No abstract

To evaluate the friction properties of new lubrication systems, two types of ammonium-based ionic liquids (ILs), N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate ([DEME][BF4]) and N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium bis(trifluoromethanesulfonyl) imide ([DEME][TFSI]), were investigated by resonance shear measurements (RSM) and reciprocating type tribotests between silica (glass) surfaces. RSM revealed that an IL layer of ca. 2 nm in thickness was maintained between the silica surfaces under an applied load of 0.40 mN ∼ 1.2 mN. The relative intensity of the RMS signal indicated that the friction of the system was lower for [DEME][BF4], 0.12, than that of [DEME][TFSI], 0.18. On the other hand, the friction coefficients μk obtained from the tribotests of [DEME][BF4] were lower than that of [DEME][TFSI] for sliding velocities in the range of 5.0 × 10(-4) m s(-1) to 3.0 × 10(-2) m s(-1) under applied loads of 196-980 mN. The friction coefficients obtained by the tribotest are discussed with reference to the RSM results.

of proteoglycans and collagen fibrils with high water content (75-80 wt%) has attracted the lubrication with gel materials possessing high mechanical strength [4][5][6][7] and stable low friction. [8][9][10][11][12] These researches aim at shedding light on the gel properties and their potential application as artificial biomaterials.However, such advantages can hardly be realized for industrial uses since solvent (ex. water) in the gels can easily evaporate to lose swelled state. For this purpose, we focused on ionic liquids (ILs) to retain their swelled state. ILs are liquid salts fully composed of cations and anions, generally so-called when their melting temperature is lower than 100 °C. [13] Their high thermal/oxidative stability, negligible volatility, low frictional properties, and facile tenability of these physicochemical properties by their molecular design, [14][15][16] which are the desired characteristics of swelling agents of low frictional gels. In addition, demand for immobilizing ILs on solid surface with keeping their physicochemical properties to fabricate functional devices [17] have opened up the development of gel materials incorporating ILs, so-called as "ion gels" or "ionogels." [18] Ion gels can roughly be classified to physical or chemical gels. [18] In case of physical gels, cross-linking point of their polymer network is formed with relatively weak interaction like hydrophobic interaction, hydrogen bond, π−π interaction and so on, while chemical gels are cross-linked with covalently bonds. There have been several reports studying on tribological behavior of physical ion gels dispersed in ILs. [19][20][21] Espejo et al. characterized the tribological properties of a bucky gel with multiwalled carbon nanotube and 1-ethyl 3-methylimidazolium tosylate. [20] These combination at a tribopair of AISI 316L stainless steel and polycarbonate under 0.98 N at 0.1 m s −1 resulted in COF of 0.025, which is lower than that of a neat IL. In contrast, study on tribological properties of chemical ion gels with sheet-like morphology has not been reported in spite of their high mechanical strength and ease for immobilizing on solid devices.We have recently reported a lubrication system based on ILs and compatible polymer brushes grafted on a Si substrate showed efficient and robust lubrication. [22] They were composed of an ionic liquid, N, N-diethyl-N-(2-methoxyethyl)-N-methylammonium bis(trifluoromethylsulfonyl)imide (DEME-TFSI), and high density ionic liquid polymer brushes composed of the derivative of DEME-TFSI with a polymerizable group, Reducing friction is important to improve the lifetime and energy efficiency of mechanical systems. Since human joints with gel-like structure possess a coefficient of friction as low as 10 -3 , gel materials are recognized as a useful example for designing low frictional materials. Ion gels incorporating ionic liquids (ILs) as swelling agent is expected to be stable gel lubricant since high thermal stability and negligible volatility of ILs can maintain swelled sta...

A polymerizable ionic liquid, N,N-diethyl-N-(2-methacryloylethyl)-N-methylammonium bis(trifluoromethylsulfonyl)imide (DEMM-TFSI), was polymerized via copper-mediated atom transfer radical polymerization (ATRP). The polymerization proceeded in a living manner producing well-defined poly(DEMM-TFSI) of target molecular weight up to about 400 K (including a polycation and an counter anion). The accurate molecular weight as determined by a GPC analysis combined with a light scattering measurement, and the molecular weight values obtained exhibited good agreement with the theoretical values calculated from the initial molar ratio of DEMM-TFSI and the monomer conversion. Surface-initiated ATRP on the surface of monodisperse silica particles (SiPs) with various diameters was successfully performed, producing SiPs grafted with well-defined poly(DEMM-TFSI) with a graft density as high as 0.15 chains/nm 2 . Since the composite film made from the silica-particle-decorated polymer brush and ionic liquid shows a relatively high ionic conductivity, we have evaluated the relationship between the grafted brush chain length and the ionic conductivity.

Among the possible solutions for achieving low friction, polymer brushes that are grafted onto surfaces are good candidates. The tribological characterization of such layers becomes more difficult when the friction is lower: the signal-to-noise ratio of the friction force that is measured with conventional tribometers impedes the precise quantification. Therefore, we have applied a new technique that has been developed at the Laboratory of Tribology and System Dynamics (LTDS), called the 'oscillating relaxation tribometer'. The advantage of this original technique is that it characterizes low friction with unequalled sensitivity. The lower the friction, the better the precision, and it permits obtaining the 'friction law' directly from robust and rapid experimental tests. In this study, the samples that have been used are the ionic liquid-type polymer brushes (ILPBs) with different thicknesses, which have been grafted onto silicon wafers and steel coupons. The counter-face is a mirror-polished steel ball. We show that (i) a thick ILPB layer on silicon is very resistant to high contact pressure, up to 555 MPa; (ii) the friction behavior that is obtained is close to that of a Newtonian viscous one, even under maximum normal loads; (iii) poorer results are obtained for the thinner sample; and (iv) the repetition, up to 5000 oscillations on the same surface, does not affect the friction damping of the contact, which demonstrates that this film provides a favorable resistance to friction under severe contact conditions. In addition, the feasibility of grafting onto steel surfaces is demonstrated. The results are then discussed, with respect to friction and dissipation.