Reinforcement of hydrogenated carboxylated nitrile–butadiene rubber with exfoliated graphene oxide

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Reinforcement of hydrogenated carboxylated nitrile–butadiene rubber with exfoliated graphene oxide Xin Bai a, Chaoying Wan b, Yong Zhang

a,*

, Yinghao Zhai

a

a

School of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, PR China b Department of Mechanical and Manufacturing Engineering, Trinity Center for Bioengineering, Trinity College Dublin, Dublin 2, Ireland

A R T I C L E I N F O

A B S T R A C T

Article history:

Graphene oxide (GO)/hydrogenated carboxylated nitrile–butadiene rubber (HXNBR) com-

Received 11 November 2010

posites were prepared by a solution-blending method. The GO monolayers with 0.9 nm in

Accepted 14 December 2010

thickness, more than 2.5 lm in width and 3 lm in length were exfoliated from natural flake

Available online 21 December 2010

graphite by a modified Hummers method and could be further dispersed homogeneously in HXNBR matrix even for the GO contents up to 1.3 vol.%. The addition of 0.44 vol.% of GO nanosheets enhanced the tensile strength and modulus at 200% elongation of HXNBR by more than 50% and 100%. This is believed to be due to strong interfacial interactions between the oxygen-containing functional groups on the surfaces of GO nanosheets and the carboxyl groups in HXNBR. Moreover, this general observation is further supported by the increase of the glass transition temperature of HXNBR from 23.2 to 21.6 C, at a GO content of 1.3 vol.%. The results indicated that GO efficiently reinforced HXNBR due to the good dispersion and strong interfacial interactions.  2010 Elsevier Ltd. All rights reserved.

1.

Introduction

Carbon nanotubes (CNTs) and graphene have been intensively studied and explored in various applications for advanced technologies in recently years due to their fascinating properties, such as high Young’s modulus (1.0 TPa), fracture strength (63–125 GPa) [1], transport conductivity [2–4] and thermal conductivity (5000 W m1 K1) [5]. Graphene composed of several planar sheets of sp2-hybridized carbon atoms in the dimensions of micrometers in diameter and 1 nm in thickness, rolls up to form CNTs with diameters ranging from 1 to 100 nm and lengths of up to millimeters [6]. Such high specific surface area (2630 m2 g1) [7] and high aspect ratio (>1000) as well as the intrinsic mechanical properties of graphene and CNTs make them be attractive reinforcing phases for ceramics and polymers. However, homogeneous dispersion and efficient interfacial stress transfer are still the main

challenges for the effective reinforcement due to the different surface chemistry of nanoparticles as compared to polymeric matrix and nanoparticle aggregation. Surface functionalization including non-covalent or covalent modification of nanoparticles is an effective method in improving of the interfacial interactions between nanoparticles and polymer matrix. Oxidative treatment by concentrated acids has been widely applied as the initial step to break CNT bundles or to delaminate graphene nanosheets as well as to improve the chemical functionality of the nanoparticles. Chemically modified CNTs exhibit the best reinforcement effects on polymers as compared to other modification methods [8]. The resultant graphene oxide (GO) fabricated through chemical exfoliation of graphite has abundant functional groups on the surface including hydroxyls, epoxides, diols, ketones and carboxyls, which are even more promising for reinforcing polymers. Meanwhile, the chemical

* Corresponding author: Fax: +86 21 54741297. E-mail address: [email protected] (Y. Zhang). 0008-6223/$ - see front matter  2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2010.12.043

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functionality significantly alters the Van der Waals interactions among the particle aggregates, makes them easy to be dispersed in organic matrix. Take CNT (GO)/polyvinyl alcohol (PVA) composites for an instance, the Young’s modulus, tensile strength and toughness of the CNT/PVA composites containing 0.6 vol.% of nanotubes were found 3.7, 4.3, and 1.7 times higher than those of bulk PVA [9]. A 76% increase in tensile strength and a 62% improvement of Young’s modulus were achieved by the addition of only 0.4 vol.% of GO to PVA. The improved mechanical properties were mainly ascribed to the successful stress transfer through strong hydrogen bonding between PVA and GO [10]. CNTs reinforced polymer composites have been widely investigated in recent years, while the application of graphene or GO as a reinforcing phase has been mainly focused on thermoplastic [10,11] or thermoset polymers [12], in which the strength and modulus or electrical conductivity of polymers have been greatly increased but often with flexibility decreasing. For polyurethane elastomers with a glass transition temperature (Tg) below 30 C, the addition of 1 wt.% graphene improved both the tensile strength and modulus [13–15]. For hydrogenated carboxylated nitrile–butadiene rubber (HXNBR), it was found that the graphite were intercalated or exfoliated to a high extent, and the graphite nanosheets effectively improved the tensile strength and modulus at 100% up to 11.8 and 11.5 MPa when the addition content is 10 phr [16]. In our previous work, we found multi-walled CNTs enhanced the tensile strength in styrene–butadiene–styrene tri-block copolymer [17] and HXNBR [18]. In this work, we used a modified Hummers method to produce chemical

