hard Coupled Amphiphilic Polymer Nanospheres for Water

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Soft/hard Coupled Amphiphilic Polymer Nanospheres for Water Lubrication Zhaoxia Li, Shuanhong Ma, Ga Zhang, Daoai Wang, and Feng Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00405 • Publication Date (Web): 22 Feb 2018 Downloaded from http://pubs.acs.org on February 23, 2018

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ACS Applied Materials & Interfaces

Soft/hard Coupled Amphiphilic Polymer Nanospheres for Water Lubrication

Zhaoxia Lia,b, Shuanhong Maa, Ga Zhang,a Daoai Wanga,c*, Feng Zhoua State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese

a

Academy of Sciences, Lanzhou 730000, China. University of Chinese Academy of Sciences, Beijing, 100049, China

b

c

Qingdao Center of Resource Chemistry and New Materials, Qingdao 266100, China

KEYWORDS: amphiphilic polymer nanospheres; lubricant additives; hydration layer; water lubrication; anti-wear

ABSTRACT: Amphiphilic polymer nanospheres of poly (3-Sulfopropyl methacrylate potassium salt -co-styrene) [P(SPMA-co-St)] were prepared by a simple soap-free emulsion polymerization method and used as efficient water lubrication additive to enhance the anti-wear behaviors of Ti6Al4V alloy. The monodisperse and flexible P(SPMA-co-St) bi-component copolymer nanospheres were synthesized with a controllable manner by adjusting the mass fraction ratio of the monomers, with the hydrophobic polystyrene (PSt) as the hard and skeletal carrier component, and the hydrophilic PSPMA with hydration layer structure as the soft lubrication layer in the course of friction. The influences of the monomers concentration, the copolymer nanospheres additive content, the load and the frequency of the friction conditions on their tribological properties were studied in detail, and a probable anti-wear mechanism of the soft/hard coupled copolymer nanospheres under water

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lubrication was also proposed. The results show that compared with pure PSt the P(SPMA-co-St) polymer nanospheres exhibited better anti-wear property as additive for water lubrication, and the friction coefficient and the wear volume were first decrease and then increase with the increase of the SPMA content, indicating the hydrophilic SPMA has a significant effect on lubrication properties owing to its hydration performance. Furthermore, with the increase of polymer nanospheres concentration, the friction coefficient and wear amount also decreased to a stable and low value at the saturation concentration of 1 w%. The flexible polymer nanospheres with hydrophilic and soft SPMA shell and rigid PS core exhibited good friction-reduction and anti-wear performance as lubrication additive, indicating promising and potential applications in water lubrication and biological lubrication.

1. INTRODUCTION In nature, some biological systems have very good lubrication or super slippery phenomena with the mucus on the surface of plants or the synovial fluid in animals, which are mostly the water-based lubrications.1-5 Inspired by this, some water lubrications with very low friction coefficient were widely applied in human production to solve the increasing production safety and environmental protection issues6-7. Moreover, compared with the traditional oil lubrication technology, water as the most common lubrication medium, is green, environmentally friendly, cheap, safe, and can be also used as effective cooling agent. In the relatively sliding tribological interface, non-associative liquids such as oil and some organic solvents in the friction process were easy to be changed into a solid state, while the water could always keep a fluid state.8It is well known that the hydration lubrication is a key factor in the formation of hydration layer around its polar groups due to the presence of


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strong dipoles in water molecules. This hydration layer can support both the enormous pressure and the mobility of the external shear.9-10While, the low viscosity, poor lubricity, and easy to cause material corrosion properties for pure water lubricant and usually hinder the applications of it in some friction interfaces. Therefore, design and development of some high-performance water lubrication additives to simulate the biological mucus and joint lubricant, will be an effective strategy to enhance the water lubrication performance in tribology. Generally, it is one of the most important strategies to improve water lubrication by adding lubricant additives. In biological bodies, there are some biological macromolecules and other soft materials in the biological mucus and joint lubricant worked as the efficient additives between the sliding friction interfaces. Inspired by this, many synthetic soft materials, such as surfactant11-13, polymer brush,2, 14-15hydrogel,16-17 emulsion18-19and composite particles20-22 have been used as water lubricant additives in previous reports. Although there were a lot of researches about the exploration of new water lubrication additives, there is still challenge to combine the lubrication with bearing capacity perfectly. For an ideal water lubricant additive, it should be a “soft” material which has suitable water-solubility to make it easy to disperse the additives uniformly or form an intact and hydrophilic lubrication layer. On the other way, the lubricant additive should also be a “hard” material which has certain load carrying capacity and extreme-pressure resistance. For example, the natural joint lubricants with the special composite of load bearing constituent and lubrication portion in human body, can make the joints continuously work in a heavy load with very low friction and wear volume, while the lubrication and abrasion resistance of most of the present artificial joints are still much lower than that of the natural joints, owing to the difficulty in preparation of the artificial bionic additives with soft/hard coupled structure for efficient lubrication.



