(609c) Ultra-Permeable Mixed Matrix Membranes Based on PTMSP and Polymer Porous Networks with Enhanced Physical Aging Resistance

Several issues need to be addressed to make membrane processes fully competitive with traditional thermal-based separations, above all the selectivity-permeability trade-off and physical aging¹. The latter is related to the collapse of conformational excess free volume of glassy polymers, which is originated from inefficient chain packing². Physical aging causes a decrease in gas permeability over time, which, in turn, detriments the membrane commercial attractiveness. Some ultra-microporous, highly permeable glassy polymers, such a poly[1-(trimethylsilyl)-1-propyne] (PTMSP), suffer from severe physical aging³. In recent years, researchers have demonstrated that incorporation of 10% wt microporous PAFs (Porous Aromatic Frameworks) microparticles in PTMSP and other high free volume glassy polymers can successfully slow down the aging rate, while enhancing gas permeability by 25% and retaining selectivity³. However, PAFs are expensive and their synthesis pathway is not straightforward, which makes their industrial scale-up difficult.

To address the issues discussed above, in this study we incorporate hyper-crosslinked triptycene-isatin Porous Polymer Network (PPN) in PTMSP via the solution-casting method, to create new free standing, mechanically robust mixed matrix membranes. PPNs benefit PTMSP transport properties and their stability in several ways:

• Triptycene, one of the PPNs monomers, contains configurational free volume, which is known to exhibit much better resistance to physical aging, while promoting gas permeability and selectivity⁴;
• The hyper-crosslinked structure of PPN and the presence of triptycenes exert an “interlocking” or “threading” action, which freezes PTMSP chains and slows down the aging process⁴;
• PPNs are fully compatible with the polymer, which eliminates any defect at the polymer-filler interface.

Finally, PPNs are much cheaper than PAFs, and can be obtained in quantitative yield by reacting triptycene and isatin in superacidic media in very mild conditions⁵.

Addition of only 5% wt PPN into PTMSP results in 3-fold increases in pure gas permeability (N₂, CH₄ and CO₂) without scarifying selectivity at 35°C within the pressure range of 2-20 bar. Equally important, a slower aging rate is observed for 5% PPN + PTMSP membrane, along with the improvement in gas permeability, compared with the neat PTMSP membrane. For example, N₂ permeability of 5% PPN+PTMSP, about 50 μm thick, remains 57% of its initial value 1 week after fabrication, while the neat PTMSP membrane retains 32% N₂ permeability within the same time frame.

The membrane structure and morphology was investigated using a variety of techniques, including FTIR, DSC, TGA and BET.

The effects of higher PPN loadings (up to 20% wt) on gas permeability and aging behavior of PTMSP-based membranes, as well as the behavior of ultra-thin membranes fabricated via spin-coating, are under investigation.

References

[1] M. Galizia et al., Macromolecules 2017, 50, 7809-7843

[2] Y. Huang, D.R. Paul, Polymer 2004, 45, 8377-8393

[3] C.H. Lau et al., Ang. Chem. 2014, 53, 5322-5326

[4] J.R. Weidman, R. Guo, Ind. Eng. Chem. Res. 2017, 56, 4220-4236

[5] C. Aguilar Lugo et al., Ind. Eng. Chem. Res. 2019, 58, 9585-9595