Measurement and prediction of kinematic viscosity for linear ethers

Using the methods of equilibrium and non-equilibrium molecular dynamics alongside capillary viscometer experiments, we explore differences between united and all-atom models of a series of linear ethers. The models are based on two transferable force fields, and changes in viscosity and diffusion are studied across a wide range of temperatures and shear rates. We analyze diffusivity and viscosity data by means of the rotational relaxation time and Arrhenius equation. Rotational relaxation times are calculated explicitly from the ether chain's end-to-end vectors, and self-diffusion values are calculated from the mean square displacement. We find an increase in orientational alignment as temperature drops in both models and consistent differences in activation energies across the models and experiment. A clear relationship is observed between viscosity, rotational relaxation time, and diffusion time. These time constants also impact the reliability of the viscosity value determined by the Green-Kubo method. We also study the trends in zero-shear viscosity as chain length increases and force field performance relative to experiment as this length changes.