Billions of tons of plastics have been produced and only a small fraction of this has been recycled. Tennakoon et al. [Nat Catal 3, 893–901 (2020)] developed a catalyst that repeatedly cleaves C10-C30 hydrocarbons from the end of a polyethylene chain. The reaction occurs at a Pt nanoparticle at the base of a cylindrical silica mesopore of diameter 2nm and length 100nm. NMR can distinguish anti, gauche, and mobile polymer configurations, and thereby reveal the amount of polymer adsorbed (with anti-configuration) in the narrow pores. Moreover, 2D EXSY NMR can reveal the time scale and extent of fluctuations as polymer chains move in and out of the pores. We construct a linear model for the free energy as a function of the leading polymer segment position in the pore. We then construct and solve a Fokker-Planck equation for the overdamped dynamics of the polymer position in the pore. The solutions allow us to predict the intensities of the 2D NMR peaks as a function of time. We use the solutions to extract a polymer diffusivity at each temperature and to estimate per-segment free energy, enthalpy, and entropy values for desorption.

Polyethylene in dead-end silica nanopores: Forces and mobility from non-equilibrium statistical mechanics and EXSY NMR

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ACS Fall 2022

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