Mechanical and Thermal Properties of Octadecylamine-Functionalized Graphene Oxide Reinforced Epoxy Nanocomposites

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1 Fibers and Polymers 2017, Vol.18, No.10, DOI /s z ISSN (print version) ISSN (electronic version) Mechanical and Thermal Properties of Octadecylamine-Functionalized Graphene Oxide Reinforced Epoxy Nanocomposites Sara Jahandideh, Mohammad Javad Sarraf Shirazi*, and Mitra Tavakoli Department of Chemical and Polymer Engineering, Faculty of Engineering, Yazd University, Yazd , Iran (Received May 11, 2017; Revised July 21, 2017; Accepted August 11, 2017) Abstract: In this study, octadecylamine-functionalized graphene oxide (GO-ODA)/epoxy nanocomposites were fabricated via vacuum shock technique and effect of GO functionalization on thermal and mechanical properties of the nanocomposites was examined. In this case, for characterization of functionalization of GO nanosheets, Fourier transform infrared spectroscopy (FTIR) and Raman analysis were used. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses indicate uniform dispersion of nanoparticles throughout the matrix. Despite this, the results of SEM analysis confirmed the significant effect of the vacuum shock technique on bubbles reduction, which affected thermal and mechanical properties of composites. Thermogravimetric analysis (TGA) showed the increase in onset degradation temperature and better thermal stability for the GO-ODA/epoxy nanocomposites in comparison to GO/epoxy nanocomposite and neat epoxy. The maximum thermal degradation temperature for epoxy resin was increased from 356 o C to 365 o C. Besides, the addition of 0.5 wt% GO-ODA within the matrix increased the tensile and flexural strength of epoxy resins by 104 % and 75 %, respectively, due to the well dispersion and strong interfacial interactions between GO-ODA and epoxy resin through covalent functionalization. The toughening effect of GO-ODA was explored in epoxy nanocomposites and found to be significant (~251 %) in improving the impact strength of specimens. Keywords: Nanocomposite, Graphene oxide, Octadecylamine, Vacuum shock, Functionalization Introduction Epoxy resins are thermosetting polymers and one of the most high-performance polymers that are widely used in structural complexes, coatings, adhesives, hardware, semiconductor encapsulation and electronic circuit board materials due to a unique combination of properties such as low cure shrinkage, compatibility with a great number of materials, high mechanical strength, adhesion and chemical resistance [1-4]. However, the brittleness and low resistance to crack initiation and propagation, resulting in low impact strength, which limits their applications [5-7]. Incorporation of reinforcing fillers is one of the effective ways to compensate for this lack of pure epoxy resins [8]. The performance of epoxy resin is expected to further increase, due to the smaller size, higher surface area and better interfacial bonding of nano fillers [9-11]. In recent years, many types of carbon nanomaterials, in particular, graphene and its derivatives have been used as a filler for polymer composites [12]. Graphene due to the unique properties has attracted a lot of scientific interest [13]. However, the uniform dispersion is still the major challenge due to the strong tendency of graphene to accumulate due to a high surface area, the Van der Waals interaction and vacuum filtration in the preparation process [14,15]. In addition, the atomic structure of graphene is smooth and has no interfacial bonding which limits transfer the load from the polymer matrix to the graphene. In order to achieve the desired improvements in the properties of graphene/polymer *Corresponding author: jsarraf@yazd.ac.ir composites, several major issues such improve the dispersion of graphene in the polymer matrix and surface modification of graphene for good interaction must be resolved [16-18]. Graphene oxide includes many functional groups, such as hydroxyl, carboxyl and epoxide, which are the sites for covalent functionalization reaction [19,20]. GO due to similar groups such as epoxy groups and hydrogen groups is considered as an ideal nano filler for epoxy resin [21]. The role of functional groups on the surface of graphene oxide in reinforcing the epoxy resin has been investigated by Young et al. [18]. They used two kinds of graphene oxide, with functional groups (ago) and without them (bwgo) in different loading, 0.25 wt.%, 0.5 wt.% and 0.75 wt.%. It is noteworthy that the best mechanical properties achieved by 0.5 wt.% ago (24 % and 11 % increase in Young s modulus and tensile strength, respectively) in comparison with bwgo/epoxy nanocomposite, while the properties reduced for nanocomposite containing 0.75 wt.