Bioinspired Surface Functionalization of Nanodiamonds for Enhanced

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Interface-Rich Materials and Assemblies

Bio-inspired Surface Functionalization of Nanodiamonds for Enhanced Lubrication Yaoyu Jiao, Sizhe Liu, Yulong Sun, Wen Yue, and Hongyu Zhang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02441 • Publication Date (Web): 24 Sep 2018 Downloaded from http://pubs.acs.org on September 26, 2018

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Bio-inspired Surface Functionalization of Nanodiamonds for Enhanced Lubrication

Yaoyu Jiao,† Sizhe Liu,‡ Yulong Sun,‡ Wen Yue,†,* Hongyu Zhang‡,*

†

School of Engineering and Technology, China University of Geosciences (Beijing),

Beijing 100083, China ‡

State Key Laboratory of Tribology, Department of Mechanical Engineering,

Tsinghua University, Beijing 100084, China *Corresponding authors: [email protected] (W. Yue) [email protected] (H. Zhang)

1

ACS Paragon Plus Environment

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ABSTRACT The addition of nanoparticles to water-based lubricants is a commonly used method to improve lubrication, but to the best of our knowledge few studies have been reported to investigate the lubrication property of surface modified nanodiamonds (ND) with polyzwitterionic brushes. In this study, a bio-inspired copolymer containing dopamine and 2-methacryloyloxyethyl phosphorylcholine (MPC) was synthesized (DMA-MPC), and then spontaneously grafted onto ND surface (ND-MPC) through simple stirring in order to enhance lubrication. The characterization of transmission electron microscopy, Fourier transform infrared spectroscopy and thermogravimetric analysis indicated that the DMA-MPC was successfully modified onto the ND surface. Furthermore, a series of tribological experiment was performed on a universal materials tester using glycerol, glycerol+ND and glycerol+ND-MPC as the lubricants. It was found that the addition of ND to the lubricant (i.e. glycerol+ND and glycerol+ND-MPC) significantly reduced wear with a smaller wear scar and wear track on the tribopairs, and the coefficient of friction further decreased by about 40% when using glycerol+ND-MPC as the lubricant, which could be attributed to the hydration lubrication of the polyzwitterionic brushes modified onto the ND surface and the rolling effect of nanoparticles. In conclusion, in this study a universal and versatile surface modification method was proposed based on the synthesis of a bio-inspired copolymer DMA-MPC, which remarkably enhanced the lubrication property of ND nanoparticles when added to water-based lubricants. Keywords: nanodiamonds; bio-inspired surface modification; 2-methacryloyloxyethyl 2


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phosphorylcholine; dopamine; hydration lubrication

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INTRODUCTION Friction and wear are the main modes of energy dissipation and components failure in moving mechanical assemblies. It is estimated that about one third of the world's energy is consumed in various forms of friction,1,2 and as a result the study of reducing friction and wear is of significant importance. Many methods have been proposed, which can be generally divided into two categories. One is to enhance the surface properties of the materials, for example surface nanocrystallization, surface coating, etc.3,4 The other is to introduce different kinds of lubricants, e.g. lubricating oil and grease, water-based lubricant or solid lubricant.5 Among these methods, water-based lubricant is considered to be valuable due to its excellent cooling performance, low price and particularly low pollution. Recently, the use of nanoparticles as water-based lubricant additives has been widely investigated and attracted the attention of more and more researchers.6,7 Nanodiamonds (ND), with typical features including biocompatible inertness, high mechanical strength and strong molecule-absorbing capability,8 has been applied in various fields such as imaging,9 drug delivery10 and specifically friction.11,12 For example, Chen et al.11 added ND to water-based lubricant, and observed that it reduced the diameter of the wear scar by 32.5% when the content of ND was only 0.01 wt%. Tortora et al.12 investigated the lubrication property of glycerol aqueous solution with 0.1 wt% ND in both rotation and reciprocating tests, and they founded that wear was significantly reduced following the addition of ND, but coefficient of friction (COF) between the tribopairs was basically unchanged. Although ND has 4


