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

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.

Keywords: block copolymer nanoparticles; boundary lubrication; polymerization-induced self-assembly; reversible addition-fragmentation chain transfer polymerization; tribology.

Scheme 1. Synthesis of Poly(stearyl methacrylate)–Poly(benzyl methacrylate)–Poly(ethylene glycol dimethacrylate) (S₃₁–B₂₀₀–E₂₀) Triblock Copolymer Spheres via (i) RAFT Dispersion Polymerization of Benzyl Methacrylate (BzMA) in Mineral Oil at 90 °C Followed by (ii) Core Cross-Linking via Addition of Ethylene Glycol Dimethacrylate (EGDMA)

Figure 1

GPC curves [refractive index detector; THF eluent; expressed relative to a series of poly(methyl methacrylate) calibration standards] recorded for the poly(stearyl methacrylate) (S₃₁) macro-CTA synthesized via RAFT solution polymerization in toluene (dashed line) and the linear poly(stearyl methacrylate)–poly(benzyl methacrylate) (S₃₁–B₂₀₀) diblock copolymer precursor prepared via RAFT dispersion polymerization in mineral oil (solid line).

Figure 2

Transmission electron micrographs recorded for (a) linear poly(stearyl methacrylate)–poly(benzyl methacrylate) (S₃₁–B₂₀₀) spheres and (b) core cross-linked poly(stearyl methacrylate)–poly(benzyl methacrylate)–poly(ethylene glycol dimethacrylate) (S₃₁–B₂₀₀–E₂₀) spheres. (c) DLS particle size distributions obtained for 0.10% w/w dispersions of linear S₃₁–B₂₀₀ spheres prepared using n-heptane as diluent (black data) and core cross-linked S₃₁–B₂₀₀-E₂₀ spheres prepared using either n-heptane (red data) or THF (blue data) as diluent. (d) SAXS patterns recorded for 1.0% w/w dispersions of linear S₃₁–B₂₀₀ spheres in mineral oil at 25 °C (black squares) and core cross-linked S₃₁–B₂₀₀-E₂₀ spheres in mineral oil at 25 °C (red circles) and 100 °C (open red circles). Dashed lines represent data fits using an established spherical micelle model. For clarity, SAXS patterns are offset by an arbitrary factor, as indicated.

Figure 3

Stribeck curves showing the change in friction coefficient with entrainment speed for a lubricating base oil alone (black squares), for 0.5% w/w glyceryl monooleate (GMO, green triangles) in the same base oil, and for a 0.5% w/w dispersion of 48 nm diameter S₃₁–B₂₀₀–E₂₀ spheres dispersed in the same base oil (red circles). Data were recorded at a 20% slide-to-roll ratio (SRR) under an applied load of 35 N at 100 °C.