Polymer Testing 43 (2015) 182e192 Contents lists available at ScienceDirect Polymer Testing journal homepage: www.elsevier.com/locate/polytest Mate...

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Polymer Testing 43 (2015) 182e192

Contents lists available at ScienceDirect

Polymer Testing journal homepage: www.elsevier.com/locate/polytest

Material properties

Multifunctional nanocomposites based on tetraethylenepentamine-modified graphene oxide/epoxy lio Ribeiro a, Wellington Marcos da Silva a, Juliana Cardoso Neves a, He llen Daniel Resende Calado a, Roberto Paniago b, Luciana Moreira Seara c, Ha ^s Camarano d, Glaura Goulart Silva a, * Denise das Merce a

Departamento de Química, Universidade Federal de Minas Gerais, Belo Horizonte, Brazil Departamento de Física, Universidade Federal de Minas Gerais, Belo Horizonte, Brazil Centro de Microscopia, Universidade Federal de Minas Gerais, Belo Horizonte, Brazil d Centro de Desenvolvimento da Tecnologia Nuclear, Belo Horizonte, Brazil b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 February 2015 Accepted 11 March 2015 Available online 19 March 2015

Composites based on epoxy/graphene were investigated for thermal-mechanical performance. Initially, few-layer graphene oxide (GO) was modified with tetraethylenepentamine (GO-TEPA) in a reaction assisted by microwave radiation. GO and GO-TEPA samples were characterized for their structure and morphology. Composites containing 0.1, 0.3 and 0.5 wt.% of GO and GO-TEPA were prepared, and the effect of fillers on the morphology of cryofractured regions of epoxy matrix was observed through electron microscopy images. Dynamic mechanical thermal analysis (DMA) tests revealed increases of approximately 20  C in glass transition. Moreover, when compared to neat polymer, composites containing 0.5 wt.% of GO-TEPA gained up to 103% in thermal conductivity (obtained by flash laser). Finally, nanoindentation analyses showed increases of 72% in Young's modulus and 143% in hardness for the same sample. The system is characterized as multifunctional nanocomposites because of the simultaneous gains in thermal and mechanical properties. The best results of the multifunctional composites were strongly associated with the chemical modification of the GO by TEPA. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Graphene oxide Epoxy Nanocomposite Nanoindentation Flash laser

1. Introduction There is a growing need in various industrial sectors for the production of high-performance composite materials. Graphene has been widely investigated [1] in the fabrication of different types of composites, and great improvement is expected as this type of material shows exceptional physical properties [2,3]. For example, graphene thermal

* Corresponding author. E-mail addresses: [email protected], (G.G. Silva).

[email protected]

http://dx.doi.org/10.1016/j.polymertesting.2015.03.010 0142-9418/© 2015 Elsevier Ltd. All rights reserved.

conductivity is ~5,000 W m1K1 [4] and as a monolayer presents high superficial area (2,630 m2 g1) [5]. Furthermore, graphene presents Young's modulus of ~1,100 GPa and tensile strength of ~125 GPa, ~200 times higher than steel [6]. Graphene will soon allow sectors such as electronics, automotive, naval and aeronautics [2] to use materials with improved thermal, electrical and mechanical properties [8,9]. It has been introduced in many polymeric systems [2], such as polyurethanes [10], poly (ethylene vinyl acetate) [11] and epoxy [12,13]. Epoxy systems are traditionally required by the industry because they belong to a class of thermosetting polymers featuring high chemical stability, excellent adhesive properties, good

