Graphene enhanced and in situ-formed alginate hydrogels for reducing friction and wear of polymers

Graphene enhanced and in situ-formed alginate hydrogels for reducing friction and wear of polymers

Journal Pre-proof Graphene enhanced and in situ-formed alginate hydrogels for reducing friction and wear of polymers Liangfei Wu (Conceptualization) (...

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Journal Pre-proof Graphene enhanced and in situ-formed alginate hydrogels for reducing friction and wear of polymers Liangfei Wu (Conceptualization) (Writing - original draft), Zhaozhu Zhang (Supervision), Mingming Yang (Funding acquisition) (Writing - review and editing), Junya Yuan (Writing - review and editing), Peilong Li (Writing - review and editing), Xuehu Men (Funding acquisition)

PII:

S0927-7757(20)30026-1

DOI:

https://doi.org/10.1016/j.colsurfa.2020.124434

Reference:

COLSUA 124434

To appear in:

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Received Date:

27 October 2019

Revised Date:

24 December 2019

Accepted Date:

6 January 2020

Please cite this article as: Wu L, Zhang Z, Yang M, Yuan J, Li P, Men X, Graphene enhanced and in situ-formed alginate hydrogels for reducing friction and wear of polymers, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2020), doi: https://doi.org/10.1016/j.colsurfa.2020.124434

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Graphene enhanced and in situ-formed alginate hydrogels for reducing friction and wear of polymers

Liangfei Wua,b, Zhaozhu Zhanga,b,, Mingming Yanga,*, Junya Yuana, Peilong Lia,b, Xuehu Menc

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics,

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a

Chinese Academy of Sciences, Lanzhou 730000, PR China

Center of Materials Science and Optoelectronics Engineering, University of Chinese

Academy of Sciences, Beijing 100049, PR China

School of Physical Science and Technology, Lanzhou University, Lanzhou, 730000, P.

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R. China

Corresponding author. Fax: 86-931-4968098.

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E-mail: [email protected] (Z.Z. Zhang); [email protected] (M.M. Yang)

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Graphical Abstract

Abstract Alginates are natural unbranched polysaccharide and have been extensively

investigated because of their biocompatibility and superior gel-forming properties. However, the tribology functions of alginate hydrogels still remain largely unexplored. Here, we develop a facile method for reducing friction and wear between polymer-metal friction pairs. The using of gluconic acid-δ-lactone (GDL) is important to regulate gelation time and induce complete, uniform gelation of alginates by in situ release Ca2+. After the addition of graphene, the friction coefficient and wear volume

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decline with ratios of 37 % and 50 %, respectively. The effective isolation of friction pairs and the release of water molecule owning to the self-decomposition of alginate hydrogels together contribute to reduce friction and wear.

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Keywords: alginate hydrogels, antiwear, polymer, grapheme

1. Introduction

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In modern industrial production, friction and wear are the primary causes of

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materials loss and power expenditure [1]. It is reported that nearly one-third global energy are estimated to be dissipated owning to friction and wear [2]. In order to

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acquire higher service lives and energy efficiencies of mechanical components, using lubricants is an alternative solution to minimize wear and friction between two

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relatively moving surfaces. Water as one kind of common lubricants is a recoverable natural resource and has the further advantages of environment friendliness, low cost and high cooling capacity. Conversely, the lubrication performance of water is much weaker than that of base oil due to its lower viscosity and surface tension, which adversely impact the development of hydrodynamic film effects even at low speed [3,

4]. As for base oils, most of them are derived from petroleum oil resources. With the concern of resources shortage and environment issues caused by the massive use of these oil based lubricants, some promising alternatives for petroleum oils was put forward, such as biodegradable and renewable vegetable oil [5]. However, the load-carrying capacity of these biological oils was poor. Though different chemical additives could be blended into the oil to optimize the lubricating performance, they

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usually need to be further modified to achieve better dispersion properties [6]. Therefore, it is an urgent demand for new method to reduce friction and wear with simple operation procedures.