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Herein, a simple and soft/hard coupled polymer nanospheres of poly (3-Sulfopropyl methacrylate potassium salt-co-styrene) [P(SPMA-co-St)] were synthesized and used as water lubricant additive to improve the tribological behaviors of some biomedical materials, such as Ti6Al4V alloy which is usually used as a typical and biocompatible artificial joint materials.23-26The flexible and bi-component copolymer nanospheres have a rigid and hydrophobic styrene skeletal structure as the load bearing component, and a hydrophilic and soft 3-sulfopropyl methacrylate with negatively charged sulfonate ions as the lubrication component, were synthesized by a simple soap-free emulsion polymerization method. Furthermore, by adjusting the mass fraction ratio of the hard PSt and soft PSPMA and the content in water lubricant environment, the coefficient of friction and wear volume of Ti6Al4V alloy could be controlled and optimized, indicating promising and potential applications of this hard/soft coupled polymer additive in biological joint lubrication for high load bearing and low friction.

2. EXPERIMENT SECTION 2.1 Materials 3-Sulfopropyl methacrylate potassium salt (SPMA, 95 %, TCI Co., Ltd.) and styrene (St, 99 %, Aldrich) were used as received. Ammonium persulfate (APS, AR) and ammonium bicarbonate (NH4HCO3, AR) were commercially available and used without any treatment. The Ti6Al4V alloy (φ= 24 mm, thickness of 7.9 mm) and steel ball (φ=10 mm) were used as friction pairs materials in this study. The elemental composition of the Ti6Al4V was Al (6.6 %), V (4.1 %), Fe (0.25 %), C (0.06 %), N (0.04 %), O (0.18 %), H (0.011 %), Ti (88.759 %). Prior to the measurement, the Ti6Al4V specimens were first polished with a series of emery papers (up to 2000 grit), then degreased in acetone, ethanol for 10 minutes, respectively. Finally it was washed with deionized water (DI) and


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dried under a stream of nitrogen. In all experiments, the resistivity of the deionized water was 18.2 MΩ · cm. 2.2 Preparation of P(SPMA-co-St) polymer nanospheres P(SPMA-co-St) polymer nanospheres were synthesized by a modified soap-free emulsion polymerization method as follows27: 0.4 g of ammonium persulfate, 0.8 g of ammonium bicarbonate and a certain amount of SPMA monomers (0.2 g, 0.4 g, 0.6 g) were dissolved in 10 mL of deionized water (DI) by magnetic stirring at room temperature under nitrogen protection, which was denoted as solution A. Then, 22 mL of new-distilled styrene (20.0 g) and 110 mL DI were added into a 250 mL three-necked flask (denoted as solution B). The mixed solution B was bubbled with nitrogen under mechanical agitation till 70 ºC, and the solution A was quickly added into solution B and maintained stirring for 8 h at 70 ºC. Finally, the obtained homogeneous solution was placed on the rotary evaporator to remove the solvent and obtained the P(SPMA-co-St) polymer nanospheres products, which were marked as P(SPMA1-co-St100),P(SPMA2-co-St100), P(SPMA3-co-St100), respectively. The pure polystyrene (PSt) was synthesized with the same processes and materials just by removing the SPMA monomers. 2.3 Characterization In order to investigate the relationship between the content of hydrophilic SPMA monomer and the size of polymer nanospheres, the morphologies of the polymer nanospheres were detected by a field emission scanning electron microscopy (FESEM, JSM-6701F, Japan) and a transmission electron microscopy (TEM, FEI, Tecnai, G2 TF20). The elemental composition of polymer P(SPMA-co-St) nanospheres was evaluated by an energy dispersive spectrometer (EDXS) and X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). For XPS test, Al Kα was used as the



