Synthesis of poly(ionic liquid)s brush-grafted carbon dots for high-performance lubricant additives of polyethylene glycol

Abstract

Poly(ionic liquid)s brush-grafted carbon dots (CDs-PILs-X) were synthesized for the first time by surface-initiated atom transfer radical polymerization, where X representing polymerization time was 1, 3, 6, 12, and 18 h. Their tribological properties as lubricant additives of polyethylene glycol (PEG200) were evaluated under the steel/steel contact and linear reciprocating mode. The CDs-PILs-1 exhibited the best friction-reducing and antiwear performance at a concentration (c) of 1.5 wt%. Specifically, the friction coefficient and wear volume of PEG200 were reduced by 61.9% and 82.5%, respectively. At a concentration (c) of 1.5 wt%, the performance of CDs-PILs-1 apparently outperformed that of PILs-1, reflecting the particularly important role of carbon cores in boundary lubrication. In addition, the CDs-PILs-1 was comparable to CDs-PILs-3 and much superior to CDs-PILs-6-18, suggesting that the carbon cores and PILs shells of CDs-PILs played a synergistic lubrication effect and an optimal degree of polymerization of 3.0 (close to that of CDs-PILs-1) should exist. Wear scar surface analyses revealed that robust boundary tribofilms with a thickness of approximately 190 nm, that is, the tribochemical film embedded with carbon cores, were formed on the sliding surfaces lubricated by CDs-PILs-1 dispersion, directly accounting for the excellent tribological properties of CDs-PILs-1.