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corrosion resistance and low shrinkage in the curing process [7]. Different types of nanofillers have been added to epoxy to improve their properties, for example, the incorporation of silicon and alumina reduced cost and increased the stiffness. Nevertheless, a major obstacle found is that the addition of rigid particles promotes reduction of the epoxy ductility, making it more brittle [14]. To overcome this problem, more flexible and tough carbon nanomaterials (CNMs) such as carbon nanotubes (CNTs), graphene and carbon nanofibers [9,15e17] are used. Moreover, these composites show low thermal expansion and relatively high thermal conductivity [18,19]. For example, Martin-Gallego et al. obtained significant increase of 40  C in Tg for a UV cured epoxy composite containing 1.5 wt.% of graphene by DMA [20]. Chatterjee et al. observed increases of up to 10% of Young's modulus and hardness in nanoindentation tests by using 1.5 wt.% of GO functionalized with dodecylamine [14]. Tang et al. explored different types of dispersion of thermally reduced GO (RGO) in epoxy, which were verified by tensile and flexural tests. They obtained a 52% increase in quasi-static fracture toughness (KIC) for samples containing 0.2 wt.% of RGO dispersed in the matrix [21]. Li et al. observed increases of 75% in Young's modulus and 30% in hardness by nanoindentation testing for samples containing 5 wt.% of CNTs functionalized with methyl 4-aminobenzoate in epoxy [22]. The groups that attained good final properties, in general, had improved wettability between the graphene and the polymer matrix to produce good dispersion [23,24]. The agglomeration of graphene, due to its low compatibility with polymer matrices, has been one of the main barriers limiting its potential use as a mechanical reinforcing agent. In order to increase the dispersibility of CNMs in different chemical environments, a large number of routes to chemical modification of its surface have been proposed [15,19,24,25]. Many strategies adopted were based in initial oxidation processes [26e28] and, usually, the oxidised surface of the CNMs had different functional groups such as epoxy, alcohols, ketones, carboxylic, ethers and/or presence of debris [29]. Depending on the nature of the functional groups found in the walls of CNMs as carboxylic acid or hydroxyl groups, they can be derivatized to ester, amide and/or carbamates, opening new possibilities of reaction routes [24]. Several studies have demonstrated that the presence of the amine group on CNMs can increase the adhesion of the nanofiller to epoxy, since these functional groups have great compatibility with this polymer system [14,19,22]. For example, Niyogi and co-workers described an effective way to obtain GO covalently linked with octadecylamine: after the oxidation of graphite in sulfuric acid, there followed treatment with thionyl chloride (SOCl2) and octadecylamine, which forms GO-ODA [30]. Hu et al. functionalized different types of CNMs with 4,40 -diaminodiphenyl sulphone (DDS) in the presence of SOCl2 at 130  C under an inert atmosphere and refluxed for 6 days [19]. Different processes of amino functionalization of CNMs have been reported in the literature and most of them use, in one of the chemical steps, SOCl2 as intermediate reagent [19,24,31]. In general, the use of SOCl2 limits the functionalization process of CNMs because of its dangerous nature and high level of toxicity. Another


drawback found in this kind of synthesis is the time spent on extended reflux lasting days, making it a very costly process. In this work, we investigated the influence of GO and GO-TEPA on the thermal and mechanical properties of epoxy based composites. The GO-TEPA were synthesized rapidly by microwave in comparison to methods commonly used and without the use of SOCl2. Through a three roll mill, these nanofillers were mixed into resin, obtaining homogeneous dispersion. 2. Experimental 2.1. Materials Expanded graphite (EG) and GO, were both supplied by Cheaptubes USA, and TEPA from Sigma Aldrich. Commercial epoxy resin DER-331 liquid (based on diglycidyl ether of bisphenol A (DGEBA)) and the curing agent DEH-24 liquid based on triethylenetetramine (TETA), were supplied by Dow Chemical Brazil. 2.2. GO functionalization 100 mg of GO were added to 150 ml of tetraethylenepentamine (TEPA) in a round bottom flask. This flask was coupled to a microwave reactor programmed to work at 120  C, increasing from 0 to 200 W for 30 min. Throughout the reaction period, the system remained under magnetic stirring. After the microwave reaction, the mixture was cooled to room temperature and transferred to a beaker containing 300 ml of anhydrous ethanol. The final mixture was dispersed in ultrasound bath for 30 minutes, filtered under vacuum and washed exhaustively with anhydrous ethanol to remove excess TEPA. The filtrate was dried in an oven at 100 C for 12 hours to yield of GO-TEPA, as shown in the schematic model in Fig. 1. 2.3. Preparation of GO and GO-TEPA/epoxy composite Composites were prepared containing 0.1, 0.3 and 0.5 wt.% of GO and GO-TEPA. The nanofillers were mixed manually with the epoxy resin at room temperature and dispersed at 80  C employing a three-roll mill (Exakt80E) with zirconium oxide rollers. The distance between the rolls was fixed at 5 mm, with a speed of 250 rpm. After this step, the material was degassed and mixed slowly with TETA (Phr 15) to avoid the formation of bubbles. The mixture was transferred to silicone molds and cured for 12 hours at room temperature and, subsequently, at 120  C for 4 hours. 2.4. Characterization To evaluate the degree of exfoliation of GO and GOTEPA, samples were dispersed in isopropyl alcohol for 1 hour in an ultrasonic bath. The resulting suspensions were dropped onto a silicon substrate and dried for atomic force microscopy (AFM) studies. The same suspensions were dropped onto a grid of carbon and copper (Holey Carbon Copper Grids) for TEM studies. The TEM micrographs were obtained on a FEI Tecnai G2 equipment operating in


