Self-Constraint Gel Lubricants with High Phase Transition Temperature

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Self-constraint gel lubricants with high phase transition temperature Yurong Wang, Qiangliang Yu, Yanyan Bai, Liqiang Zhang, Feng Zhou, Weimin Liu, and Meirong Cai ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04365 • Publication Date (Web): 29 Sep 2018 Downloaded from http://pubs.acs.org on September 29, 2018

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ACS Sustainable Chemistry & Engineering

Self-constraint gel lubricants with high phase transition temperature Yurong Wang,a,b Qiangliang Yu,a Yanyan Bai, a,b Liqiang Zhang, a,b Feng Zhou,a* Weimin Liu, a Meirong Cai,a* a State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, 18 Middle Tianshui Road, Lanzhou 730000, China b University of Chinese Academy of Sciences, Beijing 100049, China *Corresponding Author: Tel: (86) 931-4968079, Fax: (86)931-4968466, Email: [email protected] (M. Cai), [email protected] (F. Zhou).

ABSTRACT Self-constraint gel lubricant with high phase transition temperature (HTG) is reported and their physicochemical properties, tribological performances and lubrication

mechanism

are

assessed

in

the

paper.

The

HTG

is

(2R)-2-(3,4-dichlorophenyl)-6-(1,2-dihydroxyethyl)-5-hydroxy-N-octyl-1,3-dioxane4-carboxamide and it can assemble through multiple intermolecular interaction (hydrophobic interaction plus hydrogen bonding interaction and π-stacking interaction) to form fibrous structure that can effectively trap base oils. The gel can greatly enhance the phase transition temperature up to 190 °C and reduce the friction heat from 47.5 °C (base oils) to 38 °C at a rotating rate of 1450 rpm at 392 N for 30 min. When used in fully formulated oil, the gel lubricants show excellent lubricating and antiwear properties, which is believed the introduction of chlorine makes significant contribution. Excitingly, the gel having thermal reversibility, creep recovery and thixotropic properties can be infiltrated into porous poly-based bearing materials, which not only reduces the problem of throwing oil but achieve self-lubrication. These advantages of the gel help keep the environment green and clean, and save energy. So, the gel is expected to be applied in peculiar machine components and condition where temperature increase facilitates oil spreading and makes sealing much more challenging.

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KEYWORDS Self-constraint gel lubricant, thermoreversible, high-temperature resistance, lubrication, anti-wear

INTRODUCTION The supramolecular gels are formed in a selected solvent medium by the interplay of various noncovalent interactions among a large number of small molecules, involving hydrogen bonding, van der Waals force, π−π stacking, London dispersion

forces,

dipole–dipole,

hydrophobic

interaction

and

electrostatic

interactions.1-4 Such supramolecular gels has aroused substantial interest, as a kind of soft materials applied in widespread fields such as biology and medicine,5-10 material science,11-13 chemistry and energy,14-15 chemical ecology,16 and so on. Recently, the gel formed by supramolecular assembly of a part of small molecules in base oils has been used as lubricant additives in the field of lubrication.17-19 This gel lubricant effectively improves the lubrication performance of the base oil and has an advantage over greases in solving these problems of oil separation, complex production process and poor lubrication caused by thickener. So, the efficient gel lubricant formed by environmentally benign, naturally derived low molecular weight organogelators is an important technical means for saving energy and it contributes to the sustainable development on the environment and the ecosystem. Remarkably, the gel lubricant not only solves the problem of plugged bearing bores in grease lubrication applications, but also inhibits the leakage of the lubricant through confining the lubricating oil in the supramolecular network structure. These advantages help to keep environment green and clean. In addition, the gel as a new type of semi-solid lubricating material displays remarkable

lubricity,

excellent

corrosion

resistance,

good

thermal

reversibility and thixotropic properties, etc.20-22 These advantages of gel lubricant make it extreme potential to develop into an efficient lubricant to be applied in oil bearing lubrication market and so the gel gets the majority of attention. Recently, our group has reported many thermoreversible gel lubricants by supramolecular assembly


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of low-molecular-weight organic gelator (LMWG) in different base lubricating liquids, such as ionic liquid gel lubricants, in situ zwitterionic supramolecular gel lubricants, nonionic surfactant gel lubricants.17-18, 20-21, 23-24 These supramolecular gel lubricants greatly improve their tribological performance and have good conductivity and corrosion resistance. However, the phase transition temperature from the gel to the sol is relatively low (60-80°C). This greatly limits the application of gel lubricants in high temperature. To solve the problems, a novel supramolecular gel lubricant with high phase transition temperature is prepared in this work. The LMWG with special molecular design self-assembles into a network or fiber structure through hydrogen bonds, van der Waals forces, π–π stacking interactions, thereby forming cavities. Thus, different base oils are firmly constrained in the three-dimensional network structure, turning the liquid into a pectin-like semi-solid. To our delight, the prepared gel lubricant can greatly increase the phase transition temperature from 80 °C to 190 °C and was evaluated to have good lubricity and high-temperature resistance. In addition, the gels have thermal reversibility, creep recovery and thixotropic properties, which characteristics enable them to be successfully infiltrated into porous bearing materials, which not only reduces the problem of throwing oil but achieve self-lubrication. These novel gel lubricants will play an important role in energy saving and emission reduction by prolonging the mechanical life and thus contributing to the development of a sustainable society.

