Characterization and tribological behavior of polymer brush functionalized with ionic liquid moiety

Since the use of room-temperature ionic liquids as novel lubricants was first reported by Liu and coworkers in 2001 [1-3], ionic liquids have attracted considerable attention as a new class of lubricants [4,5]. This is because of their unique properties such as negligible volatility, low flammability, high thermal stability [6], a low melting point, and a wide liquid range [7]. These characteristics are derived from ionic interactions between large organic cations and inorganic or organic noncoordinating anions, which together form salts with melting points below room temperature [8]. Ionic liquids are expected to be ideal candidates for new lubricants under severe conditions such as ultrahigh vacuum and extreme temperatures [9,10]. The tribological properties of some ionic liquids have been evaluated extensively. Ionic liquids exhibited excellent friction reduction, better wear resistance, and high load capacity than conventional lubricants such as synthetic hydrocarbons and fluoroether polymers [1]. The tribological properties of ionic liquids largely depend on the chemical structure of the organic cations and anions [11]. Liu and coworkers proposed that ionic liquids could be easily adsorbed on the sliding surfaces of frictional pairs because of their polar structure. These liquids can form an effective boundary film, which would reduce friction and wear [1-3]. Xiao and coworkers measured the film thicknesses of ionic liquids at high pressures up to 3 GPa in real time employing the relative optical interference intensity measurement method [12]. The film thicknesses of ionic liquids were greater than those of silicone oils of similar viscosities, indicating better film-forming ability of ionic liquids. Liu and coworkers further suggested that a tribochemical reaction between an ionic liquid and a friction surface under severe contact conditions forms surface-protective films, which was confirmed by Mori and his coworkers [13-15]. However, some tribochemical reactions involving the decomposition of the ionic liquid would cause corrosive wear. In the case of an N-alkyl imidazolium-derived ionic liquid, it has been reported that imidazolium with a short alkyl chain and a reactive anion, such as tetrafluoroborate (BF4-) or hexafluorophosphate (PF6-), increased wear through tribocorrosive attack on steel and aluminum surfaces [16]. One of the reasons for the increased wear is that BF4- and PF6- produce corrosive hydrogen fluoride upon hydrolysis, which can damage frictional systems [17]. Jiménez et al. reported that trifluoromethanesulfonate or 4-methylbenzenesulfonate anions can reduce tribocorrosion and, consequently, friction and wear, despite the presence of the short alkyl chains of the imidazolium cation [18]. In contrast, BF4- of imidazolium cations with longer chains results in lower friction and wear compared with the corresponding PF6- salt [18,19]. The pressure viscosity coefficient of ionic liquids is also an important factor in the formation of a lubrication film [20]. Therefore, the lubrication film for hydrodynamic lubrication and the boundary layer on the friction surface are important for reducing friction and wear using ionic liquids as well as conventional oil-based lubricants. In this study, we propose another type of boundary film prepared by grafting a polymer film consisting of an ionic liquid moiety on the friction surface to improve the tribological properties of ionic liquids. It is expected that the grafted polymer bearing ionic liquid moiety would assist the retention of ionic liquids between the sliding pairs and work as an effective lubrication layer in combination with an ionic liquid.