Effect of Polymer Structure and Chemistry on Viscosity Index, Thickening Efficiency, and Traction Coefficient of Lubricants

The chemistry and structure of base oil and polymer additive molecules in lubricants directly affect key performance metrics such as viscosity index, thickening efficiency, and traction coefficient. However, the relationship between molecular properties and these metrics is still not fully understood, inhibiting the design of new fluids with potentially improved performance. This study used molecular dynamics simulations to identify structure–property-function relationships for model lubricants consisting of branched and linear polymers with chemistries consistent with commercially available products. First, fluids with similar kinematic viscosities at 100 °C were formulated with five different polymers. Then, the simulation-calculated Newtonian viscosities at 40 and 100 °C, viscosity index, thickening efficiency, and traction coefficient in full film lubrication at 40 °C were validated by direct comparison to experimental data. Next, the molecular origins of differences in the viscosity index, thickening efficiency, and traction coefficient between the fluids were investigated by calculating multiple structural properties of the simulated polymers. Finally, the simulations were used to develop simple empirical models using the best subset linear regression analysis to rapidly predict viscosity index, thickening efficiency, and traction coefficient. The atomistic simulations and empirical models developed in this work can ultimately guide the design of new lubricants or additives.