EXPERIMENTAL SECTION Materials. All feedstocks for the synthesis of the gelator including D-Gluconic acid, 3,4-dichlorobenzaldehyde,

methanol,

hydrochloride

acid,

dichloromethane,

n-octylamine and DMAP (4-dimethylaminopyridine) were purchased from Energy Chemical and TCI, respectively. The base oils, polyester (Esterex A51), polyalphaolefin (PAO10), ZDDP (a common lubricating additive) and dimethicone were purchased from ExxonMobil chemical and Sinopharm chemical reagent Co., Ltd, respectively. Market-oriented oils including HELIX v 30 and 10w-40 were purchased



from Shell. MACs oil (Macs) and lithium complex grease were synthesized by our laboratory. In tribological test, The hardness of AISI 52100 bearing steel ball was 700-800 HV and its diameter was 10 mm. The lower stationary disc was ø 24 mm × 7.9 mm and made up of AISI52100 bearing steel with hardness of 750-850 HV. The mean roughness (Ra) of the lower steel discs and steel ball were polished to about 0.02 µm with CW400 -CW2000 SiC abrasive paper. Synthesis of the HTG and Preparation of Gel Lubricants. The gelator (HTG) was (2R)-2-(3,4-dichlorophenyl)-6-(1,2-dihydroxyethyl) -5-hydroxy-N-octyl-1,3-dioxane-4-carboxamide and was synthesized according to the reported method.25 The different mass concentrations of HTG was dissolved in a variety of base oils by heating until complete dissolution in a sample vessel, and then the gel lubricants were rapidly formed when the hot mixed solution slowly cooled. 500SN, one of the most used and commercially available mineral oil, was chosen as the base oil. Under heating, 3 wt% HTG was dissolved in 500SN until completely dissolved and then the 3% 500SN gel was formed when slowly cooled. The 0.2% 500SN gel, 0.5% 500SN gel, 1% 500SN gel, 2% 500SN gel, 1% Macs gel, 2% Macs gel, 3% Macs gel, 0.2% A51 gel, 0.2% dimethicone gel, 0.2% gel PAO10 gel, 0.2% MACS gel, 3% 5w-30 gel and 3% 10w-40 gel were obtained in the same manner, respectively. Simulation Method. The GROMACS 2016.4 package26 was used to complete the molecular dynamics (MD) simulations in order to study the clustering of HTG molecules in solvents (acetonitrile) or base-oils. The GROMOS 54A727 was used as simulation force field and it was captured by the ATB28 (Automated Topology Builder). All MD simulations were accomplished under the NPT ensembles at a temperature of 298K and a pressure of 1atm, and it was applied as Isotropic and Berendsen barostat29 with the coupling time constant of 2.0 and 0.1 ps. The MD simulation was carried out by a step of 2 fs, which all directions were under the periodic boundary conditions. The bond lengths were constrained to the LINCS.30 It was set as 1.2 nm that the cut-off distances of Van ACS Paragon Plus Environment

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der Walls and electrostatic forces. The long-range electrostatic interactions adopted the particle mesh Ewald method31 and the VMD1.9.232 was the motion trajectory of the simulation molecules. Characterization. The proton and carbon nuclear magnetic resonances were used to confirm the structure of HTG. These results of 1H NMR and

13

C NMR in chloroform-d were

shown in Figure S1 and Figure S2. The thermal stability of 500SN gel and MACS gel were carried out on an STA 449 C Jupiter simultaneous TG-DSC in N2 (rate: 10 °C/min). The gel-to-sol transition temperature of the gel lubricants were measured on Mettler Toledo DSC822 series. Samples were first cooled to about −20 °C with liquid nitrogen and then heated to 230 °C at a rate of 10.0 K/min. The weight loss and heat flow values were monitored in the TG-DSC analysis. The rheological property of gel in base oils was measured on a RS6000 Rheometer. The PHI-5702 multifunctional XPS was used to detect the chemical composition on the surfaces (exciting source: Al-Kα radiation). Tribology Test. Tribological properties of different gel lubricants, base oils and lithium complex grease were accomplished on the Optimol SRV-IV oscillating reciprocating friction and wear tester with a ball sliding against a lower stationary disc. The testing parameters were presented in Table S1. Three repetitive measurements were performed for each data point. The wear volume of the lower disk was analyzed by a MicroXAM 3D noncontact surface mapping profiler. A JSM-5600LV scanning electron microscope (SEM) was used to observe the morphology of the worn surfaces. A macroscopic friction test of oil-impregnated and gel-impregnated polyimide bearing material was performed on conventional ball-on-disk reciprocating tribometer by recording the friction coeﬃcient using a Stift-Scheibe-Tribometer (TRB).

