Synthesis of Graphene Oxide and Functionalized CNT Nanocomposites Based on Epoxy Resin

ABSTRACT

Lately, nanomaterials have been largely studied as reinforcements for epoxy resin. Although their usage is highly promising, the literature has reported some drawbacks regarding the improvement of mechanical properties in nanocomposites. These difficulties are usually due to dispersion of nanomaterials and its adhesion to the polymeric matrix. One approach to this problem is the functionalization of nanomaterials such as carbon nanotubes (CNTs) and graphene. In this work, we have studied the synthesis and functionalization process of CNTs and graphene oxide (GO) to be used as reinforcements for epoxy resin nanocomposites. CNTs were synthesized at 850 °C in a quartz furnace, from hexane and ferrocene vapor, and functionalized by acids and ethylenediamine treatments. GO was obtained by graphite exfoliation through a modified Hummer's method. The nanomaterials were characterized by Raman spectrum, FT-IR, XRD, and SEM images. Nanocomposites were prepared using these nanomaterials and evaluated by DMA. While both nanomaterials showed an improvement in mechanical properties, suggesting a chemical bond between nanomaterial and the epoxy matrix, it was clear that GO reinforced samples presented a higher storage modulus.

KEYWORDS:
Carbon nanotubes; Graphene oxide; Functionalization; Nanocomposites; Epoxy resin

INTRODUCTION

Epoxy resins are high performance thermoset polymers widely used in various industrial applications and largely studied as nanocomposites (Chen et al. 2007; Saeb et al. 2015). The development of nanocomposites has become of great interest in materials science, and it has attracted the attention of aeronautics industries because of its potential to reduce weight of metallic structures that can be replaced by nanocomposites (Francisco et al. 2015). Its superior strength-to-weight ratio can lead to the reduction of fuel consumption, which is a challenge for aerospace engineering and industry (Gohardani et al. 2014).

Nanomaterials are generally regarded as high potential fillers to act as reinforcements and improve mechanical properties of polymers (Gojny et al. 2005; Wang and Liew 2015). Carbon nanotubes (CNTs) are carbon allotropes discovered by Iijima in 1991 that have attracted great interest from both scientific and industrial communities due to its outstanding properties (Iijima 1991; Kumar et al. 2014). CNTs have frequently been added to polymers to increase the mechanical, electrical and thermal properties of nanocomposites. But as-grown CNTs usually present a mixture of various diameters, lengths, and chiralities, as well as impurities and defects. Also, CNTs aggregation can hinder the mechanical properties of nanocomposites, making its dispersion a keynote in the manufacturing process (Disfani and Jafari 2013).

Graphene is virtually a one-atom-thick molecule, with a structure of two dimensional (2D) planar sheet of sp² bonded carbon; it may present extraordinary properties such as high surface area (2630 m²g^-1), excellent electronic conductivity, high chemical and thermal stability, and high surface chemistry (Kumar et al. 2017). In practice, graphene may present more than one layer due to the difficulties to separate graphite layers and to keep them apart. Several studies about graphene deal with graphene oxide, which consists of various oxygen containing functional groups (epoxy, hydroxyl, carbonyl, etc.) attached to its surfaces and edges (Zhu et al. 2010). Graphene sheets, due to its two-dimensional structure and high aspect ratio, have been perceived as an ideal reinforcement for nanocomposites (Viculis et al. 2005; Si and Samulski 2008). According to Rafiee et al. (2009; 2010), graphene structure is expected to present better stress transfer from its platelets to the composite matrix during loading than CNTs.

However, for both CNTs and graphene materials, potential applications have become limited due to CNTs easy entanglement and agglomeration (large aspect ratio), and because graphene platelets tend to restack (van der Waals and strong π-π interactions) (Li et al. 2008; Yang et al. 2011). Therefore, one of the hardest challenges for the nanocomposites area is to achieve homogeneous dispersions of CNTs and graphene materials into the polymer matrix (Gojny et al. 2005). Also, obtaining a good interfacial interaction between filler and polymer is necessary for an effective load transfer to occur (Li et al. 2013).

