(750g) Microporosity Characterization in Ultra-High Free Volume Glassy Polymers

The development of innovative materials for gas separation membranes application is mainly focused on high free volume glassy polymers, due to their peculiar features that provide excellent permeability and separation performances [1]. Among the others, the new class of polymers with intrinsic microporosity (PIMs) has been extensively studied for a variety of molecular separations, counting on the large excess of free volume produced by the poor packing of the macromolecules, purposely designed as rigid and contorted. For this reason, PIMs show ultra-high gas permeabilities and interesting selectivity values for several gas pairs [2].

In view of their very high free volume, such systems are often treated as porous media, as if permanent pores are present in the glassy matrix, and their characterization typically include nitrogen or argon sorption at cryogenic temperatures (77 or 87 K, respectively). The data obtained are treated by the BET model, in the same way as for rigid solid adsorbents, aiming to obtain material information about the alleged pore size and distribution. However, this method has been questioned for the characterization of microporous materials, as the central assumptions of the BET theory are fail in the case of narrow pore size, approaching the nanometer scale. Several works revealed the inappropriateness of the BET method for the characterization of microporous media, such as MOFs, zeolites, and polymer networks [3].

It is also questionable if the same physical interpretation is applicable also for glassy polymers, even in the case they are endowed with a large excess of free volume or, in other words, if a porous structure with continuous solid wall can be envisaged in these materials. Interestingly, indeed, the experimental N₂ sorption at 77 K in several high free volume polymers (including PIMs) is characterized by the same features observed for conventional dense glassy polymers that represent the typical footprints of non-equilibrium systems. Large sorption/desorption hysteresis are obtained for most of the materials, as well as apparent effects given by sample previous history.

A different approach has been recently proposed for the description of cryogenic sorption data in glassy polymers [4], both conventional commodities (e.g. PS, PMMA or PVC) as well as high free volume systems (e.g. PTMSP, PMP or AF2400). To such aim, the non-equilibrium thermodynamics for glassy polymers (NET-GP) [5], suitable to evaluate the penetrant solubility in polymer phases, treated as macroscopically uniform and homogeneous phases out of their equilibrium conditions, is applied, with no need for any information about the size and the distribution of the existing “free volume elements”.

The NET-GP approach provides a description of the thermodynamic behavior of penetrant/polymer mixtures below T_g, accounting for the polymer density as an additional state variable, to describe the non-equilibrium state of the glass. The model thus represents a proper extension of conventional equilibrium equation of state approaches to the non-equilibrium glassy states. In particular, the lattice fluid theory by Sanchez and Lacombe is considered, thus leading to the non-equilibrium (NELF) model.

The sorption of different penetrants, namely N₂, Ar, and CO₂ in cryogenic conditions in a series of high free volume glassy polymers, and in PIMs in particular, has been analyzed and described by the NELF approach. The case of PIM-1 has been inspected more in detail, together with a series of novel PIMs recently developed (e.g. PIM-EA-TP, PIm-6FDA-OH, or PIM-Trip-TB). The model provides a very effective and accurate representation of the experimental isotherms, in wide ranges of temperature and penetrant activity, identifying a significant volume swelling of the polymer matrix associated to gas sorption, very relevant in case of large penetrant uptakes [6]. The model allows the evaluation of the gas solubility in the same glassy polymer up to room temperature or above, with no need of any additional parameters and in a purely predictive fashion. Therefore, the analysis suggests that the same mechanism is driving the sorption process in the entire broad temperature range, from cryogenic to above room temperature.

Interestingly, two main regions can also be identified in the cryogenic isotherm, the first one, in the low activity range, is associated to an iso-volumic sorption in the excess of free volume, while the second, located in the central portion of the isotherm, indicates that the physical dissolution is accompanied by a significant swelling of the polymer. These peculiar features can be associated to specific material properties of the glassy polymers examined, such as the fractional free volume and the tendency to plasticization of the membrane.

Based on these findings, an alternative use of the sorption isotherms in cryogenic conditions is proposed, which may serve as a more appropriate metric for comparison between different glassy polymers.

References

[1] N.B. McKeown, P.M. Budd, K.J. Msayib, B.S. Ghanem, H.J. Kingston, C.E. Tattershall, S. Makhseed, K.J. Reynolds, D. Fritsch, Polymers of intrinsic microporosity (PIMs): bridging the void between microporous and polymeric materials, Chemistry 11 (2005) 2610−2020.

[2] Y. Wang, X. Ma, B.S. Ghanem, F. Alghunaimi, I. Pinnau, Y. Han, Polymers of intrinsic microporosity for energy-intensive membrane-based gas separations, Materials Today Nano 3 (2018) 69-95.

[3] M.F. de Lange, L.-C. Lin, J. Gascon, T.J.H. Vlugt, F. Kapteijn, Assessing the surface area of porous solids: limitations, probe molecules, and methods, Langmuir 32 (2016) 12664–12675.

[4] M. Minelli, D.R. Paul, G.C. Sarti, On the interpretation of cryogenic sorption isotherms in glassy polymers, J. Membr. Sci. 540 (2017) 229–242.

[5] M.G. De Angelis, G.C. Sarti, Solubility of gases and liquids in glassy polymers, Annu. Rev. Chem. Biomol. Eng. 2 (2011) 97–120.

[6] M. Minelli, B.R. Pimentel, M.L. Jue, R.P. Lively, G.C. Sarti, Analysis and utilization of cryogenic sorption isotherms for high free volume glassy polymers, Polymer 170 (2019) 157–167.