Effect of Base Oil Polarity on the Functional Mechanism of a Viscosity Modifier: Unraveling the Conundrum of Coil Expansion Model

Viscosity improvement property of a lubricant additive is commonly attributed to polymer coil expansion with increasing temperature, although only some polymer chemistries show conformance to this conceptual mechanism. Herein, we show that the polarity of base oil governs whether this mechanism underlies the action of a viscosity modifier (VM) by combining experimental and computational studies. Poly(butyl methacrylate) (PBMA) dissolved in diethylene glycol diethyl ether (DGDE) or a mixture of DGDE (polar solvent) and squalane (SQ, nonpolar solvent) was used as a model lubricant oil system. Specific viscosity of the polymer solutions measured over a wide range of additive concentrations and temperatures revealed that thickening efficiency of the VM decreased with decreasing base oil polarity. While the VM counteracted temperature-induced thinning of the low polarity base oil, in the polar solvent, the polymer did not enhance the solution viscosity at higher temperatures. Aiming to unravel the molecular mechanism underlying viscosity improvement at elevated temperatures in the different solvent systems, the polymer conformation and size in the dispersing oil were determined by combining solution viscosity, small-angle X-ray scattering measurements, and coarse-grained molecular dynamics simulations. Collectively, the experimental and simulation results show that the coil-swelling model underpins viscosity improvement of the polymer solution in DGDE, and the viscosity of PBMA in DGDE/SQ solution increased with temperature due to polymer association in solution. However, the thermoresponsive behavior of the polymer is more pronounced in the mixed solvent system due to their higher propensity to aggregate at elevated temperatures