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Recently, polysaccharide-based hydrogels, such as chitosan and extracellular

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polysaccharide salecan, have been extensively investigated in the fields of adsorbents, tumor therapy and wastewater treatments [7-10]. Alginates are natural unbranched

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polysaccharide composed of liner block copolymers of 1-4 linked β-D-mannuronic

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acid (M) and α-L-guluronic acid (G) [11-13]. They are typically extracted from marine brown algae and soil bacteria. Alginates form gel matrix in the presence of

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divalent (Ca2+, Ba2+, Cu2+, etc.) or trivalent (Fe3+, Al3+, etc.) metal cations, generating three dimensional networks, popularly described by the “egg-box” model [14-16].

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Owning to their relatively low cost and biodegradability, biocompatibility, as well as superior gel-forming properties, alginates have been extensively investigated in various fields, including environmental engineering and biomedical engineering [17-19]. In the tribology field, Tian et al. prepared calcium alginate capsules containing tung oil and incorporated the nanocapsules into a polyester resin-based

powder to analyzed corrosion-resistance and friction-wear mechanisms of the self-healing coatings [20]. So far, the tribology functions of alginate hydrogels still remain largely unexplored. On the one hand, the gel formed by adding Ca2+ into sodium alginate solution has poor uniformity and stability, which is ascribed to the boundary of alginates rapidly and tightly bound when contact with crosslink solution [21, 22]. On the other hand, alginate gelation triggered by the addition of cations is so

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quick that the hydrogels can not be smeared onto the friction interfaces freely and evenly, thus the friction-reduce effect is limited. Based on these considerations, the in situ release Ca2+ and utilizing functional additives could be feasible approaches to

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make alginate hydrogels satisfy tribology requirements.

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Herein, we use GDL as an acidifier and calcium carbonate (CaCO3) as calcium ion substitute to induce complete and uniform gelation of alginates. The gelation time

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could be controlled by changing the content of GDL in the mixed solution of sodium

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alginate and CaCO3. Moreover, graphene is also incorporated into the solution mentioned above to enhance the mechanical strength of the obtained hydrogels. The

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tribological tests indicate that the hybrid hydrogels are ideal lubricants for polymer-metal friction pairs. It is expected that this work maybe helpful to the

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tribology applications of alginate hydrogels. 2. Experimental section 2.1. Materials. GDL were purchased from Meryer Chemical Tech. Co. Ltd. Graphene was supplied by Nanjing XFNANO Materials Tech. Co. Ltd. Polyimide (YS-20) powders

were used with particles less than 75 μm .CaCO3 and sodium alginate were both of analytical grade. Prior to experiments, CaCO3 powders were treated with ball-milling machine to get better dispersion in deionized water. Sodium alginate and calcium chloride (CaCl2) was used as received. 2.2. Preparation of alginate hydrogels. Sodium alginate was dissolved in deionized water to achieve a concentration of

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1 % (w/v). Then 0.1 g CaCO3 powders were dispersed in 20 ml aqueous alginate (CAA) by intense ultrasonic treatment for 30 min, followed by the addition of GDL.

The molar ratios of GDL: CaCO3 were 0.5:1, 1:1, 2:1, 4:1 and 6:1, respectively. The

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hybrid alginate hydrogels were created with different contents of graphene, namely

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0.1 mg/ml (G1), 0.2 mg/ml (G2), 0.3 mg/ml (G3), 0.4 mg/ml (G4) while keep the molar ratio of GDL: CaCO3 constant at 6:1.

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2.3. Preparation of polymer plate.

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4 g polyimide powders were compressed and heated to 350 ℃ in a mold for 1.2 h. The pressure was held at 10 MPa and the samples had a dimension of 2 × 40 ×

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35 mm3.