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excitation source and the binding energy was calibrated with contaminant carbon (C1s, 284.6 eV) as a reference. The number average weight (Mn) and polydispersity index (PDI) of P(SPMA-co-St) were measured a Gel permeation chromatography (GPC) measurements (Waters 1515, Waters Corporation, USA). Tetrahydrofuran was used as eluent at 35 ºC. The thermal stability was measured by thermogravimetric analysis (TGA) measurement, which was performed on a STA 449C Jupiter simultaneous TG-DSC instrument under air atmosphere with a heating rate of 10 ºC/min. The hydrodynamic size and Zeta potential of the composite polymer nanospheres were tested by dynamic light scattering (Zeta sizer Nano ZS, Malvern Instruments, UK). Zeta-potential measurements were carried out in the standard capillary electrophoresis cell at 25ºC, and a 1 mg/mL of samples were diluted with the concentration of measured specimen was 0.1 mg/mL, which was dispersed in the DI. The measurement of hydrodynamic size was conducted in quartz cuvette. All data were average values calculated from eight runs. The swelling ratio (SR) is calculated by the following formula: SR = Vaqueous / Vdry = (Daqueous / Ddry)3, in which Daqueous and Ddry represent wet and dry conditions, respectively. The Vaqueous and Vdry represent the volume of the nanospheres in the wet state and the dry state, respectively. 2.4 Tribological Performance The tribological tests were performed on an oscillating friction and wear tester (Optimol SRV-IV) under the ball-disc test mode with Ti6Al4V (φ=24 mm) and steel ball (φ=10 mm) as friction pairs, and the P(SPMA-co-St) composite polymer nanospheres as the water lubrication additive with different monomer ratio and concentration. The testing amplitude was 1 mm and the duration was 30 min. All tests were performed at room temperature and a relative humidity of 40-50%. In all the friction tests, 0.5 mL lubricant with or without additives was dropped onto the ball-disk contact area,


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and the friction coefficient was recorded in succession each lubricant for three times. After friction test, the wear volumes of the disks were estimated by using an interferometric non-contact surface profilometer (MicroXAM-3D).The surface morphologies of wear scars on the Ti6Al4V surfaces lubricated in various lubricants were conducted on a low field scanning electron microscope (SEM, 5601LV, Japan). 3. RESULTS AND DISCUSSION

Figure 1. (a) The synthesis process of P(SPMA-co-St) polymer nanospheres by soap-free emulsion polymerization, and (b) the illumination of the hydration and dehydration processes of P(SPMA-co-St) nanospheres with hydrophilic PSPMA and hydrophobic PSt. The preparation process of P(SPMA-co-St) composite polymer nanospheres is shown in Figure 1a, which is mainly synthesized by a simple one-step soap-free emulsion polymerization method. In Figure S3, the number average molecular weight (Mn) were in the range of (0.2-0.5) × t

with

Mw/Mn between 1.729 and 4.0. According to relative literature28-30, the high polymer polydispersities were common in soap-free emulsion polymerization system. However, sulfonate monomers are



