Formation of organized films with fluorocarbon-modified inorganic nanoparticles and their nanodispersion behavior in solvent
Graphical abstract
Introduction
Fluorocarbon chains [1] represented by polytetrafluoroethylene (PTFE) [2] form a helical conformation at 13 monomers/6 rotation pitch [3] that deviates slightly from the trans zigzag conformation [4] because the van der Waals radius of the fluorine atom is slightly larger than that of the hydrogen atom [5]. Any compounds containing fluorocarbons form a water-repellent surface [6] and exhibit phase separation with compounds containing hydrocarbons [7]. In addition, it is known that thin films of compounds containing fluorocarbon chains exhibit low friction [8]. Furthermore, since perfluorinated compounds do not absorb infrared light owing to the lack of CH stretching vibrations [9], these compounds exhibit infrared light transmission [10]. In addition, fluorinated crystalline polymers generally have high melting points [11], and many of them are super engineering plastics [12]. These fluoropolymers are chemically stable [13] and have excellent wear [14] chemical resistances [15]. An interesting feature of fluorocarbon chains reported in recent years is the formation of a low-polarity surface by canceling the dipole moment based on the helical structure [16].
Research on lubricants using organic solvents and/or oils containing nanoparticles is attracting attention [17,18]. Lubricants are in great industrial demand for micro applications, such as the surface protective layers of storage media [19] and macro applications, such as those used in private cars [20]. When nanoparticles are included in the lubricating oil, it is expected to achieve low friction by rolling the particles [21] and/or filling surface defects [22]. The use of inorganic nanoparticles whose surfaces are modified with organic substances is considered to be effective owing to their high affinity for the surrounding organic media [23]. Especially in the development of new lubricants, it is expected that additives such as nanoparticles would be uniformly dispersed in nonpolar solvents such as normal alkanes. Surface modification technology may provide a breakthrough that can accomplish this goal.
In previous studies, nanodispersion in organic solvent [24], formation of interfacial single-particle layers [25], and nanohybridization with organic polymers [26] were investigated using organo-nanoparticles obtained from the surface modification of inorganic particles having a 5-nm diameter with amphiphilic long-chain compounds [27]. This study focuses on nanodiamonds [28] with surface modifications by amphiphiles, including fluorocarbons with different chain lengths [29]. Nanodiamonds modified with fluorocarbon chains can be expected to disperse in nonpolar solvents [30]. In addition, since nanodiamonds have a high Mohs hardness, they exhibit a polishing effect when the agglomeration size is large [31], and a lubricating effect can be expected in the nanodispersed state [32]. In addition, if low-friction fluorocarbon is used, this will lead to future surface-friction reduction effects. This study also examines changes in the dispersion properties of nanoparticles in solvents by systematically changing the chain length and bonding functional group sites. Further, the monolayer behavior on a water surface of modified inorganic nanoparticles with excellent dispersibility is evaluated (Fig. 1). In previous studies, the correlation between the dispersion behavior of organo-nanodiamond modified with hydrocarbon chains in solvent and the behavior of single-particle layers on a water surface was investigated [33]. The purpose of this study is to construct the relationship between these two kinds of chemical events in a fluorocarbon-modified system.
Section snippets
Modified agents and their modification reaction to nanodiamond surface
Nanodiamonds (average particle diameter 4.8 ± 0.7 nm, DAICEL Co., Ltd.) prepared by the detonation method were used [34]. The detonation method was performed in a special private facility (owned by Daicel Co. Ltd., Kobe, Japan) using trinitrotoluene (TNT) and/or 1,3,5-trinitrohexahydro-s-triazine (Research Department Explosive: RDX) explosives as carbon sources. Utilizing the pressure and heat from the explosion at that time, soot containing the nanodiamond was synthesized, and a nanodiamond
Desorption behavior of modified chains and estimation of surface modification rate
Figure S1(a) in supporting information shows the TG curve of the neat nanodiamond used in this study. Although the neat nanodiamond depends on the amount of adsorbed water on the surface, it showed a weight loss of only around 10 wt% by its desorption in the temperature-rising region up to 500 ℃. Figures S1(b)–(d) show the TG curves for F10P-ND, F12-ND, and F18-ND, respectively. In all cases, although the tendency of the weight reduction start of the data was not necessarily unified, the
Conclusion
In this study, fluorosurfactants were used as modified chains with different chain lengths of inorganic nanoparticle. The bidentate bond with the phosphonic acid derivative improved the surface modification rate. And, the effect of dispersing the nanodiamond as inorganic nanoparticle used in this study was made stronger by lengthening the chain. Since it finds that nanoparticles that can be dispersed in nonpolar solvents exist, this is an epoch-making discovery in the field of lubricant. The
Acknowledgements
This study was supported by JSPS KAKENHI Scientific Research on Innovative Areas “MSF Materials Science” (Grant Number JP 19H05118) and JSPS KAKENHI Grant Number (C) JP 17K05988. Further, the authors thank Dr. Yuji Shitara, Mr. Akira Tada, Mr. Takumi Yamamoto, Mr. Tatsuki Nakajima, and JXTG Energy Co., Ltd., for useful discussion. The authors also appreciate the assistance of Mr. Koichi Umemoto and Dr. Daisuke Shiro, DAICEL Co., Ltd., in providing nanodiamond samples.
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