Influence of PMA on Rheological, Viscosity–Temperature and Lubrication Properties of Base Oils

To investigate the regulation mechanism of poly (alkyl methacrylate) (PMA) additives on the viscosity–temperature characteristics and tribological performance of different types of base oils, Group III mineral base oils and Group IV PAO synthetic base oils were selected as the research objects. Rheological and boundary-lubrication tests were systematically conducted at different PMA addition levels, with emphasis on comparatively analyzing the polymer conformational evolution, interfacial adsorption behavior, and lubrication-performance response induced by differences in the solvent polarity of the base oils. The results showed that the modification effect of PMA on base oils exhibited pronounced matrix dependence and non-monotonic concentration characteristics, and its lubrication-regulating behavior was dominated by the coupled trade-off among polymer solubility, molecular conformational stability, and interfacial competitive adsorption ability. In the mineral-oil system, where the base oil acts as a good solvent, the solubility parameters of PMA and the base oil are well matched, allowing the polymer molecular chains to sufficiently swell and extend and providing excellent adsorption and film-forming ability. With increasing PMA concentration, the viscous-flow activation energy of the oil continuously decreased, while the viscosity–temperature performance and boundary-lubrication stability were simultaneously improved, resulting in stable and reliable modification effects. In contrast, in the PAO synthetic-oil system, where the base oil acts as a poor solvent, the PMA molecular chains tend to adopt coiled conformations, with their conformations being highly sensitive to temperature and shear rate, while their interfacial adsorption ability is weaker than that of the base-oil molecules. An optimum critical PMA concentration of 1.0 wt% was observed in this system. Above this concentration, intramolecular friction increased, resulting in deterioration of both viscosity–temperature characteristics and friction performance. This study clarifies the differentiated modification mechanisms of PMA in base oils with different polarities and reveals the dominant role of solvent effects in polymer rheological and tribological behaviors, thereby addressing the insufficient understanding in existing studies of the non-monotonic modification behavior of PMA and its multi-factor coupled mechanism. The findings provide a theoretical basis for PMA structural selection and precise concentration formulation in lubricating oils under different operating conditions and have important engineering application value for optimizing viscosity–temperature performance over a wide temperature range, improving service stability under boundary lubrication, and balancing lubrication reliability with formulation economy.