Poly(Cn-acrylate

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Covalently grafted graphene oxide-poly-(Cn) acrylate nanocomposites by surface-initiated ATRP: An efficient anti-friction, anti-wear and pour point depressant lubricating additive in oil media Arvind Kumar, Babita Behera, Gananath D. Thakre, and Siddharth Sankar Ray Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00848 • Publication Date (Web): 15 Jul 2016 Downloaded from http://pubs.acs.org on July 16, 2016

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Industrial & Engineering Chemistry Research

Covalently grafted graphene oxide-poly-(Cn) acrylate nanocomposites by surface-initiated ATRP: An efficient anti-friction, anti-wear and pour point depressant lubricating additive in oil media Arvind Kumarab, Babita Behera,ab Gananath D. Thakreb and Siddharth S. Rayab* a*

Academy of Scientific and Innovative Research (AcSIR), CSIR-Indian Institute of Petroleum,

Dehradun, 248005, India. b

CSIR-Indian Institute of Petroleum, Dehradun, 248005, India.

*Fax: +91-135-2660202; Tel: +91-135-2525771; *E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

Graphene oxide-polyacrylate nanocomposites with varying carbon numbers from 10 to 18 were prepared by surface-initiated atom transfer radical polymerization (SI-ATRP), thereby, controlling the density of polymer grafting through regulated molecular weight with narrow polydispersity. Dispersivity of these nanocomposites in oil medium depends on the grafted density of polyacrylates on graphene surfaces, and the degree of dispersion and stability increase with increase in chain length. After detail characterization of these nanocomposites, the lubrication properties of the nanocomposites in term of anti-friction and anti-wear behaviour in base oil and polyol were evaluated. Optimized loading of 0.04 mg/ml of graphene oxide-C18polyacryalate nanocomposite in base oil and polyol shows improved tribological properties in terms of significant reduction of friction and wear to around 42 % and 34 % respectively with improvement on pour point, thus qualifying this nanocomposite as a potential anti-friction, antiwear and pour point depressant additive for lubricating oils and polyol.

KEYWORDS: Graphene oxide-polyacrylate nanocomposites, SI-ATRP, Lubricating additives, anti-wear, anti-friction.


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INTRODUCTION Graphene based polymer nanocomposites (GPNCs) is an interesting and fast growing area of research in material science owing to its diversified applications .1–3 Particularly, Graphene oxide (GO), an oxidized form of graphene has emerged as a promising precursor of nano-material in designing a wide range of composite materials due to oxygenated functionalities like hydroxyls and acid groups at the edges and basal planes.4,5 These oxy functional groups of GO can be used as

active

sites

for

polymerization

by

means

of

non-covalent

and

covalent

interactions/approaches.6–9 The non-covalent route of functionalization includes hydrogen bonding or electrostatic forces, and π-π stacking of aromatic compound to graphene sheets.10, 11 However, the non-covalent method is very difficult to control along with tailoring and quantifying the growth of polymer due to the inherent instability, thus, limits the non-covalent functionalization methods for graphene oxide. On the other hand, the covalently functionalized approach is more versatile in designing and tailoring, easy in controlling polymers on GO surfaces. Covalent functionalization of polymers is classified into two categories; the “grafting to” and “grafting from” in terms of preparation method.7,8,12 Out of these, “grafting from” methods are much versatile in terms of controlled polymerization.13 Generally, in this technique initiator molecules are covalently attached with the hydroxyl groups of graphene oxide to generate macro-initiators on graphene oxide surface.14 In recent past, a number of papers have been published on controlled radical polymerization (CRP) utilizing “grafting from” strategy includes SI-ATRP,15–17 SET-LRP18 and RAFT.19 In a recent paper, we reported a novel visible light induced SI-ATRP method for grafting of poly methyl methacrylate (PMMA) on titania/reduced graphene oxide nanocomposite.20 Among these CRP methods, surface initiatedatom transfer radical polymerization (SI-ATRP) has been proved to be a powerful technique to


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achieve high degree of control over polymer chain density and thickness of grafting along with controlled molecular weight and polydispersity.21 Lee et al.,22 demonstrated polymer brushes of styrene, methyl methacrylate, or butyl acrylate by using SI-ATRP system and reported significant improvement in solubility of graphene oxide. On the other hand, the development of wear resistant and low friction nanomaterial additives has been a big challenge in the area of nano-tribology and mechanical engineering.23 Nowadays, polymer functionalized graphene based nanocomposites as lubricant additives are being explored as lubricant additives that can substitute the traditional bulk materials due to their nanodimensions and high mechanical and thermal properties. Owing to its unique structure graphene has been used as fillers in very low concentration in the lubricating base oils to improve their friction and wear properties.24 Tribological studies of graphene have primarily been focused on nano and micro-scale aspects since their discovery.25 However, some earlier reports have reported the tribological studies of GO/polymer composites on macro-level.

