform and homogeneous phases out of their equilibrium conditions.⁷ The model is able to provide an accurate description of all the measured data with a limited number of parameters. The model is also used to evaluate the solubility of the same gases in PIM-1 above room temperature⁸ in a purely predictive fashion.

The analysis reveals that the same mechanism is driving the sorption process in the entire broad T range, from cryogenic to above room temperature. Two main regions are also identified in the cryogenic isotherm, the first is associated to an iso-volumic sorption in the excess of free volume in the low activity range, while in the second the physical dissolution is accompanied by a significant swelling of the polymer in the central portion of the isotherm. This talk will discuss both the validity of BET surface area analysis and alternative classification metrics for this group of materials.

References

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(4) Pang, S. H.; Jue, M. L.; Leisen, J.; Jones, C. W.; Lively, R. P. ACS Macro. Lett. 2015, 4, 1415.

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