Graphene-Polysiloxane

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In Situ Synthesis of Monomer Casting Nylon-6/ Graphene-Polysiloxane Nano-composites: Intercalation Structure#Synergistic Reinforcing and Friction-reducing Effect Chengjie Li, Meng Xiang, Xiaowen Zhao, and Lin Ye ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11399 • Publication Date (Web): 07 Sep 2017 Downloaded from http://pubs.acs.org on September 11, 2017

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ACS Applied Materials & Interfaces

In Situ Synthesis of Monomer Casting Nylon-6/Graphene-Polysiloxane Nano-composites: Intercalation Structure, Synergistic Reinforcing and Friction-reducing Effect Chengjie Li

Meng Xiang

Xiaowen Zhao

Lin Ye*

State Key Laboratory of Polymer Materials Engineering Polymer Research Institute of Sichuan University, Chengdu 610065, China *: Corresponding author: Lin Ye Address: State Key Laboratory of Polymer Materials Engineering Polymer Research Institute of Sichuan University, Chengdu 610065, China E-mail: [email protected] Tel: 86-28-85408802 Fax: 86-28-85402465

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In Situ Synthesis of Monomer Casting Nylon-6/Graphene-Polysiloxane Nano-composites: Intercalation Structure, Synergistic Reinforcing and Friction-reducing Effect Chengjie Li

Meng Xiang

Xiaowen Zhao

Lin Ye*

State Key Laboratory of Polymer Materials Engineering Polymer Research Institute of Sichuan University, Chengdu 610065, China ABSTRACT: Based on the industrialized graphene nanosheets (GN) product, MC PA6/GN-3-aminopropyl-terminated poly(dimethylsiloxan) (APDMS) nano-composite was in situ synthesized through anchoring effect of APDMS onto GN surface. APDMS/PA6 molecules were confirmed to intercalate into GN layers by formation of strong interfacial interactions. The intercalation ratio and the average layer thickness of the grafted GN sample decreased in presence of APDMS. Moreover, for MC PA6/GN-APDMS nano-composite, GN-APDMS was uniformly distributed in the matrix and no phase separation was observed. The size of spherical APDMS particles was obviously reduced compared with MC PA6/APDMS composite, revealing the strong interaction between APDMS and GN and the enhancement of compatibility in the composite system. Compared with neat MC PA6, addition of GN-APDMS resulted in 12% increase in tensile strength and 37% increase in impact strength, meanwhile the storage modulus (E′) and the glass transition temperature (Tg) increased, indicating the synergistic reinforcing and toughening effect of GN-APDMS on MC PA6. Furthermore, over 81% and 48% reduction in friction coefficient and specific wear rate were achieved for the nano-composite, and the worn surface displayed flat and smooth features with uniform depth distribution, the low annealing effect and reduced friction heat, further confirming the synergistic friction-reducing effect of GN-APDMS on MC PA6. KEYWORDS: monomer casting nylon-6 (MC PA6), graphene nanosheets (GN), polysiloxane, intercalation structure, synergistic reinforcing and friction-reducing effect 1. INTRODUCTION Monomer casting nylon-6 (MC PA6), as an important engineering plastic, is synthesized 2


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through anionic ring-opening polymerization of ε-caprolactam (CL), using sodium hydroxide (NaOH) as catalyst and diisocyanates as co-catalyst. Because of its high molecular weight, high crystallinity, superior mechanical and self-lubricating performances, and ease of manufacturing,1,2 MC PA6 has been extensively investigated and widely applied in automotive and transportation industry to replace metallic materials. However, under high load and harsh PV conditions, MC PA6 presents high wear rate and poor self-lubrication property, which can’t satisfy the application requirement completely. The tribological performance of MC PA6 was usually improved by addition of various fillers such as graphite (G), carbon black (CB), fibers (F) and lubrication oil (O).3-6 These solid or liquid lubricants have been proved to be beneficial for enhancement of the tribological performance; however, the mechanical performances of the composites might deteriorate due to high fillings and poor interfacial interactions. Graphene nanosheets (GN), as a typical two-dimensional material with one-atom-thick planar sheet of sp2 carbon atoms arranged in a hexagonal lattice, has drawn great attention in academic and industry owing to its intriguing and unique performances, like superior mechanical strength, high electrical and thermal conductivity, excellent self-lubricating property and high load-bearing capacity,7-10 which promises its potential in widespread applications, including for supercapacitors, field-effect transistors, energy devices, sensors, lubricants and so on.11-13 Especially, as an effective and efficient solid lubricant, GN exhibits extraordinary self-lubricating and anti-wear properties ascribed to its easy shear ability and atomically smooth surface through its intrinsic lamellar structure.14 Plenty of researches have been dedicated to the tribological behavior of graphene in both microscale and macroscale aspects,15-21 and proved graphene’s capability in reducing wear rate. However, the wear resistance of some reported polymer/graphene composites was far less than expected, and the friction coefficient even increased, which has been a major challenge for its utilization in such

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high-performance composites. Kalin et al.22 evaluated the effect of graphene on the tribological performaces of poly(ether-ether-ketone) (PEEK), and the result showed that 2wt% graphene addition could result in a higher friction coefficient than pure PEEK. Shen et al.23 prepared epoxy/graphene oxide (GO) composites, which exhibited the lowest wear rate but more than 100% increase in friction coefficient at 0.05wt% GO content. Tai and An et al.18,24 studied the tribological behaviors of UHMWPE/GO composites, which showed that at 1wt% GO, the friction coefficient increased by ~15% and the wear rate decreased by ~35% for UHMWPE. Moreover, the graphene or graphene oxide used in these studies were small-scale prepared in the laboratory, and it is far away from large production and application in industrial scale. Polysiloxane consists of flexible Si-O-Si backbones, and the higher bond energy of Si-O bond (~460 kJ/mol vs. ~348 kJ/mol for C-C bond), the longer Si-O bond length (0.164 nm vs. 0.153 nm for C-C bond) and weak intermolecular attractions lead to lower steric hindrance to rotation.25 Such structure characteristics endow it with intriguing performances, like high chain flexibility, superior heat-resistance and self-lubricating property with extensive range of applications.26-28 Presently, many studies have shown that the friction coefficient was significantly reduced when liquid polysiloxane lubricants were impregnated into polymers; however, the mechanical strength, elastic modulus and wear resistance of the composites severely decreased.3 Meanwhile, polysiloxane has a very low solubility parameter (15.3 J1/2·cm-2/3), which makes it immiscible with PA6 (27.8 J1/2·cm-2/3).29 In this work, 3-aminopropyl-terminated poly(dimethylsiloxane) (APDMS) with primary amine as end group was selected due to its improvement of interfacial interaction with PA6. Meanwhile, it is well known that, based on the large production in industrial scale, oxygen groups on such GN surface can’t be removed completely. Therefore, reactive sites of oxygen groups on its surface were easy to be designed and controlled to obtain the tailored interface