For example,

GO/PTFE,26 MGO/Polyimide,27 GO/nitrile rubber,28 GO/Polyacrylonitrile29 composites have been investigated for their enhanced lubricant performance. In a recent paper, we reported a fast and efficient approach for the synthesis of nanocomposites of polyacrylamide-graftedfunctionalized graphene oxide (FGO-PAM) through microwave-assisted surface initiated-redox polymerization as aqueous lubricant additive.30 Recently, Koratkar et al.26 reported suppression of wear in graphene/polymer composite. They found that graphene fillers have 10-30 % lower wear rate than graphite in polymer matrix. It is further supported by report of Huang et al.27 where modified graphene/polyimide nanocomposites exhibit reinforcing and lubrication behaviour. They suggested that modified graphene form a protective coating against the contacting surface thus, contributing to the enhanced anti-wear and anti-frictional properties.


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However, these reports utilized conventional route of free radical polymerization to synthesis of GO/Polymer composites. Hence, further efforts are required in the direction of fabrication of advanced graphene based polymer nanocomposites to explore their enhanced tribological performance vis-a-vis the tailoring the microstructure of polymer. Lubricating additive should have long chain alkyl moieties for stability and compatibility in non-polar medium. In order to have GO-polymer nanocomposite as lubricating additive in oil medium, special strategies are to be adapted to incorporate long alkyl chain polymers on GO with controlled chain length and molecular weight. We proposed a new method for functionalization of graphene oxide by long alkyl polyacrylate chains that would achieve the goal of improving the solubility and compatibility properties of graphene in base oil media with exhibiting tribological properties. Here we aimed at synthesis of non-aqueous lubricant additive from functionalized graphene oxide that has compatibility with base oil. Polymer functionalization of graphene not only enhances the solubility of graphene derivatives but also offer multifunctional hybrid materials. More important, attachment of long alkyl chain polymers with graphene by controlled polymerization provides highly dispersed graphene based nanocomposites in non-polar media. In this work, we have successfully prepared GO-Cn-polyacrylate nanocomposite adapting ‘‘grafting from’’ approach for the growth of poly (Cn-acrylates) chains from the surface of initiator-immobilized graphene oxide. Initially, the ATRP initiator was covalently attached on the graphene oxide by performing esterification reaction between hydroxyl groups of GO and αbromoisobutyryl bromide as proposed by Lee et al.22 These GO based macro-initiators were then used to initiate polymerization of (Cn)-acrylate monomers to obtain nanocomposites by SI-ATRP technique. Since graphene being the derivative of graphite, therefore, it inherits lubricating properties from graphite’s solid lubricant behaviour along with enhanced shearing force between


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planes of sp2 carbon sheets. Polyacrylate is well known as lubricating additive in non-polar solvents in lubricating industry. Therefore, these GO-polymeric nanocomposites were investigated for enhancement of efficiency of tribological properties such as coefficient of friction (CoF, µ) and wear reduction in oil medium. Since GO-polymer nanocomposites have stability and compatibility problem in oil medium, therefore, for synthesized nanocomposites with different carbon numbers were investigated for their compatibility and stability. EXPERIMENTAL: Materials: Graphite flakes, α-bromoisobutyryl bromide (BiBB) (98%), triethylamine (TEA, 99%), N, N, N’, N’’, N’’’- pentamethyldiethylenetriamine (PMDETA, 99%), copper (I) bromide (CuBr), ethyl α-bromoisobutyrate (EBiB) were procured from Sigma Aldrich, India. Potassium permanganate 99.0%, sodium nitrate 99.0%, concentrated sulphuric acid, hydrogen peroxide (30%), hydrochloric acid, acrylic acid, p-toluene sulfonic acid (p-TSA), hydroquinone (HQ), 1decanol, 1-dodecanol, 1-hexadecanol, 1-octadecanol, methanol and chloroform were procured from Merck, India. All chemicals and solvents were of analytical grade and used as received. Cu(I)Br was purified by washing with glacial acetic acid, ethanol and diethyl ether before use. The procedure for synthesis of GO by modified Hummers method was followed as reported in our previous publication.30,31 Preparation of ATRP initiator-functionalized graphene oxide (GO-Br): In a typical method, GO powder (0.5 g) was dispersed in DMF (50 ml) by using ultrasonication. Then, 10 ml TEA was added to the dispersed GO solution. Subsequently, 10 ml ATRP initiator BiBB was added drop-wise using pressure equalizing funnel to the above suspension under ice bath condition and reaction was stirred for 24 h in nitrogen atmosphere.


