(606e) Exploring the Effect of Intra-Chain Rigidity on Mixed-Gas Separation Performance of a Triptycene-Tröger’s Base Ladder Polymer By Atomistic Simulations

Polymers with intrinsic microporosity (PIMs) have been a focal point of polymeric membrane materials research, since the introduction of the prototype PIM-1 to the literature. PIMs combine high gas permeabilities with moderate-to-high gas-pair selectivities making them a promising membrane material for gas separation applications.¹ Despite their promising intrinsic properties, PIM materials are not exempt from performance limitations. Along with their susceptibility to physical aging, loss of gas separation properties of PIMs in mixed-gas feed conditions due to dilation have been reported in numerous reports, even at low feed gas pressures.² Molecular simulation tools have been utilized in the study of PIMs to analyze their structural and gas transport properties.³ In a previous study we successfully reproduced pure- and mixed-gas transport properties of PIM-1 for CO₂/CH₄ gas pair using molecular simulation techniques and explored the effect of CO₂ on the structure resulting in the loss of perm-selectivities.⁴ In a previous experimental study from our group, the effect of intra-chain rigidity on the prevention of mixed-gas dilation behavior in PIMs was studied with a triptycene-Tröger’s base ladder polymer (PIM-Trip-TB).⁵ In the present study, we utilized molecular simulations to test the validity of our approach in reproducing pure- and mixed-gas transport properties of the CO₂/CH₄ gas pair in PIM-Trip-TB and explore the structural dynamics resulting in the mixed-gas separation behavior despite intra-chain rigidity.

Monte Carlo (MC) and Molecular Dynamics (MD) methods were employed to produce pure- and mixed-gas sorption and permeation data for CO₂ and CH₄ in PIM-Trip-TB. The simulated polymer density was calculated to be 1.04 ± 0.02 g/cc and compared very well with the reported experimental geometric density of 1.02 ± 0.02 g/cc.⁴ Experimental data availability for both sorption uptakes and gas permeability at pure- and mixed-gas conditions allowed extensive validation of our approach and models before performing further structural analysis. Accurate reproduction of both sorption and permeability data were accomplished using the all-atomistic methodology. Grand Canonical Monte Carlo (GCMC) simulations were combined with MD simulations to be able to reproduce the CO₂-induced dilation of the polymer matrix during sorption. Gas transport diffusion coefficients were calculated using Maxwell-Stefan relationships, where n-th order algorithm was employed to obtain the diffusion components.⁶ It should be noted that, in mixed-gas conditions, ~ 3-fold increase in CO₂/CH₄ solubility selectivities were observed compared to the pure-gas values, as demonstrated in the experimental report.⁴ When gas-polymer interaction energies were analyzed, significant decrease in CH₄-polymer interactions in mixed-gas conditions were observed, mainly due to preferential sorption of the CO₂ molecules in the polymer. Furthermore, previously reported loss in experimental perm-selectivities under mixed-gas conditions was consistently found to be reproducible by the simulations. Diffusion simulations in mixed-gas conditions proved that diffusion selectivity of PIM-Trip-TB decreases to values as low as ~ 1, resulting in either equal, or lower perm-selectivities compared to those obtained by pure-gas simulations. Despite the intra-chain rigidity of PIM-Trip-TB, co-permeation of CO₂ in mixed-gas conditions led to an increase in the CH₄ diffusivity, which resulted in the decrease in perm-selectivity.

After the accuracy of our methods was extensively proved by comparing physical and pure- and mixed-gas transport properties of PIM-Trip-TB with experimental values, in-detail structural analyses were performed. Initial structural properties of PIM-Trip-TB and structural dynamics induced by CO₂ were tracked and compared. Our simulations indicated that at 10 atm CO₂ partial pressure, FFV of PIM-Trip-TB increases by ~13% to 0.32 compared to the initial value of 0.28. Such an increase can directly be related to the observed increase in diffusivities considering diffusivities of gases are known to be exponentially related to FFV. The significance of the increase in free volume was demonstrated by PSD analysis. CO₂-induced dilation created new free volume elements with diameters larger than CH₄ in the structure which resulted in an overall shift of PSD toward larger pore sizes. Combined numerical and graphical observations led to the conclusion that FFV elements in the system were large and connected enough for the polymer to lose any size sieving capability for the CO₂/CH₄ gas pair. Torsion Distribution Function and Radial Distribution Function analyses proved that the intra-chain rigidity claimed for PIM-Trip-TB was correct, yet CO₂-induced dilation in the structure appeared despite preserving the initial intra-chain rigidity. By combining all effects, it is clear that despite the rigid structure of the PIM-Trip-TB monomer, the polymer matrix was unable to sustain its initial packing, hence, its ideal pure-gas CO₂/CH₄ diffusion selectivity under mixed-gas conditions.

The results of this study highlight the importance of inter-chain rigidity — future studies focusing on the effect and possible manipulation of inter-chain rigidity should be performed to optimize advanced membrane materials for more efficient mixed-gas separation processes.

REFERENCES

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(5) Genduso, G.; Wang, Y.; Ghanem, B. S.; Pinnau, I. Permeation, Sorption, and Diffusion of CO₂-CH₄ Mixtures in Polymers of Intrinsic Microporosity: The Effect of Intrachain Rigidity on Plasticization Resistance. J. Membr. Sci. 2019, 584, 100–109. https://doi.org/10.1016/j.memsci.2019.05.014.

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