% ago, which is consistent with the findings of Kuila et al. [7]. However, GO-based composites are thermally unstable due to their oxygen groups [22,23]. In order to overcome this problem, GO can functionalize with the surface modifying agent. It is expected that covalently functionalized GO not only increases thermal resistance, but also makes the better dispersion of GO as well as interfacial interactions between the GO and the matrix becomes stronger [24]. Tang et al. reported the effects of the GO and silane functioned GO (silane-f-go) on the mechanical and thermal properties of the epoxy composites. Composite containing silane-f-go indicates higher tensile and flexural strength, modulus, fracture toughness (K IC ), fracture energy (G IC ) and thermal 1995

2 1996 Fibers and Polymers 2017, Vol.18, No.10 Sara Jahandideh et al. stability compared to composites containing GO [17]. The void is one of the most common types of defects caused by the production process of composite that is dependent on manufacturing techniques and has a destructive effect on all properties. The presence of voids, even at very low levels, can significantly decrease the properties of composites. For example, for every 1 % increase in the volume fraction of voids, reducing the 5-15 % in shear strength have been reported [25]. The presence of a large number of voids with the large diameter, can act as the stress concentration points, leading to decrease in mechanical properties such as tensile, flexural, shear, bending, tension properties and the fracture toughness [26,27]. In a work by Hashemi and Mousavi [26], they used two different methods to produce epoxy composite samples with and without voids. They reported that, voids can lead to a significant reduction in the total amount of electrical conductivity and electromagnetic wave absorption of the composite. They also showed that the vacuum shock is a highly efficient technique to reduce bubbles in the composite sample during the preparation process. In recent years, a variety of production methods for the preparation of nanocomposites are provided to improve the dispersion of nano-fillers in polymer matrices and reduce defects [16]. In this study, we propose a new strategy to address these issues. For this purpose, octadecylamine-functioned graphene oxide/epoxy nanocomposite was prepared based on the several steps. Vacuum shock technique to remove bubbles and improve the dispersion of nano fillers in the epoxy matrix and impact of the functioned GO on mechanical and thermal properties of composites were investigated. Experimental Materials Graphite powders and ODA (99 %) were obtained commercially from the Merck Co., Germany. Epoxy resin based on diglycidyl ether of bisphenol-a (NPEL-128S) modified by CARDURA E10 (glycidyl ester of versatic acid) as a reactive diluent, with an epoxy equivalent weight of g/eq was purchased from NAN YA Plastics Co., Taiwan. EPIKURE TM F205 (cycloaliphatic amine) was used as a curing agent and supplied by Hexion, America. Ethanol, Acetone, concentrated sulfuric acid, phosphoric acid, hydrogen peroxide, potassium permanganate, sodium nitrate were purchased from the Merck, Germany and were used as received. Preparation of GO GO was synthesized from purified natural graphite by modified Hummers method [28]. In our preparation method, in brief, 10 g of graphite powder and 50 g of KMnO 4 were added to 1 l H 2 SO 4 and 110 ml of H 3 PO 4 and stirred for 72 hours at 50 o C. The resulting mixture was transferred to an ice bath (made from deionized water) and then 10 ml H 2 O 2 was added. After a while, the vacuum Erlenmeyer flask was filled with deionized water to reduce the time of reaction. At this stage, the color of the sample becomes dark brown. The resulting suspension was kept without shaking for 48 hours for further deposition of filler particles. Afterward, the mixture was repeatedly filtered and washed with HCl to remove any non-oxidized graphite to reach ph=7. The resulting GO was dried overnight at 100 o C to produce the GO powder. Preparation of GO-ODA For functionalization of GO with ODA, at the first, 0.6 g of GO powder was dispersed in 300 ml of deionized water by sonication for 2 hours and then was added to the mixture of 0.9 g of ODA in 90 ml of ethanol. The resulting mixture was refluxed for 24 hours at 85 o C and then vacuum filtered and rinsed with ethanol to remove excess ODA. This rinsing filtration process was repeated for another 3 times. Finally, the resulting powder was dried in an oven at 60 o C for 48 hours. Fabrication of Epoxy Nanocomposites The process of synthesis of GO/epoxy and GO-ODA/ epoxy