One promising approach to address this matter is the chemical modification of the filler surface prior to its dispersion into the prepolymer (Xie et al. 2005; Zhu et al. 2003; Abdalla et al. 2007). Literature shows that, for CNTs, amine treatment can weaken the interactions between the tubes, resulting in a better dispersion and stronger interfacial adhesion to the matrix (Meng et al. 2008; Cividanes et al. 2013). In this work, we have synthesized and functionalized carbon nanotubes and synthesized oxidized graphene to minimize issues of entanglement and restacking in epoxy nanocomposites. Mechanical properties of the nanocomposites were analyzed by DMA.

EXPERIMENTAL

CNT SYNTHESIS AND FUNCTIONALIZATION

CNTs were synthesized via chemical vapor deposition (CVD) in a quartz furnace at 850 °C with gas flow of nitrogen (1000 sccm) carrying hexane and ferrocene vapor (16:84 %wt.) for 30 min. The tubes were grown in vertically aligned form over quartz plates positioned at the middle of the furnace. They were scraped off from the plates surfaces with the help of a stainless-steel spoon.

Then, CNTs were annealed at high temperature (1800 °C) in a graphite oven under inert atmosphere for 3 h, followed by two chemical treatments. In the first treatment, 0.9 g of CNTs were drowned in sulphuric and nitric acids (30 : 90 mL) for 20 h to promote its oxidation; the solution was sonicated for 2 h. To conclude this procedure, CNTs were filtered, washed with 500 mL of water and dried at 80 °C for 24 h. Following oxidation, the CNT sample underwent treatment with ethylenediamine (525 mL) in a condenser, kept at 110 °C and stirred at 700 rpm for 150 h. Afterwards it was filtered, washed with 500 mL of methanol and dried once more. The functionalized CNTs were then designated as CNT- Am.

GO SYNTHESIS

GO was synthesized through a modified Hummer's method (Wu et al. 2009): 6 g of natural graphite powder (Graphene Laboratories Inc.), 4.5 g sodium nitrate and 207 mL sulphuric acid were added in a reaction flask, kept at 10 °C, and stirred for 30 min, followed by the addition of 27 g potassium permanganate. The solution was stirred for 45 min and then 414 mL of water was added. After 12 h, 1260 mL of warm water and 45 mL oxygen peroxide (30%) were added. The suspension was filtered, washed several times and finally dried at 60 °C in a vacuum oven.

NANOCOMPOSITES FABRICATION

The epoxy resin used in this study was Araldite GY-260 (diglycidyl ether of bisphenol A, DGEBA) and the hardener was Aradur 972 (diaminodiphenylmethane, DDM), both supplied by Huntsman. Nanocomposite samples were prepared without nanomaterials (i.e. neat epoxy resin), with GO and with CNT- Am. They all received the same amount of filling material, 0.25 %wt., and the same amount of hardener, 27 %wt.

To improve CNT- Am and GO dispersion, these fillers were each mixed with acetone through bath sonication (25 kHz) for 30 min. Then the solutions were added to warm samples of epoxy resin (65 °C) and suffered tip sonication (42 kHz) for another 30 min. The resulting mixtures were degasified at 80 °C for 24 h to eliminate acetone and avoid bubbles in the final nanocomposites. The hardener was added to each mixture and heated to 90 °C to melt down. Curing process was held in two stages: at 80 °C for 1 h and at 120 °C for 2 h, according to the manufacturer's instructions.