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difficult to homopolymerize into nanospheres. To improve its polymerization activity, the styrene monomer was added and mixed with SPMA monomer to form a flexible and amphiphilic copolymer nanospheres, for which is easy to co-polymerize to form polystyrene spheres with PSPMA together in the mixture solution. The as-prepared P (SPMA-co-St) copolymer nanospheres consist of a rigid and hydrophobic PS skeletal structure, and a negatively charged and hydrophilic PSPMA chains as shown in Figure 1b, which has a reversible hydration and dehydration processes in water and dry state. The morphologies of P(SPMA-co-St) polymer nanospheres were detected by FESEM and TEM, as shown in Figure 2. Figure 2a-d show the top views of pure PSt nanospheres and these P(SPMA-co-St) copolymer nanospheres with different mass fraction ratio of SPMA/St (1:100, 2:100, 3:100). It was found that the size of the prepared polymer nanospheres was uniform, and the diameter of the polymer nanosphere decreases with the increase of the content of SPMA monomer, which were varied from 230 nm of pure PSt to 106 nm for P(SPMA3-co-St100) copolymer nanospheres in dry state. Figure 2e shows the TEM image of P(SPMA2-co-St100) copolymer nanospheres with the size of 158 nm, matching well with the result in the FESEM images in Figure 2d. Figure 2f shows the distribution of the average particle size of pure PSt and these P(SPMA-co-St) nanospheres varied with the content of SPMA in statistics. This question about decreasing trend of the size of these polymer nanospheres with the increase of SPMA monomer content, can be explained by the nucleation mechanism of the particle formation. According to related report31, increasing the amount of the ionic comonomers tends to decrease the particle size due to better surface stabilization. The emulsifier-free emulsion polymerization of St and SPMA follows the homogeneous nucleation mechanism32. In the early stage of the polymerization of St and SPMA, due


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to the existence of aqueous medium, ammonium persulfate initiator firstly initiated the polymerization of the water-soluble SPMA monomer, resulting in the formation of PSPMA oligomeric radicals which can capture comonomer St, that to say initiate the polymerization of water-insoluble styrene monomer to form a polymer chain. And the primary latex particles would be precipitated from the water till reaching the critical size, resulting in the formation of stable latex particles. Therefore, with the increasing of content of water-soluble SPMA monomer, the number of nucleis increased, resulting in the reduction of the copolymerization of per nuclei with PS and the decrease of the diameter of the final co-polymer nanospheres33-34, this process can be illustrated by Figure S4. According to the relevant literature31, the amount of ionic comonomers leads to differences in size, shape and uniformity of the particles. In this article, we obtained normal spherical shape at monomer mass fraction ratio with 1:100, 2:100; 3:100. Therefore, based on the literature31, we predict that if the ionic comonomer content is increased, the shapes of the particles are deformed, maybe become an empty hollow in a particle.

Figure 2. SEM images of (a) pure PSt nanospheres and (b-d) P(SPMA-co-St) co-polymer nanospheres synthesized by varying the content of SPMA monomer (b) P(SPMA1-co-St100); (c)



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P(SPMA2-co-St100); (d) P(SPMA3-co-St100). (e) TEM image of P(SPMA2-co-St100), scale bar was 50 nm; (f) The distribution of particle size varied with the content of SPMA. Figure 3a shows the element distribution of P(SPMA-co-St) nanospheres which was analyzed by a high-angle annular dark-field scanning transmission electron microscopy (HAAD-STEM). The HAAD-STEM mapping was further employed to demonstrate the composition and the elements species of SPMA in P(SPMA-co-St) nanosphere. It is clear that the O, S, and K elements from PSPMA distributed uniformly in the total polymer nanospheres as shown in Figure 3a, indicating these polymer nanospheres were copolymerized with the St and SPMA chains.XPS was also used to clarify the composition and chemical state of the surface elements of these polymer nanospheres. As shown in Figure 3b, there are sharp C 1s, K 2p, O 1s, and S 2p signal peaks, which are consistent with the elemental composition of PSt and PSPMA, while there is only a C1s peak for the pure PSt material. Figure 3c shows the C 1s spectrum and its fitting curves analysis, which consists of four peaks according to four states of C1s in the copolymer nanospheres. The fitted peaks at 284.7 eV and 285.1 eV are attributed to C=C bond and C-C bond in polystyrene. The fitted peaks located at 286.9 eV and 283.6 eV are attributed to C-O bond and C-C bond in PSPMA, further demonstrating the copolymer of P(SPMA-co-St) nanospheres. In addition, due to the different reactivity ratio of the two monomers during soap-free emulsion polymerization, so the molar ratio of the two monomers in the resulting copolymer nanospheres is different. According to 1H NMR spectra (as shown in Figure S1), we can calculate the molar ratio of SPMA / St was 0.219, the calculation method reference to our previous work35. Meanwhile, to fully illustrate and demonstrate this question, the molar ratio of SPMA/St monomer in the as-prepared P(SPMA-co-St) nanospheres can be calculated roughly by EDX analysis (in Figure S2), about was 0.241. The result was in agreement with that calculated by


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NMR data. Specific spectral characterization and analytical interpretation, attached to the note below.

