Unveiling Local\nDynamics of a Triptycene-Based Porous\nPolymer by Solid-State NMR

Membrane-based technologies for gas\nseparation and capture are\npromising low-energy alternatives to the most common energy-consuming\nprocesses such as distillation and absorption. In this frame, porous\npolymers are attracting considerable interest, but issues related\nto a trade-off between permeability and selectivity as well as to\nthe long-term stability of the membrane performances need to be overcome.\nTo this end, the study of local dynamics is crucial as it directly\ncorrelates with the transport and separation characteristics of polymer-based\nmembranes while also shedding light on plasticization and physical\naging phenomena. This work presents a comprehensive characterization\nof the dynamic properties of a triptycene-based porous polymer with\npotential application in membrane-based gas separation technology\nby means of molecular dynamics (MD) simulations and solid-state NMR\n(SSNMR). The investigated polymer has triptycene-based structural\nrepeating units bearing <i>t</i>-butyl groups that are connected\nby perfluorinated biphenyl repeats. The combination of different SSNMR\nvariable temperature experiments including measurements of <sup>1</sup>H, <sup>13</sup>C, and <sup>19</sup>F spin–spin and spin–lattice\nrelaxation times, <sup>1</sup>H–<sup>13</sup>C and <sup>19</sup>F–<sup>13</sup>C dipolar chemical shift correlation experiments,\nand <sup>2</sup>H experiments provided selective and detailed information\non the molecular motions involving the <i>t</i>-butyl, triptycene,\nand perfluorinated biphenyl groups. A synergistic analysis of the\nacquired data, employing theoretical dynamic models and comparisons\nwith MD simulations and calculated potential energy scans (PES), has\nenabled the determination of motion parameters, including activation\nenergies and correlation times. This approach also yielded insights\ninto the motion amplitudes and geometry. These findings can be valuable\nfor future research aimed at elucidating the molecular origins of\nmembrane performance, not only for the polymer under investigation\nbut also for similar polymer-based membranes.