Effects of star-shaped poly(alkyl methacrylate) arm uniformity on lubricant properties

Star-shaped poly(alkyl methacrylate)s (PAMAs) were prepared and blended into an additive-free engine oil to assess the structure-property relationship between macromolecular structure and lubricant performance. These additives were designed with a comparable number of repeating units per arm and the number of arms was varied between 3 and 6. Well-defined star-shaped PAMAs were synthesized by atom transfer radical polymerization (ATRP) via a core-first strategy from multi-functional head-groups. Observations of the polymer-oil blends suggest that stars with less than four arms are favorable as a viscosity index improver (VII), and molecular weight dominates viscosity-related effects over other structural features. Star-shaped PAMAs, as oil additives, effectively reduce the friction coefficient in both mixed and boundary lubrication regime. Several analogs outperformed commercial VIIs in both viscosity and friction performance. Increased wear rates were observed for these star-shaped PAMAs in the boundary lubrication regime suggesting pressure-sensitive conformations may exist.