2.4. Tribological tests.

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Tribological tests were conducted using a ball-on-disc test mode and the alginate

hydrogels were introduced into friction interfaces before tests. Polyimide plates rubbed at room temperature with GCr15 bearing steel ball with diameter of 6 mm. The applied load was 15 N and liner sliding speed was 0.157 m/s. Each test lasted for 0.5 h and three repeat times were performed to evaluate the average wear volume and

friction coefficient. 2.5. Characterization. The morphologies of freeze-dried alginate hydrogels and worn surfaces were observed by the Scanning Electron Microscopy (SEM, JEOL JSM-5600LV). The wear volume was measured using an interferometric noncontact surface profilometer (MicroXAM-3D). A DHR-2 stress-controlled rheometer (TA Instruments) was applied

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to perform dynamic rheology experiments. Transmission electron microscopy (TEM)

measurements were carried out with a TecnaiTF20 (FEI). Raman spectroscopy (LabRAM HR Evolution) was utilized to inspect carbon-based materials on the

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friction pairs.

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3. Results and discussion 3.1. Characterizations of Alginate Hydrogels.

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The general method for preparing alginate hydrogels was shown in Fig. 1.

Fig. 1. (a) Schematic illustration of the formation of alginate hydrogels; (b) Snapshots of the gelation process. GDL was an appropriate dissociating agent for CaCO3, which could hydrolyze into

gluconic acid and release protons [23]. Thus the G fragments in the alginate chains would chelate with Ca2+, forming physically crosslinked alginate hydrogels. This approach was analogous to photogelation scheme reported by Raghavan et al [23]. The difference there was that acid formation was triggered by UV irradiation via photoacid generator. Aside from the in situ release Ca2+, the gelation time could be also regulated by using GDL. In this paper, the gelation time was determined to be at

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the point where the alginate did not flow upon lean. Fig. 2 displayed the gelation time

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with molar ratios of GDL: CaCO3 varied from 0.5:1 to 6:1.

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Fig. 2. The gelation time with different molar ratios of GDL: CaCO3.

It was clearly seen that the gelation time reduced with the loading of GDL. When the

molar ratio reached 6:1, the gelation occurred within 2 min. The gelation time played a significant role in following friction tests. If the gelation time was long enough, aqueous alginates failed to transform themselves into hydrogels throughout the whole friction tests. With the aim to make alginate function in the form of hydrogels, the

time spent for gelation should be as short as possible. However, alginate hydrogels could not be smeared onto the friction interfaces freely and uniformly if aqueous alginates gelled immediately. The perfect situation was aqueous alginates containing GDL can keep their fluidity for a while before friction tests and then they transformed into elastic gelatin in the early time of friction tests. Based on the analysis above, gelation time of 2 min was optimal. Instantaneously gelling alginate was also

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prepared by either adding dilute hydrochloric acid into CAA/ Sodium alginate

solution or by addition of aqueous alginate to CaCl2 solution. As shown in Fig. 3f, the instantaneous crosslinked gels could not maintain their geometries and wept when the

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bottle was flattened. It was because that the bound arose from rapid and tight

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crosslink destroyed the integrity of alginate hydrogels. Some alginate beads also produced due to the diffusion-limited of gelation. The structural stability indicated

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GDL was in favor of a more uniform and complete crosslink. Fig. 3a-b illustrated

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TEM graphs and Raman spectroscopy of graphene. It was evident that graphene exhibited a typically thin and sheet-like structure. The G band represented the E2g

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vibrational mode of sp2 carbon atoms while the D band came from lattice distortions and structural defects, respectively [24, 25]. The digital images of the cylindrical

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alginate hydrogels were shown in Fig. 3c. The color of the hydrogels changed from light yellow to black due to the introduction of graphene. The microstructures of freeze-dried hydrogels with and without the addition of graphene were observed using SEM. As shown in Fig. 3d, the neat alginate hydrogels displayed interconnected hierarchical porous network architechture with pore sizes ranging from several

micrometers to several hundred micrometers. After the incorporation of graphene, the network structures were rougher than that for neat alginate hydrogels. The smooth surfaces of cells disappeared and more coarse strands emerged in the pores. Graphene, a two-dimensional, one-atomic thick carbon material, was famous for its exceptional physical and mechanical properties as well as large surface area [26-28]. It was presumed that the interaction between alginate chains and Ca2+ would be blocked by

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graphene. This confined effect promoted chelation between Ca2+ and G sequences of alginates to be more discontinuous and random, producing more stands with local

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connectivity.

