Unveiling Local Dynamics of a Triptycene-Based Porous Polymer by Solid-State NMR

Membrane-based technologies for gas separation and capture are promising low-energy alternatives to the most common energy-consuming processes such as distillation and absorption. In this frame, porous polymers are attracting considerable interest, but issues related to a trade-off between permeability and selectivity as well as to the long-term stability of the membrane performances need to be overcome. To this end, the study of local dynamics is crucial as it directly correlates with the transport and separation characteristics of polymer-based membranes while also shedding light on plasticization and physical aging phenomena. This work presents a comprehensive characterization of the dynamic properties of a triptycene-based porous polymer with potential application in membrane-based gas separation technology by means of molecular dynamics (MD) simulations and solid-state NMR (SSNMR). The investigated polymer has triptycene-based structural repeating units bearing <i>t</i>-butyl groups that are connected by perfluorinated biphenyl repeats. The combination of different SSNMR variable temperature experiments including measurements of ¹H, ¹³C, and ¹⁹F spin-spin and spin-lattice relaxation times, ¹H-¹³C and ¹⁹F-¹³C dipolar chemical shift correlation experiments, and ²H experiments provided selective and detailed information on the molecular motions involving the <i>t</i>-butyl, triptycene, and perfluorinated biphenyl groups. A synergistic analysis of the acquired data, employing theoretical dynamic models and comparisons with MD simulations and calculated potential energy scans (PES), has enabled the determination of motion parameters, including activation energies and correlation times. This approach also yielded insights into the motion amplitudes and geometry. These findings can be valuable for future research aimed at elucidating the molecular origins of membrane performance, not only for the polymer under investigation but also for similar polymer-based membranes.