CHARACTERIZATION METHODS

Raman spectrum was used to show the graphitic ordering before and after functionalization treatments on CNT samples. It was acquired on a Renishaw 2000 Micro-Raman, with Ar laser (λ = 514.5 nm) and range of 500 - 3500 cm^-1 (only first order spectrum is shown in results). XPS high-resolution spectra were obtained to determine atomic composition of CNT- Am and GO in a UNI-SPECS UHV spectrometer (5 × 10^-7 Pa, hν = 1253.6 eV). FT-IR was used to characterize the presence of chemical groups on CNT- Am and GO surfaces. Infrared spectra were recorded on a Perkin-Elmer Spectrum GX, in the range of 4000 - 400 cm^-1 with 4 cm^-1 resolution, 12 scans and KBr pellet method. XRD analyses were acquired in a PANalyticalX'Pert PRO X-Ray Diffraction Unit with Kα1 radiation (λ = 1.5406 Å) emitted by a copper target at 1800 W (45 kV; 40 mA). DMA tests were performed by a TA Instrument (DMA 2980 model) with cured samples in dual cantilever mode, with 20 µm amplitude, 1 Hz frequency and 0.56 N m torque.

RESULTS AND DISCUSSION

Figure 1 shows SEM images of CNTs as grown by CVD process and after functionalisation treatments (CNT- Am). CNTs as-grown were vertically aligned, presenting large blocks of tubes. As a result of functionalisation, CNT- Am showed a slight degree of tangling, but the blocks were clearly undone and the basic structure of the tubes was preserved.

Figure 1
SEM images of CNT: (a) a block of vertically aligned CNT as grown; (b) CNT- Am after acids and ethylenediamine treatment.

Figure 2 shows the Raman spectrum of CNTs as grown and CNT- Am. CNTs as grown have clear bands at 1348.91 cm^-1 (D band) and 1575.99 cm^-1 (G band), and a shoulder near 1591.21 cm^-1 (D' band). After functionalization, CNT- Am presents two bands at 1350.33 cm^-1 (D band) and 1581.96 cm^-1 (G band) and a shoulder near 1614.64 cm^-1 (D' band). These bands are characteristic of multi-walled CNTs (Kuan et al. 2005; Wang et al. 1998; Belin and Epron 2005). The higher intensity of the G band for CNTs as grown indicates a higher degree of graphitisation/crystallinity, while D band is typically attributed to disordered structures (defective CNTs and non-crystalline carbon) and indicates lattice distortions in the tubes (Awasthi et al. 2010). The change in intensities of D and G band could be observed on the Raman spectrum of CNT- Am due to the acid and amino functionalization process. It is known that during oxidizing treatments of graphitic structures two concurring phenomena take place: the removal of amorphous carbon from the tube walls and the formation of oxygenated functional groups (Scaffaro et al. 2012), changing the atomic structure from C-C sp² to C-C sp³. Due to this change, a displacement in the position of G band and a higher intensity of D band can be observed (Maio et al. 2015). Other studies have already shown that annealing treatment is effective for reduction of defects in CNTs structure, but the acid treatment eventually damages its walls (Cividanes et al. 2013; Huang et al. 2003).

Figure 2
Raman spectrum of: (a) CNT as grown, (b) CNT- Am.

Crystallinity of carbonaceous materials can be evaluated by the ratio between D and G band intensity (I_D/I_G), as well as full width at half height (FWHM) of G band. Table 1 shows the information from the deconvoluted spectra.

Table 1
Deconvolution data of Raman spectrum of CNT as grown and CNT- Am.

Parameter	CNT as grown	CNT- Am
ID/IG	0.44	1.05
D band (cm-1)	1348.91	1350.33
G band (cm-1)	1575.99	1581.96
FWHM (D)	26.78	22.08
FWHM (G)	16.54	21.99

I_D/I_G ratio changed from 0.44 for CNTs as grown to 1.05 for functionalized CNT- Am. The increase in I_D/I_G ratio suggests that formation of oxygenated functional groups was more intense than removal of amorphous carbon (Scaffaro et al. 2012).

The presence of functional groups on CNT- Am and GO surface was confirmed by XPS analysis (Table 2). As expected, oxygen and nitrogen were found in CNT- Am, and GO exhibits high oxygen content from its oxidation. Also, exploratory scans have indicated the residual presence of sulphur and iron from growth and functionalization processes in CNT- Am (Fig. 3), as well as residual presence of sulphur in GO from its synthesis (Fig. 4).

