Molecular simulation of poly-α-olefin synthetic lubricants:: Impact of molecular architecture on performance properties

Equilibrium and nonequilibrium molecular dynamics simulations are performed at constant pressure and temperature on three structurally distinct poly-a-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 PBO 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.