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by the covalent linkage. This feature also enables GN simultaneously have strong interactions with MC PA6 and APDMS. Therefore, the way to combine APDMS and GN together was attractive to endow PA6 with inherent properties of both components. Herein, firstly, the anchoring of APDMS onto GN surface was realized by ultrasonic dispersion of both GN and APDMS in melting caprolactam matrix at 80 °C, and a stable colloidal suspension of GN-APDMS in caprolactam was obtained. Secondly, the MC PA6/GN-APDMS nano-composite was in situ synthesized by using the obtained caprolactam/GN-APDMS mixture. The interfacial interaction, intercalation structure and the synergistic reinforcing and friction-reducing effect of GN-APDMS on MC PA6 were investigated comprehensively. 2. EXPERIMENTAL SECTION 2.1 Materials Caprolactam (CL) with commercial grade was provided by Shanghai Niunuo Chemical Co. Ltd. Sodium hydroxide (NaOH) with analytical grade was commercially obtained from Shanghai Chemical Reagent Co. Ltd. (Shanghai, China). Toluene-2, 4-diisocyanate (TDI) was bought from Wanhua Polyurethane Co. Ltd. (Yantai, China). Graphene nanosheets (GN) particles with size of micron grade was provided by the Sixth Element Materials Technology Co. Ltd. (Changzhou, China). 3-aminopropyl-terminated polydimethylsiloxane (APDMS) with an average molecular weight of 6000 was purchased from the Zhong Haojing Polymer Material Co. Ltd. (Guangzhou, China). 2.2 In situ synthesis of MC PA6/GN-APDMS nano-composite The synthesis procedure for the MC PA6/GN-APDMS nano-composite was carried out as follows: firstly, the caprolactam (CL) monomer with 4 mol in stoichiometric amount was put into a three necked flask and heated to 80 °C; after the monomer was melted, a certain amount of APDMS (1wt% based on CL) and GN powders (0.5wt% based on CL) were added.

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The mixed melt was stirred and sonicated for another 1 hour. Then, the melt was vacuumed for 0.5 hour to remove trace of water, and a certain quantity of NaOH (0.05 mol) was added into the melt, and afterwards the melt was vacuumed again and TDI (0.05 mol) was added. Finally, the mixed melt was cast into a mould at 170 °C, and reacted for 1 hour, and thus the product of MC PA6/GN-APDMS nano-composite was obtained.30 For comparison, the control experiments to synthesize neat MC PA6, MC PA6/GN and MC PA6/APDMS samples were also performed by following the same procedure. The synthesis process of MC PA6/GN-APDMS nano-composite is illustrated in Figure 1.

Figure 1. Synthesis process of MC PA6/GN-APDMS nano-composite via in situ polymerization

2.3 Measurements 2.3.1 FT-IR analysis The structure analysis of GN, g-GN and g-GN-APDMS samples were conducted with a Nicolet-560 Fourier-transform infrared spectrometer (FT-IR) (U.S.A), and the scanning rate of 20 min-1 of and resolution of 4 cm-1 were applied. 6


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2.3.2 Raman analysis The skeletal structures of the GN, g-GN and g-GN-APDMS samples were investigated at room temperature with a RENISHAW Invia Raman Microprobe (UK) over the range of 1000 to 3500 cm-1. Argon ion laser excitation source at 532 nm was used. 2.3.3 XPS analysis The XPS analysis of the GN, g-GN and g-GN-APDMS samples was conducted with a XSAM 800 spectrometer (KRATOS Co., UK), and AlKa radiation (1486.6 eV) was used at a pressure of 2.0 ×10-7 Pa. 2.3.4 TGA analysis TGA analysis of the GN, g-GN and g-GN-APDMS samples was used to characterize the grafting ratio of PA6 and APDMS onto GN surface. It was conducted with a TA2950 thermobalance from TA Co. (USA) under nitrogen atmosphere. The flow rate was 50 ml/min, and the heating rate was 10 °C/min. All samples were dried in the oven at 90 °C for 3 hours before the measurement. 2.3.5 XRD analysis The XRD analysis of the GN, g-GN, g-GN-APDMS samples was conducted with Rigaku D/max ⅢB X-ray diffractometer (Japan). Cu Kα radiation (λ=0.154 nm) was used at voltage of 40 kV and current of 40 mA at room temperature in the scanning range of 2θ=5-40°. 2.3.6 AFM analysis The sheet dimensions of the GN, g-GN, g-GN-APDMS samples and the worn surfaces of MC PA6 composites were analyzed with a Shimadzu SPM-9700 Scanning Probe Microscope (Japan) and recorded with a tapping mode. The GN dispersion samples were drop-cast onto the mica substrates, and dried at ambient temperature and pressure. The worn surfaces of MC PA6 composites were directly stuck onto mica substrates for observation. 2.3.7 SEM analysis

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The morphologies of the cryogenically fractured surfaces, the impact and the tensile fractured surfaces, and the worn surfaces of MC PA6 composites were observed with a JEOL JSM-5900LV SEM (Japan) at the operating voltage of 15 kV. The samples were ion beam sputter-coated with gold with layer thickness of 1-20 nm. The energy-dispersive X-ray spectroscopy (EDS) analysis was conducted at the same time. 2.3.8 TEM analysis The micro-morphologies and structures of MC PA6 composites were observed on JEOL JEM 100CX II TEM equipment (Japan), and the acceleration voltage of 200 kV was applied. The tested TEM samples of MC PA6 composites were prepared by thin sections under cryogenic conditions. 2.3.9 Mechanical performances The tensile performances of MC PA6 composites were tested with a 4302 universal material testing machine from Instron Co. (U.S.A) by following standard ISO 527-1993. The tensile speed of 10 mm/min was applied, and the size of sample was 60 mm × 25 mm. The notched charpy impact strength of MC PA6 composites was tested with ZBC-4B impact testing machine from Xinsansi Co. (Shenzhen, China) by following standard ISO 179-1993. 2.3.10 Dynamic mechanical analysis (DMA) The DMA measurement of MC PA6 composites was conducted with a TA Instrument Q800 DMA (U.S.A). A bending mode was applied, the heating rate was 3°C/min and the frequency was 1 Hz. The size of the sample was 40 mm×10 mm×4 mm. 2.3.11 Friction and wear property The tribological performance of MC PA6 composites was measured under dry sliding by using a block-on-ring testing machine M-200 from Beijing Precision Instrument Equipment Co. (China) by following standard GB 3960-1983. The block samples were in size of 30 mm

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× 7 mm × 6 mm, and rotated against a 45# steel ring, lasting for 2 hours. A linear velocity of 0.42 m/s and a normal load of 196 N under the ambient temperature were applied. After the end of each test, the wear scar was measured with accuracy of 0.02 mm, and the wear volume ∆V (mm3) of the specimen was obtained based on the equation below:  πr 2  b  b ∆V = B  arcsin   −  2r  2 180

r2 −

b2 4

  

