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Lubricity of cholesteric liquid-crystal nanomaterials in friction of solids
Abstract
A physical model of the lubricity of cholesteric liquid-crystal nanomaterials has been proposed in accordance with which the transition from single-crystal layers of a cholesteric liquid-crystal nanomaterial to polymolecular structurally sensitive liquid-crystal films, which occurs with varying temperature and is governed by the effect of molecules of cholesteric liquid-crystal compounds contained in the lubricant, as well as the reverse transition, lead to a reversible change in the shear resistance. This change results from the formation of continuous planar-oriented and helically twisted boundary layers in the zone of dynamic contact, which consist of molecules of the cholesteric liquid-crystal nanomaterials. The pitch distance of the helix and, therefore, the thickness of cholesteric liquid-crystal films of these nanomaterials exhibit a structurally sensitive response to changes in the temperature in the friction zone, which varies the dissipation of the energy during friction.
... It's suppose that the sequence of tribochemical processes leading to the formation of boundary films is: the stage of lubricant primary oxidation with air oxygen; decomposition reactions of peroxides, the 2 products of which are ketones, aldehydes, acids and alcohols; subsequent reactions of aldol condensation or Claisen condensation with the formation of high-molecular compounds ("friction polymers") [7][8][9][10][11]. When the molecular weight of the products of such reactions reaches the solubility limit in the lubricant, they are deposited on the surface. ...
... The spectra, obtained from the friction surfaces after a longer friction time, are identical to each other and correspond to the spectrum of a fully formed antifriction film [12]. This means that in the first seconds of friction, a sufficient amount of oxidation products (in accordance with [7][8][9] -aldehydes) should be accumulated in the lubricating environment, which entered into condensation reactions and formed a surface film. At the same time, it is known that for the noticeable course of the oxidation processes of DEHS, long periods of time (hours) and high temperatures (more than 130° C) are required [10,13]. ...
... The spectra, obtained from the friction surfaces after a longer friction time, are identical to each other and correspond to the spectrum of a fully formed antifriction film [12]. This means that in the first seconds of friction, a sufficient amount of oxidation products (in accordance with [7][8][9] aldehydes) should be accumulated in the lubricant, which formed a surface film. At the same time, it is known that for the noticeable course of the oxidation processes of DEHS, long periods of time (hours) and high temperatures (more than 130° C) are required [10,13]. ...
The boundary lubrication mode is usually implemented in conditions of low sliding speeds and high loads. The formation of strong boundary lubricating films under this friction mode determines the operability and durability of the friction units. It is believed that the formation of surface boundary films during friction includes the stages of the lubricant oxidation, and the aldol condensation reaction of oxidized molecules. As a result, high-molecular substances called “friction polymers” are formed. The paper studies the formation of surface films in the presence of substances with different reactivity in the aldol condensation and Claisen condensation reactions. Sunflower oil, bis (2-ethylhexyl) sebacate (DEHS), triisodecyl benzene-1,2,4-tricarboxylate (TC) were used as lubricants. It is shown by ATR IR-spectroscopy of that the common thing for the studied oils is that the C=O and C-O groups participate in the formation of boundary films in these oils. The addition of substances, active in aldol condensation reactions, into lubricants does not accelerate the formation of boundary films. Additives that can chemically interact with iron contribute to the dissolution of the surface oxide film and accelerate the formation of boundary layers. The formation of “friction polymers” occurs when the lubricant molecules interact with the metal surface.
... 28 The influence of temperature on tribological performance has been reported in several studies. Ermakov et al. 29 found that cholesteric liquid crystal displayed the minimum coefficient of friction during the friction tests in the temperature range of mesophase. Gao et al. 30 reported that cholesteric liquid crystals presented better tribological behaviors within and close to the mesogenic phase temperature ranges as lubrication additives, and the tribological performance is influenced greatly by the length of the alkyl chain. ...
... Objects of research were dioctyl sebacate (DOS), dioctyl sebacate with 3% cholesteryl esters of caproic acid (LCKA) and dioctyl sebacate with 3% cholesteryl esters of oleic acid (LCOA). The additive concentration of 3% is selected as the most effective according to the results of previous studies [16]. ...
In the article, investigations of antifriction films formation at friction in dioctyl sebacate (DOS) medium with additives of cholesteryl esters of caproic and oleic acids. The relevance of the study is due to the development of biodegradable lubricants. The time required to achieve a friction coefficient of 0.02 in DOS medium occurs after 20,000 s from the start of the test, in DOS with 3% LCKA medium after 13,000 s, and DOS with 3% LCOA medium after 1000 s. The elemental analysis of antifriction films showed that the films contain organic and inorganic components. The organic part consists of dioctyl sebacate molecules. The inorganic part includes iron compounds. Cholesteryl esters are capable of planar orientation on friction surfaces and form layered structures in the zone of tribocontact. This provides an additional reduction in the friction coefficient and an increase in the energy-saving properties of the lubricant. This effect is achieved due to the realization in the friction zone of the mesomorphic state of a lubricant, which is characterized by an ordered oriented arrangement of the molecules of the mesogenic additive.