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Fig. 1. Representative structure model of EG (a), GO (b) and GO-TEPA (c).

vacuum with electron beam (tungsten filament) of 200 kV. SEM images were obtained on Quanta 200 equipment, Model 2006 FEG-FEI, operating under vacuum with the electron beam in acceleration between 5 and 30 kV. The samples were fixed in the sample holder with the aid of conductive carbon tape. The AFM images were obtained on a MFP-3D-AS SPM equipment (Asylum Research, CA, USA) at a temperature of 20  C and ~50% relative humidity, in tapping mode with a silicon probe (Olympus AC240 TS) used with a spring constant between 0,5e4.4 Nm1 and typical resonance frequency of 50e90 KHz. Thermogravimetric measurements (TG/DTG) were obtained in TG Q5000 equipment from TA Instruments. The samples were analyzed at a heating rate of 5  C min1 to 900  C under an atmosphere of synthetic air flow at 25 ml.min1. MicroRaman experiments were performed using a Dilor XY spectrometer that employed backscattering geometry and was equipped with a liquid nitrogen-cooled CCD detector. Samples were resolved on an OLYMPUS BH-2 optical microscope with an 80x objective using a 514.5 nm AreKr laser line and 1 mW power to avoid spectral changes due to sample heating. The spectra were collected from three different points of each sample. XPS spectra were acquired at room temperature on a VG Scientific Escalab-220 -ixL system. The base pressure in the vacuum chamber was 2.0  1010 mbar and used an anode of Mg generating Xrays in the Ka line (E ¼ 1487 eV). XPS spectra were obtained in the region between 0-1000 eV (extended spectrum) with steps of 1 eV, and high-resolution spectra with steps of 0.1 eV in the region of the photoemission peaks of C1s, N1s and O1s. The electron energy analyzer operated in large mode area (Ø ¼ 4 mm) with pass energy of 50 eV to 20 eV and extended spectra for analysis of individual lines. A Q2000 equipment from TA Instruments was used for the DSC measurements of nanocomposites. The samples were analyzed under an atmosphere of helium gas flow of 50 ml min1. We adopted the following DSC protocol: heating rate of 10  C min1 from room temperature to 200  C, isotherm of 1 minute, cooling with the same rate to

50  C, isothermal for 1 minute and a heating rate to 10  C min1 up to 200  C. To obtain significant results, the Tg is the average of three measurements having deviations with values lower than ±1  C. Nanoindentation tests were performed using Asylum Research Nanoindenter (CA, USA) equipped with a Berkovich diamond tip to determine the elastic modulus and hardness of epoxy composites. The method used was load control, keeping the maximum load for 10 seconds before unloading. The maximum load was 500 mN and the load rate was 50 mN s1. The 1 cm3 molded samples were embedded in a low contraction resin, polished successively and then mounted on a sample holder. In order to obtain reliable results, the experiment consisted of 36 indentations on a 50 mm2 area. Young's modulus (E) and hardness (H) were calculated from the loadedisplacement curves using the Oliver-Pharr method [32]. DMA analysis were performed with a heating rate of 3  C/min using a TA Instruments Model Q800 DMA in a temperature range of 50  C to 200  C at a frequency of 1 Hz. The test samples had dimensions of 56  13  3 mm. For the pure epoxy and composites, thermal conductivity measurements were performed by laser flash analysis following a method described in ASTM E1461-13. The samples were prepared in disc-shaped forms, with diameter of c.a 10 mm and thickness of 1.0 mm. All measurements were taken at (30 ± 2) C with a laser voltage power of 1538 V and laser transmission filter of 100%. A total of 5e10 shots were taken per sample set. The thermal diffusivity values (cm2/s) of the epoxy and composites were recorded. 3. Results and discussion 3.1. Structures and morphology of GO and GO-TEPA After the chemical modification process, the GO were converted into GO-TEPA. To investigate the structure and morphology of these materials, SEM, TEM and AFM images