(1)

where B is the width of specimens (mm); r is the radius of the counterpart ring (mm), and b is the width of wear scars (mm). The specific wear rate ( W ) (mm3/Nm) can be defined as:

W=

∆V ∆V = PL Pνt

(2)

where P is the applied load (N); L is the distance of sliding (m); ν is the sliding linear velocity (m/s), and t is the experimental duration time (s). 3. RESULTS AND DISCUSSION 3.1 Intercalation behavior and structure of MC PA6/GN-APDMS nano-composite Based on the industrialized graphene nanosheets (GN) product, MC PA6/GN-APDMS nano-composite was prepared through in situ polymerization and cast molding method according to the mechanism of anionic polymerization, using NaOH as catalyst and TDI as auxiliary catalyst. For research and analysis of the intercalation structures of the MC PA6/GN-APDMS nano-composite, a proper quantity of the composite sample was dissolved in formic acid, centrifugated and washed repeatedly with formic acid and toluene to remove free PA6 and APDMS molecules, and the resultant grafted GN sample was denoted as g-GN-APDMS. For comparison, the grafted sample prepared in absence of APDMS was denoted as g-GN. 3.1.1 Interfacial interaction and intercalation ratio Figure 2(a) shows the structural features of GN, g-GN and g-GN-APDMS samples. It

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can be seen that GN exhibits typical absorption peaks at 3434.5 cm-1 (O-H stretching vibration), 1629.2 cm-1 (O-H bending vibration, or C=C skeletal ring vibrations),31,32 1049.7 cm-1 and 882.0 cm-1 (C-O-C asymmetric and symmetric stretching vibrations). These vibrations revealed the existence of hydroxyl (O-H) and epoxy (C-O-C) groups on the GN surface. For the sample of g-GN, the characteristic peak at 3433.5 cm-1 was assigned to O-H stretching vibration of GN, and the slight red-shift indicated the hydrogen bonding interactions between GN and PA6 molecules. The bands at 2923.3 cm-1 and 2854.2 cm-1 were related to the asymmetric and symmetric stretching vibrations of methylene (-CH2-) groups on PA6 molecules, respectively. The characteristic peak at 1633.8 cm-1 corresponded to the stretching vibration of the C=O groups of the amide carbonyl, and the characteristic peak at 1535.4 cm-1 was designated as the bending vibration of the N-H groups of the amide. All these results undoubtedly indicated that PA6 molecular chains were effectively intercalated into GN layers. For g-GN-APDMS sample, the characteristic peaks ascribed to PA6 resembled that of g-GN sample. In addition, the new peak at 1261.4 cm-1 was ascribed to the structural vibration of Si-CH3 groups, and the bands at 1095.1 cm-1 and 1024.2 cm-1 were assigned to the stretching vibration of Si-O-Si groups.33,34 The peak at 801.5 cm-1 corresponded to the stretching vibration of Si-C groups. Moreover, the further red-shift of C-OH group indicated the hydrogen bonding interactions in the nano-composite. As a result, APDMS and PA6 molecular chains were intercalated into GN layers, and the anchoring effect of APDMS onto GN surface was realized through the reaction between amine groups of APDMS and epoxy groups of GN surface, as depicted in Figure 2(b).

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Figure 2. (a) FTIR spectra of GN, g-GN and g-GN-APDMS; (b) Anchoring effect of APDMS and PA6 molecules onto GN surface

In order to further explore the interaction between GN layers and PA6 molecules, Raman spectra of GN, g-GN and g-GN-APDMS samples are performed, as shown in Figure 3. For GN sample, two characteristic peaks at around 1350 and 1583 cm-1, corresponded to the D band and G band, which were ascribed to the breathing mode of κ-point phonons of A1g symmetry due to the disordered fractions and the first-order scattering of the E2g vibration mode, respectively.35,36 For g-GN and g-GN-APDMS samples, besides the above D/G bands, no obvious absorption peak attributed to PA6 molecules can be observed. However, for g-GN-APDMS sample, the G band shifted from 1583 cm-1 to 1575 cm-1, indicating that there was an interaction in the composition system, which was consistent with the FT-IR analysis.

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The intensity ratio of ID/IG correlates to the disorder degree and the average size of the sp2 domains of graphene.37 The ID/IG of g-GN (0.99) is the same with that of GN (0.99), while the ID/IG value of slightly increased to 1.04 for g-GN-APDMS, indicating an slight increase in the average size of sp2 domain by anchoring of APDMS.38,39

Figure 3. Raman spectra of the samples of GN, g-GN and g-GN-APDMS

The element composition of GN, g-GN and g-GN-APDMS is analyzed with the wide scanned spectra of XPS (Figure 4(a)), and the corresponding elements percentage was listed in Table 1. For GN sample, only two peaks at 286.2 eV (C1s) and 533.2 eV (O1s) were detected. For g-GN and g-GN-APDMS samples, besides the strong signals of C1s and O1s, the signal of N1s was also found at 401.3 eV, indicating the intercalation of PA6 molecular chains into GN layers. In addition, for g-GN-APDMS sample, the new peaks at 101.3 eV (Si2s) and 152.0 eV (Si2p) were observed, and the content of N element was lower than that of g-GN, revealing that relative less PA6 molecules were intercalated into GN layers due to the anchoring of APDMS. The C1s core level spectrum of GN with peak-fitting curves in Figure 4(b) showed that

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three typical chemically shifted components: sp2 C=C and sp3 C-C at 284.5 eV, C-OH at 286.0 eV and C-O-C at 288.0 eV. However, for the samples of g-GN and g-GN-APDMS, the new appearance of C-N at 285.6 eV and C=O at 288.9 eV indicated the presence of PA6 chains onto GN surfaces in Figure 4(d,g). The O1s core level spectrum of GN in Figure 4(c) showed only one component of C-O at 531.0 eV, while g-GN and g-GN-APDMS exhibited another C=O peak at 532.9 eV in Figure 4(e,h), which was caused by the repetitive -O=C-NH- bond in PA6 chains. Furthermore, the N1s spectra of g-GN and g-GN-APDMS presented C-N peak at 399.3 eV, which was assigned to -NH-C=O-, as shown in Figure 4(f,i). Meanwhile, the Si spectrum of g-GN-APDMS exhibited Si-O peak at 102.3 eV, which was ascribed to Si-O-Si bond in APDMS molecules. These above mentioned XPS results further demonstrated the intercalation of PA6 and APDMS molecular chains into GN layers, which agreed well with FT-IR analysis.

