(491e) Gas Sorption Properties of Novel Iptycene-Based Thermally Rearranged Co-Polymers: Effect of Temperature, Mixed Gas and Pre-Treatment

Iptycene-containing polymers have emerged as promising materials for gas separation applications¹. Triptycenes are three-dimensional structures formed by three aromatic rings disposed in different planes¹. Once inserted on the polymer backbone, they provide an extra, internal free volume which improves gas permeability while preserving optimal levels of selectivity. Moreover, unlike what happens in common glassy polymers, the internal free volume provided by iptycene units is not related to the non-equilibrium conformation, but to the molecular configuration. Such configuration-based free volume is intrinsic to the polymer structure and, as such, it is noncollapsible, similar to the case of inorganic molecular sieves¹. Conversely, free volume related to non-equilibrium conformation is subject to changes over time, due to the physical ageing. Although several iptycene-containing polymers have shown separation properties that exceed the upper bound², fundamental structure-property correlations for this family of polymers are scarce in the literature and, specifically, the role of sorption and diffusion coefficients to the overall permeability coefficients is still unclear.

In this study, single gas solubility isotherms for He, N₂, CH₄, CO₂ and C₂H₆ in a novel thermally rearranged co-polymer prepared from a co-polyimide precursor with controlled triptycene molar content, i.e., triptycene-dianhydride(0.25)-6FDA(0.75)-6FAP(1.0), were experimentally measured in the range 5 to 50 °C and up to 32 atm. Experimental data were analyzed in the framework of the Dual Mode Model³. Interestingly, the vast majority of penetrant molecules are sorbed in the Langmuir mode, which justifies the high resistance of iptycene-based materials to plasticization. Specifically, the Langmuir to Henry mode sorption ratio is greater than for other high performance materials. The dual mode parameters retrieved from the analysis of single gas sorption isotherms were used to estimate a priori the sorption behavior in mixed gas conditions⁴. Remarkably, the mixed gas solubility-selectivity is higher than ideal solubility-selectivity.

The analysis of sorption enthalpies provides additional information about polymer-penetrant interactions, as well as the plasticization resistance. Finally, to highlight the effects of thermal treatment and physical ageing, sorption experiments were run, for selected gases, on samples previously annealed at 120°C under vacuum for 1 week.

Modeling work is underway to interpret the experimental observables in the framework of the NELF model.

References

1. J.R. Weidman, R. Guo, Ind. Eng. Chem. Res. 2017, 56, 42240-4236
2. R. Guo, unpublished results
3. W.J. Koros, D.R. Paul, A.A. Rocha, J. Polym. Sci. B 1976, 14, 687-702
4. W.J. Koros, J. Polym. Sci. B 1980, 18, 981-992