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The mixture was then collected by centrifugation and washed several times with chloroform and de-ionized water for removal of unreacted reactant and by-product. The resultant GO-Br initiator was dried overnight at 40 oC under vacuum. The percentage yield of reaction of immobilization of ATRP initiator onto GO based on mass of product obtained after centrifugation and drying was found 34.2 %. The TGA thermogram of GO-Br (Fig. 4) shows improved thermal behaviour as compare to neat GO sample with different weight loss of initiator moieties around at 200-300 o

C (21.01 wt %), which indicates new bonding appears due to the immobilization of ATRP

initiator moieties. Synthesis of alkyl chain (C-10 to C-18) acrylate monomers: The acrylates monomers were synthesized by simple esterification reaction of acrylic acid and respective long alkyl alcohols. The esterification reaction was carried out by 1.1 mol of acrylic acid with 1 mol of respective alcohols to result in long chain acrylate monomers. Particularly for synthesis of octadecyl acrylate, 37 g of acrylic acid was mixed with 135 g 1-octadecanol, 0.25 g p-toluene sulfonic acid (p-TSA), 0.1 g hydroquinone (HQ) in toluene (300ml) as solvent in three necked round bottom flask (500ml) fitted with dean stark assembly connected to chiller. The reaction was performed at 120 oC in presence of heating beads for uniform heating and also to overcome excess heating. After the reaction half mole H2O (~ 9.0 g) was obtained that indicated in completion of esterification reaction. The resultant acrylate monomers were purified through washing of excess de-ionized water (4-5 times) to remove unreacted acid and catalyst (p-TSA) impurities. The solvent (toluene) was removed by Rota evaporator. In this esterification reaction between equimolar amount of free carboxylic acid and alcohol, the yield of all the monomer products is found above 98 %. Successful synthesis of long chain acrylates monomer was


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confirmed by 1H and 13C NMR (Fig. S1). The monomer acrylates were passed through column to remove inhibitors prior to use. Surface initiated-atom transfer radical polymerization (SI-ATRP) of long alkyl chain (C10-C18) acrylates on the surfaces of GO-Br: Initiator-functionalized GO (GO-Br) (100 mg) was dispersed in 5 ml DMF by sonication. Then, 1 g acrylate monomer with Cu(I)Br (30 mg, 0.2 mmol) was loaded into sealed schlenk flask. The vial was degassed by nitrogen purging and vacuum using three freeze-pump-thaw cycles. Approximately, 40 µL (0.2 mmol) ligand (PMDETA) and ethyl α-bromoisobutyrate (20 µL) as sacrificial initiator was added to the reaction mixture and polymerization was carried out at 70 oC for 48 hours. The overall ratio of monomer to immobilized initiator is 10:1 (wt ratio) and the overall ratio of monomer to sacrificial initiator is 23:1 (molar ratio).The resulting viscous solution was then poured into methanol to precipitate out the polymer composites. The product was washed several times with chloroform to remove occluded free polymer and then dried under vacuum at 40 oC for 24 hours. CHARACTERIZATION Instrumental characterization GO, GO-Br and GOPA nanocomposites were characterized by different analytical techniques like FTIR, Solid state NMR, Raman, X-Ray diffraction, FESEM, HRTEM and TGA. FTIR spectra were recorded with a Nicolet 8700 FTIR spectrometer, equipped with XT-KBr beam splitter and using a DTGS TEC detector in the region of 4000-400 cm-1with 4 cm-1 spectral resolution and 36 kHz scanning speed. For preparation, about 1% of sample in KBr was finely grinded using mortar pestle inside the glove box to homogenize the mixture. Now, the mixture is subjected to KBr die-set and then, 5-7 tons of pressure was applied by hydraulic press to form