Figure 4. XPS results for all samples (a) and the related C1s, O1s, N1s and Si2s, Si2p spectra of GN (b,c), g-GN (d,e,f) and g-GN-APDMS (g,h,i,j) 13



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Table 1. Percentage of atomic composition of GN, g-GN and g-GN-APDMS Samples

C (%)

O (%)

N (%)

Si (%)

GN

92.93

7.07

-

-

g-GN

77.27

14.61

8.12

-

g-GN-APDMS

75.36

15.69

5.97

2.98

TGA was conducted to quantify the intercalation ratio of PA6 molecules into GN layers, and the result was presented in Figure 5. GN showed a weight loss of 4.2% in the range of 30~800 °C, resulting from the removal of the water held on the surface and the decomposition of oxygen groups like epoxy and hydroxyl on GN surface, leaving residual carbon. In contrast, for g-GN and g-GN-APDMS samples, the degradation process has two stages. The first degradation stage in the range of 30-300 °C was caused by the removal of functional groups from GN surface. At the second degradation stage in the range of 300~450 °C, obvious weight loss was observed and the weight loss of g-GN due to the decomposition of PA6 component was higher than that of g-GN-APDMS, which was attributed to the decomposition of PA6 and APDMS components. Compared with GN, the thermal weight loss for g-GN increased by 31.69%, while the weight loss for g-GN-APDMS increased only by 21.41%.

Figure 5. TGA curves of GN, g-GN and g-GN-APDMS

Based on the above analysis, the intercalation ratio (D) of PA6 or APDMS molecules can be estimated according to the TGA data. For the sample of g-GN, the intercalation ratio (D) of 14


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PA6 molecules on the unit mass of GN (per gram) was calculated with the following equations.40 M1 = M 0 × Wg-GN - (M 0 - M1 ) × WGN

M1 =

that is,

Wg-GN - WGN 1 - WGN

(3)

× M0

(4)

D = M1 / (M 0 - M1 ) × 100%

(5)

where WGN and Wg-GN are the thermal weight loss ratio of GN and g-GN, respectively; M0 is the weight of the added g-GN, and M1 is the weight of PA6 intercalated into GN layers. For g-GN-APDMS sample, the intercalation ratio (D) of PA6 and APDMS molecules was calculated according to the same method and herein M1 is the weight of PA6 and APDMS molecules intercalated into GN layers. As shown in Table 2, it was found that the intercalation ratio of g-GN-APDMS was much lower than that of g-GN due to the anchoring effect of APDMS onto GN surface. APDMS with low molecular weight was much more easily intercalated into GN layers than PA6 with high molecular weight, and the limited compatibility between APDMS and PA6 resulted in the low intercalation of PA6 chains in GN layers. The result was in accordance with FTIR, Raman and XPS analysis. Table 2. The intercalation ratio of g-GN and g-GN-APDMS Samples

W (%)

D (%)

GN

4.20

-

g-GN

35.89

49.73

g-GN-APDMS

25.61

28.78

Note: W is the thermal weight loss ratio of the samples.

3.1.2 Intercalation structure and morphology The XRD patterns of GN, g-GN and g-GN-APDMS samples were illustrated in Figure 6. The broad peak for the (002) plane of GN at 2θ=24-28°corresponded to the disorganization 15



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order of GN. For g-GN and g-GN-APDMS samples, the characteristic peak corresponding to GN broadened, and the new obvious diffraction peaks at 2θ=19.98° and 23.96° corresponding to α1 and α2 crystalline form of PA6 were observed,41,42 indicating the intercalation of PA6 molecular chains into GN layers.

Figure 6. XRD patterns of GN, g-GN and g-GN-APDMS

The intercalation of PA6 molecules into GN layers can be intuitively characterized by AFM analysis. Figure 7 depicted the typical AFM images and the corresponding 3D view images of GN, g-GN and g-GN-APDMS. The height of the individual GN nano-sheet was measured to be ~0.51 nm, larger than that of the general ideal thickness of exfoliated grapheme (0.35 nm),43 due to the wrinkles on the GN surface caused by the inherent instability of 2D structures, the epoxy (C-O-C) and hydroxyl (O-H) groups held on the layers. For the sample of g-GN, the average thickness increased to ~3.39 nm because of the intercalation of PA6 molecules into GN layers, thicker than that of g-GN-APDMS, which was measured to be ~3.22 nm, further confirming the relative less PA6/APMDS molecules intercalated into GN layers.

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Figure 7. Typical AFM images of GN (a), g-GN (b) and g-GN-APDMS (c) and 3D view images of GN (d), g-GN (e) and g-GN-APDMS (f)

SEM and TEM were used to observe the micrographs of GN-APDMS in MC PA6 matrix at the submicrometer scale, as shown in Figure 8. For the sample of MC PA6/APDMS, it can be observed that the composite clearly exhibited a two-phase morphology structure due to the poor compatibility, where the spherical APDMS particles tended to self-aggregate (Figure 8(a)). For MC PA6/GN nano-composite, the cryogenically fractured surface was relatively smooth and GN was embedded and homogeneously distributed in the PA6 matrix (Figure 8(b)), although some aggregates may be observed in the matrix (Figure 8(c,d)). In comparison, for MC PA6/GN-APDMS nano-composite, it was found that the size of dispersed spherical APDMS particles was obviously reduced to ~8 µm from ~45 µm in MC PA6/APDMS composite. GN layer bonded with APDMS was embedded into MC PA6 matrix and no phase separation was observed (Figure 8(e,f,g)). EDS result showed the element composition of the corresponding components of the nano-composite system, further revealing the microstructure of the system, as shown in Figure 8(h). The compatibility between APDMS and MC PA6 is

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poor, but there is strong interaction between APDMS and GN, as a result, APDMS was easily anchored onto GN surface and uniformly distributed in MC PA6 matrix. The model of the formation mechanism of the molecular structure was proposed, as shown schematically in Figure 9.

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Figure 8. SEM images of cryogenically fractured surfaces of MC PA6/APDMS (a), MC PA6/GN (b) and MC PA6/GN-APDMS (e); TEM images of MC PA6/GN (c,d) and MC PA6/GN-APDMS (f,g). EDS element analysis of MC PA6/GN-APDMS (h) 19



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Figure 9. Schematic illustration of the interfacial interactions of MC PA6/GN-APDMS nano-composite

3.2 Synergistic reinforcing effect of GN-APDMS on MC PA6 The effect of GN-APDMS on the mechanical performances of MC PA6 was investigated. Figure 10(a) showed the stress-strain curves of neat MC PA6 and MC PA6 composites, and the stress-strain curves for all samples presented an untypical stress yield behavior, and neat MC PA6 showed an elastic deformation stress plateau. By incorporation of GN, APDMS or GN-APDMS, the stress plateau of the composites disappeared, which indicated that the molecular movement and deformation of PA6 matrix were restricted. Figure 10(b,c) showed the mechanical performances of neat MC PA6 and the composites. Compared with neat MC PA6, for MC PA6/APDMS composite, the tensile strength and the elongation at break decreased drastically while the impact strength increased, which was caused by the relatively poor compatibility between flexible APDMS and PA6 matrix. For MC PA6/GN nano-composite, there was no notable improvement of mechanical strength due to stress concentration and defects. However, for MC PA6/GN-APDMS nano-composite, a rough 12% increase in tensile strength, from 81.0 to 90.6 MPa, and 37% increase in impact strength was achieved, indicating the synergistic reinforcing and toughening effect of GN-APDMS on PA6 matrix by the anchored APDMS molecular chains onto GN surface.