exfoliated GO nanosheets for the reinforcement of HXNBR. The mechanical properties of HXNBR were expected to be highly improved as compared to those of the CNTs reinforced HXNBR composites due to the above reasons. The dispersion and interfacial interactions of GO in HXNBR were analyzed by scanning electronic microscopy (SEM) and dynamic mechanical analysis (DMA), and also predicted by Halpin–Tsai model.

2.

Experimental

2.1.

Materials

Flake graphite with average particle size of 6 lm was purchased from Qingdao Graphite Co. Ltd. HXNBR (TherbanXT VP KA 8889, the Mooney viscosity ML (1+4)100 C is 77 ± 7) containing 33% acrylonitrile, 5% carboxylic acid and 3.5% residual double bonds, was kindly provided by LANXESS Deutschland GmbH. The chemicals include NaNO3, KMnO4, concentrated H2SO4, concentrated HCl, dimethyl formamide (DMF), tetrahydrofuran (THF), 30% H2O2 aqueous solution and dicumyl peroxide (DCP) were all analytical grade and purchased from Sinopharm Chemical Reagent Co. Ltd., China.

2.2.

Preparation of GO

GO was prepared using a modified Hummers method from flake graphite [19]. Briefly, 5 g of flake graphite and 3 g of NaNO3 were put into a flask, then 120 ml concentrated H2SO4 was added with stirring in an ice-water bath. 22.5 g of KMnO4 were then slowly added to the above mixture over 1 h and followed by continually stirring at room temperature

(a)

(C)

(b)

(d) Height (nm)

2

0.90 nm

0

-2 0

1

2 Length (μm)

3

Fig. 1 – Typical images of GO dispersed in DMF. (a) TEM image of a GO nanosheet, the arrow indicates the folded edge of GO. (b) TEM image in larger magnification, the arrow shows the folded area. (c) A tapping mode AFM image of a GO nanosheet, and (d) the height profile of the AFM image.

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(23 C) for 2 h. Then 700 ml H2SO4 aqueous solution (5 wt.%) was slowly added with stirring and the temperature was kept at 98 C. The resultant mixture was further stirred for 2 h at 98 C. Then the temperature was reduced to 60 C and 15 ml H2O2 aqueous solution (30 wt.%) was added. The product was washed with 5% HCl solution and distilled water several times, then finally freeze-dried.

2.3.

diffraction (XRD) spectra were recorded by D/max-2200/PC (Japan Rigaku Corp.) using Cu Ka radiation. DMA was performed with DMA 7e (Perkin Elmer, Inc., USA) under a nitrogen atmosphere at a heating rate of 3 C min1 from 50 to 20 C and a frequency of 1 Hz. Tensile tests were performed on an Instron 4465 instrument at room temperature at a crosshead speed of 500 mm/min. The dumbbell shape samples were 75 mm in length, 0.1 mm in thickness and 4 mm in width.

Preparation of GO/HXNBR composites

3. GO/DMF solution was firstly prepared with the aid of ultrasonication to yield a clear solution. 5 g HXNBR was dissolved in 60 mL THF at room temperature. The GO/DMF solution was gradually added to the HXNBR solution, and sonicated for 60 min at room temperature. Finally, this homogeneous GO/ HXNBR solution was dried at 150 C until its weight kept unchanged. This composite was then mixed with 0.15 g DCP on a two roll mill at room temperature, and cured at 170 C for 20 min under pressure of 10 MPa. The GO/HXNBR composites with GO contents of 0.44 and 1.3 vol.% were prepared under the same procedure.

2.4.