Figure 3. Elemental mapping (a) and X-ray photoelectron spectroscopy analysis of survey (b) and C 1s (c) of P(SPMA-co-St) polymer nanospheres.

The hydrodynamic size and zeta potential of the composite polymer nanospheres were measured by dynamic light scattering (DLS). Figure 4 shows the hydrodynamic particle size distribution and Zeta potential of P(SPMA-co-St) polymer nanospheres with different PSPMA content, and the statistical results were listed in Table 1. In here, the measurement of P(SPMA-co-St) particle size mainly through two methods: one is measured dry particle size (Ddry) with field emission scanning electron microscopy (SEM); the other is the use of dynamic light scattering (DLS) measured hydrodynamic size (Dh). As shown in Figure 4a-c, the average hydrodynamic particle size of the composite nanospheres was at the range of 251.6-159.1 nm, which is significantly larger than that in dry state measured by scanning electron microscopy (223-106 nm, in Figure 1), owing to the



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swelling of PSPMA polymer chains in water by hydration reaction. The swelling ratio (Vaqueous / Vdry) is 1.17, 1.32, 1.50, respectively, increasing with the increase of the content of PSPMA, as shown in Table 1. Moreover, the Zeta potentials of all the composite nanospheres are negative with the values between -11.9 and -45.15 mV, indicating the PSPMA polymerization chain is wrapped around the PS sphere in the hydration state, meanwhile showing that the colloidal nanospheres dispersion in a stable state in the form of aqueous solution, which also can be seen in Figure S6. Generally, the Zeta potential is used to measure the intensity of the repulsion or attraction between the particles.36The smaller the molecular or dispersed particles, the higher the zeta potential (positive or negative), the more stable the system, that is, the dissolution or dispersion can resist aggregation. Besides, in this system, the P(SPMA-co-St) polymer nanospheres can be dispersed in DI and maintain stably for several months, without delamination or sedimentation phenomenon, as shown in Figure S6, indicating promising practical applications in water lubrication and biological lubrication.

Figure 4. The particle size distribution (a-c) and zeta potential (d-f) of P(SPMA-co-St) polymer nanospheres with different mass fraction ratio（SPMA/St）(a, d )1:100; (b, e) 2:100; (c, f) 3:100.


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Table 1. Hydrodynamic Size and ζ-Potential Analysis of polymer nanospheres. Dh (nm)

Ddry (nm)

-11.9±0.5a

251.6±0.8b

223.3±0.7b

1.17

2:100

-17.6±0.2

208.9±0.5

158±0.3

1.32

3:100

-45.15±0.7

159.1±1

106±0.9

1.50

Polymer nanospheres

SPMA/St

ζ (m V)

P(SPMA1-co-St100)

1:100

P(SPMA2-co-St100) P(SPMA3-co-St100) a

Zeta potential (mV). Data shown are the mean ± standard deviation.

b

Mean particle size(nm). Data shown are the mean ± standard deviation.

Swelling Ratio

To investigate the tribology performance of P(SPMA-co-St) nanospheres as additivesinwater, the friction coefficients and wear volumes were compared to those obtained with lubrication of DI and DI blended with PSt (PSt as lubrication additives).However, the main focus of this articleto improve the friction-reduction and anti-wear properties of soft and hard composite nanospheres through the introduction of hydrophilic SPMA. Therefore, we did not use PSPMA homopolymer as a control. As seen from Figure 5, When pure DI was used as lubricant, the friction coefficient was approximately 0.62. Blending of 1 wt% PSt nanospheres into DI as lubrication additives led to significant friction reduction. That is,the friction coefficient was reduced to around 0.3 after 30 minutes sliding. Nevertheless, fluctuation of friction coefficient was noticed from the friction tendency curve, indicating an unstable interface behavior probably caused by breakage of the nanoparticles as described below. When the P(SPMA-co-St) amphiphilic polymer nanospheres were used as additives, the friction coefficient was further decreased and maintained nearly stable at around 0.2. It is thus demonstrated that the P(SPMA-co-St) nanospheres provided excellent lubricating performance. Figure 5b-e compare the wear volumes and 3D topographies of the wear scars on Ti6Al4V alloy obtained after tests using DI, DI containing 1 wt% PSt and DI containing 1



