Covalent coupling of ionic liquid to carbon nanotubes: preparation and tribological properties
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Covalent coupling of ionic liquid to carbon nanotubes: preparation and tribological properties Triinu Taaber1,2, Kaija Põhako-Esko1,2, Urmas Joost2, Kadi Külasalu2, Mikk Antsov2 ,Maksim Antonov3, Renno Veinthal3, Kristjan Saal2, Rünno Lõhmus2, Uno Mäeorg1 1
University of Tartu, Institute of Chemistry, 14a Ravila St.,50411, Tartu, Estonia. University of Tartu, Institute of Physics and Estonian Nanotechnology Competence Center,142 Riia St., 51014, Tartu, Estonia. 3 Tallinn University of Technology, Department of Mechanical Engineering, 5 Ehitajate tee St., 19086, Tallinn, Estonia 2
ABSTRACT A new approach for functionalising oxidised MWCNTs using hydroxylated imidazolium bromide via esterification reaction is reported. The bromide anion of a functionalised MWCNTs was exchanged with bis(trifluoromethanesulfonyl) imide (TFSI) through a metathesis reaction to improve its solubility in the IL medium. Composite was characterized with IR, XPS, EDX and TGA analysis, which clearly confirmed that the MWCNTs were functionalized with IL. For potential application as lubricant, the tribological properties of the IL-functionalised MWCNTs (MWCNT-IL) were also evaluated. It was confirmed that even low concentrations of MWCNTIL composite in ILs causes a significant improvement in the anti-wear and friction properties. INTRODUCTION Carbon nanotubes (CNTs) have gained lots attention since their discovery for their unique mechanical and electronical properties. Achieving stable colloids out of CNTs can prove to be challenging, significantly hindering their practical applications. Weak van der Waals attraction between the CNTs and ILs helps to stabilize the dispersion and a gel forms [1]. Unfortunately the stability of this gel is not sufficient and different surfactants such as CTAB [2] and chemical modification of CNTs [3] has been used to increase the stability of CNT dispersions in various solvents. Wang et al. [4] functionalised MWCNTs with 1-(3-aminopropyl) imidazolium salt using thionyl chloride. Use of hazardous thionyl chloride can be avoided by functionalising MWCNTs with amines [3] or alcohols [5]. Ionic liquids (ILs) have been characterized as versatile materials and green solvents because they have a long-term thermal stability and low volatility [6]. Recently, the use of ILs as lubricants has been reported because they display remarkable lubrication and anti-wear capabilities relative to conventional lubrication oils [6]. We report the direct functionalisation of MWCNTs with hydroxylated imidazolium salt via esterification, where different alkyl chain lengths of IL cations were used. Specific cations with functional groups were chosen to achieve the adsorption of functionalized MWCNTs on the friction surfaces and to protect those surfaces from wearing. The MWCNTs were chemically oxidized using HNO 3 /H 2 SO 4 and IL was coupled to carboxylic acid groups with N,N'-dicyclo hexylcarbodiimide. To our knowledge this is the first report of a direct coupling reaction between COOH-functionalised MWCNTs and IL, earlier methods uti
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