Synthesis and Application of Oil-Soluble Polymer Brush-Grafted Silica Nanoparticles as Lubricant Additives for Friction and Wear Reduction

Although inorganic and metallic nanoparticles are potentially effective oil lubricant additives for friction and wear reduction, their high tendency to undergo aggregation and precipitation in base oils has been an obstacle for real-world applications. This dissertation research aims to develop oil-soluble, polymer brush-grafted nanoparticles (hairy NPs) for use as additives for base oils such as polyalphaolefin (PAO). Well-defined hairy NPs were synthesized by surface-initiated reversible addition-fragmentation chain transfer polymerization from chain transfer agent-functionalized, 23 nm silica NPs, and their lubrication properties were investigated in PAO by high-contact-stress ball-on-flat reciprocating sliding tribological tests.The effects of alkyl pendant groups of poly(alkyl methacrylate) brushes on oil dispersibility, stability, and tribological properties of hairy NPs were studied. It was found that hairy NPs with sufficiently long alkyl pendants ( > 8 carbon atoms, such as 12, 13, and 16) were readily dispersed in PAO and formed homogeneous, clear dispersions with long-term stability over a wide temperature range. Significant friction and wear reductions were achieved by using 1 wt% homogenous dispersions of hairy NPs in PAO. In an effort to increase the function of polymer brushes through the introduction of triboactive phosphorus into the grafted polymers, three phosphonate-containing monomers were synthesized and copolymerized with a long alkyl methacrylate. A significant amount of phosphonate can be incorporated into the brushes without comprising the dispersibility of hairy NPs in PAO. A synergistic effect of combining hairy NPs with a phosphonium-phosphate ionic liquid as PAO additives was discovered. The lubricating performance was improved significantly when the two additives were mixed at certain ratios. Analysis showed that both silica NPs and the ionic liquid participated in the tribo-chemical reaction. These research efforts have shown that hairy NPs have promise for use as lubricant additives. Lastly, as a side project, a series of isotactic and atactic polyethers with monosulfone-containing pendants were synthesized by the reaction of corresponding poly(epichlorohydrin) with various n-alkanethiols and subsequent oxidation of thioether groups with a goal of seeking ferroelectric liquid crystalline polymers for use in solid state cooling. The isotactic poly((R)-epichlorohydrin) was synthesized by ring-opening polymerization of (R)-(-)-epichlorohydrin using a commercial methylaluminoxane as the catalyst.