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wt% P(SPMA-co-St) nanospheres, respectively. It is interesting that the addition of only 1 wt% P(SPMA-co-St) nanospheres reduced the wear volume by more than 50%, when compared to that obtained with lubrication of pure water. Whereas, around 30% reduction in wear volume was achieved by using PSt as water lubrication additives. It was thus corroborated that P(SPMA-co-St) exhibited better lubrication performance than PSt. From the 3D topographies given in Figure 5c-e, after adding P(SPMA-co-St) nanospheres as water lubrication additives, the wear scar became smaller and shallower. Moreover, the worn surface became much smoother when lubricated with water containing the P(SPMA-co-St) additives, indicating mild abrasion occurred. Similarly, the depth of the wear trace curve is consistent with the above results, as shown in Figure S7.These above results manifested that the composite nanospheres as lubrication additives exhibited high frictionand wear-reduction performance.

Figure 5. Comparison of friction coefficients (a), wear resistance (b) and 3D topographies of wear scars on Ti6Al4V alloy obtained when lubricated with DI (c), DI containing 1 wt% PSt nanospheres (d) and DI containing 1 wt% P(SPMA2-co-St100) composite nanospheres (e). Applied load: 100 load, frequency: 20 Hz.


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Figure 6. SEM images of the worn scar morphologies on Ti6Al4V alloy surfaces lubricated with DI (a,d), and DI containing PSt (b,e) and DI containing P(SPMA-co-St) (c,f) nanospheres with low (a,b,c) and high magnifications (d,e,f). Applied load: 100 N, reciprocating frequency: 20 Hz.

The surface morphologies of the wear scars on Ti6Al4V plates were inspected by SEM. As consistent with above observations, the wear scar on Ti6Al4V surface with DI as lubricant was larger than those obtained when PSt and P(SPMA-co-St) nanospheres were used as water lubrication additives (Figure 6a-c). Closer examination of the worn surfaces revealed that protective films consisting of the nanospheres spread on the worn surfaces, whereas severe adhesion wear occurred when DI was used as lubricant (Figure 6d-f and Figure S7). Moreover, it is observed that some hard PSt nanospheres fragmented probably owing to their rigidity and afterwards agglomerated, as indicated by white arrows in Fig. 6e. However, the soft/hard coupled P(SPMA-co-St) nanospheres spreading on the worn surface maintained their original sphere structures and were self-assembled into a continuous and homogeneous film (Figure 6f). We believe that the homogeneous film separated the direct rubbing of the friction pair. On the other hand, a soft hydration layer can form on the surface of the composite nanospheres due to the presence of PSPMA, leading to enhance



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lubrication. These results were in good agreement with the friction and wear tendencies presented above.

Figure 7. Effect of monomer mass fraction ratio (SPMA/St =0:100, 1:100, 2:100; 3:100) on the friction coefficients (a) and wear volumes (b). SEM images of the wear scars on Ti6Al4V alloy after sliding lubricated with water containing P(SPMA1-co-St100) (c, e) and water containing P(SPMA1-co-St100) (d, f) nanospheres. Applied load: 100N, frequency: 20Hz.

In order to shed light on the structure-performance relationship of the nanospheres, we investigated the effect of the content of PSPMA in P(SPMA-co-St) on the tribology performance. The monomer mass fraction ratio of SPMA and St was controlled during the polymerization process