Characterization

Atomic force microscopy (AFM) images of GO were taken in a tapping mode by carrying out on a Nano Scope III A (Digital Instrument, USA). The GO/DMF solution was spin-coated onto freshly exfoliated mica substrates at 2000 rpm and dried under vacuum at 80 C for AFM characterization. SEM images were taken on a JSM-7401F field-emission SEM system. Transmission electron microscopy (TEM) was carried out on a JEOL JEM-2100 electron microscope at 200 kV. The GO samples were prepared by depositing a drop of GO/DMF solution on carboncoated copper grids and dried at room temperature. X-ray

Results and discussion

Fig. 1 shows the TEM and AFM images of the morphology of the chemical exfoliated GO in this study. As shown in Fig. 1a and b, a thin and large GO sheet with more than 200 nm in width and 400 nm in length was observed and the sheet edges scrolled and folded slightly. Chemical modified GO often has oxygen-containing functional groups on its surface, and the GO sheets tend to fold back thus prevent them from stacking back and regraphitization. Such folding can also maintain the high surface area of GO sheets upon drying and also allows for a cross-section view of the ordered graphitic lattices as shown in Fig. 1b. As characterized by AFM, the prepared chemical exfoliated GO sheet was 0.9 nm in thickness, more than 2.5 lm in width and 3 lm in length (Fig. 1c and d) indicating that the graphite has been successfully exfoliated into monolayers [20] during the chemical modification process. The chemical modified GO sheets with oxygen-containing functional groups were readily exfoliated in DMF through hydrogen-bonding interaction and formed a stable solution with GO concentration of 0.44 vol.% as shown in Fig. 2a, right. The solution was well mixed with HXNBR/THF solution and gave a homogeneous mixture as shown in Fig. 2a, left. After solution-casting, drying, compounding and vulcanization

(a)

(b)

(c)

Fig. 2 – (a) Photograph of GO dispersion in DMF (right) and GO/HXNBR composites in THF (left); SEM cross-sectional image of GO/HXNBR composites with GO concentration of 0.44 vol.%: (b) 250·, (c) 5000·.

process, all GO/HXNBR composites with various GO contents presented as homogeneous black films without obvious aggregated sheets. The dispersion conditions of GO sheets in HXNBR matrix were observed by SEM and shown in Fig. 2b and c. No obvious GO aggregates were observed from the cross-section of the composite in Fig. 2c, and some wrinkled GO sheets were pulled out of HXNBR matrix as pointed by the arrows in Fig. 2c. The GO sheets in HXNBR are much thicker than those original GO as compared with Fig. 1c, which could be ascribed to the surface-coatings by HXNBR [21]. The dispersion of GO sheets in HXNBR matrix was more homogeneous than those observed in CNT/HXNBR composites [18], which were prepared by mechanical mixing method. Since the CNTs we used were acid-treated, the functional groups along the surface of CNTs should be similar with those of GO. Compared with acid treated CNTs, GO was much easier to be dispersed in HXNBR matrix owing to its structure and functional groups. The dispersion of GO in HXNBR was further characterized by XRD. Fig. 3 shows the XRD patterns of graphite, GO, HXNBR and the GO/HXNBR composite. As curves a and b show, the typical diffraction peak of graphite at 2h = 26.5 shifted to 11.2 after chemical modification, indicating the interlayer spacing between graphite layers was expanded from 0.35 nm for graphite sheets to 0.80 nm for GO. A broad diffraction peak was observed for HXNBR, indicating its noncrystalline structure (Fig. 3, curve d). For the GO/HXNBR composite with 1.3 vol.% GO (curve c), only a broad diffraction peak of HXNBR was observed, indicating that the periodic structure of GO might be ruptured during compounding process and GO has been exfoliated into monolayers or few layers. GO/ HXNBR composites with other GO loadings exhibit similar results with curve c. The well dispersed GO nanosheets in HXNBR matrix ensured strong interfacial interactions between the two phases. As observed in Fig. 4, Tg of HXNBR was increased from 23.2 to 21.6 C after addition of 1.3 vol.% GO. The GO sheets have abundant oxygen-containing groups (hydroxyls, carboxyls, epoxides) on the surfaces [20] and high aspect ratio of over 1000, which could form hydrogen bonding with the carboxyl groups in the HXNBR chains [14] and large interfaces between

Relative intensity

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d

1.6

0 vol% 0.22 vol% 0.44 vol% 1.3 vol%

1.4 1.2 1.0

Tan δ

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-20

0

20

Temperature/ºC

Fig. 4 – DMA curves of GO/HXNBR composites with different loadings of GO.