H. Ribeiro et al. / Polymer Testing 43 (2015) 182e192


were taken. Fig. 2a shows a grain of GO received with irregular and thin textures. It has been reported in the literature that graphene and GO containing thickness between 0.34 and 1.2 nm can be considered as monolayers [24,33]. Fig. 2b shows an AFM image of GO-TEPA and its corresponding topographic profile. The height of this film is of ~1.5 nm between the silicon substrate and graphene, which can contain ~1e3 layers; it is possible to observe folded and/or overlapped nanosheets in other regions of the image. The lateral dimensions observed in Fig. 2b are between a few hundred nanometers to a few micrometers. The morphology of the GO-TEPA sample can also be observed by TEM image in Fig. 2c, showing the presence of thin, transparent and wrinkled sheets similar to tissue paper, characteristic for this type of material, as described in the literature [12]. 3.2. Raman spectroscopy analysis of GO and GO-TEPA Raman spectra for EG, GO and GO-TEPA samples in Fig. 3 show the major bands of graphene. The G bands are identified between 1500 and 1650 cm1, which is characteristic of graphite tangential vibrational modes (identified by rules of symmetry as belonging to the E2G group). The D band at ~1350 cm1 is associated with the vibrational modes of structural disorder arising from defects, sp3 carbons and functionalizations. The intensity of the shoulder associated with D0 band follows the same behavior as the D band [34]. The G0 band (~2670 cm1) is the second-order

Fig. 3. Raman spectra for EG, GO and GO-TEPA samples. ID/IG ratios are showed in the figure.

overtone of the D band [35]. This band was observed for the sample of graphite, however, it was not clear in the chemically modified samples. It is possible to evaluate the process of graphene surface modification through the integrated areas for the G (IG) and D (ID) band [34,35]. A very significant increase of the ID/IG ratio of GO and GO-TEPA in relation to EG was observed. This expected increase is characteristic of introducing defects, functional groups and/or sp3 carbons. Shifts in wavenumbers and intensity

Fig. 2. (a) SEM micrograph of GO, (b) AFM image of GO-TEPA and its topographic profile. (c) TEM micrograph of GO-TEPA nanosheet.


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Fig. 4. XPS extended spectra (a) for samples of EG, GO and GO-TEPA. High resolution spectrum (b) obtained in the region of binding energy of N1s for GO-TEPA.

changes of the G band provide important information about the structural changes of the graphitic plane when it is functionalized or interacts with different chemical environments [34]. The profiles of the G band of GO and GOTEPA were modified as well as their height and half width

in comparison with the G band for the EG sample; furthermore, a displacement of up to ~11 cm1 for higher wavenumbers was observed for GO-TEPA, indicating the presence of electron withdrawing chemical groups [36]. The position of the G band can also be related to the

Fig. 5. SEM micrographs for pure epoxy (a), composite containing 0.5wt.% of GO (b,c) and composite containing 0.5wt.% GO-TEPA (def).