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thin and transparent pallets. The pallets were then analyzed by FTIR spectrometer. All 13C NMR spectra were recorded on a Bruker 300 MHz solid state spectrometer using 4 mm MAS BB H8 probe, resonating at 75.46 MHz for carbon and 300.13 MHz for proton frequencies. Approximately 100 mg of dried and finely powdered samples were packed in the ZrO2 rotor fitted with Kel-F cap. The 13C CPMAS NMR of samples were carried at 10 kHz spinning speed under optimal Hartman-Hahn condition using contact time of 1 ms for efficient polarization transfer and 3 s recycle delay. Care was taken in setting MAS rate such that the spinning side bands did not overlap with the signals. All spectra were externally referenced to 132.2 ppm peak of hexa-methyl benzene as secondary reference. Bruker interactive solid-state deconvolution method was employed to obtain more detailed information about methylene peaks in the range of 24-32 ppm. Gaussian/ Lorentzian deconvolution factor was applied for the individual peaks with their respective isotropic chemical shift values (δiso). Raman analysis of GO, GO-Br and GOPA18 nanocomposite was performed on laser micro Raman spectrometer at 632.8 nm laser excitation. X-ray diffraction patterns were recorded using Cu Kα radiation (λ= 1.5406 Å) at 40 kV and 40 mA in the range of 2θ=2-60° with a XRD diffractometer (D8 advance, Bruker). Diffraction data were collected by setting proportional counter detector at 1° min⁻¹ with increment of 0.01° for 2θ values. Scanning electron micrographs (SEM) were taken by using FESEM, (Quanta 200 F, Netherlands) at a voltage of 10-30 kV. Prior to analysis by SEM the samples were dried in vacuum at 45 oC, mounted on the sample holders and coated with gold. TEM analysis was carried out on a FEI-Tecnai G2S-Twin TEM instrument operated at 200 kV. Samples were prepared by mounting a water-dispersed sample on a carbon coated Cu grids. Approximately 2-4 mg of sample was mounted on the sample holder for TGA analysis by Perkin Elmer TG/DTA diamond instrument under N2 atmosphere. The temperature range under study


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was 30-800 oC with scan speed of 5 oC min-1. Molecular weight measurements were carried out by using Agilent HPLC/GPC system comprising of 1200 Infinity series precision Pump, 1200 Series Diode Array detector and 1260 Infinity Evaporative Light Scattering detector. Tetrahydrofuran (THF) was used as an eluent and polystyrene standard was used for calibration of the system. Tribological Evaluation: The tribological tests for graphene based long alkyl chain polyacrylates nanocomposites of different chain length (GOPA18, GOPA16, GOPA12 and GOPA10) were performed on a DUCOM, India make four ball tribo-tester TR-30H, in which a bearing ball was rotated in contact with three fixed balls which were immersed in the base oil media. Wear behaviour of the samples were reported in terms of wear scar diameter (WSD) obtained on the balls and measured at the end of the tests by optical microscope. Performance behaviour of all the lubricating additive samples was evaluated as per ASTM D 4172B test method. The samples were prepared at different loading of additives in base oil (N150, API Group-II) and Pentaerythritol tetraoleate ester (polyol ester). The tests were conducted at 75 oC with sliding speed of 1200 rpm under a normal load of 40 kgf (392 N) for one hour. The calculated average coefficient of friction (CoF, µ) and WSD were then plotted by error bar. RESULTS AND DISCUSSION: The dispersibility of graphene oxide is not stable in base oil for which it could not be used as additive although certain literatures showed graphene oxide as lubricant additives for aqueous purposes.32,

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Herein, we utilized SI-ATRP technique to introduce graphene based long alkyl

chain polyacrylates nanocomposites with varying alkyl chain from C-10 to C-18 for lubricating additives in base/lubricating oil at very low concentration. The results showed that with highest


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alkyl chain (C-18) nanocomposite significantly improved not only the solubility but also antiwear as well as frictional properties; therefore, we have discussed in detail the characterization of GOPA18 composites. Synthesis of GO-(Cn)polyacrylate nanocomposites involved the three major steps: (1) preparation of GO by using modified Hummer’s method, (2) synthesis of GO-Br macro-initiator by utilizing hydroxyl groups of GO for SI-ATRP, and finally (3) SI-ATRP of long alkyl chain acrylate monomers on the surface of nanolayers of macro-initiator. The detailed procedure of synthesis is shown in Scheme 1.

Scheme 1. Proposed schematic diagram of synthesis of GOPA18 nanocomposite by SI-ATRP Step by step formation of GO-(Cn) polyacrylate nanocomposite was monitored through FT-IR and supplemented by other characterization techniques. Formation of GO was evidenced from bands at 3410 cm-1 (due to O-H stretching), 1725 cm-1 (due to -C=O from carboxylic groups), 1622 cm-1 (due to aromatic C=C stretching), bands at 1228 cm-1 and 1068 cm-1 were attributed to


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the C-O and C-O-C (epoxy) stretching as shown in Fig.1a.30 Subsequently, the synthesized GO was modified by ATRP initiator α-bromoisobutyryl bromide in the presence of triethylamine (TEA) and N, N-dimethylformamide (DMF). In this reaction, we assumed that the hydroxyl groups located on the surface of GO undergoes esterification with the acid bromide initiator as proposed by Lee et al.22