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been employed to reduce wear as water-based lubricant addictive, few studies, to the best of our knowledge, have been documented in order to further enhance the lubrication property of ND (i.e. reduction of COF) through surface modification techniques. Articular cartilage is the most typical natural system with great superlubricity under loading, and this is attributed to supramolecular synergy of the biomolecules within the cartilage, in particular to the hydration lubrication mechanism of the polyzwitterionic charges (N+(CH3)3 and PO4–) in phosphocholine lipids.13-15 Consequently,

inspired

by

superlubricity

of

articular

cartilage,

2-methacryloyloxyethyl phosphorylcholine (MPC), as a methacrylate monomer with the same polyzwitterionic charges as phosphocholine lipids, has been intensively investigated to enhance lubrication and reduce friction and wear.16-18 For example, Klein et al. modified mica with poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) brushes using atom transfer radical polymerization, and obtained an extremely low COF (as low as 0.0004 measured by surface force balance) at the pressure of 7.5 MPa.19 Ishihara et al. proposed a photo-induced polymerization approach to prepare PMPC brushes-grafted polyethylene, and observed a significantly reduced COF and wear, which could confer extended durability to artificial implants.20 However, although these surface modification methods based on grafting-from strategy can generate more robust PMPC brushes with respect to being torn off the surface, the synthesis process is generally complicated and has relatively harsh reaction conditions. Accordingly, a universal and versatile surface modification 5


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method to achieve enhanced lubrication using PMPC brushes is preferably required. Since Lee et al. first reported mussel-inspired surface chemistry for multifunctional coatings in 2007, a large number of studies have been reported to investigate dopamine, as this material, with the presence of catechol groups and amine groups, can be easily deposited onto virtually all kinds of organic and inorganic substrates.21-24 Although the detailed mechanism of spontaneous attachment is complex and not quite clear, it is well accepted that the catechol groups are susceptible to oxidation into quinone under neutral or alkaline conditions, which contributes to the excellent adhesive ability.25 Preferably, dopamine can be used to modify nanoparticles for intentional surface functionalization. For example, Barras et al. employed two types of dopamine derivatives, as a simple and feasible strategy, to graft on the ND surface and prepared successfully azide- and poly-N-isopropylacrylamide-terminated nanoparticles, which exhibited a great potential for various applications.26 Wang et al. designed a series of multifunctional polymer ligands for surface functionalization of magnetic nanoparticles, which coupled dopamine as the anchoring groups.27 In the present study, motivated by cartilage-inspired superlubricity and mussel-inspired adhesion, a novel copolymer containing MPC and dopamine was synthesized, which could be modified onto ND surface to achieve enhanced lubrication as water-based lubricant additive. Because of its universality and versatility, the method developed in the present study can be regarded as an avenue for the design of functional nanoparticles with lubrication property. EXPERIMENTAL SECTION 6


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Materials and Reagents. ND (diameter 5-10 nm) was purchased from XF-Nano Co., Nanjing, China. MPC (98%) was purchased from Joy-Nature Co., Nanjing, China. Dopamine hydrochloride (98%), azodiisobutyronitrile (AIBN) and sodium borate were purchased from Aladdin Bio-Chem Technology Co., Ltd, Shanghai, China. Methacrylic anhydride was purchased from J&K Scientific Ltd, Beijing, China. Glycerol (99.5%) was purchased from Guangfu Fine Chemical Research Institute Co., Tianjin, China. Tris(hydroxymethyl)aminomethane was purchased from Macklin Biochemical Co., Ltd, Shanghai, China. Hydrochloric acid, ethyl acetate and N, N-dimethyl formamide (DMF) were purchased from Modern Oriental Technology Development Co., Ltd, Beijing, China. The other reagents including tetrahydrofuran, n-hexane, sodium bicarbonate and magnesium sulfate were purchased from Beijing Chemical Works, China. DMA-MPC Copolymer Preparation. Dopamine methacrylamide (DMA) was initially synthesized according to the study reported by Lee et al.28 Briefly, dopamine hydrochloride (5 g, 26.5 mmol), sodium borate (10 g, 26.2 mmol) and sodium bicarbonate (4 g, 47.6 mmol) were dissolved in 100 mL of deionized water under a nitrogen atmosphere. Then, 5 mL (33.7 mmol) of methacrylic anhydride was dissolved in 25 mL of tetrahydrofuran, and added dropwise to the above solution (pH=8). The mixed solution was stirred for reaction overnight under a nitrogen atmosphere. The pH value of the solution was adjusted below 2 by hydrochloric acid (0.5 mol/L), and afterwards the solution was purged by ethyl acetate, extracted by ethyl acetate and filtered with excessive magnesium sulfate. Finally, the filtered 7