Fig. 3. (a) TEM images and (b) Raman spectroscopy of graphene; (c) The digital

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images of the cylindrical alginate hydrogels; SEM images of freeze-dried alginate

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hydrogels (d) without and (e) with graphene; (f) The digital images of alginate hydrogels prepared with different methods.

Compression tests were conducted to evaluate the mechanical properties of these alginate hydrogels. Fig. 4a showed the stress-strain curves of alginate hydrogels at maximum strain of 40%. The stress at 40% for the neat alginate hydrogels was about 9.9 kPa while it was 11.6 kPa for G3. Though graphene weakened the network

structure of alginate hydrogels, the compressive stress still increased which might owning to the excellent mechanical performances of graphene. Besides, graphene could reduce the electrical resistance of alginate hydrogels, as evidenced by the brightness of the LED bulbs (Fig. 4b). The dynamic viscoelastic properties of the alginate hydrogels were characterized by determining the frequency dependence of the shear viscous modulus (G’’) and shear elastic modulus (G’). In Fig. 4c-d, G’

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dominated the viscoelastic response over the entire frequency range and G’, G’’ were both nearly independent of frequency, indicating that elasticity played a greater role in

the contribution to viscoelasticity compared with viscosity [29, 30]. In Fig. 4e, the

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mechanical damping tan δ (tan 𝛿 = 𝐺 ′′ /𝐺′) of G3 was higher than that of neat

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alginate hydrogels. This phenomenon implied more energy went to viscosity flow in the tests of G3. Note the difference in the corresponding microstructure, it might

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because graphene inhibited Ca2+ from combing with alginate chains to some extent

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[17].

Fig. 4. (a) Compressive strain-stress curves of alginate hydrogels; (b) Conductive alginate hydrogels lighting up a LED bulb; Viscoelastic behaviors of alginate hydrogels (c) with and (d) without graphene; (e) Mechanical damping as a function of

frequency of alginate hydrogels. 3.2. The Tribological Properties of Alginate Hydrogels. Fig. 5a-b presented the influence of GDL and graphene concentrations on the friction coefficient. It could be seen that friction coefficient decreased with the increase of GDL. When the molar ratio of GDL: CaCO3 was 6:1, all the friction coefficient curves presented the same features. Namely, the friction coefficient

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reached the highest value at around 3 min first and then stared to decline. We speculated that the turning point corresponded to the time when the gel with sufficient

mechanical strength formed. Meanwhile, the friction coefficient curve tended to be

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more smooth and stable after the addition of graphene. The friction coefficient for G1

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was 0.106, which decreased by 37% compared to that of deionized water (0.169). Moreover, the mean wear volume (Fig. 5c) for G3 decreased by 50% compared to that

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of deionized water. This could be partly attributed to the outstanding self-lubricating

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and anti-wear performances of graphene.

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Fig. 5. (a) Friction coefficient curves; (b) Average friction coefficient and (c) wear volume.

The worn surface morphologies of polymers were investigated by SEM in Fig. 6. In the case of deionized water, the worn surfaces were smooth but distinct ploughed marks were observed, indicating the development of hydrodynamic film failed and the

main wear manner was severe two-body wear. In the case of CAA, the plowing grooves were more obvious and abrasion of polymer plate was significantly aggravated. It was inferred that CaCO3 was an improper lubricant additive and the dominant wear mechanism was third-body wear [31]. Interestingly, the worn surfaces in the cases of alginate hydrogels and G3 were rough but the plow effect was mitigated. According to the interpretation of Xu et al. [3], a hard steel ball would ‘cut’

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through the asperities on the soft polymer surface, resulting in polishing effect. In this work, the alginate in situ gelled could prevent the direct contact between polymer

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rougher worn tracks and smaller wear volumes.