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number of graphitic layers in the samples by turning to the empirical equation proposed by Hui et al. [37]. Using the Eq. (A.1) (described in Appendix A), the number of graphitic layers in the GO and GO-TEPA samples was identifie as ~1e3, this value agreeing with the results obtained by AFM. 3.3. XPS and TG analysis of GO and GO-TEPA Extended XPS spectra were obtained in order to determine the chemical composition of graphitic surface. Fig. 4a shows the extended XPS spectra to EG, GO and GO-TEPA. All samples showed photoemission peaks for C1s (284.5 eV) and O1s (~534.0 eV). The N1s peak is observed only for the GO-TEPA sample. The presence of the characteristic carbonyl amide functional group is identifiable in the detailed N1s spectrum (Fig. 4b). After adjustment, three significant contributions in terms of binding energy were observed: i) at 399.1 eV, attributed to nitrogen atoms of primary amines; ii) at 400.1 eV assigned to the amide group; and iii) at 401.9 eV attributed to secondary amines present in the molecule of TEPA [38,39]. The XPS results indicate the introduction of TEPA molecule covalently


linked to the surface of GO. The weight loss as a function of increasing temperature (TG) and its derivative (DTG) are shown in Fig. B.1 for the EG, GO and GO-TEPA samples and discussed in the Appendix B. 3.4. SEM images of composites Fig. 5 shows a SEM image of cryo-fractured surface of epoxy (Fig. 5a) and its composites containing 0.5wt.% of GO (Fig. 5b,c) and GO-TEPA (5def). Typically, cryofractures of epoxy samples have a smooth structure and, occasionally, have continuous lines in the direction of crack propagation [13]. Nevertheless, the composites containing GO and GOTEPA showed different morphology when compared to neat epoxy since they have very rough surfaces. The rough and rugged aspect of the cryofractured surfaces were more intensely observed for GO-TEPA sample, confirming that there is better distribution and dispersion of carbon nanofiller in the polymer matrix [13]. The sample containing GO shows larger agglomerate regions as seen in Fig. 5c. Thus, the TEPA in GO-TEPA may act as a bridge between the graphene surface and the polymer chains, leading it to a flexible interface and discouraging the formation of brittle

Fig. 6. Model of GO (a) and GO-TEPA (b) dispersed in the epoxy matrix.


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Fig. 7. Storage modulus (a,c) and Tan d (b,d) of neat epoxy and composites containing different wt.% of GO and GO-TEPA.

the results are shown in Fig. 7aed. It can be observed that the E0 values for all composites were higher compared to neat epoxy in the overall temperature range studied. Gains of up to ~20  C on Tg were observed for GO-TEPA sample as determined as the peak in Tan d of Fig. 7 (b,d). The Tg values obtained from DMA are consistent with results from DSC results in Table 1. In the conditions studied, it can be suggested that the crosslinking process was aided by the addition of GO-TEPA (and also GO in some extent), and polymer chains with free extremities were probably not generated [41]. It is likely that the amine groups introduced on the GO surface have served as anchor points for polymer chains (Fig. 6), reducing their mobility and increasing the modulus and Tg values. This is indicative of the reinforcing effect of GO-TEPA in epoxy matrix and decrease in damping

fracture (Fig. 5f.) [12,23]. The good adhesion observed between the epoxy resin and GO-TEPA may be also attributed to the presence of the amine group covalently linked on graphitic surfaces, increasing the compatibility between epoxy and nanofiller. Hopefully, this will reflect in a better thermal and mechanical performance of composite [11,40], which will be presented in following sections. Fig. 6 shows a scheme for GO and GO-TEPA immersed in the polymer matrix, illustrating that the adhesion of GO-TEPA to the epoxy matrix is greater than the GO one. 3.5. Dynamic mechanical thermal analysis of composites The storage modulus (E0 ) and Tan d were obtained by DMA in a range between 35 and 200  C for all samples, and

Table 1 Average values of the modulus of elasticity (E), hardness (H), relation hres/hmax obtained by nanoindentation and thermal conductivity (k) and Tg of neat epoxy and nanocomposites produced with GO and GO-TEPA with different mass %. Sample

% w/w

E (GPa)

Epoxy GO

0.0 0.1 0.3 0.5 0.1 0.3 0.5

(3.3 (3.0 (2.8 (3.2 (3.3 (3.5 (5.7


a b

± ± ± ± ± ± ±

0.1) 0.2) 0.1) 0.2) 0.1) 0.2) 0.1)

hmax ¼ displacement at maximum load. hres ¼ residual displacement after load removal.


H (GPa) (0.28 (0.27 (0.26 (0.38 (0.33 (0.31 (0.68

± ± ± ± ± ± ±

0.02) 0.02) 0.01) 0.03) 0.01) 0.02) 0.02)


0.81 0.79 0.78 0.74 0.77 0.78 0.72

K (W/m.K) 0.35 0.31 0.40 0.59 0.32 0.50 0.71

± ± ± ± ± ± ±

0.01 0.02 0.01 0.01 0.02 0.01 0.02

Tg (DSC) 124.2 137.1 138.3 139.4 139.0 140.4 143.4

± ± ± ± ± ± ±

0.3 0.4 0.2 0.3 0.5 0.1 0.4

H. Ribeiro et al. / Polymer Testing 43 (2015) 182e192

Fig. 8. Thermal conductivity of epoxy composites with 0.1, 0.3 and 0.5 wt.% of GO and GO-TEPA.

behavior with increasing GO-TEPA content when compared to pure polymer [50]. Tang et al. observed, using DMA, increases of 11  C in Tg and gains in storage modulus in composites containing 0.2 wt.% of GO reduced in epoxy [21]. Similar increase in Tg of 20  C determined using DMA was obtained by Martin-Gallego et al. in composites containing 1 wt.% of GO in epoxy cured by UV radiation [42]. The importance of the amino groups in graphene was also studied by Kuila et al., who observed an increase in storage modulus and gains of 22  C in Tg for composites containing 8 wt.% of GO-ODA in ethylene vinyl acetate co-polymers [11]. 3.6. Thermal conductivity Fig. 8 shows the thermal conductivities (k) at 30  C of GO/epoxy and GO/TEPA epoxy composite prepared with 0.1, 0.3 and 0.5 wt% of filler. The thermal conductivity of the pure epoxy was 0,35W/m.K, a similiar value as observed by Shahil et al. 2012 [43] and Chatterjee et al. 2012 [14] for the same system. Initially, a slight decrease in thermal conductivity was observed for both samples containing 0.1 wt.%, however, at this composition onwards the


conductivity increased considerably. At 0.5wt.% loading, the GO/epoxy and GO-TEPA/epoxy composites showed enhanced thermal conductivity of 70% and 103%, respectively, compared to pure epoxy. Kim et al. 2012 found an increase of almost 100%, twice as great as that of the neat epoxy for composite based in 3 wt.% of GO [44]. Increases of up to 32% in thermal conductivity were observed by Chatterjee et al. when compared to pristine epoxy by using 2.0 wt.% of GO functionalized with dodecylamine in epoxy resin [14]. Im et al. investigated the effect of the hybrid filler on the thermal conductivity of the composites containing epoxy/GO as function of the effect of multi-walled nanotubes (MWCNTs) addition. The synergistic effect could be observed with 0.36 wt.% of MWCNTs addition and the highest enhancement ratio (~140%) relative to GO/epoxy composite [45]. Thermal conductivity was effectively affected by carbon functionalized nanofiller within the epoxy matrix in the present work, showing that functionalized surfaces of graphene plays an important role in inhibiting their aggregation and falcilitating dispersion of nanofiller and polymer matrix. Since efficient heat propagation in graphene is mainly due to acoustic phonons [14], a homogeneous dispersion and network formation on GOTEPA composite may contribute to the steady increase in thermal conductivity. The average of numerical values of thermal conductivity is shown in Table 1. 3.7. Nanoindentation analysis of composites Nanoindentation analysis was carried out using the method described by Oliver and Pharr [32]. The hardness value (H) shows the plastic behavior of the material and can be obtained dividing the value of the applied strain by the projected indentation area [16,22,32,46]. For a Berkovich indentor, the projected area is a function of the contact depth of indentation in situ [14,32]. The Young's modulus (E) shows the elastic behavior of the material and is obtained through the slope of the stressestrain curve during the unloading process. Calculations of E and H are performed at maximum indentation depth. Typical stressestrain curves for epoxy, GO and GO-TEPA composites are shown in Fig. 9(a,b). The average of numerical values of Young's

Fig. 9. Nanoindentation typical curves for pure epoxy and composites containing 0.1, 0.3 and 0.5 wt.% of GO and GO-TEPA.