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Figure 10. Stress-strain curves (a) and mechanical properties (b,c) of neat MC PA6, MC PA6/APDMS composite, MC PA6/GN nano-composite and MC PA6/GN-APDMS nano-composite

The impact and tensile fractured surface morphologies of neat MC PA6 and MC PA6 composites were shown in Figure 11. The impact and tensile fractured surfaces of neat MC PA6 were relatively flat and smooth (Figure 11(a,b)), showing typical characteristics of brittle fracture. For MC PA6/APDMS composite, the impact fractured surface was relatively rough while the tensile fractured surface showed segregated spherical APDMS aggregates due to the phase separation (Figure 11(c,d)). For MC PA6/GN nano-composite, the fractured surfaces morphologies exhibited no notable changes compared with neat MC PA6 (Figure 11(e,f)). However, for MC PA6/GN-APDMS nano-composite, the fractured surfaces showed many yield folds and large deformation accompanied by a large amount of stress whitening phenomena (Figure 11(g,h)), presenting obvious ductile fracture characteristics. Meanwhile, 21



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the phase interface between GN-APDMS and MC PA6 was obscure without apparent phase separation or other defects, indicating excellent compatibility and strong interfacial adhesion of the two phases. As a consequence, the maximized interfacial contact resulted in the effective stress transfer and relaxation at the interfacial region.44,45

Figure 11. SEM images of impact and tensile fractured surfaces of neat MC PA6 (a,b), MC PA6/APDMS composite (c,d), MC PA6/GN nano-composite (e,f) and MC PA6/GN-APDMS nano-composite (g,h) (magnification ×500)

To examine the reinforcing efficiency of GN-APDMS on the performance of MC PA6, DMA measurement was performed to investigate the dynamic mechanical

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properties. The temperature dependent curves of the storage modulus (E′) and loss factor (tan δ) of MC PA6 composites were displayed in Figure 12, and the DMA parameters were listed in Table 3. It can be seen that the E′ of MC PA6/GN-APDMS was higher than that of the composites by only addition of APDMS or GN (Figure 12a), indicating that the stiffness and load-bearing capacity of the nano-composite was improved, which was a convincing evidence of synergistic reinforcing effect of GN-APDMS on MC PA6. Another important feature is that tan δ peak for MC PA6/GN-APDMS nano-composite shifted toward higher temperature (Figure 12b). Taking the temperature at the maximum of the tan δ peak as the glass transition temperature (Tg), compared with MC PA6/APDMS or MC PA6/GN composites, it can be seen that the Tg of MC PA6/GN-APDMS nano-composite shifted to high temperature, which reflected an enhancement in the interfacial interaction between GN-APDMS and MC PA6 matrix. The value of tan δ of MC PA6/GN-APDMS was strongly suppressed, indicating that the movement of molecular chains of PA6 became much more difficult.

Figure 12. Storage modulus (a) and tan δ (b) versus temperature for neat MC PA6, MC PA6/APDMS composite, MC PA6/GN nano-composite and MC PA6/GN-APDMS nano-composite

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Table 3. Numerical DMA data of neat MC PA6, MC PA6/APDMS composite, MC PA6/GN nano-composite and MC PA6/GN-APDMS nano-composite Sample

E′ (MPa)

Τg (°C)

tan δ

neat MC PA6

4318

56.36

0.149

MC PA6/APDMS

3098

28.62

0.160

MC PA6/GN

5453

65.57

0.141

MC PA6/GN-APDMS

7479

79.73

0.142

3.3 Synergistic friction-reducing effect of GN-APDMS on MC PA6 The impact of GN-APDMS on the tribological performance of MC PA6 was studied, as shown in Figure 13. Figure 13(a) showed the variation of friction coefficient with sliding time of neat MC PA6 and MC PA6 composites during the wear test. The friction coefficient for all the samples increased rapidly at the initial stage, and then tended to be stable gradually with the prolongation of sliding time. Compared with neat MC PA6, for MC PA6/APDMS composite, the friction coefficient decreased drastically, which demonstrated the excellent lubricating effect of APDMS. For MC PA6/GN nano-composite, there was no apparent reduction for the friction coefficient, or even slightly increased. However, for MC PA6/GN-APDMS nano-composite, the friction coefficient decreased significantly and exhibited a lower trend than MC PA6/APDMS composite. Figure 13(b) showed the friction coefficient and specific wear rate of neat MC PA6 and the composites. Compared with neat MC PA6, for MC PA6/APDMS composite, the friction coefficient exhibited an obvious downward trend while the specific wear rate increased a lot, which was mainly caused by the deterioration of mechanical strength due to poor compatibility between self-lubricating APDMS and MC PA6. For MC PA6/GN nano-composite, the impact of GN on the friction coefficient could be negligible, but the specific wear rate significantly decreased owing to excellent wear resistance of GN nano-sheets. However, for MC PA6/GN-APDMS nano-composite, both of the friction 24


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coefficient and the specific wear rate substantially decreased. Moreover, compared with neat MC PA6 sample (coefficient friction: 0.5949; specific wear rate: 3.8175×10-6 mm3/Nm), the friction coefficient and specific wear rate of MC PA6/GN-APDMS sample (coefficient friction: 0.1120; specific wear rate: 1.9920×10-6 mm3/Nm) decreased by more than 81% and 48%, respectively. GN with excellent mechanical strength and wear resistance could reinforce and protect PA6 matrix, and thus load bearing and stress transfer was achieved during the wear test; meanwhile, APDMS with self-lubricating property contributed to decreasing friction coefficient of the composite. As a consequence, the tribological performance was improved through the synergistic friction-reducing effect of GN-APDMS on MC PA6.

Figure 13. Variations of friction coefficient with time (a) and tribological properties (b) of neat MC PA6, MC PA6/APDMS composite, MC PA6/GN nano-composite and MC PA6/GN-APDMS nano-composite

Figure 14 showed SEM images of the worn surfaces of MC PA6 composites. For the neat MC PA6 sample, obvious ploughing damage and rugged furrows appeared on the worn surface, and severe matrix spalling phenomenon occurred along sliding direction (Figure 14b, marked by red arrows). For MC PA6/APDMS sample, the worn surface showed more serious damage, and the torn debris and the spherical APDMS aggregates were exposed on the worn surface (Figure 14d), revealing the severe deterioration of wear resistance. For MC PA6/GN sample, the worn surface displayed relatively flat feature due to the protection of GN for PA6 25



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matrix (Figure 14f), indicating that the wear resistance was improved. In contrast, for MC PA6/GN-ADPMS sample, the worn surface showed much smooth characteristics and no obvious rugged defects appeared (Figure 14h), resulting from the formation and antifriction effect of GN-APDMS lubricating film on the worn surface.