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plate and steel counterpart, thus more asperities could be preserved, leading to

Fig. 6. SEM images of worn surfaces with (a) water, (b) CAA, (c) alginate hydrogels and (d) hybrid alginate hydrogels as the lubricants.

Raman spectra was a powerful tool for inspecting carbon-based materials on the contact surfaces [32-34]. However, as depicted in Fig. 7a-b, the typical G and D peaks

of graphene could neither be identified on the wear tracks, nor were detected on the counterpart balls where alginate hydrogels or G3 were used as lubricant. It was speculated that most of graphene were entrapped in alginate networks and almost no free graphene could deposit on the rubbing surfaces. The surface morphologies of counterpart balls under different lubricants condition were shown in Fig. 8. The counterpart morphology lubricated with alginate hydrogels was smooth while it was

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rougher for G3, indicating that graphene exerted an influence on the transfer of alginate hydrogels. It was observed from EDS analyses that the content of C, O and Ca elements increased slightly in the case of G3, suggesting that graphene could

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promote gels matrix transfer onto the counterpart ball. Thus the direct contact

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between hard steel ball and polymer plate could be further reduced.

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Fig. 7. Raman spectra of (a) wear tracks and (b) counterpart balls under different lubricant condition.

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Fig. 8. SEM images and EDS analyses of counterpart balls lubricated with (a-c) alginate hydrogels and (d-f) G3.

Alginate hydrogels obtained by ionic crosslinking usually exhibited limited

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long-term stability, because Ca2+ would exchange with monovalent ions

(predominantly Na+) causing the destabilization and rupture of three-dimensional

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network [11, 35, 36]. This decomposition process would release water molecule

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within hydrogels gradually. In order to intuitively confirm the release of water molecule, we put alginate hydrogels on the glasses and picked it up in a moment.

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There was no drip left behind on the glasses. If the cylindrical alginate hydrogels were kept undisturbed for 2 h, some water dyed with methylene blue would emerge around

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the bottom of the hydrogels. Some liquid were also found on the glasses even though

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the hydrogels were taken away (Fig. 9a). Besides, it was revealed that repeated pressing could accelerate the release of water molecule. In Fig. 9b, alternating compression and release were performed on the hydrogels for 200 cycles (the force was about 0.2 N). The obtained phenomenon was similar to that observed previously.

Fig. 9. Water release of alginate hydrogels by (a) keeping them undisturbed, (b) alternating compression and release. Spontaneously, it was possible that alginate hydrogels were enwrapped by water

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molecule gradually because of the alternating stress during the friction process. From what has been discussed above, a lubrication mechanism scheme for alginate

hydrogels could be illustrated in Fig. 10. Alginates gelled on the interfaces between

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polymer plate and steel counterpart, thus plow effect was suppressed due to the

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effective isolation of friction pairs. The introduction of graphene enhanced the mechanical performances of alginate hydrogels and endowed them with better bearing

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capacity and wear resistance. The release of water molecule within the hydrogels

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network and self-lubricating properties of graphene together reduced the friction. Therefore, graphene reinforced alginate hydrogels exhibited superior anti-wear and

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lubricity properties.

Fig. 10. Schematic diagram of the lubrication mechanism.

4. Conclusions In summary, we used GDL as an acidifier to in situ release Ca2+ for inducing complete and uniform gelation of alginates. The gelation time could be regulated by changing the content of GDL. After the incorporation of graphene, the compression property and electrical conductivity were enhanced. The tribological experiments indicated that graphene reinforced alginate hydrogels were extraordinary lubricants

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for polymer-metal friction pairs. The predominant reasons for the results were the effective isolation of polymer plate and steel counterpart by utilizing the hydrogels as

buffer layers. The release of water molecule due to the self-decomposition of alginate

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hydrogels also contributed to decrease friction. It is expected that this work may pave

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Author Statement

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a route for applying alginate hydrogels in tribology.