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solution was precipitated with n-hexane to give a white solid product named as DMA. Note that in order to push the reaction and increase the yield of DMA, the quantity of anhydride is slightly more than amino-group. Additionally, sodium borate is used to protect the catechol groups in dopamine hydrochloride, and sodium bicarbonate acts as the acid binding agent to react with acid, which is produced during the reaction. The

DMA-MPC

copolymer

was

synthesized

based

on

free

radical

copolymerization of DMA and MPC, with AIBN as the initiator. Briefly, DMA (0.2 g), MPC (0.8 g) and AIBN (3 mg) were dissolved in 50 mL of DMF, and the reaction was allowed to proceed at 65 °C under continuously stirring and nitrogen atmosphere for 24 h. After reaction, the solution was dialyzed against deionized water and then freeze-dried for several days to obtain the solid product named as DMA-MPC. The preparation of the DMA-MPC copolymer was shown in Fig.1 (a). ND Surface Modification via DMA-MPC Copolymer. As shown in Fig.1 (b) and (c), DMA-MPC (20 mg) and ND powder (50 mg) were uniformly dispersed in 10 mL of Tris(hydroxymethyl)aminomethane buffer (10 mM, pH=8.5) in a small beaker. The suspension was stirred at room temperature for 16 h, then the resulting product was collected by centrifugation (2000 rpm, 10 min), and washed with alcohol and deionized water sufficiently. Finally, the resulting product was dried under vacuum overnight, and named as ND-MPC. Tribological Test. The tribological test was performed employing a universal materials tester (UMT-3, Centre for Tribology Inc., Campbell, California, USA). The experiments were completed in a rotation mode (rotation radius: 4 mm; rotation speed: 8


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360 rpm; normal load: 1 N) at 25 °C and 10% relative humidity, each for a duration of 30 min. As demonstrated in Fig.1 (d) and (e), a polished bearing steel GCr15 ball with a diameter of 6 mm was used as the upper specimen, which contacted with a rotating bearing steel GCr15 disk as the lower specimen. 25 mL of glycerol was placed into a 100 mL flask with deionized water to prepare the basic lubricant, i.e. glycerol-water solution with a concentration of 30% (Gly.30%). Subsequently, ND and ND-MPC were added into Gly.30% (nanoparticle concentration: 0.01 wt%), and the resulting lubricant was named as Gly.30%+ND and Gly.30%+ND-MPC, respectively. Specifically, for the lubricant of Gly.30%+ND-MPC, its lubrication property was further investigated under various experimental conditions, including rotation speed (180~360 rpm), nanoparticle concentration (0.01~0.07 wt%) and normal load (1~3 N). Additionally, the apparent maximum contact pressure was calculated based on the Hertz contact theory employing ball-on-flat configuration,29-31 as shown in the following equation, where is the apparent maximum contact pressure, is the

normal load, is the radius of the steel ball (3 mm), and / and / are the elastic modulus and Poisson’s ratio of bearing steel GCr15 (208 GPa and 0.3). Consequently, the apparent maximum contact pressure under different normal loads was calculated to be about 0.65 GPa (1 N), 0.83 GPa (2 N) and 0.94 GPa (3 N), respectively.