RESULTS AND DISCUSSION Formation of HTG Lubricant.



The HTG can assemble through multiple intermolecular interaction to form fibrous structure that can effectively trap different base oils, turning the liquid into a pectin-like semi-solid. The gelation process of 0.2% HTG in different base oils was recorded in Figure S3a, using the digital camera with 10 seconds interval. This gelation occurs immediately when the mixed hot solution cooled down. After 30 seconds, the semi-solid gel initially formed and then a dense and stable gel forms at 60 seconds (Figure S3a). By supramolecular assembly, the HTG can solidify different lubricating oils including multiple-alkylated cyclopentane (MACS), polyester (A51), polyalphaolefin (PAO10), 500SN, dimethicone, polyethylenglycol (PEG400), 150BS, etc.

Figure S3b presents only four gel lubricants in various base-oils at the least

gelation concentration (0.2%) and 2SN-1ZD gel. Under heating, 2wt% HTG and 1wt% ZDDP were dissolved in 500SN until completely dissolved. Then the 2SN-1ZD gel was obtained when slowly cooled. Figure S3c shows different concentration of 500SN gel. The ordered fibrous structure of xerogel obtained in acetonitrile by SEM is shown in Fig. 1(a, b). For the preparation of xerogel, 2wt% HTG gelator was completely dissolved in acetonitrile to prepare the gel and then the xerogel was obtained after vacuum drying for 24h. Figure 1c exhibits the molecular structure of the HTG gelator. Figure 1d indirectly proves that the weak interaction cause a red shift of 12–13 nm by the ultraviolet-visible spectroscopy. According to the above results and previous reports,25 the reason why these gels can be rapidly gelled was that a combination of intermolecular hydrogen bonding, p–p-stacking and van der Waals forces was the driving forces for the self-assembly of HTG in different base oils as indicated in Figure 1e. Small amount of fiber-like aggregates were composed of the HTG supramolecular firstly and extend further to fibrous network.4, 8 In the Figure 2, it is seen that the scattered gelators aggregate to form some fiber-like structures (figure 2a), so that realize the oils are limited freedom of movement, as shown in figure 2 (b, e). In addition, we find that there are about 100 H-bond among the gelators of pairs within 0.35 nm, and responding structures are showed in figure 2e. That is to say that the gelators are assembled by H-bond and π-π interactions and ACS Paragon Plus Environment

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formed some fiber-like structures eventually. The fiber-like aggregates further cross-link to reduce the movement of oil molecules.

Figure 1. (a, b) SEM images of the xerogel in acetonitrile (SEM magnification: the left is 600×and the right is 6000×); (c) Molecular structures of gelators HTG; (d) ultraviolet-visible spectrometry of the solution and the 500SN gels with HTG as gelator at a concentration of 0.5 wt%; (e) Self-assembled schematic of the 500SN gel.



Figure 2. Snapshots of simulation structures (a) 0ns, (b) the front view of gelators at 100 ns, (c) the frontside of simulation system at 100 ns (the pink is gelator, the light blue is oil), (d) the side elevation of gelators at 100 ns, (e) one part of (d), the dash line represent H-bond. The light blue, red, white, green and dark blue are represent carbon, oxygen, hydrogen, chloride and nitrogen, respectively. Here we apply hexane, heptane and octane to simulate as oils.

Thermal Analysis. Figure S4 exhibits the thermo-decomposition temperature of 500SN gel and MACs gel is bigger than 240 °C, which suggests all the gel lubricants have good thermal stability and provides a possibility of being used in high temperature. It is seen that the thermo-decomposition temperature of 500SN gel gets elevated with the increase of the gelator (HTG) mass concentration in Figure S4a. Figure S4b presents the thermo-decomposition temperature increase in the following sequence: Macs < 2% Macs gel, SN-1% ZD (1wt% ZDDP was dissolved in 500SN) < 2% SN-1% ZD gel (2wt% HTG and 1wt% ZDDP were completely dissolved in 500SN under heating and then the 2% SN-1% ZD gel was obtained when slowly cooled). These results imply that HTG can improve the decomposition temperatures and high-temperature resistance of base oils during the friction processes. Figure 3a shows the phase transition temperatures (Tg) of 500SN gel lubricants and their phase transition enthalpy. There is only one endothermic peak in every concentration of 500SN gel and their phase transition enthalpy(∆Η) is able to be obtained by the areas of the endothermic peak. The concentration of HTG affects the T g and ∆H of 500SN gel lubricant. It’s found that the Tg of 500SN gel increased from 185.7 and 187.9 °C to 188.8 °C and the ∆H increased from 0.0215 J/g and 0.8604 J/g to 2.468 J/g, when the concentration of HTG increases from 1% and 2% to 3%. Remarkably, the HTG gel lubricant can greatly increase the phase transition temperature from 80 °C to 190 °C compared with the reported gels. For example, a nonionic surfactant gel, ionic liquid gel and in situ zwitterionic supramolecular gel widely applied in room temperature because their phase transition temperatures are below 80 °C.18, 20-21, 23-24 Moreover, although the ∆Η has a little impact on reducing the temperature in four-ball test, the heat generated by the friction can be absorbed by the gel when the gel goes from solid to liquid. Interestingly, 3 % 500SN gel reduce the ACS Paragon Plus Environment

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friction heat from 47.5 °C (base oils) to 38 °C according to evolution of temperature with time at a rotating rate of 1450 rpm at 392 N for 30 min in Four ball from the Figure 3b. This indirectly suggests that the gel can provide superior lubricating property to reduce the friction heat and provides a possibility to apply in high temperature.

Figure 3. (a) DSC curve of different concentration of 500SN gel; (b) Evolution of temperature with time lubricated by 500SN and 3% 500SN gel at room temperature (Four ball load: 392 N, rotating rate:1450 rpm, duration: 30 min).

Rheological Characterization and Thixotropic Behavior. To understand their viscoelastic and mechanical behavior, the storage modulus G′ and loss modulus G″ of 500SN gels were measured as functions of shear stress. Figure 4a shows the stress sweep data of different mass concentrations of 500SN gels. It is found that all the 500SN gels have the plateau regions of the dynamic moduli below a critical stress value. G′ shows a substantial elastic response, and G′ is larger than G″ over the entire strain range. Increasing the mass concentration of the HTG from 1% to 3%, the value of G′ increased from 1.52 kPa to 20.08 kPa and the yield stress increased from 97 Pa to 650 Pa. The viscoelastic property and mechanical stability of the gels enhance increasing the mass concentration of the HTG. The values of G′ and G″ of different 500SN gels gradually enhance with the increase of the frequency in Figure 4b. It exhibits the G′ is bigger than G″ with this frequency range, which indicates 500SN gels have good tolerance ability to external forces and a



typical viscoelastic behavior.33 Figure 4c exhibits that the viscosity of 500SN gels decreases with the shear rate increasing, which illustrates this gel has characteristic of a pseudoplastic and shearing thinning fluid.34-35 It’s a typical character of the viscoelastic property. The viscosity of gels decreases under the high shear stress, indicating that the physical bonds and gel structures are destroyed. However, the result exhibits the gel has a rapid viscoelastic creep recovery after high-speed shear was stopped in Figure 4d. The viscosity of 500SN gels rapidly dropped under the high shear rate (γ=300s-1), resulting in a quasi-liquid state. When the shear rate changed to a lower value (γ=0.05 s-1), the viscosity recovered close to the initial value within 20s. These results are fully reproducible. As mentioned, gelators can self-assemble through hydrogen bonding, p– p-stacking and van der Waals forces to form 3D network structure effectively trapped base oils, obtaining a semi-solid gel lubricant. But, the fibrous network is destroyed because the noncovalent interaction is too weak under the high shearing or drastic shaking. So the gel’s viscosity decreased and the trapped base oils were released. After a while, the HTG is able to self-assemble to rapidly form gel again and the viscosity of this gel increased again, suggesting the gel has a rapid creep recovery property. Figure 5 shows the gel with good thermal reversibility changes to solution when heating/shearing and the gel forms again when cool/rest. The process is able to be repeated multiple times, suggesting good thixotropic behavior of gel. The gel has the excellent thermal reversibility, good creep recovery and thixotropic properties, endowing it “self-restraining” properties. That is to say, the lubrication system is like the lubricating oil when the gel is used, and appears semi-solid like grease when it is not used. So the gel is expected to solve the leakage, creep and volatile loss of lubricating oil and the poor cooling effect of grease in the process of application.


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Figure 4. Rheological data of the 500SN gel, (a) The changes of G’ and G’’ as a function of the applied shear stress of 500SN gel, and the frequency of the measurement is f = 1 Hz.(b) Frequency sweep, the applied shear stress is 10 Pa. (c) Shear rate dependency of viscosity for 500SN gel. (d) Viscosity switch of 500SN gel with a continuous step rate test (high shear rate γ = 300 s-1 and low shear rate γ = 0.05 s-1.

Figure 5. Thermoreversible and thixotropic properties of 500SN gel.

Tribological Properties. The tribological properties were investigated under the different harsh conditions in Figure 6−8 and Figure S5-9. It shows 500SN gel has good reducing friction and anti-wear capability not only at room temperatures but also at high-temperature. Furthermore, the gel provides superior lubricating properties and high-temperature



resistance, whereas the lubrication failure of the commercially available ester (lithium complex grease) occurs under the same conditions. It is also found that the HTG has good compatibility with ZDDP and better synergistic effect with existing oil additives. Figure 6a shows the mean evolution of the friction coefficient (COF) and the wear volume (WV) losses of sliding discs lubricated by different mass concentrations of 500SN gel at RT. The COF and WV losses of the lower discs decrease in the following sequence: 3% 500SN gel (0.11, 5.03×10-5mm3) < 2% 500SN gel (0.12, 6.82×10-5mm3) < 1% 500SN gel (0.16, 45.8×10-5mm3) < 500SN (0.22, 62.5× 10-5mm3). Moreover, the evolution of COF with time lubricated by different mass concentration of 500SN gel and Macs gel at room temperature was shown in Figure 6b and Figure S5, respectively. The lubrication failure of 500SN and 1% 500SN gel occurred after 3 and 10 minutes at 300 N, respectively, whereas that of 3% 500SN gel and 2% 500SN gel is very small and stable throughout the experiment. Figure S5 shows the lubrication failure of Macs oils occurred after 4 minutes at 400 N. Although the lubrication effect of 1% Macs gel gets worse after 30 minutes, 2% and 3% Macs gel have a sustained and stable COF (Figure S5). These results present the mass concentration of HTG has a noticeable effect on lubrication performance. It also demonstrates the HTG not only jellies base oils to form gels but also significantly reduces the COF and WV losses of the lower discs as the friction-reduction and anti-wear additive.

Figure 6. (a) Mean friction coefficient and wear volume of steel discs; (b) Evolution of friction coefficient with time lubricated by 500SN gel with different concentration at room temperature. 0% concentration refers to the base oils 500SN (SRV load: 300 N, frequency: 25 Hz, stroke: 1 mm, duration: 30min).


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Interestingly, 3% 500SN gel and 2% Macs gel not only present good reducing friction and anti-wear capability at room temperatures, but also have excellent lubricity at high temperatures from Figure 7a and Figure S6. Figure 7a shows 3% 500SN gel has a small and stable COF (0.107), whereas the lubrication failure of 500SN occurs at 300 N at 150 °C. It also exhibits the WV of steel discs lubricated by 500SN (107.8×10-5mm3) is much larger than that of 3% 500SN gel (13.4×10-5mm3). Figure S6(a, b) shows 2% Macs gel has much smaller COF and WV than that of Macs at 400 N at 150 °C. These data display the gel still has an apparent improvement in lubricating and anti-wear properties at high temperatures. In addition, one of the most used and commercially available ester, lithium complex grease was chosen as the comparison to further evaluate tribological properties of 500SN gel. It is found that the lubrication failure of lithium complex grease occurs within 8 minutes, while the COF of 3% 500SN gel has been very small and smooth throughout the experiment in Figure 7a. It also shows the WV of 3% 500SN gel (13.4×10-5mm3) is far less than lithium complex grease (100.1×10-5mm3). From these data, it is easy to conclude that 3% 500SN gel has better lubricating and anti-wear properties than base oils and lithium complex grease. Furthermore, the effect of the frequency on triobological performance of 500SN, lithium complex grease and 500SN gel is investigated in Figure S7(a, b). It is shown that the COF of 500SN and lithium complex grease are fluctuant during the whole frequency conversion test. On the contrary, 3% 500SN gel exhibits the best constant frictional behavior with an average COF of 0.12 during the whole frequency conversion test at 150 °C. These notable results reveal HTG gelator in base oils plays a significant role in friction-reduction and anti-wear properties. This may be because a large number of gelator molecules can strongly absorb on the metal sliding surface with its polar headgroup, a protective film forming on the sliding surface.36 Thus, the gel lubricant significantly improves the tribological properties of base oils. This further indicates the gel is likely to develop into the market. Remarkably, 3% 500SN also demonstrates excellent lubricity at a rotating rate of 1450 rpm at 392 N at 150 °C for 30 min in a standard four-ball tester in Figure 7b. It ACS Paragon Plus Environment


presents 3% 500SN gel has much lower and stable COF throughout the experiment compared with pure 500SN. It is also seen that the wear scar diameters of the steel balls lubricated by 3% 500SN gel is far less than that of pure 500SN in Figure 7b. This indicates that the gel lubricant is a good additive for improving the friction reduction and antiwear properties of 500SN. These results further exhibit the prepared gel is an effective lubricant additive in different tribological testing ways.

Figure 7. (a) Friction coefficient and wear volume of steel discs lubricated by 500SN, 3% 500SN gel and Lithium complex grease at 25 Hz, respectively (SRV temperature: 150 °C, load: 200 N, stroke: 1 mm); (b) Evolution of friction coefficient with time and wear scar diameters of steel balls (Four Ball load: 392 N, rotating rate:1450 rpm, duration: 30 min).

To further investigate the compatibility with ZDDP, the HTG was dissolved in 500SN with ZDDP. The 2wt% HTG was dissolved in 500SN with 1wt% ZDDP (denoted as SN-1% ZD) to prepare 2% SN-1% ZD gel. The prepared gel was denoted as 2% SN-1% ZD. Figure 8(a, b) displays the evolution of COF and WV lubricated by SN-1% ZD and 2% SN-1% ZD gel at a load of 300 N at 25 Hz at 150°C for 1 hour. Surprisingly, 2% SN-1% ZD gel has a superior reducing-friction and anti-wear performance with an average COF of 0.10 and an average WV of 22.9×10-5mm3 at 150°C during the whole 1 hour. However, SN-1% ZD shows an especially large COF and WV, which are much higher than that of 2% SN-1% ZD gel. It exhibits 2% SN-1% ZD gel has excellent high-temperature resistance and boundary lubricating capability. Moreover, Figure S8a shows the COF of SN-1% ZD is all extremely large and fluctuant with a load ramp test from 100 to 600N stepped by 100 N intervals with


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5-min test duration for each load at 150°C, whereas 2% SN-1% ZD gel shows a small and stable COF during the entire experiment. The signal of lubrication failure of SN-1% ZD occurs within 10 minutes when the load reaches 300N. However, the maximum load carrying ability of 2% SN-1% ZD gel surpasses 600 N. This further suggests 2% SN-1% ZD gel can significantly improve the extreme-pressure capability of 500SN. As shown in Figure S8b, the COF of 2% SN-1% ZD gel is more stable and smaller than that of SN-1% ZD during the whole frequency conversion test (from 10 to 40 Hz) at a load of 300N at 150°C, which further indicates 2% SN-1% ZD gel lubricant significantly decrease friction and wear. It means the HTG gelator in base-oil is beneficial to tribological behavior of oil and ZDDP. To further investigate the synergistic effect with existing oil additives, the HTG was dissolved in 500SN with the HELIX 5w-30 and 10w-40, respectively. The 3wt% HTG was dissolved in HELIX 5w-30 (denoted as5w-30) to prepare 3% 5w-30 gel (denoted as 3% 5w-30). The 3wt% HTG was dissolved in 10w-40 (denoted as 10w-40) to prepare 3% 10w-40 gel (denoted as 3% 10w-40). Figure S9 shows the change of COF and WV lubricated by 5w-30, 10w-40, 3% 5w-30 gel and 3% 10w-40 gel. The 5w-30 and 10w-40 have big and fluctuant COF (0.184), while 3% 5w-30 gel and 3% 10w-40 gel have relatively small and stable COF (0.126). The wear volume of 3% 5w-30 gel (18.1×10-5mm3) and 3% 10w-40 gel(17.0×10-5mm3) is less than 5w-30 (90.4×10-5mm3) and 10w-40 (96.7×10-5mm3). These contrasts show that the best friction-reduction performance was achieved when the HTG is mixed with fully formulated oil. Above these remarkable data show the HTG has good compatibility with ZDDP and better synergistic effect with existing oil additives, which is good for their business application.



Figure 8 (a) Friction coefficient, (b) Wear volume of steel discs lubricated by SN-1% ZD and 2% SN-1% ZD gel; 3D morphologies of worn scars lubricated by (c) SN-1% ZD and (d) 2% SN-1% ZD gel at a load of 300 N for 60 mins (SRV frequency: 25 Hz, stroke: 1 mm, temperature: 150 °C).

Surface Analysis. Figure S10 (a, a1) shows the worn surface is highly rough and has many wider and deeper wear scars with severe scuffing under the lubrication of 500SN at a load of 300 N at room temperature. However, the worn surfaces lubricated by 3% 500SN gel has comparatively narrow and shallow wear scars with small furrows and scratches Figure S10 (b, b1). So, the wear volume of 500SN is much larger than that of 3% 500SN gel. Figure S10 (c, c1, d, d1) exhibits the worn surface lubricated by 500SN and 3% 500SN gel at a load of 200 N at 150 °C. Under the high temperature, the worn surface lubricated by 500SN is still much rough, and has a larger number of wider and deeper wear scars with many grooves and severe scuffing. Although the worn surface lubricated by 3% 500SN gel at 150 °C has a little scuffing than that at 25 °C, the worn scar is quite narrow and shallow and displays hardly any grooves. It suggests the 3% 500SN gel lubricant has certain anti-wear performance. Figure S11(a-d) presents the wear scar of 3% 500SN gel is narrower and shallower than that of 500SN according to the corresponding 3D optical microscopic images, which further confirms the gel has excellent lubrication performance. In the Figure S12(a, a1), the worn surface lubricated by SN-1% ZD at a load of 300 N at 150 °C for 1 hour also has wider and deeper wear scars with many grooves and severe scuffing. In contrast, the worn surface of 2% SN-1% ZD gel (Figure S12(b, b1)) is quite smooth and has shallow wear scars without grooves under the same condition. This indicates that the 2% SN-1%


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ZD gel has the good anti-wear properties and this date is corresponding to the WV in Figure S9.

Lubrication mechanism The electrical contact resistance (ECR) measurements were carried out to determine whether an insulating tribofilm was generated.19, 37 Figure 9a shows the change of ECR with the friction time lubricated by 500SN and 3% 500SN gels at 200 N and 150℃. It is seen that the value of ECR lubricated 500SN is very low, implying that almost direct contact between rough sliding pairs result in friction and wear. However, a relatively large value of ECR was measured when lubricated by 3% 500SN gels. This implies that a comparatively stable insulating tribofilm forms on sliding pairs. XPS spectras of N1s, Cl2p Fe2p and O1s are accomplished in Figure 9b and Figure S13, to further investigate the lubrication mechanism of 500SN gel. The binding energies of N, O and Cl of the worn surfaces lubricated by 3% 500SN gel differ from that of HTG. We suppose a tribofilm formed on the surfaces by the tribochemical reaction.39 Figure 9b displays the peaks of Cl2p at 198.4-200.5 eV, which may correspond to FeCl2 or FeCl3 or C–Cl binding.40 Figure 9b also exhibits the N1s spectra of the worn surface at 399.8 eV, possibly corresponding to carbonitride and/or nitrogen oxide, and/or nitrogen double-bond compounds.39-40 It is likely to illustrate that HTG gelator molecules have physical adsorption on the steel surfaces. Considering the peak of Fe2p (about 710.9 and 724.4 eV) and O1s (about 531.8 eV) in Figure S13, it may be attributed to Fe2O3, FeOOH, FeO, Fe3O4.39-42 Based on the results of XPS and ECR, we suppose many gelator molecules adsorbed the metal surface first and then an effective protective film composed of Fe2O3, FeOOH, FeO, Fe3O4, nitrogen double-bond compounds, FeCl2 or FeCl3 or C–Cl binding forms during the friction process by a significant triboelectrochemical reaction in Figure 9c.18, 36, 38 The formed tribofilm obstructs the direct contact of the friction pair and thus effectively decrease the friction and wear on the metal surface. This is why the gel possesses better lubricity and extreme-pressure capability.



Figure 9. (a) The ECR value lubricated by pure 500SN and 3% 500SN gel, (b) The XPS spectra of N1s and Cl2p on the steel worn surfaces lubricated by 3% 500SN gel and neat HTG, (c) Schematic of the proposed lubrication mechanism of gel (temperature:150℃, load: 200 N, frequency: 25 Hz, stroke: 1 mm).

Tribological property of gel impregnated bearing materials At high speed, there are still some problems at present that most of lubricating oil in the bearing will be thrown out in large quantities because of its low viscosity. On the one hand, the large loss of lubricating oil causes the friction and wear of the bearing, which greatly reduces the service life of the bearing. On the other hand, if the lubricating oil is thrown out or splashed onto the parts, it may react with it. This may cause some problems such as hardening, swelling, softening, and pollution, etc. Excitingly, this supramolecular gel lubricant with excellent creep recovery and thixotropic properties can be poured into porous iron-based and poly-based bearing materials, which can reduce the problem of throwing oil though improving their viscosity. Figure 10a shows the impregnated oil or gel in porous polyimide bearing material used for friction or centrifugation experiments. The centrifugation experiments of the oil-impregnated and gel-impregnated porous polyimide bearing were performed by the TG16-WS high speed centrifuge. Figure 10b presents the change of centrifuged mass loss of the oil-impregnated and gel-impregnated porous


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polyimide bearing with a speed test from 1000 to 7000 stepped by 1000 r/min intervals with 15-min test in the centrifugation experiments. The specific data about the mass loss of lubricant are shown in the Table1. It is found that the centrifuged mass loss of 3% 500SN gel-impregnated porous polyimide bearing is less than 500SN-impregnated porous polyimide bearing at every speed in Figure 10b and Table1. The results further explain that the gel can effectively reduce the mass loss of base oil, such as throwing, leakage or volatilization and improves the oil storage capacity of porous bearing. Figure 10c exhibits the COF of oil-impregnated and gel-impregnated porous polyimide bearing by ball-on-disk at TRB (load: 8N, frequency: 1Hz, stroke: 10 mm, temperature: 25oC, duration: 32 min). It is seen that the gel-impregnated porous polyimide bearing always has a lower COF than oil-impregnated porous polyimide bearing. Under the environment of frictional heat and high contact pressure, the gel changes from a gel state to a liquid and then oozes out of the micropores in the bearing to achieve self-lubrication. So, the porous polyimide bearing impregnated gel has a better lubricating property. It is possible to be applied in peculiar machine components and condition where temperature increase facilitates oil spreading and makes sealing much more challenging.



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Figure 10. (a) Impregnated oil or gel in porous polyimide bearing material used for friction or centrifugation; (b) Evolution of centrifuged mass loss of the oil-impregnated and gel-impregnated porous polyimide bearing material with the speed in the centrifugation experiments; (c) Friction coefficient of oil-impregnated and gel-impregnated porous polyimide bearing (TRB load: 8N, frequency: 1Hz, stroke: 10 mm, temperature: 25oC, duration: 32 min).