Figure 14. SEM images of the worn surfaces of neat MC PA6 (a,b), MC PA6/APDMS composite (c,d), MC PA6/GN nano-composite (e,f) and MC PA6/GN-APDMS nano-composite (g,h) (a,c,e,g: magnification ×200; b,d,f,h: magnification ×1000)

Figure 15 showed the AFM 3D view images and the corresponding depth distribution of the worn surfaces of MC PA6 composites. In Figure 15(a,e), the neat MC PA6 sample exhibited relatively rough worn surface, with depth distribution ranging from 53 to 196 nm. By comparison, MC PA6/APDMS sample showed a much rough worn surface, with depth 26


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distributing ranging from 34-285 nm, as shown in Figure 15(b,f). For MC PA6/GN sample, the worn surface was relative smooth, with depth distribution ranging from 48-156 nm, as shown in Figure 15(c,g). And for MC PA6/GN-APDMS sample, the worn surface was more smooth and more uniform depth distribution ranging from 26 to 82 nm was observed, as shown in Figure 15(d,h). The result demonstrated that a protection layer resisting deformation formed through the synergistic tribological effect of GN-APDMS during sliding process, leading to lower friction coefficient and the specific wear rate for the nano-composite.

Figure 15. AFM 3D images and depth distribution of the worn surfaces of neat MC PA6 (a,e), MC PA6/APDMS composite (b,f), MC PA6/GN nano-composite (c,g) and MC PA6/GN-APDMS nano-composite (d,h)

DSC curves and the comparison of crystallinity of the bulk and the worn surface of MC 27



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PA6 composites were illustrated in Figure 16. Non-friction part of the samples was designated as the bulk part. For the neat MC PA6, the melting and crystallization temperature, and crystallinity of the worn surface were 2.6 °C, 7.8 °C, and 3.55% higher than that of the bulk part, which indicated that the crystal was more perfect after friction. Besides, the molecules of the worn surface could be fully annealed due to the shear and stretching effects of the surface, and the friction heat was produced under the circular movement of the steel ring, thus the crystallization ability was improved.40 For MC PA6/APDMS sample, the melting and crystallization temperature, and crystallinity of the worn surface were 5.3 °C, 8.3 °C, and 20% higher than that of the bulk part, also indicating the obvious improvement of crystallization ability. However, for MC PA6/GN and MC PA6/GN-APDMS samples, the variation of the crystallization ability of the worn surfaces was lower than that of neat MC PA6. The result indicated that the inclusion of GN-APDMS could effectively separate the action of the steel ring on PA6 matrix, and reduce generated friction heat. The annealing degree of the worn surfaces for MC PA6/GN-APDMS nano-composite exhibited much lower than that of neat MC PA6, and thus the wear resistance was improved.

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Figure 16. DSC curves of the bulk and the worn surfaces of neat MC PA6 (a,b), MC PA6/APDMS composite (c,d), MC PA6/GN nano-composite (e,f) and MC PA6/GN-APDMS nano-composite (g,h); the comparison of crystallinity of the corresponding composites (i) 29



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A simplified schematic model provided an illustration for the enhanced tribological performance endued by synergistic reinforcing and friction-reducing effect of GN-APDMS, as illustrated in Figure 17. The appearance of GN-APDMS on the worn surface protected MC PA6 matrix against the external friction force and endowed the matrix with low resistance to shearing. And the strong interfacial interactions in the composite system effectively reinforced the matrix, which improved the load bearing ability and suppressed the generation of wear debris.

Figure 17. Schematic representation of the friction-reducing mechanism of MC PA6/GN-APDMS sample

4. CONCLUSIONS MC PA6/GN-APDMS nano-composite was synthesized via in situ polymerization. APDMS/PA6 molecules were confirmed to intercalate into GN layers, and strong interfacial interactions formed in the composite system. The intercalation ratio and the average thickness for g-GN-APDMS decreased compared with g-GN. APDMS was easily anchored onto GN surface and the size of APDMS was obviously reduced, meanwhile GN-APDMS was uniformly distributed in MC PA6 matrix and no phase separation morphology was observed. Compared with neat MC PA6, addition of GN-APDMS resulted in 12% increase in tensile 30


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strength and 37% increase in impact strength, superior to that of MC PA6/APDMS and MC PA6/GN composites, and E′ and Tg of MC PA6/GN-APDMS nano-composite also increased, indicating the synergistic reinforcing and toughening effect of GN-APDMS on MC PA6. Moreover, more than 81% and 48% reduction in friction coefficient and specific wear rate were achieved for the nano-composite, and the worn surface displayed smooth and flat features with uniform depth distribution, the lower annealing effect and reduced friction heat, demonstrating the synergistic friction-reducing effect of GN-APDMS on MC PA6. The present work shows a promising perspective in developing PA6 nano-composites with excellent mechanical and tribological performance in industrial scale.

REFERENCES (1) Pan, B.; Zhang, S.; Li, W.; Zhao, J.; Liu, J.; Zhang, Y.; Zhang, Y. Tribological and Mechanical Investigation of MC Nylon Reinforced by Modified Graphene Oxide. Wear 2012, 294, 395-401. (2) Wang, W.; Meng, L.; Huang, Y. Hydrolytic Degradation of Monomer Casting Nylon in Subcritical Water. Polym. Degrad. Stab. 2014, 110, 312-317. (3) Kang, S.; Chung, D. Improvement of Frictional Properties and Abrasive Wear Resistance of Nylon/Graphite Composite by Oil Impregnation. Wear 2003, 254, 103-110. (4) Zheng, L. Y.; Zhao, L. X.; Zhang, J. J. The Effect of Fiber Oxidation on the Tribological Behavior of 3D-Braided Carbon Fiber/Nylon Composites. Wear 2007, 262, 1026-1030. (5) Li, D. X.; Deng, X.; Wang, J.; Yang, J.; Li, X. Mechanical and Tribological Properties of Polyamide 6-Polyurethane Block Copolymer Reinforced with Short Glass Fibers. Wear 2010, 269, 262-268. (6) Ning, L.; Jian, L.; Yang, S.; Wang, J.; Ren, J.; Wang, J. Effect of Carbon Black on

31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 39

Triboelectrification Electrostatic Potential of MC Nylon Composites. Tribol. Int. 2010, 43, 568-576. (7) Liu, H.; Huang, W.; Yang, X.; Dai, K.; Zheng, G.; Liu, C.; Shen, C.; Yan, X.; Guo, J.; Guo,

Z.