Table 2
Atomic concentration from high resolution XPS analysis of CNT- Am and GO (± 5% precision).

Element	Atomic concentration (%)
	CNT- Am	GO
Carbon (C1s)	94	63.8
Oxygen (O1s)	5.1	35.1
Nitrogen (N1s)	0.9	1.1

Figure 3
CNT- Am XPS scanning spectrum (a) and XPS high-resolution survey scans of: (b) C1s spectrum; (c) N1s spectrum; (d) O1s spectrum.

Figure 4
GO XPS scanning spectrum (a) and XPS high-resolution survey scans of: (b) C1s spectrum; (c) N1s spectrum; (d) O1s spectrum.

FT-IR analysis was used to investigate the structure and functional groups of the fillers. For CNT- Am sample (Fig. 5), FT-IR spectrum shows the following bands and peaks of interest: at 1670 cm^-1, corresponding to the amide carbonyl (C=O) stretching (Yang et al. 2007; Naumkin et al. 2012); at 3728 cm^-1, due to -NH stretching (Naumkin et al. 2012); at 1587 cm^-1, because of N-H in-plane bending (Naumkin et al. 2012; Xiong et al. 2006); and at 1220 and 1047 cm^-1, assigned to C-N stretching (Yang et al. 2007; Ma et al. 2010). These bands prove the presence of amide groups and lead to the conclusion that carboxylic groups on CNTs surface were modified by amine. Therefore, CNTs were indeed functionalized through the acid and amine treatments.

Figure 5
FT-IR spectrum of CNT- Am.

For GO sample (Fig. 6), FT-IR spectrum shows adsorption bands at 1723 cm^-1, due to the C=O stretch of COOH group (Song et al. 2014); at 1621 cm^-1, for stretch of C=C groups (Guo et al. 2009); at 1220 cm^-1, for C=C skeleton vibration (Thema et al. 2013); and at 1043 cm^-1for alkoxy C-O groups (Song et al. 2014). Although graphite had been oxidized into GO, C=C groups led to the conclusion that the main structure of graphite layer was retained. The presence of oxygen-containing functional groups confirmed that the graphite was greatly oxidized into GO and was in agreement with the literature (Song et al. 2014; Guo et al. 2009; Thema et al. 2013; Shahriary and Athawale 2014). The assignments for FT-IR spectra of CNT- Am and GO are listed on Table 3.

Figure 6
FT-IR spectrum of GO.

Table 3
Assignments for FT-IR spectra of CNT- Am and GO.

Sample	Peak (cm-1)	Assignment
CNT- Am	3728	NH stretch (Awasthi et al. 2010)
	3360, 3296	OH stretch of COOH groups (Scaffaro et al. 2012)
	2945, 2929, 2856	CH stretch (Scaffaro et al. 2012)
	1670	C=O stretch of amide carbonyl groups (Belin and Epron 2005; Awasthi et al. 2010)
	1587	in plane N-H bending (Awasthi et al. 2010; Maio et al. 2015)
	1518	C=C skeleton vibration (Maio et al. 2015; Song et al. 2014)
	1220, 1047	C-N stretch (Belin and Epron 2005; Huang et al. 2003)
GO	3306	stretch of -OH groups from adsorbed water (Yang et al. 2007)
	2930	CH2 symmetrical stretch (Naumkin et al. 2012)
	2849	CH2antisymmetrical stretch (Naumkin et al. 2012)
	1723	C=O stretch of COOH groups (Yang et al. 2007)
	1635	-OH bending from adsorbed water (Ma et al. 2010)
	1621	C=C stretch of carbonyl groups (Naumkin et al. 2012)
	1385	C-O stretch of carboxylic acid groups (Naumkin et al. 2012)
	1220	C=C skeleton vibration (Xiong et al. 2006)
	1110	stretch of C-OH groups of alcohols (Naumkin et al. 2012)
	1043	alkoxy C-O groups (Yang et al. 2007)
	740	C=O stretch of carboxylic acid groups (Naumkin et al. 2012)