Figure 1. FTIR spectra of (a) GO, (b) GO-Br and (c) GOPA18 nanocomposite Formation of GO-Br initiator was confirmed from the appearance of FT-IR bands (Fig. 1b) at 1731 cm-1 due to ester group (O-C=O) and the disappearance of C-OH and C-O-C peaks of GO with a new peak around 1180 cm-1 that could be assigned for C-O stretching of ester. The formation of GO-Br further supported by solid state NMR as discussed below and SEM elemental mapping with EDAX results (Figure S2 of supporting information). Long alkyl polyacrylate chains were covalently attached to initiator functionalized GO through SI-ATRP technique as evident from FTIR spectrum of the nanocomposite (Fig. 1c). Here we only showed


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GOPA18 nanocomposites while the FTIR spectra of others are available from supporting files (Figure S3). The characteristic bands were observed at 1737 cm-1 (O-C=O stretching); 2917 cm-1 and 2850 cm-1 (C-H stretching of sp2 and sp3 carbon respectively); 1469 cm-1 (C-H bending); 1162 cm-1 (C-O stretching of ester) and 721 cm-1 (C-Br stretching) due to polyacrylate chain and simultaneous retention of aromatic stretching of GO-Br at 1629 cm-1. Formation of the nanocomposite is further supported by other analytical techniques like SSNMR, Raman, XRD, SEM, HR-TEM etc. as discussed below. The SI-ATRP reaction of acrylates on GO-Br surface is confirmed by

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C CP MAS NMR

analysis as given detailed discussion in supplementary data (Figure S4). In addition, the gauche and trans conformations of grafted long polyacrylates chains tails also calculated from the chemical shift range of 24-32 ppm due to inter-methylene groups (refer to supplementary section, Table S1). The nanocomposite formation is further evidenced from the Raman analysis (see in supplementary section, Figure S5). The wide angle XRD results of GO, GO-Br and nanocomposite materials are shown in Figure S6 (refer to supplementary section). As reported earlier,30 we employed Bragg’s relation in calculating layer distance between aromatic sheets from the graphitic band and further lattice parameters such as inter-lamellar distance, average height of stacking layers (Lc), layer diameter (La) and number of aromatic layers (M) in GO and GOPA18 are computed from XRD spectra are reported in Table-S2 (supplementary section). In addition, small angle XRD pattern provide an important information of variation in 2θ and interlayer distance ‘d’ of GO sheets after modification with polyacrylate chains using ATRP technique as shown in Figure S7 (supplementary section). The ‘d’ value of nanocomposites is calculated from the small angle XRD pattern. It has been found that with increased the chain length of grafted polyacrylate nanocomposites from C10 to C18, the ‘d’ value also increased consequently from 4.52 nm to 5.69 nm. The increased ‘d’ value of nanocomposites (4.52-5.69 nm) as compare to neat GO (0.81 nm) is evidenced the successful grafting of polyacrylate chains on the surface of GO sheets. The morphology is very important to understand the surface behaviour for the nanocomposite which was studied from FESEM and HRTEM images. As presented in Fig. 2a, the SEM image


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of GO showed a bunch of papery sheets, whereas the SEM image of GO-Br (Fig. 2b) showed a lot of wrinkles and crumpled features. This results due to covalently attached Br-initiator moieties on the surface of GO sheets. Moreover, the uniform distribution of Br atoms over the surface of GO sheets as envisaged from SEM image is further supported by EDAX elemental mapping (Figure S2). Going from image (Fig. 2b) to image (Fig. 2c) it is observed that the crumpleness is relatively reduced due to arranged orientation of polymer chains along with their crystal packing. This behaviour is further observed in high resolution SEM image of the nanocomposite (Fig. 2d).

Figure 2: SEM images (a) GO, (b) GO-Br, (c) GOPA18 nano-composite and (d) GOPA18 with high resolution. From the TEM images (Fig. 3) of graphene oxide and nanocomposites, it is observed that the GO sheets have layered structure which is almost retained in GOPA18. Further, the TEM image of GOPA18 shows few nanolayers with some dark features at high resolution (2nm) (Fig. 3c).


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The dark spots are attributed to grafted long alkyl polyacrylate chains densely packed on the surface of GO.

Figure 3: TEM images of (a) GO, (b) GOPA18 nanocomposite and (c) GOPA18 nanocomposite at higher magnification.

Figure 4: TGA thermogram of GO, GO-Br, GOPA10, GOPA12, GOPA16 and GOPA18 nanocomposites.