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ranging from 1:100 to 3:100. Note that we also tried a higher mass fraction ratios of 4:100 and 5:100 to prepare the P(SPMA-co-St) copolymer, however, no regular spherical polymers were obtained. Figure 7a and 7b shows the effects of the monomer mass fraction ratio on friction coefficient and wear volume. As seen from Figure 7a, the increase of the monomer mass fraction ratio (SPMA/St) decreased the friction coefficient. Nevertheless, when compared to the lubrication with pure PSt nanospheres as additives, more stable friction coefficients was obtained even when the mass fraction ratio of SPMA/St was as low as 1:00. That is, even rather low fraction of SPMA play a role in the friction coefficient. When the mass fraction ratio of SPMA/St was 2:100, the friction coefficient reached the minimum value of 0.2. Further increase the mass fraction ratio to 3:100 led to a slightly increased friction coefficient, whereas the wear volume decreased monotonically with increasing the fraction ratio (Figure 7b). Thus, it was demonstrated that the lubrication performance was closely related to the proportion of PSPMA part in P(SPMA-co-St). The morphologies of the wear scars obtained respectively with lubrication of P(SPMA1-co-St100) and P(SPMA3-co-St100) are given in Figure 7c-f. It was verified that that wear scar became much smaller with enhancing the fraction ration from 1:00 to 3:100 (cf. Figure 7c and d). With regard to lubrication with P(SPMA1-co-St100), small amount of nanospheres were broken owing to the rubbing stress on the interface (Figure 7e). However, we can see that nearly all the nanospheres sustained the original morphology after termination of the tests (Figure 7f and Figure S8).It seems that in the range concerned, an increment in the mass fraction ratio of SPMA enhanced the ductility of nanospheres. In this case, breakage of nanospheres entrapped on the rubbing interface can be mitigated or even avoided. Thus, the load-carrying capability of the polymer film was improved. On the other hand, a soft hydration layer can easily form on the surface of the nanospheres containing relatively high fraction of SPMA,



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yielding better lubrication performance. The content of additives was deemed to be another important factor affecting the lubrication performance. Figure 8 shows the effect of concentration of P(SPMA2-co-St100) nanospheres in DI. As the concentration increased from 0 to 1 wt%,

both the coefficient of friction and wear volume

decreased continuously. However with further increasing the concentration to 3 wt%, the coefficient of friction and wear volume slightly increased (Figure 8a and 8b). It was thus demonstrated that 1 wt% of P(SPMA2-co-St100) in DI is the optimal concentration for friction-reduction and anti-wear performance. SEM graphs of the worn surfaces lubricated with water containing 0.1%, 0.3%, 0.5% and 3% P(SPMA2-co-St100) nanospheres were given in Figure 8c-j. Apparently, the size of the wear scar on Ti6Al4V surface varied as the additive concentration increased, i.e. the higher concentration of P(SPMA2-co-St100)in DI, the smaller wear scar size. Besides, as seen from Figure 8g-j, with increasing the concentration of P(SPMA2-co-St100) nanospheres in water, the density of the nanospheres spreading on the worn surface increased. It is inferred that the dense layer consisting of the nanospheres protected effectively the rubbing surface. Nevertheless, when the concentration was increased to 3 wt %, aggregates of polymer nanospheres were noticed from the worn surface, which can be caused by compaction of the nanospheres during the rubbing process.


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Figure 8. Effect of concentrations of P (SPMA2-co-St100) nanospheres in DI lubricant on the friction coefficient (a) and wear volume (b). SEM morphologies of the wear scars on Ti6Al4V alloy surface lubricated with water containing various concentrations of P(SPMA2-co-St100), i.e. 0.1 wt% (c, g ), 0.3wt% (d, h ), 0.5wt% (e, i ) and 3wt% (f, j ). Applied load: 100 N, frequency:20 Hz. In order to explore the load bearing capability of the composite nanospheres, effect of applied load on the tribological characteristics was studied. It is seen from Figure 9, an increase of the load from 20 to 100 N decreased the friction coefficient and wear volume. Further increase of the load from 100 to 200 N did not lead to significant change of the tribological characteristics. It was thus demonstrated that, the composite nanospheres as additives in water exhibited a high load-bearing capability. That is, the nanospheres have a potential for applications under harsh running conditions, e.g. highly loading and boundary lubrication conditions.



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Figure 9. Friction coefficients (a) and wear volumes (b) obtained whenP(SPMA2-co-St100) nanospheres were used as water lubrication additives at different loads.