=

9


(1)

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Figure 1: Schematic representation of (a) synthesis of DMA-MPC copolymer; (b) and (c) surface modification of ND via DMA-MPC for the preparation of ND-MPC; (d) and (e) tribological test with a ball-on-flat configuration, using ND-MPC as the water-based lubricant addictive.

Characterizations. The 1H NMR and

13

C NMR spectra of DMA were measured

using a nuclear magnetic resonance (NMR) spectrometer (AVANCE III HD 400 MHz, Bruker, Switzerland) to confirm its structure, with dimethyl sulfoxide (DMSO) as the solvent. The morphology of ND and ND-MPC was observed using a transmission 10


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electron microscopy (TEM, JEM-2100F, FEI, JEOL, Japan) associated with an energy dispersive spectroscopy (EDS) to enable the quantitative comparison of elemental composition. Fourier transform infrared spectroscopy (FTIR) of ND, DMA-MPC and ND-MPC was recorded with a Nicolet 6700 spectrometer (Thermo Scientific, USA). Additionally, thermogravimetric analysis (TGA) of ND, DMA-MPC and ND-MPC was conducted using a Q5000IR instrument (TA Instruments, USA) at a heating rate of 10 °C /min from 25 °C to 500 °C. The viscosity of the three lubricants (Gly.30%, Gly.30%+ND and Gly.30%+ND-MPC) was measured by a rotary rheometer (Physica MCR301, Aaton Paar, Austria) with a cone-on-plate geometry. After the tribological test, an optical microscope (BX51M, Olympus, Japan) was used to analyze the wear scar on the ball and the wear track on the disk, and an optical interferometer (NanoMap-D,

AEP

Technology,

USA)

was

employed

to

evaluate

the

three-dimensional surface topography of the wear track on the disk. REAULTS AND DISCUSSION ND Surface Modification via DMA-MPC Copolymer. The 1H NMR and

13

C

NMR spectra of DMA are displayed in Fig.2. The d peak at 7.93 ppm (t, -NHCO-) in 1

H NMR spectrum and d peak at 168 ppm in 13C NMR spectrum (-NHCO-) indicate

that -NHCO- is produced by the reaction from dopamine hydrochloride and methacrylic anhydride. Additionally, the i peak at 8.74 ppm and j peak at 8.62 ppm (s, ArOH) in 1H NMR spectrum indicate that the catechol groups (phenolic hydroxyl groups) are still present in DMA.

11


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Figure 2: (a) 1H NMR and (b) 13C NMR spectra of DMA.

Fig. 3 shows the TEM images of ND and ND-MPC under different magnifications and the in situ EDS analysis. Although there is no obvious difference in morphology and size between ND and ND-MPC, it can be observed from the elemental composition that the C, N and P elements exhibit the same distribution as that of ND-MPC. Therefore, a preliminary conclusion can be drawn that the DMA-MPC copolymer has been successfully coated onto the ND surface.

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Figure 3: TEM images of (a) and (b) ND at different magnifications; (c) and (d) ND-MPC at different magnifications; (e)-(h) EDS analysis of ND-MPC with the distribution of C, N and P elements.

The evaluation of FTIR and TGA was performed in order to further verify that ND was modified by the DMA-MPC copolymer. As shown in Fig.4, the stretching vibration of C-C for ND appears at about 1100 cm-1, and both DMA-MPC and ND-MPC exhibit the characteristic absorption peaks of benzene skeleton vibration at 1400-1600 cm-1 and phosphate (P-O and P=O groups) stretching vibration at about 960 cm-1, 1060 cm-1 and 1244 cm-1. It is considered that benzene skeleton and phosphate are solely derived from dopamine and MPC in the DMA-MPC copolymer, thus the inclusion of these peaks in the spectrum of ND-MPC can act as further evidence indicating of successful surface modification of ND by the DMA-MPC copolymer.

13


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Figure 4: (a) FTIR spectra of ND, DMA-MPC and ND-MPC, and their FTIR spectrum labeling characteristic absorption peaks in (b), (c) and (d), respectively.

The TGA results of ND, DMA-MPC and ND-MPC are demonstrated in Fig.5 (a). It can be observed that ND has a weight loss of about 6%, which is caused by the removal of the organic groups (i.e. hydroxyl group, carboxyl group, etc.) during the heating process. The weight loss for DMA-MPC and ND-MPC is about 71% and 20%, from which the content of the DMA-MPC copolymer contained in ND-MPC is calculated to be about 21.5%. Assuming that all the DMA and MPC are involved in the reaction for preparing DMA-MPC, the molar ratio of MDA to MPC can be obtained (the molecular weight of DMA and MPC is 211 and 295) and as a consequence the content of P element in ND-MPC is calculated as about 1.8%. This is slightly higher than that measured by EDS (0.78%), as shown in Fig.5 (b). However, 14


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it should be noted that the measurement of P element by EDS may be compromised because Cu element originating from the copper mesh has also been collected using this method, which is considered to result in the lower content of P element. Consistent with the conclusion of FTIR result, the TGA data not only confirm that the DMA-MPC copolymer has been successfully modified onto the ND surface, but also provide a quantitative evaluation for each component of the ND-MPC nanoparticle.

Figure 5: (a) TGA profiles of ND, DMA-MPC and ND-MPC; (b) EDS analysis of ND-MPC with typical elemental composition.

Tribological Test and Characterization. The viscosity-shear rate plot and shear stress-shear rate plot of the three lubricants (Gly.30%, Gly.30%+ND and Gly.30%+ND-MPC) are illustrated in Fig.6 (a) and (b). It is clear that the addition of nanoparticles, either ND or ND-MPC, slightly increases the viscosity of the basic lubricant Gly.30%. Besides, the shear stress of the lubricant is proportional to the shear rate, indicating that all the lubricants are Newtonian fluid. Fig.6 (c) shows the comparison of COF for the three lubricants, from which it can be seen that the COF is quite similar (0.028) when using Gly.30% and Gly.30%+ND as the lubricants, indicating that the addition of ND has almost no significant influence on reducing the 15


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COF. Additionally, the COF reduced dramatically to 0.017 (~40%) when using Gly.30%+ND-MPC as the lubricant, which is attributed to the hydration lubrication mechanism of the polymer brushes coated on the ND surface. Fig.6 (d), (e) and (f) show the influence of rotation speed, nanoparticle concentration and normal load on the COF for the lubricant of Gly.30%+ND-MPC. It is obvious that the COF value exhibits only relatively slight fluctuation using different nanoparticle concentrations, indicating that nanoparticle concentration has little effect on the COF. Furthermore, the COF decreases with the increase of rotation speed, and it also shows an increasing trend with the increase of normal load, which conforms to the mixed lubrication regime in the Stribeck curve.32

16


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Figure 6: (a) and (b) Viscosity-shear rate plot and shear stress-shear rate plot of three lubricants Gly.30%, Gly.30%+ND and Gly.30%+ND-MPC; (c) Comparison of COF of three lubricants Gly.30%, Gly.30%+ND and Gly.30%+ND-MPC from tribological test, and a greatly reduced COF (0.017 vs 0.028) is observed for Gly.30%+ND-MPC; (d)-(f) The influence of rotation speed (180~360 rpm), nanoparticle concentration (0.01~0.07 wt%) and normal load (1~3 N) on COF of Gly.30%+ND-MPC.

