Silica nanoparticles grafted with poly(methyl acrylate) (PMA) chains anchored by a maleimide-anthracene cycloadduct were synthesized to demonstrate mechanochemically selective activation of mechanophores at heterogeneous interfaces. By quantifying the anthracene-containing cleaved PMA polymers, which are generated via retro-[4 + 2] cycloaddition reactions, the first-order kinetic coefficient was determined. Activation characteristics of mechanophores anchored to a nanoparticle exhibit behavior similar to mechanophore-linked polymers, e.g., threshold molecular weight and linear increase in rate coefficient with molecular weight above the threshold. This model system is thus valuable as a probe to test stress activation of interfacially bonded mechanophores relevant to the design of fiber-reinforced polymer composites.

The development of high performance lubricants has been driven by increasingly growing industrial demands and environmental concerns. Herein, we demonstrate oil-soluble polymer brush-grafted inorganic nanoparticles (hairy NPs) as highly effective lubricant additives for friction and wear reduction. A series of oil-miscible poly(lauryl methacrylate) brush-grafted silica and titania NPs were synthesized by surface-initiated atom transfer radical polymerization. These hairy NPs showed exceptional stability in poly(alphaolefin) (PAO) base oil; no change in transparency was observed after being kept at -20, 22, and 100 °C for ≥55 days. High-contact stress ball-on-flat reciprocating sliding tribological tests at 100 °C showed that addition of 1 wt % of hairy NPs into PAO led to significant reductions in coefficient of friction (up to ≈40 %) and wear volume (up to ≈90 %). The excellent lubricating properties of hairy NPs were further elucidated by the characterization of the tribofilm formed on the flat. These hairy NPs represent a new type of lubricating oil additives with high efficiency in friction and wear reduction.

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We report a systematic study on the effect of molecular weight (MW) on microphase separation of mixed poly(tert-butyl acrylate) (PtBA)/polystyrene (PS) brushes grafted on 172 nm silica particles. The brushes were synthesized by sequential surface-initiated atom transfer radical polymerization (ATRP) of tBA at 75 °C and nitroxide-mediated radical polymerization (NMRP) of styrene at 120 °C from silica particles functionalized with an asymmetric difunctional initiator bearing an ATRP initiator and an NMRP alkoxyamine. The MWs of the two polymers in each sample were controlled to be similar to each other. A series of mixed brush samples with different average MWs, from 13.8 to 33.1 kDa, but comparable overall grafting densities were made, and their microphase separation was studied by transmission electron microscopy (TEM). The TEM samples were prepared by drop-casting the dispersions of mixed brush-grafted particles in CHCl3 and also in water (stabilized by a surfactant) and staining the brushes with RuO4. While CHCl3 is a good solvent for both PtBA and PS, making the brushes spread out on the carbon films of TEM grids, water is a nonselective poor solvent for the two polymers, causing the brushes to collapse uniformly on silica core. All samples exhibited lateral microphase separation, forming nearly bicontinuous rippled nanopatterns. The average ripple wavelength D increased with increasing the MW. For samples directly cast from chloroform, D scaled with MW0.70 in the studied MW range. For uniformly collapsed mixed brushes cast from water, D was proportional to MW0.56. The latter is close to the theoretical prediction of D ∼ MW0.5 for perfect Y-shaped brushes in the melt and in nonselective poor solvents. We further compared the phase separation of mixed PtBA/PS brushes grafted on silica particles and PtBA-b-PS diblock copolymers and found that the microphase separation of mixed brushes was weaker than that of diblock copolymers.

This article reports on the synthesis of a series of poly(alkyl methacrylate) brush-grafted, 23 nm silica nanoparticles (hairy NPs) and the study of the effect of alkyl pendant length on their use as oil lubricant additives for friction and wear reduction. The hairy NPs were prepared by surface-initiated reversible addition-fragmentation chain transfer polymerization from trithiocarbonate chain transfer agent (CTA)-functionalized silica NPs in the presence of a free CTA. We found that hairy NPs with sufficiently long alkyl pendant groups (containing >8 carbon atoms, such as 12, 13, 16, and 18 in this study) could be readily dispersed in poly(alphaolefin) (PAO), forming clear, homogeneous dispersions, and exhibited excellent stability at low and high temperatures as revealed by visual inspection and dynamic light scattering studies. Whereas poly(n-hexyl methacrylate) hairy NPs cannot be dispersed in PAO under ambient conditions or at 80 °C, interestingly, poly(2-ethylhexyl methacrylate) hairy NPs can be dispersed in PAO at 80 °C but not at room temperature, with a reversible clear-to-cloudy transition observed upon cooling. High-contact-stress ball-on-flat reciprocating sliding tribological tests at 100 °C showed significant reductions in both the coefficient of friction (up to 38%) and wear volume (up to 90% for iron flat) for transparent, homogeneous dispersions of hairy NPs in PAO at a concentration of 1.0 wt % compared with neat PAO. The formation of a load-bearing tribofilm at the rubbing interface was confirmed using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy.