Mechanism of Superlubricity Conversion with Polyalkylene Glycol

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Mechanism of Superlubricity Conversion with Polyalkylene Glycol Aqueous Solutions Wenrui Liu, Hongdong Wang, Yuhong Liu, Jinjin Li, Ali Erdemir, and Jianbin Luo Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b01857 • Publication Date (Web): 21 Aug 2019 Downloaded from pubs.acs.org on August 23, 2019

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Mechanism of Superlubricity Conversion with Polyalkylene Glycol Aqueous Solutions Wenrui Liu, †, # Hongdong Wang, †, ‡, # Yuhong Liu, †, * Jinjin Li, † Ali Erdemir, ‡ Jianbin Luo † † State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China ‡ Energy Systems Division, Argonne National Laboratory, Argonne, Illinois 60439, United States # W.L. and H.W. contributed equally to this work

KEYWORDS: superlubricity, PAG aqueous solution, threshold concentration, hydrated concentration, adsorption layer

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ABSTRACT: In this study, ultralow friction coefficient (COF, μ < 0.01) was obtained through polyalkylene glycol (PAG) aqueous solutions with different molecular weights (MWs) ranging from 270 to 3930 g·mol-1 under ambient conditions. With increases in the MWs of PAG molecules, the threshold concentration to obtain this type of superlubric behavior gradually changed from 90 wt% to 60 wt%. This phenomenon was closely related to the interaction between PAG chains and water molecules, and the state of chemical binding. In the superlubricity system, the superior loadbearing capacity was achieved at optimal threshold concentrations of all PAG aqueous solutions wherein multilayered adsorption layers that consisted of fully hydrated PAG molecules were formed on the sliding solid surfaces. With respect to the concentration below the threshold value, the existence of a shearing layer was indicated to play a significant role. Thus, the synergetic effect of sufficient adsorption of molecules and unique shear rheology of PAG aqueous solution were essential to achieve superlubricity.

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INTRODUCTION Friction and wear are extremely common in our daily life and can lead to poor surface quality and significant losses of material and energy. Lubricants are widely used to avoid unnecessary friction and wear. However, lubricating properties were widely used in mechanical systems given in-depth research and further development of ultra-precision manufacturing technology, micro/nano processing technology, and nano science.1,2 Superlubricity is considered as an ideal method to solve these problems since it was first proposed by Hirano and Shinjo in 1990.3 It mainly attracted significant attention in the field of tribology because it involves negligible friction between two sliding solid surfaces and an ideal condition where wear is minimized.2,4 With respect to liquid superlubricity, several types of water-based lubricants (especially biolubricants) are proved as significantly effective in reducing coefficients of friction (COF) to near zero levels and especially when used on sliding ceramic surfaces with water,5,6 phosphoric acid solutions,7,8 polyethylene glycol aqueous solutions with boric acid additive,9 ethanediol with graphene oxide nanoflakes,10 polysaccharide solutions extracted from plants,11-13 and hyaluronic acid and its complexes14,15. Similarly, Klein’s group conducted extensive research on hydrated polymer brushes16-19 where they indicated that the fluidity of hydration layers was considered as a key reason for ultralow COF in biological systems. Specifically, polymers with charged, polar, dipolar, or zwitterionic groups are most ideal for hydration, and thereby for the formation of superlubricating boundary films.20-24 Polyalkylene glycols (PAGs) are generally used as synthetic lubricants in industry and are copolymers manufactured via a combination of ethylene oxide (EO) and propylene oxide (PO). Highly polar PAGs can adsorb on solid surfaces due to the presence of oxygen atoms. Based on these specific properties, PAGs are generally used as lubricants in all sorts of mechanical systems 3


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and also widely used for other purposes such as household cleaning.25 Previously, our group indicated the considerable superior tribological property of PAG aqueous solutions in terms of significantly decreasing friction and wear.26 Specifically, the results indicated that a stable COF as low as 0.002 in the full immersion state can be obtained over a wide range of concentrations. The molecular-level hydrated top layer and a suitable amount of free water molecules were considered as the main reason for the observed superlubricity. The molecular weight (MW) specifies the length of the polymer chains and is a crucial factor in lubrication by macromolecules. Long-tailed phospholipids as lubricant additives were reported as significantly better when compared to shorttailed ones.27 The results also revealed that higher MW directly improves the ability to form a thicker lubricating film to reduce the probability of asperity–asperity collisions and thereby related friction.25 However, the increase in MW leads to stronger interactions and larger entanglements between polymer chains, and this may fundamentally affect the superlubric behavior of PAGs. In the present study, we broaden the scope of PAGs superlubricity system with different MWs and explored the effect of MW at maximum aqueous concentrations to obtain superlubricity. The results demonstrated that achieving superlubricity was highly dependent on the concentration of PAGs (w(PAG)) with different MWs in aqueous solutions. The hydration effects of polymer chains with various lengths were considered as significantly related to the final loading capacity and COF. Furthermore, a systematic investigation of the relationship among contact pressure, theoretical film thickness, and viscosity indicated that two different states of liquid (adsorption and shearing layer) exist in the contact region. Finally, three different lubrication models for PAG aqueous solutions are proposed.

EXPERIMENTAL SECTION Materials. PAGs used in our tests were the commodities termed as UCONTM 50-HB, a type 4


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of organic lubricants available in the market that are produced by Dow Chemical Company with a high purity exceeding 99% (the chemical composition of PAGs is shown in Figure S1). Given different synthesis processes, PAGs include a family of products with different degrees of polymerization and molecular chain lengths. Seven types of 50-HB series products are selected to prepare aqueous solutions for experiments and include 50-HB-55, 50-HB-100, 50-HB-260, 50HB-660, 50-HB-2000, 50-HB-3520, and 50-HB-5100. The trademarks are related to MWs, and the viscosities of pure liquids are also shown in Table S1. Additionally, all PAGs in the 50-HB series can be completely dissolved in water at any concentration. Thus, the types of PAGs are mixed with deionized water in a range of concentrations from 10 wt% to 100 wt% and then placed into an ultrasonic bath for 30 min to obtain well-proportioned and stable sample solutions. The changes in viscosity with a variety of concentrations is shown in Figure S2. Tribological Evaluation. Tribological tests were performed on a Universal MicroTribotester (UMT-3) from Bruker (USA) under the reciprocating ball-on-disk mode. The COF was recorded every 0.04 s. A ball composed of silicon nitride (Si3N4) with a diameter of 4 mm and a sapphire disk composed of sapphire were selected as solid sliding surfaces. The roughness of the ball and disk did not exceed 10 nm. Prior to testing, ball and disk were cleaned in acetone, ethanol, and deionized water for 10 min in an ultrasonic bath and subsequently dried via compressed air. The liquid was introduced between the ball and disk in a full immersion state. Additionally, given PAG aqueous solutions with various concentrations and MWs, tribological tests were performed under a constant load of 3 N (initial Hertzian pressure corresponded to a maximum of 1.68 GPa), and a constant frequency of 4 Hz with a stroke of 3 mm. The normal force and friction force were measured via a 2-D sensor with a precision of 0.25 g. Furthermore, all experiments were performed at least twice to ensure repeatability with the temperature of approximately 25 °C and the relative 5


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humidity of approximately 20 – 40 %. Characterization of the Solid Surfaces. Following the tribological tests, the contact surfaces of ball and disk were cleaned by deionized water. The morphology of the wear scar on ball was observed through an optical microscope (Olympus BX60). Furthermore, X-ray photoelectron spectroscopy (XPS) was also performed to obtain the spectra of elements on the worn surface lubricated by PAG aqueous solutions. Analysis of PAG Aqueous Solutions. The shear viscosity of the solutions was measured via a standard cone-and-plate rheometer (MCR301, Anton Paar Physica) at 25 °C. Thermal gravity analysis (TGA) and differential thermal analysis (DTG) were performed by the TGA/DSC, STARe system (Mettler Toledo), which was measured under a nitrogen atmosphere with a temperature ramp rate of 10 °C min−1.

RESULTS AND DISCUSSION Here, three types of PAGs with different MWs were selected and their aqueous solutions were prepared to explore the tribological properties. The results of the tests are shown in Figure 1a and the images of the worn region are shown in Figure S3. It takes nearly 10000 s for 70 wt% HB-100 aqueous solution to exhibit a sharp decrease and enter the superlubricity regime while it takes approximately only 4000 s for a 70 wt% HB-260 aqueous solution to reach the same regime. However, with respect to the 70 wt% HB-5100 aqueous solution, the COF decreases from 0.1 and finally stabilizes at approximately 0.03. Although the concentration is controlled, the tribological processes of aqueous solutions of PAGs with different MWs are variant. The results suggest that in addition to the content of water molecules,26 MW is possibly another fundamental factor that dominates the lubrication behavior of PAGs. The concentration range of PAGs to achieve 6


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superlubricity is potentially related to their MWs.

Figure 1. (a) Tribological evaluation of three types of 70 wt% PAG aqueous solutions in the full immersion state. (b) Threshold concentration and corresponding COF of PAG aqueous solutions with different MWs.

With the aim to further confirm the assertion, several additional tribological tests are supplemented. Specifically, “threshold concentration” is defined as the maximum boundary w(PAG) to achieve superlubricity. Additionally, “threshold COF,” “threshold pressure,” “threshold viscosity,” and “threshold film thickness” are defined as COF, final contact pressure, viscosity, and film thickness at the threshold concentration. Figure 1b shows the threshold concentrations for each type of PAGs to reach the superlubricity regime. Evidently, the threshold concentration decreases from 90 wt% to 60 wt% when MW increases from 270 to 1590 g·mol-1 and then remain unchanged. This indicates that the concentration range of superlubricity is definitely related to MW of PAGs. Additionally, the threshold COF is stable at approximately 0.0025 when MW does not exceed 1590 g·mol-1. However, the average value of the threshold COF exhibits an upward trend with higher MW and finally approaches the 0.01 level. As shown in the results, variation trends of threshold concentration and threshold COF are clearly distinct in low MW regions (MW ≤1590 g·mol-1) and high MW regions (MW ＞1590 g·mol-1). Therefore, it is 7


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considered that there are significantly different mechanisms of superlubric behaviors at the threshold concentration in the two regions.

Figure 2. (a) Contact pressure, viscosity and (b) theoretical film thickness of PAG aqueous solutions at the threshold concentration.

In order to investigate the superlubric behavior of various PAG aqueous solutions (with different MWs) at threshold concentrations, we conducted more systematic studies on threshold pressure and threshold viscosity. The results of the tests are shown in Figure 2a. Generally, there is a sharp decrease in the contact pressure with evident chemical wear in the contact region to enter the superlubricity region (images of the worn region on Si3N4 ball are shown in Figure S4). Similarly, distinguishable variation trends are observed in low and high MW regions. The viscosity–MW curve exhibits a linear trend and values of threshold pressure remain stable at approximately 37 MPa in the low MW region while the viscosity and pressure rapidly increase with increases in MW in the high MW region. The viscosity-MW curve in the high MW region also significantly deviates from the originally linear trend, and this indicates that a few changes can occur in the contact area. Therefore, the results indicate that the states of the PAGs in the contact area are significantly similar in the respective MW region. In order to conduct further analysis, the theoretical threshold film thickness is calculated. 8


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Based on Hamrock–Dowson theory,32,33 theoretical threshold center film thickness is calculated as follows: ℎ𝑐 = 2.69

𝑈0.67𝐺0.53R 𝑊0.067

(1 ― 0.61𝑒 ―0.73𝑘)

(1)

where 𝑈 = 𝜂0𝑢/𝐸𝑅, 𝐺 = 𝛼𝐸, 𝑊 = 𝐹/𝐸𝑅2. Additionally, 𝜂0 denotes the shear viscosity of the fluid, 𝑢 denotes the average relative velocity of sliding solid surfaces, 𝛼 denotes the pressureviscosity coefficient of the lubricant, 𝐹 denotes the normal load, and k (≈1) denotes the ellipticity of the ball. Furthermore, 𝑅 denotes the radius of equivalent ball that is described by the Hertzian contact theory as given in eq (2) as follows: 𝑅=

𝐸𝐷3

(2)

6W

where 𝐷 denotes the diameter of the worn region of the ball. Additionally, 𝐸 denotes the effective modulus of elasticity of the friction pairs and is calculated as follows: 𝐸 = 1/[

1 ― 𝑣21 𝐸1

+

1 ― 𝑣22 𝐸2

(3)

]

where 𝜈𝑖 denotes the Poisson’s ratio for material 𝑖, and 𝐸𝑖 denotes the elasticity modulus of material 𝑖. The results are shown in Figure 2b. The results reveal that the theoretical threshold film thickness and MW initially increase simultaneously. As MW continues to increase and it enters the high MW region, the theoretical threshold film thickness significantly decreases. The XPS spectra and QCM-D method are utilized to successfully confirm the absorption of hydrated PAGs on the ceramic surfaces as shown in Figure S5-7. 28-31 Hence, it is assumed that the morphology and behaviors of the absorbed hydrated PAG layers can play a role in the lubrication. The significant decrease in theoretical film thickness is mainly attributed to entangled chains in adsorption layer, which cannot extend completely like the hydrated layer. In order to further 9


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confirm the suspect, HB-260 aqueous solutions over a wide range of concentrations are selected to perform tribological tests and the final contact pressure and theoretical film thickness are calculated as shown in Figure 3a. Remarkably, it is observed that a significant decrease in film thickness also occurs when the polymer concentration increases further to 80 wt%. It is demonstrated that the proportion of entanglements increases when the polymer concentration exceeds approximately 70 wt%, and a fundamental restraining factor is the lack of water molecules.26 Thus, the sharp decrease in theoretical film should be closely related to the lack of water molecules and entanglements of polymers. The result demonstrates that with the increases in MW, the change in the hydrated state at the threshold concentration is the reason for the unique tribological behaviors of PAGs. To distinguish between the lubrication state in different MW regions in depth, HB-5100 aqueous solutions over a wide range of concentrations are also selected to perform tribological tests when compared to HB-260 in the low MW region.

Figure 3. (a) Theoretical film thickness and contact pressure of the HB-260 aqueous solutions with a range of concentrations. (b) TGA profile of the 20 wt% HB-260 aqueous solution and its DTG profile.

With respect to HB-260 in the low MW region in Figure 3a, the highest value of film thickness is obtained at the threshold concentration. When the concentration decreases, the 10


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thickness of the theoretical film decreases. Additionally, the loading capacity also exhibits a significant decline. Specifically, thermal gravity analysis (TGA) of 20 wt% HB-260 aqueous solution is conducted from 30 to 500 °C to determine the state of water molecules. As shown in Figure 3b, a weight loss occurs prior to 100 °C based on differential thermal analysis (DTG), thereby indicating the existence of a large quantity of free water in the aqueous solution. A total weight loss of ca. 74.49% occurs at approximately 100 °C, and this indicates that remaining water molecules are tightly combined with PAG chains. The calculation reveals that the water content in the residue at this moment is approximately 73.44 wt% (the calculation process is shown in the Supporting Information). Thus, when w(PAG) is lower than 73.44 wt%, PAG HB-260 macromolecules are transformed into a completely hydrated state in the aqueous solution. The concentration is considered as the “hydrated concentration.” Significantly, the threshold concentration of HB-260 is found to be similar to the hydrated concentration, and the regulation is followed by other PAG macromolecules with low MW (shown in Figure S8). Thus, all PAGs with low MW are assumed to form intermolecular hydrogen bonds with water molecules below the threshold concentration, and a shearing layer is potentially formed in the contact region via free water molecules in the bulk solution. 33,34

Figure 4. (a) Theoretical film thickness and contact pressure of the HB-5100 aqueous solutions with a range of concentrations. (b) TGA profile of the 20 wt% HB-5100 aqueous solution and its 11


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DTG profile.

With respect to HB-5100 in high MW in Figure 4a, the thickness of the theoretical film is on an upward trend when the concentration decreases from 60 wt% to 40 wt%. The phenomenon significantly differs from the PAG in the low MW region. The results for TGA and DTG of 20 wt% HB-5100 are shown in Figure 4b. When compared to the result of HB-260, a more significant weight loss occurs at approximately 120 °C from the DTG curve and a total weight loss of ca. 51.02% occurs at approximately 100 °C from the TGA curve. The result suggests that a significantly higher number of water molecules are assumed as bound to polymer chains. The hydrated PAG macromolecules are expected to be the main molecular entities in the contact region as strong adsorption layers. Additionally, the hydrated concentration of HB-5100 is approximately 40.83 wt%, and this is evidently lower than the threshold concentration. There are more oxygen atoms on the longer polymer chains, and thus it is necessary for more water molecules to be bound to PAGs with higher MW to form a complete hydrogen bonding network. Thus, given the extremely high MW, HB-5100 is gradually transformed into a completely hydrated state when the concentration decreases from 60 wt% to 40 wt%. The film thickness increases when the polymer chains gradually swell during the period, and this is attributed to more binding sites on the long chains. Furthermore, based on the TGA profiles of other PAGs (shown in Figure S8), hydrated concentration of PAG macromolecules with high MW (HB-2000, HB-3520 and HB-5100) are slightly lower than the threshold concentration. Specifically, increases in the PAG molecular weight decrease the hydrated concentration. Thus, with respect to PAGs with high MW (for e.g., PAG HB-5100), the threshold concentration actually corresponds to a relatively wide range. Extremely strong intermolecular hydrogen bonds between PAG macromolecules and water molecules guarantee the achieved superlubricity. When the concentration decreases, the 12


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appearance of shearing layer results in a thinner lubrication film. With increases in the concentration, it is not possible to ignore interactions between polymer chains due to the shortage of water proportion, and this causes entanglements between chains in the adsorption layers. Based on the previous study on molecular dynamics simulation of relevant aqueous solution systems,35 the uniform distribution of atoms in the bulk solution is broken by solid surfaces in the contact region. A few lubricating molecules are adhered to two solid surfaces to form adsorption layers, and this significantly determines the load-bearing capacity and COF value. Simultaneously, a shearing layer is proven to exist in between, thereby maintaining the nature of lubricants similar to that in the bulk solution. Given the highly polar nature of PAG molecules, an adsorption layer can be favorably formed in the contact region during the sliding process. Water molecules bound with PAG chains are assumed to form a hydration layer covering the adsorption layer. Thus, in the low MW region, the ultralow COF as obtained by solutions at threshold concentration is attributed to the low shear strength of the hydration layer of extending polymer chains. Additionally, given that the theoretical film thickness significantly exceeds the length of PAG chains (shown in Figure S7), it is inferred that adsorption layer consists of multilayer PAG chains on each solid surface. In the high MW region, it is not possible for polymers to stretch completely, and thus the entanglement occurs commonly. However, high quantities of water molecules are easily bound to them given the existence of more binding sites on their long chains. This suggests that hydrated layers with excellent lubricating performance are established to further decrease shear strength, and this is the main reason for superlubricity obtained in the high MW region at the threshold concentration.36,37

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Figure 5. Schematic illustration of the proposed lubrication model between two sliding solid surfaces. Model for PAG aqueous solutions (a) below the threshold concentration; (b) at the threshold concentration; (c) beyond the threshold concentration.

Based on the aforementioned analysis, the ultralow COF obtained by PAG aqueous solutions is attributed to the formation of the adsorption layer and shearing layer in the contact region. Specifically, superlubricity obtained by the solutions at threshold concentration mainly depends on the adsorption layer. Therefore, three possible lubrication models (as illustrated in Figure 5) are proposed. First, when w(PAG) is below the threshold concentration, the adsorption layer and shearing layer are established in the contact region. The absorbed PAG molecules near the surface exhibit extremely strong interactions with the solid surface. The farther polymers exhibit weak adsorption and can be easily washed by the flowing fluid. Thus, the shearing layer is assumed to consist of a few hydrated PAG chains and free water molecules. The shear action occurs between the fixed hydration layer and shearing layer. Second, when w(PAG) is at threshold concentration, most water molecules are assumed as bound with PAG macromolecules, and the remaining free water molecules are insufficient to form an effective shearing layer. Multilayered adsorption layers are the main structure in the contact region. Low shear strength of hydration layer constitutes the key factor to obtain ultralow COF. Finally, large entanglements occur in the bulk solution when 14


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w(PAG) exceeds the threshold concentration. Strong interaction occurs between the polymer chains in contact region, and thus high shear strength prevents COF from decreasing further.

Figure 6. Variation in theoretical film thickness and contact pressure relative to the viscosity by PAG aqueous solutions that can achieve superlubricity. Theoretical film thickness and final contact pressure of PAG aqueous solutions at threshold concentration are highlighted in red. Solutions with different MWs or concentrations are all classified by the viscosity.

Generally, the final contact pressure and theoretical film thickness at the threshold concentration are closely related to the viscosity of PAG aqueous solution as shown in Figure 6. Specifically, their viscosity is directly affected by the weight fraction and MW of PAGs. PAG aqueous solutions with high MW or at high concentration exhibit high viscosity. However, it should be noted that a linear growth relationship is absent between the film thickness and viscosity of the lubricant. Remarkably, the theoretical film thickness of solution at the threshold concentration is considerably lower than that at other concentrations while their final contact pressure is significantly higher. Given that most water molecules are bound with PAG molecules to from hydrogen bonds, the absorbed hydrated polymers are assumed as the main structure in the 15


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contact region. This unique feature is attributed to the molecular level structure, and this is expected to sustain more contact pressure.

CONCLUSIONS In summary, superlubricity is the phenomenon that approaches the ideal state of lubrication, which is also significantly sensitive. Thus, it is meaningful to obtain stable superlubricity lubricated by a PAG aqueous solution under an ambient condition. In the study, ultralow COF is obtained by aqueous solutions of PAGs with a wide range of MWs, and this suggests that superlubricity is the basic property of hydrated PAGs. However, the concentrations to achieve superlubricity are distinct for PAGs with different MWs. The threshold concentration decreases from 90 wt% to 60 wt% when MW increases from 270 to 3930 g·mol-1. As confirmed by TGA, the phenomenon is highly dependent on the binding state between the structure of polymer chains and water molecules. Furthermore, we consider the threshold film thickness and pressure and elements in the worn region, and multilayered adsorption layers are considered as the main structure in the contact region. The layers consist of hydrated PAG chains and are considered as the primary reason to maintain ultralow COF and superior load-bearing capacity. Subsequently, investigations on PAG aqueous solutions at a range of concentration gradients reveal that a shearing layer exists and plays a vital role when the concentration of solutions is below the threshold concentration. Therefore, two different states of liquids (molecular adsorption and shearing layer) are demonstrated to act synergistically to obtain superlubricity with PAG aqueous solution systems. Furthermore, the wide application of the superlubricity system can exhibit significant potential value in industrial applications.

ASSOCIATED CONTENT 16


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Supporting Information Chemical composition of PAGs in 50-HB series, basic properties of PAGs and their aqueous solutions in 50-HB Series, images of the worn region on Si3N4 ball lubricated with PAG aqueous solutions, XPS spectra of the elements in the worn region on Si3N4 ball after lubrication, QCM-D analysis of the PAG absorbed surface layers on the solid surfaces and the TGA and DTG profiles of PAG aqueous solutions.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] Authors Contributions # W.L. and H.W. contributed equally. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant No. 51875303, Grant No. 51527901) and China Postdoctoral Science Foundation (2019M650654). Hongdong Wang wishes to acknowledge Chinese Scholarship Council (CSC) and the China Postdoc Innovation Talent Support Program (BX20180168) for the support.

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Seror, J.; Merkher, Y.; Kampf, N.; Collinson, L.; Day, A. J.; Maroudas, A.; Klein, J. Articular Cartilage Proteoglycans as Boundary Lubricants: Structure and Frictional Interaction of Surface-Attached Hyaluronan and Hyaluronan-Aggrecan Complexes. Biomacromolecules 2011, 12 (10), 3432–3443.

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Schematic illustration of superlubricity obtained by polyalkylene glycol aqueous solutions (for Table of Contents use only)

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Figure 1. (a) The tribological evaluation of three kinds of 70 wt% PAG aqueous solutions in full immersion state. (b) The threshold concentration and corresponding COF of PAG aqueous solutions with different MWs. 538x190mm (150 x 150 DPI)


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Figure 2. (a) The contact pressure, viscosity and (b) the theoretical film thickness of PAG aqueous solutions at threshold concentration. 640x235mm (150 x 150 DPI)


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Figure 3. (a) The theoretical film thickness and contact pressure of the HB-260 aqueous solutions with a range of concentrations. (b) The TGA profile of the 20 wt% HB-260 aqueous solution and its DTG profile. 538x190mm (150 x 150 DPI)


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Figure 4. (a) The theoretical film thickness and contact pressure of the HB-5100 aqueous solutions with a range of concentrations. (b) The TGA profile of the 20 wt% HB-5100 aqueous solution and its DTG profile. 538x190mm (150 x 150 DPI)


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Figure 5. Schematic illustration of the proposed lubrication model between two sliding solid surfaces. Model for PAG aqueous solutions (a) below threshold concentration; (b) at threshold concentration; (c) beyond threshold concentration. 835x268mm (150 x 150 DPI)


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Figure 6. Variation in theoretical film thickness and contact pressure relative to the viscosity by PAG aqueous solutions that can achieve superlubricity. Theoretical film thickness and final contact pressure of PAG aqueous solutions at threshold concentration are highlighted in red. Solutions with different MWs or concentrations are all classified by the viscosity. 272x208mm (300 x 300 DPI)


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Schematic illustration of superlubricity achieved by polyalkylene glycol aqueous solutions (for Table of Contents use only) 848x475mm (150 x 150 DPI)


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Figure S1. Chemical composition of PAGs in 50-HB series 296x209mm (300 x 300 DPI)


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Figure S2. The rheological behavior of different PAGs and their aqueous solutions with concentrations from 10 to 100 wt%. 296x209mm (300 x 300 DPI)


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Figure S3. The wear scar on the top of Si3N4 ball lubricated by (a) 70% HB-100 aqueous solution, (b) 70 wt% 50-HB-260 aqueous solution and (c) 70 wt% 50-HB-5100 aqueous solution. 388x129mm (150 x 150 DPI)


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Figure S4. The wear scar on the top of Si3N4 ball lubricated by PAG aqueous solutions under threshold concentration. (a) 80 wt% 50-HB-100 aqueous solution; (b) 70 wt% 50-HB-260 aqueous solution; (c) 60 wt% 50-HB-660 aqueous solution; (d) 60 wt% 50-HB-2000 aqueous solution; (e) 60 wt% 50-HB-3520 aqueous solution; (f) 60 wt% 50-HB-5100 aqueous solution. 239x166mm (96 x 96 DPI)


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Figure S5. XPS spectra of (a) C 1s and (b) Si 2p in the worn region on Si3N4 ball after lubrication with PAG aqueous solutions. 457x171mm (150 x 150 DPI)


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Figure S6. QCM-D test results of the PAG aqueous solution on the (a) Al2O3-coated quartz crystal surface and (b) Si3N4-coated quartz crystal surface. 523x190mm (150 x 150 DPI)


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Figure S7 The fitted thickness of the remaining adsorption layer on the Al2O3-coated quartz crystal surface and Si3N4-coated quartz crystal surface after being rinsed by the DI water. 272x208mm (300 x 300 DPI)


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Figure S8. TGA of 20 wt% (a) HB-55, (b) HB-100, (c) HB-660, (d) HB-2000, and (e) HB-3520 aqueous solutions and their DTG profiles. (f) Hydrated concentration of various PAGs. 808x396mm (150 x 150 DPI)


Mechanism of Superlubricity Conversion with Polyalkylene Glycol

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