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Fig. 10. Elastic Modulus (a) and hardness (B) of the pure epoxy and composites containing 0.1, 0.3 and 0.5% w/w GO and GO-TEPA; force applied of 500 mN.

modulus and hardness are shown in Table 1 and Fig. 10, and have experimental uncertainty of ±0.2 GPa for modulus and ±0.03 GPa for hardness. The E and H values for the epoxy found in this study are in agreement with those of Shen et al. for a DGEBA system based on DER 332 supplied by Dow Chemical [47]. It can be seen that for composites containing 0.5 wt.% of GO, the H value increased of 36% in relation to neat epoxy. For the composite containing 0.5 wt.% GO-TEPA, significant increases of 72% of E and 143% of H were observed when compared to neat epoxy. Chaterrjee et al. also observed significant increases in E and H for composites containing 1.5 wt.% of GO nanoplatelets functionalized with dodecylamine (DDA-GO) in epoxy [14]. Table 1 shows the calculated values of the parameter (hres/hmax), which is the ratio between the residual depth and the maximum depth observed in the nanoindentation test. This parameter has a range between 0 (completely elastic) to 1 (completely rigid plastic) [10]. It was observed that the composite containing GO and GO-TEPA showed increased elastic tendency when compared with neat epoxy. The results reported in the present work strongly support the idea that the presence of modified graphene in epoxy effectively improves the thermal and mechanical performance of the composite leading to multifunctional ability. 4. Conclusions Few-layer GO was functionalized using TEPA by efficient microwave assisted reactions without the use of SOCl2. The morphological and structural characteristics, such as the presence of covalently attached functional groups linked to the GO, were then investigated. The GO and GO-TEPA were incorporated into epoxy at three different wt.% and, in particular for the composites containing GO-TEPA, the process resulted in homogeneous dispersions. Nanoindentation measurements revealed increases of 72% in

elastic modulus and 143% in hardness for the composite containing 0.5 wt.% of GO-TEPA. Improved thermal performance of the composites was confirmed by DMA and DSC measurements, demonstrating gains in Tg and storage modulus for all composites studied, with the best results for the 0.5 wt.% GO-TEPA. For the same composite, gains of up to 103% in thermal conductivity were also registered. This work clearly indicates that to improve the thermalmechanic properties of composites based on graphene/ epoxy, the type and extent of chemical modification of graphene is of central importance. Acknowledgements This research was supported by Petrobras. H. Ribeiro is grateful to the Brazilian agency CNPq for financial support. The authors are also thankful to the Instituto Nacional de ^ncia e Tecnologia em Nanomateriais de Carbono, Cie Departamento de Química da UFMG and Centro de Microscopia/UFMG. Appendix A The position of the G band (uG) can be related to the number of graphitic layers (n) in the samples through the empirical equation proposed by Hui et al. [37].

uG ¼ 1581:6 þ

11 1 þ n1:6

(A 1)

Appendix B The curves of weight loss as a function of increasing temperature (TG) and its derivative (DTG) are shown in Fig. B.1 (a,b) for the EG, GO and GO-TEPA sample. It was observed that the thermal decomposition of EG occurs at 710  C, while in GO and GO-TEPA it occurs at a lower range

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Fig. B.1. . TG (a) and DTG (b) curves for EG GO and GO-TEPA samples.

(555e540  C), the same range reported in the literature [30,48]. This decrease in thermal stability of GO and GOTEPA samples in relation to expanded graphite can be associated with the introduction of functional groups or defects in the surface of EG during its oxidation process [38,48]. For the GO sample a peak at 80  C (19 wt.% of mass loss) is observed in Fig. B.1b, and can be associated with the thermal desorption of water molecules on the hydrophilic GO surface [49]. A second peak at 224  C (34 wt.%) can be observed for the DTG for GO sample, and attributed to the loss of oxygen's functional groups linked to the graphene [48]. For GO-TEPA a weight loss of 28 wt.% at 320  C was observed (see Fig. B.1b), which is assigned to the oxidative decomposition of TEPA molecules and residual oxygenated groups covalently bound to the GO [30,38].







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