Impact of molecular architecture on the high-pressure rheology of hydrocarbon fluids

Molecular dynamics simulations are conducted on three C-18-poly-alpha-olefi n isomers under extreme conditions typical of traction fluids or lubricants under elastohydrodynamic lubrication conditions. The viscosity, self-diffu sivity, and rotational relaxation times of the molecules are computed at pr essures ranging from atmospheric to as high as 1 GPa. The dynamics of all t hree isomers are slowed as pressure increases, but a highly branched isomer shows a more dramatic reduction in mobility with pressure than does a line ar or singly branched isomer. In particular, the viscosity of the highly br anched molecule exhibits a much larger increase with pressure than does the viscosity of the other isomers, indicating that the highly branched molecu le should exhibit more favorable traction properties than the other isomers . An explanation for the differences in dynamic properties between the isom ers 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 th e pressure dependence of diffusivity and viscosity than commonly used engin eering models.