(692d) Estimation and Analysis of the Rheological Properties of Perfluoropolyethers

Perfluoropolyethers (PFPEs) form a class of lubricants, which are broadly applied in oxygen service, in aircraft instrument bearings, in reactive chemical environments, in vacuum pumps, in sealed-for-life electric motors, in computer hard drives, and as high-temperature greases. They possess numerous advantageous properties including chemical inertness, thermal, oxidative and hydrolytic stability, nonflammability, radiation resistance, shear stability, low vapor pressure, broad insolubility, and excellent lubricity under normal, severe, and starved operation conditions such as under heavy loads, at high speeds, and at elevated temperatures. Several research studies through molecular dynamics were performed to study the tribological behavior of branched and linear PFPEs confined by two rigid surfaces of a face-centered lattice [1,2]. In this work, equilibrium and nonequilibrium molecular dynamics simulations of several short-chain perfluoropolyethers are reported using an atomistic interaction potential. The bulk rheological properties of the perfluoropolyethers are investigated through molecular dynamics simulations as functions of both temperature and shear rate. The effect of molecular structure on viscosity is explored in detail. The rotational relaxation time is reported as a function of temperature. Structural properties, including the mean-square end-to-end chain length, the mean-square radius of gyration of chains, and the distribution functions of bond lengths, bond angles, and bond torsional angles are collected and analyzed as functions of shear rate. After an initial plateau, both mean-square end-to-end chain length and mean-square radius of gyration decrease monotonically with increasing shear rate. The behavior of the rheological and structural properties is explained through an analysis of the individual contributions due to bond stretching, bond bending, and bond torsion, as well as both intramolecular and intermolecular non-bonded interactions. A further analysis is possible through a meticulous breakdown of each contribution into a specific type of mode; e.g., the total bond stretching is comprised of CC, CO, and CF bond stretching terms. In this way, one can relate the shear viscosity to the specific chemical structure. The results from molecular dynamics are also compared with the experimental data. The structural effects on the rheological properties are analyzed in detail through group theory.

Reference: 1. A. Koike, J. Phys. Chem. B, 1999. 103: p. 4578. 2. D. Kamei, et al., Tribology international, 2003. 36: p. 297.