(704a) Polyethylene in Dead End Silica Nanopores: Forces and Mobility from Non-Equilibrium Statistical Mechanics and Exsy NMR

Billions of tons of plastic have been produced and only a small fraction of this has been recycled. Tennakoon et al. [Nature Catalysis 3, 893 (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. ¹³C NMR can distinguish anti, gauche, and mobile polymer configurations, and thereby reveal the amount of adsorbed 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.