Liangfei Wu: Conceptualization, Writing - Original Draft

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Zhaozhu Zhang: Supervision

Mingming Yang: Funding acquisition, Writing - Review & Editing

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Junya Yuan: Writing - Review & Editing Peilong Li: Writing - Review & Editing Xuehu Men: Funding acquisition

Conflict of Interest

We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

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Acknowledgments

The authors gratefully acknowledge the financial support from the National

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Nature Science Foundation of China (Grant Nos. 51805516 and 51675252).

References

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1. Wu P, Chen X, Zhang C, Luo J, Synergistic tribological behaviors of graphene

177-184.

na

oxide and nanodiamond as lubricating additives in water, Tribol. Int. 132 (2019)

ur

2. Ye M, Cai T, Shang W, Zhao L, Zhang Y, Liu D, Liu S, Friction-induced transfer of carbon quantum dots on the interface: Microscopic and spectroscopic studies on the

Jo

role of inorganic–organic hybrid nanoparticles as multifunctional additive for enhanced lubrication, Tribol. Int. 127 (2018) 557-567. 3. Xu Y, Liu Z, Dearn K. D, Dong Y, You T, Hu X, Thermo-tribological behaviour of microgels for improved aqueous lubrication for steel/UHMWPE contact, Tribol. Int. 130 (2019) 63-73.

4. Fan M, Du X, Ma L, Wen P, Zhang S, Dong R, Sun W, Yang D, Zhou F, Liu W, In situ preparation of multifunctional additives in water, Tribol. Int. 130 (2019) 317-323. 5. Zhang G, Xu Y, Xiang X, Zheng G, Zeng X, Li Z, Ren T, Zhang Y, Tribological performances of highly dispersed graphene oxide derivatives in vegetable oil, Tribol. Int. 126 (2018) 39-48.

[email protected] microspheres, Tribol. Int. 129 (2019) 427-435.

ro of

6. Huang J, Li Y, Jia X, Song H, Preparation and tribological properties of core-shell

7. Sun J, Guo Y, Xing R, Jiao T, Zou Q, Yan X, Synergistic in vivo photodynamic and

photothermal antitumor therapy based on collagen-gold hybrid hydrogels with

-p

inclusion of photosensitive drugs, Colloids Surf. A 514 (2017) 155-160.

re

8. Qi X, Liu R, Chen M, Li Z, Qin T, Qian Y, Zhao S, Liu M, Zeng Q, Shen J, Removal of copper ions from water using polysaccharide-constructed hydrogels,

lP

Carbohydr. Polym. 209 (2019) 101-110.

na

9. Mei E, Li S, Song J, Xing R, Li Z, Yan X, Self-assembling collagen/alginate hybrid hydrogels for combinatorial photothermal and immuno tumor therapy, Colloids Surf.

ur

A 577 (2019) 570-575.

10. Jiao T, Zhao H, Zhou J, Zhang Q, Luo X, Hu J, Peng Q, Yan X, Self-assembly

Jo

reduced graphene oxide nanosheet hydrogel fabrication by anchorage of chitosan/silver and its potential efficient application toward dye degradation for wastewater treatments, ACS Sustainable Chem. Eng. 3 (2015) 3130-3139. 11. LeRoux M. A, Guilak F, Setton L. A, Compressive and shear properties of alginate gel: Effects of sodium ions and alginate concentration, J. Bio. Mater. Res. 47 (1999)

46-53. 12. Tong D, Fang K, Yang H, Wang J, Zhou C, Yu W, Efficient removal of copper ions using a hydrogel bead triggered by the cationic hectorite clay and anionic sodium alginate, Environ. Sci. Pollut. Res. 26 (2019) 16482-16492. 13. Camacho D. H, Uy S. J. Y, Cabrera M. J. F, Lobregas M. O. S, Fajardo T. J. M. C, Encapsulation of folic acid in copper-alginate hydrogels and it's slow in vitro release

ro of

in physiological pH condition, Food Res. Int. 119 (2019) 15-22.

