Cholesteryl Liquid Crystals as Oil-Based Lubricant Additives: Effect of Mesogenic Phases and Structures on Tribological Characteristics

Abstract

Mechanical operation could be seriously affected by friction and controlling it by oil lubrication has been considered as an effective way. Good lubricant additives are very necessary to avoid the friction damages, and to find or design new additives is always a challenge. In this study, a systematic investigation of using cholesteryl liquid crystals (LCs) as lubricant additives to obtain exceptional tribological behaviors was performed. In total, four cholesteryl LC compounds were synthesized targetedly and their thermal and mesogenic properties were studied to see the inherent relationship between the mesogenic phases and antifriction and antiwear performance. Through a series of tribological and related tests, including the UMT TriboLab test, three-dimensional optical microscopy, oil film thickness and viscosity tests, etc., the effect of the mesogenic phases and structures of the synthesized cholesteryl LCs on their tribological properties as lubricant additives was investigated and a related mechanism was analyzed. The result showed that within and close to the mesogenic phase temperature ranges, which we called as effective temperature ranges of LC additives (T EF), the LCs presented better tribological behaviors, meaning they could be used in special lubrication applications that need to be confined in certain temperature scopes; however, the ester groups with long alky tails could help dissolve in base oils and adsorb onto the friction pairs. Among the four LCs, LC-D with a long perfluoroalkyl tail brought widest mesogenic phase with considerably enhanced lubrication performance and increased oil film thickness, viscosity, and thermal stability, indicating that the perfluoroalkyl group could be well used in the structural modification of LC additives to give unexpected tribological performance. This study, in conjunction with our experimental data, suggested that the liquid crystals may be evaluated as potential friction modifiers for temperature-controllable lubrication and also shed a fresh light on the development of novel liquid crystal lubrication materials.

Tribological capabilities of chiral nematic liquid crystal additives in mineral motor oil

“…The beneficial effect without benefitting from liquid crystal phases at higher temperatures could be explained by larger length of molecules of the liquid crystals (typical molecules forming a liquid crystal have lengths of approximately 3 nm and widths of about 0.5 nm) compared with oil and its additives’ molecules and by structuring effect of solid surface. As shown in the works of Yermakov 4 (for individual liquid crystals) and Gao et al 3 (for liquid crystals used in mixture with oil), molecules of the liquid crystals in boundary layer exhibit higher degree of arrangement as compared with bulk material and arrangement remains even if compound temperature exceeds clearing point.…”

“…Although excellent tribological properties of pure liquid crystals have been widely demonstrated, 4,7,[10][11][12][13] the high cost of liquid crystals, designed and synthesized with the high purity required for display applications, has prevented until now the commercial development of liquid crystalline lubricants for mechanical machines and devices. 3,4,7 A strategy to reduce costs can be the use of the liquid crystals as additives of mineral and synthetic oils rather than as the neat lubricants. 3 In such a case, it is also possible to realize 'liquid crystalline' state of lubricant on rubbing surfaces.…”

“…1 They exhibit fluidity as liquid and elasticity of crystalline solid body in direction perpendicular to the flow. 2,3 The remarkable rheological properties of these materials are due to the flow-induced evolution of molecular configurations. 1 It was also established that liquid crystals can change their orientation state in the vicinity of a solid surface.…”

“…4,8 The experiments revealed that the low friction of cartilages results from liquid crystalline cholesteryl compounds in the synovial lubricant. 3,9 Artificial synovial fluids containing a mixture of cholesteric liquid crystals have been developed 4 as the result of these experiments. Cholesteric liquid crystals have clearly layered structure.…”

Ionic Liquid Crystals in Tribology

“…In order to form stable mesophases, alkyl chain lengths should contain more than 12 carbon atoms, as shown in Figure 4 for imidazolium-derived ILCs. In the study of cholesteric mesophases as additives of base oils, Gao et al [11] have very recently shown that longer perfluoroalkylated chains enlarge the mesogenic temperature range. These additives show higher solubility in base lubricant oils and improve tribological performance due to their high thermal stability and formation of thicker lubricant films.…”