Organic

Vapor

Sensing

Behaviors

of

Conductive

Thermoplastic

Polyurethane-Graphene Nanocomposites. J. Mater. Chem. C 2016, 4, 4459-4469. (8) Sham, A. Y.; Notley, S. M. A Review of Fundamental Properties and Applications of Polymer-Graphene Hybrid Materials. Soft Matter 2013, 9, 6645-6653. (9) Lee, S. K.; Rana, K.; Ahn, J. H. Graphene Films for Flexible Organic and Energy Storage Devices. J. Phys. Chem. Lett. 2013, 4, 831-841. (10) Huang, X.; Qi, X.; Boey, F.; Zhang, H. Graphene-Based Composites. Chem. Soc. Rev. 2012, 41, 666-686. (11) Chen, D.; Zhang, H.; Liu, Y.; Li, J. Graphene and Its Derivatives for the Development of Solar Cells, Photoelectrochemical, and Photocatalytic Applications. Energy Environ. Sci. 2013, 6, 1362-1387. (12) Choudhary, S.; Mungse, H. P.; Khatri, O. P. Dispersion of Alkylated Graphene in Organic Solvents and Its Potential for Lubrication Applications. J. Mater. Chem. 2012, 22, 21032-21039. (13) Berman, D.; Erdemir, A.; Sumant, A. V. Graphene: A New Emerging Lubricant. Mater.

Today 2014, 17, 31-42. (14) Zhang, L.; Pu, J.; Wang, L.; Xue, Q. Synergistic Effect of Hybrid Carbon Nanotube-Graphene Oxide as Nanoadditive Enhancing the Frictional Properties of Ionic Liquids in High Vacuum. ACS Appl. Mater. Interfaces 2015, 7, 8592-8600.

32


Page 33 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(15) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films.

Science, 2004, 306, 666-669. (16) Kim, K. S.; Lee, H. J.; Lee, C.; Lee, S. K.; Jang, H.; Ahn, J. H.; Kim, J. H.; Lee, H. J. Chemical Vapor Deposition-Grown Graphene: The Thinnest Solid Lubricant. ACS Nano 2011, 5, 5107-5114. (17) Huang, T.; Xin, Y.; Li, T.; Nutt, S.; Su, C.; Chen, H.; Liu, P.; Lai, Z. Modified Graphene/Polyimide Nano-composites: Reinforcing and Tribological Effects. ACS Appl.

Mater. Interfaces 2013, 5, 4878-4891. (18) Tai, Z.; Chen, Y.; An, Y.; Yan, X.; Xue, Q. Tribological Behavior of UHMWPE Reinforced with Graphene Oxide Nanosheets. Tribol. Lett. 2012, 46, 55-63. [19] Shen, X. J.; Pei, X. Q.; Liu, Y.; Fu, S. Y. Tribological Performance of Carbon Nanotube-Graphene Oxide Hybrid/Epoxy Composites. Composites, Part B 2014, 57, 120-125. (20) Young, R. J.; Kinloch, I. A.; Gong, L.; Novoselov, K. S. The Mechanics of Graphene Nanocomposites: A Review. Compos. Sci. Technol. 2012, 72, 1459-1476. (21) Li, S.; Li, Q.; Carpick, R. W.; Gumbsch, P.; Liu, X. Z.; Ding, X.; Sun, J.; Li, J. The Evolving Quality of Frictional Contact with Graphene. Nature 2016, 539, 541-545. (22)

Kalin,

M.;

Zalaznik,

M.;

Novak,

S.

Wear

and

Friction

Behaviour

of

Poly-Ether-Ether-Ketone (PEEK) Filled with Graphene, WS2 and CNT Nanoparticles. Wear 2015, 332, 855-862. (23) Shen, X. J.; Pei, X. Q.; Fu, S. Y.; Friedrich, K. Significantly Modified Tribological

33



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 39

Performance of Epoxy Nano-Composites at Very Low Graphene Oxide Content. Polymer 2013, 54, 1234-1242. (24) An, Y.; Tai, Z.; Qi, Y.; Yan, X.; Liu, B.; Xue, Q.; Pei, J. Friction and Wear Properties of Graphene

Oxide/Ultrahigh-Molecular-Weight

Polyethylene

Composites

Under

the

Lubrication of Deionized Water and Normal Saline Solution. J. Appl. Polym. Sci. 2014, 131, 3-11. (25) Lewis, G. N. The Atom and the Molecule. J. Am. Chem. Soc. 1916, 38, 762-785. (26) Mark, J. E. Some Interesting Things about Polysiloxanes. Acc. Chem. Res. 2004, 37, 946-953. (27) Burnside, S. D.; Giannelis, E. P. Nanostructure and Properties of Polysiloxane-Layered Silicate Nanocomposites. J. Polym. Sci. Part B Polym. Phys. 2000, 38, 1595-1604. (28) Zheng, Y.; Cheng, J.; Zhou, C.; Xing, H.; Wen, X.; Pi, P.; Xu, S. Droplet Motion on a Shape Gradient Surface. Langmuir 2017, 33, 4172-4177. (29) Xue, C. H.; Zhang, Z. D.; Zhang, J.; Jia, S. T. Lasting and Self-Healing Superhydrophobic

Surfaces

by

Coating

of

Polystyrene/SiO2

Nanoparticles

and

Polydimethylsiloxane. J. Mater. Chem. A 2014, 2, 15001-15007. (30) Xiao, M.; Sun, L.; Liu, J.; Li, Y.; Gong, K. Synthesis and Properties of Polystyrene/Graphite Nanocomposites. Polymer 2002, 43, 2245-2248. (31) Szabó, T.; Berkesi, O.; Forgó, P.; Josepovits, K.; Sanakis, Y.; Petridis, D.; Dékány, I. Evolution of Surface Functional Groups in a Series of Progressively Oxidized Graphite Oxides. Chem. Mater. 2006, 18, 2740-2749. (32) Nethravathi, C.; Rajamathi, M. Chemically Modified Graphene Sheets Produced by the

34


Page 35 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Solvothermal Reduction of Colloidal Dispersions of Graphite Oxide. Carbon 2008, 46, 1994-1998. (33) Yi, L.; Zhan, X.; Chen, F.; Du, F.; Huang, L. J. Synthesis and Characterization of Poly [Styrene-b-Methyl (3,3,3-Trifluoropropyl) Siloxane] Diblock Copolymers Via Anionic Polymerization. J. Polym. Sci. Part A Polym. Chem. 2005, 43, 4431-4438. (34) Palsule, A. S.; Poojari, Y. Enzymatic Synthesis of Silicone Fluorinated Aliphatic Polyesteramides. Polymer 2010, 51, 6161-6167. (35) Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; Geim, A. K. Raman Spectrum of Graphene and Graphene Layers. Phys. Rev. Lett. 2006, 97, 187401. (36) Guo, J.; Ren, L.; Wang, R.; Zhang, C.; Yang, Y.; Liu, T. Water Dispersible Graphene Noncovalently Functionalized with Tryptophan and Its Poly (Vinyl Alcohol) Nanocomposite.

