This article reviews viscosity modifiers, additives that increase the viscosity of lubricating oils. Viscosity modifiers are high molecular weight polymers whose functionality is derived from their thickening efficiency, viscosity-temperature relationship, and shear stability. There are now many different additive chemistries and architectures available, all of which have advantages and disadvantages, and affect solution viscosity through different mechanisms. Understanding these mechanisms and how they impart additive function is critical to the development of new viscosity modifiers that enable lubricants to function more efficiently over a wide range of temperatures.

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The molecular mechanisms by which mechanical energy accelerates a chemical reaction at sliding solid−solid interfaces are not well understood because of the experimental difficulties in monitoring chemical processes and their rates, and in controlling parameters such as interfacial temperature. These issues are addressed by measuring the shear-induced rate of methane formation from the decomposition of adsorbed methyl thiolate species on copper in ultrahigh vacuum (UHV), where the frictional heating is negligible. The effect of a constant force F on the energy profile for thiolate decomposition from density functional theory calculations is modeled by superimposing a linear potential, V(x) = −Fx. This enables the change in activation barrier to be calculated as a function of force. The mechanically induced reaction rate is measured by sliding a ball over a methyl thiolate-covered copper surface from the methane yield measured by a mass spectrometer placed in the UHV chamber. Molecular dynamics simulations reveal that a wide distribution of forces are exerted on the thiolates and comparing the measured methyl thiolate decomposition rate with the rate calculated by assuming a wide force distribution reproduces the experimental data. This reveals that only a small proportion of the adsorbed thiolates experience sufficiently high forces to reduce the activation barrier to reproduce the experimentally measured rate constant.

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In an effort to find correlations between size changes with temperature of lipophilic polymers in solution and viscosity index trends, the determination of the size of thermoresponsive polymers of various architectures (linear, comb-like, star, and hyperbranched) using two experimental techniques under infinite dilution conditions (0.5% w/w) – dynamic light scattering and small angle neutron scattering, and predictive molecular dynamics simulations is described herein. Viscosity index is an important parameter for lubricants and other rheological applications. The aim of this work was to predict polymer behavior as viscosity index improvers (VIIs) using tools which require minimal amounts of material, as opposed to measuring kinematic viscosities, which require multigram quantities. There were no significant correlations between changes in polymer size with temperature and viscosity index (VI). The polymers with the highest VI (polyalkyl methacrylate - PAMA and Star PAMA) had polar backbones in contrast to the nonpolar backbones of the linear and hyperbranched (OCP and HBPE, respectively), so the disparity in solubility of the backbone and solvent medium appears to correlate with the observed VIs. It was concluded that none of the aforementioned techniques can entirely predict the polymer behavior as VIIs, at least in the temperature range studied (40–100 °C).