Equilibrium and nonequilibrium molecular dynamics simulations are performed at constant pressure and temperature on three structurally distinct poly-R-olefin (PAO) isomers representative of a major component in synthetic motor oil basestock. In agreement with empirical observations, the temperature dependence of viscosity, as characterized by the viscosity number (VN), is reduced as the degree of branching is lowered. A molecular-level explanation for this behavior is given in terms of the energy barriers for intramolecular reorientation. Other dynamic properties, such as the diffusivity and rate of tumbling, were also computed and found to have similar dependencies on temperature as the viscosity. Based on these calculations, it appears that PAO molecules with long, widely spaced branches should yield a higher VN than those with short, closely spaced branches. The impact of shear rate on PAO properties is also investigated. High shear rates and shear-thinning increases the VN because the behavior of the fluid is largely dominated by the flow field rather than by the thermodynamic state point. Contrary to what has been observed with linear alkanes, it is observed that molecular alignment with the flow field does not always correlate with enhanced shear-thinning. These observations are explained in terms of a competition between the shear forces responsible for aligning the molecules and intermolecular forces that resist shear-thinning. The results of the present work provide molecular-level explanations for the favorable lubricant properties exhibited by "star-like" molecules and suggest an important strategy for assisting in a more rational approach toward the development of improved lubricants and additives.

Molecular dynamics simulations are conducted on three C18 poly-α-olefin isomers under extreme conditions typical of traction fluids or lubricants under elastohydrodynamic lubrication conditions. The viscosity, self-diffusivity, and rotational relaxation times of the molecules are computed at pressures ranging from atmospheric to as high as 1 GPa. The dynamics of all three isomers are slowed as pressure increases, but a highly branched isomer shows a more dramatic reduction in mobility with pressure than does a linear or singly branched isomer. In particular, the viscosity of the highly branched molecule exhibits a much larger increase with pressure than does the viscosity of the other isomers, indicating that the highly branched molecule should exhibit more favorable traction properties than the other isomers. An explanation for the differences in dynamic properties between the isomers is given in terms of a reduction in liquid void volume coupled with the greater backbone stiffness of the highly branched molecule. A free volume analysis is conducted and shown to provide a better means of correlating the pressure dependence of diffusivity and viscosity than commonly used engineering models.