TEMPO-Functionalized Core-Cross-Linked Micellar Nanoparticles Obtained by Group Transfer Polymerization-Induced Self-Assembly (GTPISA): A Novel Approach to Antioxidant Lubricant Additives

This study presents the use of core-cross-linked and non-cross-linked micellar nanostructures incorporating 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) motifs in their core as antioxidant additives for lubricants. These functionalized micelles are constituted of self-assembled poly(lauryl methacrylate)-b-poly(methyl methacrylate-co-TEMPO methacrylate) block copolymers and have been rationally synthesized by group transfer polymerization-induced self-assembly (GTPISA) at room temperature, initiated by 1-methoxy-1-(trimethylsiloxy)-2-methylprop-1-ene (MTS) and catalyzed by tetra-n-butylammonium bis-benzoate (TBABB), performed either in n-heptane or directly in a mineral oil serving as base component of engine lubricants. Core-cross-linking was achieved by incorporating ethylene glycol dimethacrylate (EGDMA) as the bifunctional comonomer. Electron paramagnetic resonance (EPR) spectroscopy evidenced both the accessibility and the reactivity of the TEMPO sites within the core of the micelles. Furthermore, these micellar additives were evaluated for their efficacy in lubricants. In comparison with the same formula with no antioxidants, lubricants based on TEMPO-functionalized nanostructures produced fewer oxidation end products after oxidation tests. Delayed oxidation peaks were obtained in pressure differential scanning calorimetry (PDSC) analyses for lubricants containing TEMPO-based micelles. The antioxidative efficiency of the micellar additives correlates with the concentration of TEMPO radicals, indicating potential for further enhancement by increasing TEMPO loading. Notably, non-cross-linked micelles provided higher short-term antioxidant efficiency owing to greater TEMPO accessibility, whereas cross-linked micelles offered sustained protection under oxidative stress due to their enhanced structural stability. Overall, these findings highlight the potential of TEMPO-functionalized micellar structures synthesized via GTPISA for enhancing the performance and longevity of lubricants, particularly in the context of electric vehicles.