The Hydrodynamic Radii of Viscosity Index Improvers for Lubricant Oils

Viscosity index improvers are polymeric additives that are commonly used to control the viscometrics of lubricants. Nevertheless, there have been few studies on the molecular size of these additives, which is the origin of their viscosity-increasing effects. We herein show the fundamental differences of the topological features of two types of viscosity index improver molecules: olefin copolymers and polyalkylmethacrylates. The hydrodynamic radii of these viscosity index improvers were estimated from their intrinsic viscosity with varying molecular weights. The measured relationships between the hydrodynamic radii and molecular weight were compared to the theories of polymer chain mechanics. The behaviors of olefin copolymers likely correspond to the classical theory of polymer chains with excluded-volume effects. Polyalkylmethacrylates showed features of both an ideal chain and an excluded-volume chain, depending on the temperature, molecular weight, and side chain heterogeneity. The hydrodynamic radii of olefin copolymers decreased with increasing temperature independently of their molecular weight. Meanwhile, the hydrodynamic radii of polyalkylmethacrylates increased with increasing temperature in a molecular weight-dependent manner. The coil expansion of polyalkylmethacrylates might result from the extension of an effective bond angle and transition from an ideal to an excluded-volume chain