the two phases. Then efficient reinforcement of GO on HXNBR was expected. The mechanical properties of GO/HXNBR composites were summarized in Table 1 and Fig. 5. As shown in Table 1, the modulus at 200% elongation (M200) of HXNBR was consistently increased from 1.7 to 6.5 MPa as GO contents increased at the expense of elongation at break. The tensile strength of HXNBR was enhanced from 14.8 to 22.4 MPa, increased as much as 50% after addition of 0.44 vol.% GO then decreased to 10 MPa when the GO content was 1.3 vol.%. Compared with our previous work [18], GO/HXNBR composites showed higher tensile strength (22.4 MPa) than those of CNT/HXNBR composites (15.7 MPa) with the filler loading of 0.44 vol.% indicating better reinforcement effect of GO on HXNBR matrix. The GO/HXNBR composite with 0.44 vol.% of GO has a balanced mechanical properties, which could be ascribed to the much more homogeneous distribution of GO in HXNBR matrix compared with CNTs. Halpin-Tsai model [22–24] is used to predict the modulus of unidirectional or randomly distributed filler-reinforced composites. It was also used in this study to predict the dispersing state of GO in HXNBR. The Young’s moduli of the composites with randomly oriented (Er) and or unidirectional (Eu) GO sheets in HXNBR matrix were calculated using Eq. (1) and (2), respectively,   3 1 þ gL nVg 5 1 þ 2gT Vg þ ð1Þ Er ¼ Em 8 1  gL V g 8 1  gT Vg Eu ¼ Em

1 þ gL nVg 1  gL Vg

ð2Þ

c

Table 1 – Mechanical properties of GO/HXNBR composites.

b a 5

10

15

20

25

30

35

40

2θ / deg.

Fig. 3 – XRD patterns (a) graphite, (b) GO, (c) GO/HXNBR (1.3 vol.%) and (d) HXNBR.

GO contents (vol.%)

Tensile strength (MPa)

0 0.22 0.44 1.3

14.8 ± 0.8 21.7 ± 1.8 22.4 ± 1.3 10.3 ± 0.5

M200 (MPa) 1.7 ± 0.2 2.2 ± 0.2 3.4 ± 0.1 6.5 ± 0.3

Elongation at break (%) 534 ± 13 485 ± 10 419 ± 18 248 ± 8

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of the composites with different distribution of GO sheets were calculated and compared with the experimental data as shown in Fig. 6. It shows that the experimental results are close to the theoretical modeling results for the composite with random distribution of GO sheets, and then the dispersion of GO sheets in HXNBR is mostly random throughout the composites.

24

0 vol% 0.22 vol% 0.44 vol% 1.3 vol%

22 20

Tensile Stress (MPa)

18 16 14 12 10

4.

8 6 4 2 0

0

100

200

300

400

500

600

Strain (%)

Fig. 5 – Representative stress–strain behavior for GO/HXNBR composites with different GO volume ratio. gL ¼

Eg =Em  1 Eg =Em þ n

ð3Þ

gT ¼

Eg =Em  1 Eg =Em þ 2

ð4Þ

n¼

2l 3d

ð5Þ

where Er and Eu represent M200 of the composites with randomly distributed GO sheets and aligned GO sheets parallel to the surface of the sample, respectively. Eg and Em are the Young’s modulus of the GO and M200 of HXNBR. l and d refer to the average length and thickness of GO sheet, which were about 1.5 lm and 0.9 nm, respectively, as determined by AFM analysis. Vg is the volume fraction of GO in the composites. The Young’s modulus of the chemically reduced GO sheet was reported to be around 0.25 TPa [25] and used for the calculation. The M200 of unfilled HXNBR was 1.7 MPa from the experimental data in Table 1. The density of HXNBR is 0.97 g cm3 and the density of GO is 2.28 g cm3 [10]. M200

20 Experimental Halpin-Tsai(random) Halpin-Tsai(unidirectional)

Modulus/ MPa

15

10

5

0.0

0.3

0.6

0.9

1.2

1.5

GO/Vol%

Fig. 6 – Young’s moduli of the composites and Halpin–Tsai theoretical models; the theoretical simulations were taken as two cases: the random orientation and unidirectional distribution of GO sheets in the HXNBR matrix.

Conclusions

Chemical exfoliated GO nanosheets with high aspect ratios were successfully prepared by a modified Hummers method. Such GO nanosheets were well mixed with HXNBR by a simple solution-mixing method and produced GO/HXNBR composites. The GO nanosheets were dispersed homogeneously and randomly in HXNBR matrix as observed by SEM and XRD, which was also in coincidence with those predicted by Halpin–Tsai model results. The strong interfacial interactions between the functional groups of GO and the carboxyl groups of HXNBR were accounted for the increase of Tg of HXNBR from 23.2 to 21.5 C, and the increase of 50% and 100% in tensile strength and M200 of the HXNBR when the GO content was 0.44 vol.%.

Acknowledgements Financial supports from the National Natural Science Foundation of China (Project Numbers 50773036 and 51073092) and Shanghai Leading Academic Discipline Project (Project Number, B202) are gratefully acknowledged.

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