Poly(alkyl methacrylate)-Grafted Polyolefins as Viscosity Modifiers for Engine Oil: A New Mechanism for Improved Performance

Lubricant additives based on graft copolymers with polyolefin backbones and poly(alkyl methacrylate) side chains were examined for their viscosity modifying behavior in base oil. A combination of ring-opening metathesis polymerization, atom transfer radical polymerization of alkyl methacrylates, and hydrogenation was used to prepare the target materials. Viscometric measurements reveal that larger side chain molar mass provides better thickening efficiency. More importantly, increasing the side chain polarity favorably impacts the performance of these graft copolymers as viscosity modifiers. While competitive modifiers must simultaneously meet several technical requirements, the most promising graft copolymer exhibits similar or larger viscosity index at a lower concentration (treat rate) compared to state-of-the-art polymeric additives. This feature bodes well for future additive design. Dynamic light scattering and pulsed field gradient NMR on dilute solutions of the graft copolymer in dodecane (a model oil) consistently show a significant (∼30 nm) decrease in the hydrodynamic radius of the copolymer upon heating from 40 to 100 °C, in conflict with the prevailing assumed viscosification mechanism of coil expansion upon heating. Small angle neutron scattering experiments suggest that the graft copolymer chains may associate to give clusters containing methacrylate-rich domains at lower temperatures due to the solubility of the hydrocarbon backbone and insolubility of the polymethacrylate side chains. We posit that, at elevated temperatures, deaggregation into single chains occurs due to the increased solubility of the side chains. This declustering process results in an overall increase in the hydrodynamic volume of the dispersed polymers, which presumably leads to an improved viscosity modification behavior in engine oil.