Concentration and Polarity Effects on the Mechanism of Comb-Polymethacrylate Viscosity Modifiers

Viscosity modifiers (VMs) are widely applied in lubricants. Advanced VMs can modulate viscosity in response to various stimuli, such as pH, light, salinity, and temperature. This article explores comb-shaped macromolecules that exhibit thermoresponsive behavior, with potential applications as oil additives for use in transportation. Copolymers were prepared by free-radical polymerization of up to four methacrylate-based monomers with different polarities into sparsely branched comb-shaped macromolecules. The side chains of the comb comprised hydrogenated poly(butadiene), whereas the backbone contained lauryl, benzyl, and butyl methacrylates. Rheological measurements of concentrated solutions in a Yubase oil revealed an initial increase followed by a decrease in viscosity as temperature was increased from 25 to 105 °C. The magnitude of this effect was dependent on the concentration, between 10 and 20 wt % polymer, and the fraction of benzyl methacrylate versus butyl methacrylate in the copolymers. Cryogenic transmission electron microscopy (cryo-TEM) and small-angle X-ray scattering (SAXS) revealed polymer aggregates at low temperatures, which first swelled and then dissolved as individual solvated molecules on heating. This aggregation-expansion-dissolution phenomenon qualitatively accounts for the changes in viscosity.