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Thermal behaviour is vital for nanocomposites to under-stand not only the stability but also the tribological behaviour. In TGA thermogram, the curve from GO indicates initial weight loss near 108 oC, due to evaporation of water molecules trapped in the material as shown in Fig. 4. The second significant weight loss is observed in the range of 180–230 oC, is attributed to the thermal decomposition of oxygen carrying functionalities.40 The thermal stability of GO-Br initiator has improved over GO as observed is due to introduction of the acid bromide initiator at the hydroxyl sites replacing hydrogen atoms during esterification process on GO sheets.22 The decomposition profile of GOPA nanocomposites is rather different from those of GO and GOBr. The major weight loss is observed around at 400 oC in all of the nanocomposites (GOPA10, GOPA12, GOPA16 and GOPA18) as shown in Fig. 4. This weight loss is mainly attributed to the decomposition of long alkyl polyacrylate chains grafted on the GO and shifting to higher decomposition temperature may be due to polymer chain crystal packing. From the thermodecomposition profile of these nanocomposites, it is found that there is a difference with their residual mass in respect to the grafted density. The GOPA18 nanocomposite exhibited higher decomposition and less residual mass due to the high grafting densities of long polymer C-18 chains as compared to the other nanocomposites at 48 h polymerization time. Considering that the temperature range of 300-500 oC responsible for the degradation of grafted polymeric chains on the GO-Br initiator support, we use this range to calculate the graft density ratio of polymer chains as following the literature report.41 The graft density of GOPA18, GOPA16, GOP12 and GOPA10 has been calculated to be 51.92 %, 44.44 %, 40.25 % and 33.59 %, respectively. In addition, we have detached the polymeric chains to object GPC analysis of grafted polymers that also supplement the above TGA graft results. The percentage grafting via simple mass balance is calculated for GOPA18, GOPA16, GOPA12 and GOPA10 to be 46.6 %, 40.2 %, 36.2 % and


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30.2 %, respectively. This high grafting density of polymer nanocomposites is responsible for the improvements in targeted tribological properties as discussed below. Furthermore, for the controlled molecular weight measurements of grafted polymeric chains GPC analysis was carried out and results are discussed in supplementary section (Figure S8). It has been concluded that the grafted polymers shows narrow polydispersity (below 1.4) with controlled molecular weight. Evaluation of lubrication properties: Generally, GO has a very poor solubility and dispersibility in lube oil media as it has inherited from its predecessor graphene. Like pristine graphene it has a tendency of agglomeration and restoration into stacked graphitic carbon due to strong π–π interactions between aromatic layers. Hence, poor solubility and dispersibility factors make tribological evaluation of GO very difficult in non-polar/base-oil medium. This is mainly caused by the hydrophilic nature of GO due to a presence of polar oxy-functionalities on the basal planes and edges of nanolayers. Thus, it is strongly desired to improve its solubility as well as dispersibility in oil medium by functionalizing GO, especially by polymers/functional groups so that these groups render solubility. Here, we prepared nanocomposites with C10-C18 alkyl polyacrylate chains on GO to enhance the solubility in oil media by inducting alkyl chains and simultaneously hindering the agglomeration effect owing to steric effect of long alkyl groups. This grafting of alkyl acrylates on GO has been achieved through SI-ATRP in order to control the molecular weight (refer GPC results, Fig. S8), resulting in good dispersibility in base oil and polyol oil with the help of steric repulsive force and Van der Waals interaction originating from long alkyl polyacrylates chains on the surface of graphene sheets. It is well reported in literature that (i) semi-crystalline packing of polymer chains makes the bulk polymer soluble though it does have amorphous back bone


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structure and (ii) the van der Waals interaction occurs between long alkyl chains of polyacrylates and alkyl chains of lubricant base oil leading into good dispersion of nanocomposite.36,

42

Therefore, we studied both the solubility as well as dispersivity of all alkyl chain variants of nanocomposites in base oil and polyol media. Among these nanocomposites, GOPA18 exhibited best solubility as well as dispersivity due to longer carbon chain. Figure 5 shows the digital images of GOPA18 nanocomposite dispersed in the base oil at different concentration. It is observed that GOPA18 shows great dispersibility in base oil and polyol near around a month.