GO was characterized by XRD analysis (Fig. 7). It showed a very strong peak at 2θ = 10.6°, which is in accordance with those in previously reported literatures (Marcano et al. 2010; Fan et al. 2010) and corresponds to (002) crystalline plane. The peak at 2θ = 43.3° corresponds to the turbostratic band of disordered carbon materials, indicating disorder between planes (101) and (012) (Zhu et al. 2010) and indicating that the initial graphite was not completely oxidized, which is in accordance with FT-IR results. XRD results indicated the successful synthesis of GO, although part of the original graphite remained.

Figure 7
XRD profile of graphene GO.

Figure 8 shows the dynamic storage modulus (E') versus temperature. Results showed that storage modulus decreases with temperature for all the samples. With the increase of temperature there is an increase in molecular motion of polymer chains, which decreases the polymer rigidity and hence the storage modulus (Tiwari et al. 2017). It is clear that mechanical strength is very dependent on the presence of CNT- Am and GO in the nanocomposites. The incorporation of CNT- Am and GO increased E' of the epoxy resin, both presenting some stiffening effect. This effect can be attributed to a better adhesion of the filler to the resin and to a more homogeneous dispersion (Meng et al. 2008). Observing glass transition temperatures (Tg) from loss module (E") and loss tangent (tan δ) peaks, they clearly shift towards high temperature when compared to the neat resin (Table 4). Tg rising reflects a better adhesion to the matrix and can also be an indicator of a higher cure degree. Kalakonda and Banne (2017) have studied the thermomechanical properties of poly(methyl methacrylate) composites reinforced with functionalized single-walled CNTs, and observed a similar increase in Tg for nanocomposite samples. GO nanocomposites presented a better result than CNT- Am reinforced samples, leading to the conclusion that it had a stronger interfacial adhesion with DGEBA matrix and/or more homogeneous dispersion. According to Tiwari et al. (2017), high surface area of GO increases the polymer nanomaterial interaction area and improve the stress transfer thus enhancing the nanocomposite storage modulus.

Figure 8
DMA curves of the various samples.

Table 4
Glass transition temperature (Tg) from loss module (E") and loss tangent (tan δ) peaks.

Sample	Tg (°C)
	E"	tan δ
Neat resin	137.1	160.3
CNT- Am + epoxy	147.5	165.0
GO + epoxy	149.7	166.5

CONCLUSIONS

In this study, we achieved the synthesis and functionalization of carbon nanotubes and graphene oxide. Acid and amine treatments could effectively induce polar oxygen-containing groups on CNTs surface. The modified Hummer's method successfully oxidized graphite to GO (although not completely). The morphology of functionalized CNTs was observed by SEM images and Raman spectrum. The presence of functional groups was confirmed by FT-IR and XPS spectra. These materials were studied as reinforcement fillers to DGEBA epoxy resin by dynamic mechanical analysis and effectively improved the observed mechanical properties, suggesting a chemical bond between them and the epoxy matrix. GO nanocomposite presented higher storage modulus and higher glass transition temperature than CTN-Am nanocomposite.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge FAPESP and CNPq for financial support and LAS/INPE, LEFE, and IP&D/UNIVAP for collaboration.

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Edited by

Section Editor: Mariana Fraga

Data availability

Data citations

Naumkin AV, Kraut-Vass A, Gaarenstroom SW, Powell CJ (2012) NIST X-ray photoelectron spectroscopy database. NIST Standard Reference Database 20:4.1. Gaithersburg: NIST.

Publication Dates

• Publication in this collection
  16 July 2018
• Date of issue
  2018

History

• Received
  28 Jan 2017
• Accepted
  30 Oct 2017