TRAPDOR NMR Reveals How Surface Fluorination Changes the Conformation of Polyethylene Adsorbed on γ-Alumina

Surface fluorination greatly accelerates acid cracking of polyethylene catalyzed by γ-Al2O3. The effect of this modification on interactions between the polymer and the catalyst surface was investigated for poly(ethylene-rand-1-dodecene) using 13C{27Al} TRAPDOR NMR with sensitivity enhancement by dynamic nuclear polarization (DNP). Selective 13C-labeling of the polymer at the methine and adjacent α-methylene makes it possible to compare surface contacts of the comonomer and ethylene subunits. On both unmodified and fluorinated surfaces, adsorbed comonomer backbone carbons are located at the same effective C-Al distance as ethylene monomer carbons, indicating that the short-chain branches do not disrupt adsorption significantly. However, the effective C-Al contact distance is slightly longer (ca. 0.2 Å) when the surface is fluorinated, consistent with weaker dispersive interactions between the hydrocarbon and the fluorine-rich surface. Curiously, the fraction of polymer backbone carbons in close contact with the fluorinated surface is significantly higher, consistent with full extension of the adsorbed chains that, when adsorbed on γ-Al2O3, exist in a combination of trains, loops and tails.. The change in conformation may be a consequence of the decreased surface hydroxyl density and/or the increased density of strong acid sites. This work demonstrates that DNP-enhanced 13C{27Al} TRAPDOR NMR can resolve atomic-scale polymer-surface contacts for polyethylene interacting weakly with a solid catalyst. The resulting structural information shows how surface fluorination influences polymer conformation in a system relevant to polymer upcycling.