Composites, Part B 2011, 42, 2130-2135. (37) Paredes, J. I.; Villar-Rodil, S.; Solís-Fernández, P.; Martínez-Alonso, A.; Tascon, J. M. D. Atomic Force and Scanning Tunneling Microscopy Imaging of Graphene Nanosheets Derived from Graphite Oxide. Langmuir 2009, 25, 5957-5968. (38) Ji, J.; Zhang, G.; Chen, H.; Wang, S.; Zhang, G.; Zhang, F.; Fan, X. Sulfonated Graphene as Water-Tolerant Solid Acid Catalyst. Chem. Sci. 2011, 2, 484-487. (39) Chen, W.; Yan, L.; Bangal, P. R. Preparation of Graphene by the Rapid and Mild Thermal Reduction of Graphene Oxide Induced by Microwaves. Carbon 2010, 48, 1146-1152. (40) Li, C.; Xiang, M.; Ye, L. Intercalation Structure and Highly Enhancing Tribological

35



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Performance of Monomer Casting Nylon-6/Graphene Nano-Composites. Composites, Part A 2017, 95, 274-285. (41) Li, C.; Xiang, M.; Ye, L. Structure and Tribological Performance of Monomer Casting Nylon-6/Colloidal Graphite Composites Synthesized Through In Situ Polymerization.

Polym.-Plast. Technol. Eng. 2017, DOI: 10.1080/03602559.2016.1275685. (42) Boscolo Boscoletto, A.; Trezza, G.; Andreis, B.; Milan, L.; Tavan, M.; Furlan, P. Anionic Polyamides Modified with Poly (Oxypropylene) by "One-Shot" RIM Technology: Structural and Morphological Characterization. Macromolecules 1992, 25, 5752-5758. (43) Gu, J.; Du, J.; Dang, J.; Geng, W.; Hu, S.; Zhang, Q. Thermal Conductivities, Mechanical and Thermal Properties of Graphite Nanoplatelets/Polyphenylene Sulfide Composites. RSC Adv. 2014, 4, 22101-22105. (44) Zhang, X.; Xue, X.; Yin, Q.; Jia, H.; Wang, J.; Ji, Q.; Xu, Z. Enhanced Compatibility and Mechanical Properties of Carboxylated Acrylonitrile Butadiene Rubber/Styrene Butadiene Rubber by Using Graphene Oxide as Reinforcing Filler. Composites, Part B 2017, 111, 243-250. (45) Sainsbury, T.; Gnaniah, S.; Spencer, S. J.; Mignuzzi, S.; Belsey, N. A.; Paton, K. R.; Satti, A. Extreme Mechanical Reinforcement in Graphene Oxide Based Thin-Film Nanocomposites Via Covalently Tailored Nanofiller Matrix Compatibilization. Carbon 2017, 114, 367-376.

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Caption Figure Captions Figure 1. Synthesis process of MC PA6/GN-APDMS nano-composite via in situ polymerization Figure 2. (a) FTIR spectra of GN, g-GN and g-GN-APDMS; (b) Anchoring effect of APDMS and PA6 molecules onto GN surface Figure 3. Raman spectra of the samples of GN, g-GN and g-GN-APDMS Figure 4. XPS results for all samples (a) and the related C1s, O1s, N1s and Si2s, Si2p spectra of GN (b,c), g-GN (d,e,f) and g-GN-APDMS (g,h,i,j) Figure 5. TGA curves of GN, g-GN and g-GN-APDMS Figure 6. XRD patterns of GN, g-GN and g-GN-APDMS Figure 7. Typical tapping-mode AFM images of GN (a), g-GN (b) and g-GN-APDMS (c) and 3D view images of GN (d), g-GN (e) and g-GN-APDMS (f) Figure 8. SEM images of cryogenically fractured surfaces of MC PA6/APDMS (a), MC PA6/GN (b) and MC PA6/GN-APDMS (e); TEM images of MC PA6/GN (c,d) and MC PA6/GN-APDMS (f,g). EDS element analysis of MC PA6/GN-APDMS (h) Figure 9. Schematic illustration of the interfacial interactions of MC PA6/GN-APDMS nano-composite Figure 10. Stress-strain curves (a) and mechanical properties (b,c) of neat MC PA6, MC PA6/APDMS composite, MC PA6/GN nano-composite and MC PA6/GN-APDMS nano-composite Figure 11. SEM images of impact and tensile fractured surfaces of neat MC PA6 (a,b), MC PA6/APDMS composite (c,d), MC PA6/GN nano-composite (e,f) and MC PA6/GN-APDMS

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nano-composite (g,h) (magnification ×500) Figure 12. Storage modulus (a) and tan δ (b) versus temperature for neat MC PA6, MC PA6/APDMS composite, MC PA6/GN nano-composite and MC PA6/GN-APDMS nano-composite Figure 13. Variations of friction coefficient with time (a) and tribological properties (b) of neat MC PA6, MC PA6/APDMS composite, MC PA6/GN nano-composite and MC PA6/GN-APDMS nano-composite Figure 14. SEM images of the worn surfaces of neat MC PA6 (a,b), MC PA6/APDMS composite

(c,d),

MC

PA6/GN

nano-composite

(e,f)

and

MC

PA6/GN-APDMS

nano-composite (g,h) (a,c,e,g: magnification ×200; b,d,f,h: magnification ×1000) Figure 15. AFM 3D images and depth distribution of the worn surfaces of neat MC PA6 (a,e), MC PA6/APDMS composite (b,f), MC PA6/GN nano-composite (c,g) and MC PA6/GN-APDMS nano-composite (d,h) Figure 16. DSC curves of the bulk and the worn surfaces of neat MC PA6 (a,b), MC PA6/APDMS composite (c,d), MC PA6/GN nano-composite (e,f) and MC PA6/GN-APDMS nano-composite (g,h); the comparison of crystallinity of the corresponding composites (i) Figure 17. Schematic representation of the friction-reducing mechanism of MC PA6/GN-APDMS sample

Table Captions Table 1. Percentage of atomic composition of GN, g-GN and g-GN-APDMS Table 2. The intercalation ratio of g-GN and g-GN-APDMS Table 3. Numerical DMA data of neat MC PA6, MC PA6/APDMS composite, MC PA6/GN nano-composite and MC PA6/GN-APDMS nano-composite

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Table of Content graphic 59x35mm (300 x 300 DPI)


Graphene-Polysiloxane

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