Based on above investigations on the lubrication behaviors of P(SPMA-co-St) composite nanospheres, an interface model regarding the friction-and wear-reduction mechanisms of the nanospheres was proposed and was given in Figure 10a. P(SPMA-co-St) polymer nanospheres mainly consist of three parts: hydrophobic styrene component, hydrophilic SPMA and hydration layers. Thanks to the presence of the hydration layers on P(SPMA-co-St) nanosphere surface, the hydration layers played an important role in friction-reduction according to the mechanism of hydration lubrication.37-39 Meanwhile, under the combined actions of perpendicular load and tangential shearing stress, the polymer nanospheres underwent significant elastic deformation, which reduced the Hertz contact pressure and played the role of buffering. More importantly, the continuous self-assembled film consisting of the composite nanospheres separated the direct rubbing between the friction pair and thus greatly reduced the friction and wear. In this case, the soft hydrophilic PSPMA triggering formation of the hydration layer and the hard bearing component PS which were organically combined in the composite nanospheres can play a synergistic role. On the other hand, the polymer nanospheres entrapped on the rubbing interface can play a rolling lubrication


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role. Nevertheless, with respect to pure PS nanospheres, they exhibited a lower load carrying capability because of its fragility. As schematically shown in Figure 10 b, PS nanospheres were broken, and thereafter obvious aggregation of the broken nanospheres occurred. In this case, the lubrication performance was inhibited in comparison to that of the composite nanospheres.

Figure 10. Schematic diagram of friction-reduction and anti-wear mechanisms when P(SPMA-co-St) nanospheres (a) and PS nanospheres (b) are used as water lubricant additives, respectively.

4. CONCLUSIONS In this paper, the amphiphilic P(SPMA-co-St) polymer nanospheres with soft/hard coupled structures were prepared and used as an efficient water lubrication additive for friction-reduction and anti-wear, which consist of hydrophobic styrene (St) skeletal structure as a hard carrier component and the hydrophilic SPMA composite as the soft lubrication layer. Compared with pure PSt nanospheres as water lubrication additive, the P(SPMA-co-St) polymer nanospheres as the water lubrication additive has more stable and lower coefficient of friction and smaller wear volume, owing to the synergistic effect of the hard PSt nanospheres and the good PSPMA water lubricating layer.



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Different from the PSt nanosphere additive, P(SPMA-co-St) polymer nanospheres can endure huge load in the friction process, not cracked or deformed severely. Furthermore, due to the designability of P(SPMA-co-St), the friction coefficient can be adjusted by controlling the mass fraction ratio of SPMA and St, and also the additive content in water lubricant. The composite soft/hard coupled polymer nanospheres as a lubricant additive not only have the performance of friction-reduction, but also has the role of anti-wear, indicating promising applications in in water lubrication and biological lubrication with high load bearing and low friction.

ASSOCIATED CONTENT Supporting Information The 1H NMR spectrum (Bruker Avance III HD, 400 MHz, CDCl3, ppm) of P(SPMA-co-St) nanospheres (Figure S1).The EDX spectrum and content analysis of P(SPMA-co-St) nanospheres (Figure S2) GPC trace of P(SPMA-co-St)with different monomer mass fraction ratio (SPMA/St =0:100, 1:100, 2:100; 3:100) (Figure S3).The schematic of nucleation formation of polymer nanoparticles with different particle Size (Figure S4). The thermogravimetric curves (TG) of polystyrene spheres (PS) and P(SPMA-co-St) nanospheres (Figure S5). The stability and dispersibility of P(SPMA2-co-St100) polymer nanospheres in aqueous environment at different concentrations (Figure S6). Grinding depth curves of different samples (a) DI; (b) PS nanospheres; (c) P(SPMA-co-St) as lubricant additives (Figure S7). SEM Images of the grinding fluid of additive, (a) PSt nanospheres; (b) P(SPMA1-co-St100); (c) P(SPMA2-co-St100); (d) P(SPMA3-co-St100) at 100 load for 30min (Figure S8). SEM Images of the grinding fluid of different concentration (0.1wt%; 0.3wt%; 0.5wt%; 3wt%) P(SPMA2-co-St100) additives at 100 load for 30min (Figure S9). This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Tel: +86-931-4968169. ACKNOWLEDGMENT

Thanks for the financial support of the NSFC (No. 51722510, 21573259), the outstanding youth fund of Gansu Province(1606RJDA31), Qingdao science and technology plan application foundation research project (17-1-1-70-JCH) and the “Hundred Talents Program” of Chinese Academy of Sciences (D. Wang).

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hard Coupled Amphiphilic Polymer Nanospheres for Water

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