Block Copolymer Nanoparticles Prepared via Polymerization-Induced Self-Assembly Provide Excellent Boundary Lubrication Performance for Next-Generation Ultralow-Viscosity Automotive Engine Oils.

Matthew J Derry,Paul S O’Hora,Steven P Armes,Timothy Smith

doi:10.1021/acsami.9b12472

Abstract

Core cross-linked poly(stearyl methacrylate)–poly(benzyl methacrylate)–poly(ethylene glycol dimethacrylate) [S31–B200–E20] triblock copolymer nanoparticles were synthesized directly in an industrial mineral oil via polymerization-induced self-assembly (PISA). Gel permeation chromatography analysis of the S31–B200 diblock copolymer precursor chains indicated a well-controlled reversible addition–fragmentation chain transfer dispersion polymerization, while transmission electron microscopy, dynamic light-scattering (DLS), and small-angle X-ray scattering studies indicated the formation of well-defined spheres. Moreover, DLS studies performed in THF, which is a common solvent for the S and B blocks, confirmed successful covalent stabilization because well-defined solvent-swollen spheres were obtained under such conditions. Tribology experiments using a mini-traction machine (MTM) indicated that 0.50% w/w dispersions of S31–B200–E20 spheres dramatically reduce the friction coefficient of base oil within the boundary lubrication regime. Given their efficient and straightforward PISA synthesis at high solids, such nanoparticles offer new opportunities for the formulation of next-generation ultralow-viscosity automotive engine oils.

Highlights

The automotive industry has seen a concerted drive toward ultralow-viscosity oils because such fluids enable greater fuel economy to be achieved.[1]
The copolymer chains were prepared via atom transfer radical polymerization (ATRP) using a copper catalyst the deliberate addition of this transition metal is normally considered unacceptable for automotive engine oil applications because its Received: July 16, 2019
In 2015, we reported that linear block copolymer nanoparticles can be prepared directly in mineral oil using a high-yielding one-pot synthetic route based on methacrylic comonomers, which possess sufficient thermal stability for automotive engine oil applications.[30]

Summary

■ INTRODUCTION

The automotive industry has seen a concerted drive toward ultralow-viscosity oils because such fluids enable greater fuel economy to be achieved.[1]. This problem can be addressed by adding polymers in the form of sterically stabilized spherical nanoparticles, which make a negligible contribution to the overall oil viscosity.[2] In this context, Liu and co-workers reported that crosslinked acrylic block copolymer nanoparticles can act as boundary lubricants for base oil.[3] Lubrication tests performed in a mini-traction machine (MTM) indicated a remarkable reduction in the friction coefficient within the boundary lubrication regime, in which opposing surfaces become sufficiently close to produce frequent metal-on-metal asperity contacts.[4,5] This typically occurs under high loads and/or when the rate of flow of the lubricating fluid through such contacts is sufficiently low,[3] e.g., for engine start-up at relatively low temperatures. The solution viscosity was measured as a function of shear rate at a fixed temperature of either 25 or 100 °C

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES