Impact of Molecular Architecture on the High-Pressure Rheology of Hydrocarbon Fluids

Molecular dynamics simulations are conducted on three C 18 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.