... В последнее время активно изучается проблема влияния на граничное трение присадок холестерических жидкокристаллических наноматериалов в различных условиях [4][5][6][7]. Благодаря способности к образованию самоупорядоченных граничных смазочных пленок вблизи трущихся поверхностей указанные материалы существенно снижают силы трения механических сопряжений. ...
The lubricity of 4-acryloyloxyphenyl ester of 4'-n-octyloxybenzoic acid and the system composed of p-npropyloxybenzoic acid and p-n-propyloxy-p'-cyanobiphenyl in the conditions of boundary friction was investigated. The studies were carried out both for pure substances and their solutions in the liquid paraffin base. It was established that the studied additives possess good tribological properties. The tribological activity is defined by chemical structure and concentration of the additive in solution. The presence of the triboactive additives in lubricant stabilizes tribological properties of the lubricant on heating.
This study employs the Landau-de Gennes theory to investigate the behaviour of chiral liquid crystals (CLCs) between concentric and eccentric cylinders under different flow conditions. This theory was implemented in dynamic finite element simulations to solve the evolution of the microstructure of CLCs and couple it with linear momentum balance equation (modified Navier–Stokes equation) to capture the structure of CLCs. The study focused on the microstructure formation of CLCs and their performance as lubricants under various chirality strengths (θ), Deborah numbers (De), and eccentricity of eccentric cylinders. The hexagonal structure of the CLCs was found between cylinders at low De (0.001), where the chiral term predominates. Moreover, the number of hexagonal structures between cylinders was shown to be significantly correlated to the chiral strength. We showed that increasin De resulted in vanishing hexagonal pattern of CLCs and the De value at which the hexagonal pattern vanishes depends on the chiral strength. We also observed that a higher eccentricity ratio significantly increased the pressure force per length on the inner cylinders and caused more non-uniformity in the structure. Based on the findings, we concluded that the performance of CLCs as lubricants is greater at higher θ and eccentricity and lower De.
The hydrotalcites were dispersed in the hexagonal lyotropic liquid crystals (LLCs) composed of 1-hexadecyl-3-alkyl imidazolium bromide ([C16imCn]Br, n = 1, 2 and 3) and H2O. The effects of the hydrotalcite concentration and the alkyl chain length of the [C16imCn]Br on the properties and the lubricant performance of the [C16imCn]Br/hydrotalcite hexagonal LLCs were studied by small angle X-ray diffraction measurements, rheology and tribology. Compared with the pure hexagonal LLCs, the incorporation of hydrotalcite into the [C16imCn]Br hexagonal LLCs can increase the interlayer distances and the viscoelasticity of the hybrid hexagonal LLCs gradually. However, the friction coefficient and wear volume decrease firstly and then increase with the increasing of hydrotalcite content. With the increase of the alkyl chain length of the [C16imCn]Br, the viscoelasticity, the interlayer distance and the water permeability of the mixed LLCs gradually increase which decreases the friction coefficient and wear volume at the moderate hydrotalcite content. The mechanism of the effective improvement of lubrication performance of the hybrid hexagonal LLCs is the tribofilm formed by the physical adsorption and tribochemical reaction of [C16imCn]Br/hydrotalcite LLCs on the steel surface.
Fluorescent liquid crystal polymers have attracted much attention because of the combination of fluorescence, liquid crystallinity and processability. In this study, we designed and synthesised two series of photoluminescent cholesteric liquid crystal polysiloxanes, which contained cholesterol mesogens and fluorescein methyl ester with different side chain lengths. The chemical structures and liquid crystal properties of the polymers were investigated by the usual methods. The photoluminescent property were characterised by fluorescence spectroscopy. At the same time, the effects of the lengths of the fluorescent group chains on the liquid crystal and fluorescence properties of the polymers were studied. The results showed that the synthesised polymers possessing very good fluorescent and liquid crystal properties, which may has certain practical value and application prospect.
With polymethylhydrogensiloxane (PMHS) as the main chain, two series side-chain cholesteric liquid crystal polymers (ChLCPs) with different liquid crystal side chain lengths were prepared by hydrosilylation reaction. Then two series of cholesteric fluorescent liquid crystal polymers (FL-ChLCPs) were obtained by etherification reaction. The obtained FL-ChLCPs have both excellent mesomorphic behavior, and great photoluminescent property. We confirmed their chemical structures by FT-IR, and ¹H NMR spectroscopy, and found them with excellent thermal stability by TGA. We characterized their mesomorphic behavior and thermal performance by POM, DSC and XRD, and confirmed all the polymers belonged to cholesteric phase. FL-ChLCPs emitted different-intensity fluorescence under UV light, and we found some regularity by PL spectrum.