14. Bajpai S. K, Sharma S, Investigation of swelling/degradation behaviour of

alginate beads crosslinked with Ca2+ and Ba2+ ions, React. Funct. Polym. 59 (2004)

-p

129-140.

re

15. Ochbaum G, Davidovich-Pinhas M, Bitton R, Tuning the mechanical properties of alginate–peptide hydrogels, Soft Matter. 14 (2018) 4364-4373.

lP

16. Zhao X, Xia Y, Zhang X, Lin X, Wang L, Design of mechanically strong and

na

tough alginate hydrogels based on a soft-brittle transition, Int. J. Biol. Macromol. 139 (2019) 850-857.

ur

17. Song Y, Yang L.-Y, Wang Y.-g, Yu D, Shen J, Ouyang X.-k, Highly efficient adsorption

of

Pb(II)

from

aqueous

solution

using

amino-functionalized

Jo

SBA-15/calcium alginate microspheres as adsorbent, Int. J. Biol. Macromol. 125 (2019) 808-819. 18. Xing L, Sun J, Tan H, Yuan G, Li J, Jia Y, Xiong D, Chen G, Lai J, Ling Z, Chen Y, Niu X, Covalently polysaccharide-based alginate/chitosan hydrogel embedded alginate microspheres for BSA encapsulation and soft tissue engineering, Int. J. Biol.

Macromol. 127 (2019) 340-348. 19. Reakasame S, Boccaccini A. R, Oxidized Alginate-Based Hydrogels for Tissue Engineering Applications: A Review, Biomacromolecules 19 (2018) 3-21. 20. Sun J, Wang Y, Li N, Tian L, Tribological and anticorrosion behavior of self-healing coating containing nanocapsules, Tribol. Int. 136 (2019) 332-341. 21. Zheng J, Zeng R, Zhang F, Kan J, Effects of sodium carboxymethyl cellulose on

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rheological properties and gelation behaviors of sodium alginate induced by calcium ions, LWT 103 (2019) 131-138.

22. Ribeiro H, Trigueiro J. P. C, Silva W. M, Woellner C. F, Owuor P. S, Cristian

-p

Chipara A, Lopes M. C, Tiwary C. S, Pedrotti J. J, Villegas Salvatierra R., Tour J. M,

re

Chopra N, Odeh I. N, Silva G. G, Ajayan P. M, Hybrid MoS2/h-BN Nanofillers As Synergic Heat Dissipation and Reinforcement Additives in Epoxy Nanocomposites,

lP

ACS Appl. Mater. Interfaces 11 (2019) 24485-24492.

na

23. Growney Kalaf E. A, Flores R, Bledsoe J. G, Sell S. A, Characterization of slow-gelling alginate hydrogels for intervertebral disc tissue-engineering applications,

ur

Mater. Sci. Eng. C 63 (2016) 198-210. 24. Baek S, Kim J, Kim H, Park S, Ban H. W, Gu D. H, Jeong H, Kim F, Lee J, Jung

Jo

B. M, Choa Y.-H, Kim K. H, Son J. S, Controlled Grafting of Colloidal Nanoparticles on Graphene through Tailored Electrostatic Interaction, ACS Appl. Mater. Interfaces 11 (2019) 11824-11833. 25. Zhang X, Zhao J, He X, Li Q, Ao C, Xia T, Zhang W, Lu C, Deng Y, Mechanically robust and highly compressible electrochemical supercapacitors from nitrogen-doped

carbon aerogels, Carbon 127 (2018) 236-244. 26. Shi R., Zhao J, Liu S, Sun W, Li H, Hao P, Li Z, Ren J, Nitrogen-doped graphene supported copper catalysts for methanol oxidative carbonylation: Enhancement of catalytic activity and stability by nitrogen species, Carbon 130 (2018) 185-195. 27. Wang Z, Shen X, Akbari Garakani M, Lin X, Wu Y, Liu X, Sun X, Kim J.-K, Graphene Aerogel/Epoxy Composites with Exceptional Anisotropic Structure and

ro of

Properties, ACS Appl. Mater. Interfaces 7 (2015) 5538-5549.