Figure 5: Digital photographs of dispersed GOPA18 nanocomposites at (i) 0.02, (ii) 0.04, (iii) 0.06, (iv) 0.08 and (v) 0.1 mg/ml concentration in base oil and polyol oil showing stability and dispersivity after 20 days. In order to understand the tribological behaviour of nanocomposites having alkyl chains, we synthesized composites having different chain length from C=10 to C=18 and designated them as GOPA10, GOPA12, GOPA16 and GOPA18 according to their carbon chain length. The triboperformance behaviour such as CoF, µ and WSD of all these polyacrylate nanocomposites as lubricant additive was evaluated in a standard four ball Tribo-test machine as per ASTM D


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4172B test method in base oil (N150, API Group II) and polyol as discussed in experimental section. Fig. 6 shows variation in average CoF, µ and WSD of these nanocomposites blended in base oil and polyol at optimum loading (0.04 mg/ml) of additives under load of 392 N with 1200 rpm speed for 60 minutes. The reason for taking this optimal loading of additive is obvious from the discussion of effect on GOPA18 in the next paragraph. Figure 6 (iii) shows the friction pattern of these nanocomposites in comparison to the base oil. It is observed that the CoF, µ gradually decreases as the alkyl chain length in nanocomposite increases. Among these nanocomposites, GOPA18 demonstrate lowest friction pattern as can be seen in Figure 6 (iii), thus compelling us to undertake a detail tribological study both in base oil and polyol.


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Figure 6. Relative comparison of (i) Coefficient of friction, µ (ii) wear scar diameter and (iii) CoF, µ pattern of GOPA18, GOPA16, GOPA12 and GOPA10 with base oil (N-150). Fig. 7 shows variation in average CoF, µ and WSD of GOPA18 nanocomposite blended in base oil and polyol oil at different loading of additives under same test conditions of load of 392


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N with 1200 rpm speed for 60 minutes. The average CoF, µ and WSD of base oil is found to be 0.121 and 0.825 mm, respectively, whereas in case of polyol it is found to be 0.105 and 0.600 mm, respectively as shown in Figure 7. The trend for both CoF, µ and WSD of GOPA18 nanocomposite (Fig. 7 (i) and (ii)) illustrates significant reduction with respect to base oil

Figure 7. (i) Friction and (ii) wear behaviour of GOPA18 nanocomposite blended in base oil (N150) and polyol oil at different concentration.


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and polyol till an optimal condition is reached and thereafter increment in trend is observed with gradual increase in loading of additive. The lowest value of average CoF, µ is found to be 0.070 and 0.085 respectively for base oil and polyol loaded with optimal 0.04 mg/ml of GOPA18 nanocomposite, whereas lowest value of WSD is found to be 0.545 mm and 0.529 mm respectively for the same systems. Loading of GOPA18 nanocomposite beyond 0.04 mg/ml CoF, µ and WSD started increasing (Fig. 7 (i) and (ii)). This inclined trend of CoF, µ and WSD at higher loading of GOPA18 beyond 0.04 mg/ml, may be due to agglomeration or non-dispersivity of the functionalized GO sheets after a certain compositional content in the composite. It can be concluded that at an optimum concentration of GOPA18 in media, uniform tribo film between the counter surfaces of steel balls is formed and further increase in content may disrupt the film surface. Thus, the GOPA18 nanocomposite play a significant role at the optimum dose for enhancement of lubrication properties of base oil and polyol in terms of CoF, µ as well as WSD under a high temperature and pressure tribo test conditions. We found that GOPA18 nanoadditive shows around 42 % and 34 % significant reduction in CoF, µ and WSD, respectively with respect to base oil. Similarly, GOPA18 nanocomposite shows improvement in anti-friction and anti-wear properties of polyol as well as, around 19 % and 12 % reduction of CoF, µ and WSD respectively as shown in Fig. 7. Physico-chemical properties of neat base oil, polyol and their blend with additive can provide correlation to the tribological effects as well as the most influencing parameter that plays the role in enhancing tribo behaviour. Some physico-chemical properties such as kinematic viscosity, viscosity index, pour point and density are determined for base oil and polyol along with 0.04mg/ml loaded GOPA18 are shown in Table 1.


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Table 1. Physico-chemical properties of base oil, polyol and GOPA18 nanocomposite (0.04 mg/ml) blended in base oil and polyol oil.

Sample Description

Kinematic Viscosity, Viscosity mm2s-1 Index (VI) 40 oC

Base oil

Pour Point, o C

100 oC

Density, g cm-3 (at 40 oC)

30.85

5.76

125

-18

0.834

GOPA18 in 31.44 base oil

5.85

126

-24

0.835

Polyol

71.24

13.2

146

-21

0.912

GOPA18 in polyol

65.08

13.3

152

-24

0.913

From these data, it is found that these polymeric nanocomposites, particularly GOPA18 acts as a pour point (PP) depressant additive besides CoF and WSD enhancer. This additive improves PP by 6 oC and 3 oC for base oil and polyol media. Importantly, this GOPA18 nanocomposite does not enhance the VI much but by just 1 value at the third decimal point. In case of GOPA18 blended in polyol oil the VI value is increased by 6 point as compare to neat polyol oil. This property is also vital in designing any lubricating additive. Further understanding of the effect of GOPA18 on the friction and wear behaviour (WSD) in both media is gathered from morphology of worn surfaces of steel balls using SEM (Fig. 8). The GOPA18 nano-additive at different doses showed a notable reduction in WSD as compare to neat base oil. A number of deep plowing grooves along with scratches are found on the worn surface of steel balls lubricated with the base oil. This can be clearly identified at higher


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magnification of SEM image (Fig. 8 a-c). These ruptured features point out the high metal-tometal contact during four ball tribo mechanism indicating poor lubricating properties.