The effect of temperature on the friction coefficient under boundary lubrication by cholesteric liquid crystal nanomaterials, as well as by their solutions in industrial oil, is studied. It is established that the minimum friction coefficient for the investigated substances is reached in the mesomorphic state. In the isotropic state, an insignificant monotonic increase in the friction coefficient with increasing temperature is observed. The friction coefficient also decreases with the increasing molecular weight of the lubricant. Under friction in solutions, an abrupt increase in the friction coefficient occurs when a certain critical temperature is reached, which could be caused by the temperature destruction of the adsorbed layer.
Development of the efficient lubricants can improve durability of tribosystems and save energy in machinery. An optimum and cost-efficient trend is to improve tribological performance of the available lubricants by introduction of suitable additives. The experience shows that liquid-crystal cholesterol compounds (LCCC) can acquire planar orientation on the bearing surfaces and thus reduce friction of lubricated surfaces and increase their load-bearing capacity. It can be assumed that lubricant turns into a mesomorphous state where the molecules of the additive and lubricant acquire the ordered position.
The influence of the number of carbon atoms in the alkyl radical of cholesteric liquid-crystal nanomaterials and their concentration
on the optical properties of mineral and synthetic lubricants is investigated. It is found that adding cholesteric liquid-crystal
nanomaterials to lubricants changes their optical properties, as indicated by a switch from right rotation to left rotation
of the polarization plane of transmitted light. It is shown that the angle of rotation of the polarization plane depends practically
linearly on the concentration of cholesteric liquid-crystal nanomaterials, in semilogarithmic coordinates.
Changes or variations of the amplitude have been established more informative from the viewpoint of pendulum tribometry than the amplitude of oscillations itself. Principles of valid assessment of the amplitude of oscillations and its changes are proposed. It is shown that gated integration of the signal and assessment of its amplitude values through values proportional to the areas of semiwaves of oscillations followed by determination of their variation within a period ensure high accuracy of numerical averaging of experimental data. A new method has been developed to register fine variations of the amplitude of oscillations and some aspects of its application in pendulum tribometry are considered.
The terms nanotechnology, nanoparticles, and nanomaterials are now well known to a wide circle of researchers. Indeed, manipulations at the level of individual atoms have made it possible to design new "structured" materials and devices that possess unique preset properties. Recently, there have appeared such scientific notions as nanobiotechnology and nanomedicine. This means that new fields of nanotechnology, in which molecules of biological origin are used as "building blocks" for designing nanostructures, are being developed. In this article, issues related to nanodesign using nucleic acid molecules and opportunities for the practical application of such structures are discussed.
269 Nowadays, ever increasing attention of experts in the field of the triboengineering and physics of condensed state is focused on the problem of realization of boundary layers structured at the nanolevel in contact with solids. It is actual under conditions of both static and dynamic contact with solids, when the presence of boundary layers with a special structure in the microgap between them defines the degree of friction-energy dissipation [1‐4]. The cholesteric liquid-crystal substances, which quite justifiably can be referred to the class of nanoobjects due to the size of their molecules and the structural features, occupy a special place in these events. Some authors cited them as an example of compounds that are widely involved in the production of unique nanoconstructions, for example, nucleinic acids, etc., in nature [5]. However, despite the very large number of experimental data revealing the mechanism of lubricant action of such liquid-crystal compounds, the regularities of their adsorption on friction surfaces between solids remain unclear. The theoretical and experimental data concerning the interaction of such nanomaterials with a solid surface, in particular, the adsorption of their molecules, describe the behavior of individual mesogenic substances and mainly consider the sphere of their electrooptical application [6, 7]. At the same time, the data on the interaction of solutions of nanocompositions having liquid-crystal properties with friction surfaces represent the clear interest in the tribologic aspect.
Tribologiya zhidkokristallicheskikh nanomaterialov i sistem (Tribology of Liquid-Crystalline Nanomaterials and Systems)
- S F Ermakov
- S.F. Ermakov
Optika kholestericheskikh zhidkikh kristallov (Optics of Cholesteric Liquid Crystals
- V A Belyakov
- V.A. Belyakov
Vvedenie v fiziku zhidkikh kristallov (Introduction into Liquid Crystal Physics
- A S Sonin
- A.S. Sonin
Molekulyarnaya fizika granichnogo treniya (Molecular Physics of Boundary Friction)
- A S Akhmatov
- A.S. Akhmatov