28. Ren G, Zhang Z, Song Y, Li X, Yan J, Wang Y, Zhu X, Effect of MWCNTs-GO hybrids on tribological performance of hybrid PTFE/Nomex fabric/phenolic

-p

composite, Compos. Sci. Technol. 146 (2017) 155-160.

re

29. Romo-Uribe A, Albanil L, POSS-Induced Dynamic Cross-Links Produced Self-Healing and Shape Memory Physical Hydrogels When Copolymerized with

lP

N-Isopropyl acrylamide, ACS Appl. Mater. Interfaces 11 (2019) 24447-24458.

na

30. Bai Y, Yu Q, Zhang J, Cai M, Liang Y, Zhou F, Liu W, Soft-nanocomposite lubricants of supramolecular gel with carbon nanotubes, J. Mater. Chem. A 7 (2019)

ur

7654-7663.

31. Qi H, Guo Y, Zhang L, Li G, Zhang G, Wang T, Wang Q, Covalently attached

Jo

mesoporous silica–ionic liquid hybrid nanomaterial as water lubrication additives for polymer-metal tribopair, Tribol. Int. 119 (2018) 721-730. 32. Wu L, Xie Z, Gu L, Song B, Wang L, Investigation of the tribological behavior of graphene oxide nanoplates as lubricant additives for ceramic/steel contact, Tribol. Int. 128 (2018) 13-120.

33. Ruiz V, Yate L, Langer J, Kosta I, Grande H, Tena-Zaera R, PEGylated carbon black as lubricant nanoadditive with enhanced dispersion stability and tribological performance, Tribol. Int. 137 (2019) 228-235. 34. Yang J, Chen Y, Xu P, Li Y, Jia X, Song H, Fabrication of compressible and underwater superoleophobic carbon/g-C3N4 aerogel for wastewater purification, Mater. Lette. 254 (2019) 210-213.

Microbeads, Biomacromolecules 7 (2006) 1471-1480.

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35. Mørch Ý. A, Donati I, Strand B. L, Effect of Ca2+, Ba2+, and Sr2+ on Alginate

36. Agüero L, Zaldivar-Silva D, Peña L, Dias M. L, Alginate microparticles as oral

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colon drug delivery device: A review, Carbohydr. Polym. 168 (2017) 32-43.

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Figure captions

Fig. 1. (a) Schematic illustration of the formation of alginate hydrogels; (b) Snapshots

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of the gelation process.

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Fig. 2. The gelation time with different molar ratios of GDL: CaCO3.

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Fig. 3. (a) TEM images and (b) Raman spectroscopy of graphene; (c) The digital

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images of the cylindrical alginate hydrogels; SEM images of freeze-dried alginate hydrogels (d) without and (e) with graphene; (f) The digital images of alginate hydrogels prepared with different methods.

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Fig. 4. (a) Compressive strain-stress curves of alginate hydrogels; (b) Conductive alginate hydrogels lighting up a LED bulb; Viscoelastic behaviors of alginate

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hydrogels (c) with and (d) without graphene; (e) Mechanical damping as a function of

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frequency of alginate hydrogels.

Fig. 5. (a) Friction coefficient curves; (b) Average friction coefficient and (c) wear

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volume.

Fig. 6. SEM images of worn surfaces with (a) water, (b) CAA, (c) alginate hydrogels

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and (d) hybrid alginate hydrogels as the lubricants.

Fig. 7. Raman spectra of (a) wear tracks and (b) counterpart balls under different

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lubricants condition.

Fig. 8. SEM images and EDS analyses of counterpart balls lubricated with (a-c)

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alginate hydrogels and (d-f) G3.

release of alginate hydrogels by (a) keeping them

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undisturbed, (b) alternating compression and release.

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Fig. 9. Water molecule

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Fig. 10. Schematic diagram of the lubrication mechanism.