Figure 8. FESEM images of worn surfaces of steel balls lubricated with base oil (a-c) and GOPA18 nano-additive blended in base oil (d-f) after tribo test. The arrow is indicating sliding direction duration test. The GOPA18 nano-additives improved the anti-wear properties of base oil by reduction of WSD around 34 % as compare to base stock. The worn surfaces of GOPA18 nanocomposite shows comparatively quite smooth and uniform nature of wear track at optimum dose (0.04 mg/ml) of nano-additive as illustrated in Figure 8 d-f. Possibly uniform nano film formation of GOPA18 on surfaces avoids direct contact of the steel balls and therefore reduces friction and improves wear resistance. The higher magnification of SEM image clearly shows the miniscule worn surface at the operating conditions that well support our hypothesis of tribo film formation for anti-friction and anti-wear activities. The longer alkyl chain of GOPA18 nano-additive has more capability to form uniform tribo film during metal to metal contact owing to their ordered


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packing as discussed in XRD section. Thus, optimal quantity of GOPA18 nanocomposite can be a better non-aqueous lubricating additive in improving the anti-friction and anti-wear properties. CONCLUSIONS A facile and efficient strategy of covalent grafting of long alkyl acrylate polymers onto GO layers is achieved for tribological applications in oil media which make its industrial importance. The resulting nanocomposite materials were characterized and confirmed by several analytical techniques. The presence of long alkyl polyacrylate chains in GOPA18 nanocomposite exhibits stable dispersion in both lube oil and polyol that qualifies it to be used as a lubricating additive in first principle. The GOPA18 nano-additive form a conformal protective coating on the sliding contact interfaces to facilitate shear that drastically reduce the wear scar as well as friction. Thus, the GOPA18 nanocomposite proved to be a potential lubricating nano-additive with enhanced tribological properties of base oil and polyol.

Supporting Information. Figure S1 NMR spectra of acrylate monomers, Figure S2 EDX-elemental mapping of GO-Br and Figure S3 FTIR spectra of GOPA10, GOPA12 and GOPA16 nanocomposites, Figure S4 13C CP MAS NMR spectra of GO, GO-Br and nanocomposites, Figure S5 Raman spectra of GO, GO-Br and GOPA18 nanocomposite, Figure S6 XRD pattern of GO, GO-Br and nanocomposites, Figure S7 small angle XRD spectrum of GOPA10 and GOPA18 nanocomposites , Figure S8 GPC results of detached polymers, Figure S9 Tribo results of GOPA10 nanocomposite in polyol, Figure S10 FESEM images of worn surface of GOPA10 nanocomposite, Table S1 NMR derived structural parameters of GOPA nanocomposites and Table S2: XRD derived structural parameters of GO and GOPA18 nanocomposite.


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AUTHOR INFORMATION Corresponding Author *Siddharth S. Ray Email: [email protected] CSIR-Indian Institute of Petroleum, Dehradun, 248005, India. Fax: +91-135-2660202; Tel: +91-135-2525771; Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources A part of work is funded by DST, New Delhi under project GAP-5219. CSIR-IIP is acknowledged for providing financial support through grant OLP-0694.

ACKNOWLEDGMENT A. Kumar acknowledges UGC, Govt. of India for providing the Senior Research Fellowship. Raghuvir Singh, Sandeep Saran, K. L. N. Siva Kumar are acknowledged for recording FTIR, XRD and SEM, respectively. Dr P. Rajamohanan (CSIR-NCL) is duly acknowledged for


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providing SSNMR. IISER Mohali is acknowledged for Raman analysis. Finally, the work is funded by CSIR under project OLP-0694 and DST, New Delhi under project GAP-5219. ABBREVIATIONS GO, graphene oxide; GOPA, graphene oxide polyacrylate nanocomposites; SI-ATRP, surface initiated-atom transfer radical polymerization; CoF, coefficient of friction; WSD, wear scar diameter; PP, pour point; VI, viscosity index.

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Poly(Cn-acrylate

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