Fig. 7 shows the optical images of wear scar on the steel ball and corresponding wear track on the steel disk using different lubricants. It is obvious that the addition of 17


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ND and ND-MPC plays an important role in the reduction of wear on the tribopairs, which is considered to be caused by the rolling effect of nanoparticles. The wear rate of the steel ball can be calculated based on the Achard wear equation as shown below33, where is the wear volume of the steel ball and it can be calculated as

= ℎ3

+ ℎ "/6, $ is the wear rate per unit normal load and per unit sliding

distance, is the normal load, % is the sliding distance,

is the radius of the wear

scar on the steel ball, is the radius of the steel ball, and ℎ = − √ −

.

Accordingly, the wear rate of the steel ball using Gly.30%, Gly.30%+ND and Gly.30%+ND-MPC as the lubricants are calculated to be about 8.38×10-15, 1.55×10-16 and 4.89×10-16 m3/N·m, respectively. Although the wear rate and wear scar on the steel ball are larger for Gly.30%+ND-MPC than that of Gly.30%+ND, which may be caused due to the relatively less amount of nanoparticles added (the mass of ND and ND-MPC added to Gly.30% is the same, but the molecular weight of ND-MPC is larger), the wear track on the steel disk is much shallower when using Gly.30%+ND-MPC as the lubricant, as shown in Fig.7 (g) and (h), indicating of a slight damage on the surface of the steel disk.

=$∙∙%

18


(2)

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Figure 7: Optical images showing the wear scar on the steel ball lubricated with (a) Gly.30%, (b) Gly.30%+ND and (c) Gly.30%+ND-MPC, respectively. Optical images showing the wear track on the steel disk lubricated with (d) Gly.30%, (e) Gly.30%+ND and (f) Gly.30%+ND-MPC, respectively. Three-dimensional surface topography of the wear track on the steel disk lubricated with (g) Gly.30%+ND and (h) Gly.30%+ND-MPC, measured by the optical interferometer.

It is demonstrated from the results of the tribological test that the addition of ND and ND-MPC nanoparticles to water-based lubricant Gly.30% significantly reduces wear of the tribopairs, and the COF further decreases by about 40% (from 0.028 to 0.017) when using Gly.30%+ND-MPC as the lubricant. The mechanism is attributed to the hydration lubrication of the polyzwitterionic brushes modified onto the ND 19


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surface and the rolling effect of nanoparticles, as shown in the schematic lubrication model of Fig.8. Because the steel ball and steel disk are not absolutely smooth and there are peaks and valleys present on the surface, the actual contact between the tribopairs during relative motion of the tribological test is illustrated in Fig.8 (a), and the rolling effect of nanoparticles contributes significantly to the reduction of wear both on the steel ball and on the steel disk. Additionally, the water molecule H2O is overall neutral, but possesses a large electric dipole due to the residual charges on the H and O atoms, consequently water molecules can surround charges in aqueous media such as ions or zwitterions. As shown in Fig.8 (b) and (c), the positively charged ions (N+(CH3)3) and negatively charged ions (PO4–) of the MPC polymer brushes tend to form a hydrated layer around them, and the water molecules within the hydration shell are not only very tenaciously held due to the interaction of the large water dipole with the enclosed charge but also exchange very rapidly with surrounding water molecules, up to 109/s depending on the nature of the enclosed charge.15 Consequently, it is extremely difficult to squeeze the hydration water out between two surfaces even under a large compressive load and, more importantly, the hydration shell will respond in a fluidlike manner when sheared, resulting in the reduction of COF and enhancement of lubrication as the two surfaces can slide very easily past each other.

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Figure 8: Schematic illustration of the lubrication model: (a) The contact between the tribopairs during relative motion of the tribological test, and wear reduction due to the rolling effect of the nanoparticles; (b) and (c) The hydrated layer formed surrounding the positively charged ions (N+(CH3)3) and negatively charged ions (PO4–) of the MPC polymer brushes, and COF reduction due to the hydration lubrication mechanism.

Theoretical Lubrication Regime Analysis. The lubrication regime when using Gly.30%+ND-MPC as the lubricant is theoretically analyzed based on the calculation of ) (i.e. )

Bioinspired Surface Functionalization of Nanodiamonds for Enhanced

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