Table1. The specific data about the loss of lubricant. speed

the loss of oil (g)

the loss of gel (g)

1000

0

0

2000

0

0

3000

0.0022

0

4000

0.0162

0.0077

5000

0.0348

0.016

6000

0.0737

0.032

7000

0.1116

0.06

CONCLUSION A novel self-constraint gel lubricant with high phase transition temperature


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(HTG) is prepared. The HTG can not only assemble through multiple intermolecular interaction to form fibrous structure that can effectively trap various base-oils, but also plays a key in friction reducing and anti-wear as an effective lubrication additive. The HTG has greatly increased the phase transition temperature from 80 °C to 190 °C, which provides a potential for high-temperature friction. What’s more, the gel lubricant exhibits excellent lubricating and anti-wear properties than base oils and lithium complex grease at both room temperature and high temperatures. Interestingly, the gel can effectively reduce the friction heat from 47.5 °C (base oils) to 38 °C at a rotating rate of 1450 rpm at 392 N for 30 min. In addition, the gel lubricant has good compatibility with ZDDP and fully formulated oil, the mixed gel has an apparent improvement in lubricating properties, extreme-pressure and abrasion resistance behaviors at even harsh conditions. Remarkably, the gel poured into bearing materials changes from a gel state to a liquid due to frictional heat and then oozes out of the micropores to achieve self-lubrication during the friction process. At the end of the friction, the gel can be condensed again and stored in the micropores of bearing, which can effectively reduce the loss of base oil and improves the oil storage capacity of bearing. These advantages of the gel play an important role in keeping the environment clean and save energy. So, the gel is possible to be applied in peculiar machine components and condition where temperature increase facilitates oil spreading and makes sealing much more challenging.

ASSOCIATED CONTENT Supporting Information Nuclear magnetic resonance spectroscopy (1H NMR and

13

C NMR) of HTG and

corresponding NMR data and digital pictures of gels and thermal decomposition properties of gel lubricants and tribological data of different gel lubricants and 3D optical microscopic images of 500SN gel. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. Phone: +86 0931 4968079. ACS Paragon Plus Environment


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*E-mail: [email protected]. Phone: +86 0931 4968466. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The

authors

acknowledge

the

financial

support

from

the National Key Research and Development Program of China (2018YFB0703802), the National Natural Science Foundation of China (Grant Nos. 51675512 and 51705504), and the Youth Innovation Promotion Association of Chinese Academy of Sciences (Grant No. 2018454). The authors would like to thank Ms Feng Wang and Nannan Mai in Luoyang Bearing Science & Technology CO., LTD for providing the polyimide bearing materials. REFERENCES (1) Bhattacharya, S.; Samanta, S. K., Soft-Nanocomposites of Nanoparticles and Nanocarbons with Supramolecular and Polymer Gels and Their Applications. Chem. Rev.2016, 116 (19), 11967-12028. (2) Du, X.; Zhou, J.; Shi, J.; Xu, B., Supramolecular Hydrogelators and Hydrogels: From Soft Matter to Molecular Biomaterials. Chem. Rev.2015, 115 (24), 13165-307. (3) de Loos, M.; Feringa, B. L.; van Esch, J. H., Design and Application of Self-Assembled Low Molecular Weight Hydrogels. Eur. J. Org. Chem. 2005, 2005 (17), 3615-3631. (4) Babu, S. S.; Praveen, V. K.; Ajayaghosh, A., Functional pi-gelators and their applications. Chem. Rev. 2014, 114 (4), 1973-2129. (5) Truong, W. T.; Su, Y.; Meijer, J. T.; Thordarson, P.; Braet, F., Self-assembled gels for biomedical applications. Chem--Asian J. 2011, 6 (1), 30-42. (6) Tomatsu, I.; Peng, K.; Kros, A., Photoresponsive hydrogels for biomedical applications. Adv. drug deliver Rev. 2011, 63 (14-15), 1257-66. (7) Xu, B., Gels as functional nanomaterials for biology and medicine. Langmuir 2009, 25 (15), 8375-7. (8) Skilling, K. J.; Citossi, F.; Bradshaw, T. D.; Ashford, M.; Kellam, B.; Marlow, M., Insights into low molecular mass organic gelators: a focus on drug delivery and tissue engineering applications. Soft Matter 2014, 10 (2), 237-56. (9) Wang, H.; Luo, Z.; Wang, Y.; He, T.; Yang, C.; Ren, C.; Ma, L.; Gong, C.; Li, X.; Yang, Z., Enzyme-Catalyzed Formation of Supramolecular Hydrogels as Promising Vaccine Adjuvants. Adv. Funct. Mater 2016, 26 (11), 1822-1829. (10) Borre, E.; Bellemin-Laponnaz, S.; Mauro, M., Amphiphilic Metallopolymers for Photoswitchable Supramolecular Hydrogels. Chemistry 2016, 22 (52), 18718-18721. (11) John, G.; Jadhav, S. R.; Menon, V. M.; John, V. T., Flexible optics: recent developments in molecular gels. Angew. Chem. 2012, 51 (8), 1760-2. (12) Amabilino, D. B.; Puigmartí-Luis, J., Gels as a soft matter route to conducting nanostructured organic and composite materials. Soft Matter 2010, 6 (8), 1605. (13) Brassinne, J.; Gohy, J.-F.; Fustin, C.-A., Orthogonal Control of the Dynamics of Supramolecular Gels


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The gel with superior lubricating properties save energy and keep the environment clean by reducing the problems of throwing oil.

64x53mm (300 x 300 DPI)


Self-Constraint Gel Lubricants with High Phase Transition Temperature

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