nanocomposites is shown schematically in Figure 1. A multi-stage process was used for the production of composite samples without or with the low amount of voids and defects [26]. Initially, GO and GO-ODA were dispersed in acetone by horn sonication for 1 hour at 45 o C in a water bath to prevent the increase in temperature. Then, a mixture of epoxy resin and acetone was added and mixed for 3 hours by a magnet under a vacuum shock with the pressure between cmhg. For evaporating the acetone, the mixture was heated at 70 o C and mixed again with a magnet under vacuum shock. Then mixture and a curing agent with low viscosity were placed for 2 hours in a vacuum oven under a vacuum shock. After that, the curing agent was added to the mixture and then the suspension was cast into the silicone molds in different shapes for various types of measurements. The amount of curing agent should be exactly the 58:100 ratio because of the addition of more than a specified amount, can lead to an increase in a number of bubbles and makes the composite more brittle. After that, the samples were cured for 24 hours at room temperature and various analyses were carried out on samples after a week of curing. In the vacuum shock by adding and removing the negative vacuum pressure, bubbles in different parts of the suspension forced to move to the surface, thus all the trapped air in the bubbles can be removed. After removing the negative vacuum pressure, due to the high viscosity of the resin, bubbles can not return to their initial position and thus by this technique, step by step bubbles are removed from the suspension [26]. All the samples were fabricated in a similar way and condition. The nanocomposites specification and filler

3 Functionalized Graphene Oxide/Epoxy Nanocomposites Fibers and Polymers 2017, Vol.18, No Table 1. Specification of nanocomposites Sample code Filler loading (wt.%) GO GO-ODA ER*/GO ER/GO-ODA *Pure epoxy resin. loadings can be seen in Table 1. Characterizations A Nicolet 5700 Fourier transform infrared spectroscopy (FTIR) was used to characterize the chemical structure of GO and GO-ODA in the wavenumber range of cm -1. The powdered samples of pure GO and GO-ODA were mixed with KBr and pressed into thin pellets for FTIR study. Raman spectroscopy was carried out using a Teskan Takram P50C0R10 spectrometer with a 532 nm YAG laser as the excitation source. X-ray diffraction (XRD) of GO, GO-ODA, epoxy nanocomposites were carried out using a STOE-STADV diffractometer with Cu radiation (λ=15.4 nm) at 40 kv and 40 ma. The diffraction angle was increased from 1 o to 80 o. The fracture surfaces of the cured composites, which obtained through breaking them in liquid nitrogen were characterized using a VEGA3 SB TESCAN SEM with an operating voltage of 15 kv. Furthermore, the specimens were previously coated with a conductive layer of gold. Transmission electron microscopy (TEM) image was recorded with a Zeiss EM 900 TEM (Carl Zeiss AG) at 80 kv. The nm thick ultrathin section of epoxy nanocomposite for TEM was obtained using a Reichert Om- U3 ultramicrotome. The tensile properties of the composites were investigated according to ASTM D1807, at a constant crosshead speed of 1 mm/min, using an Instron Universal Tensile Tester. Flexural tests were also measured using the same machine, at a crosshead speed of 2 mm/min, following the ASTM D790. For tensile and flexural tests, five specimens were tested and the final values were reported as the average. The Izod impact test of the composites was performed with a Zwick Izod Impact Tester according to the ASTM D Five specimens from each sample were measured. Thermogravimetric analysis (TGA) was conducted by Mettler Toledo TGA1 at a heating rate of 10 o C/min from room temperature to 600 o C under N 2 atmosphere. Results and Discussion Characterization of GO and GO-ODA FTIR Spectroscopy Figure 2(a) to 2(c) shows the FTIR spectra of GO, ODA and GO-ODA. As presented in Figure 2(a), there are the number of characteristic peaks of graphene oxide, a broad peak between 3550 cm -1 and 2500 cm -1 (O-H stretching from COOH and OH groups), around 1733 cm -1 attributed to C=O stretching vibration group of (carboxyl), 1625 cm -1 (C=C stretching in aromatic ring), 1390 cm -1 (C-OH stretching), 1066 cm -1 and 1174 cm -1 (C-O-C stretching vibration in epoxide) [29]. After functionalization with ODA (Figure 2(c)), the decrease of intensity at 1066 cm -1 and 1390 cm -1 also disappear peaks at 1174 cm -1 and 1733 cm -1 and a new peak at 3297 cm -1 refer to the N-H stretch mode of amide carbonyl is observed. Besides, the bands at 1590 cm -1 (N-H bending of amide), 1467 cm -1 (C-N stretch of amide) appear in the spectrum. Peaks at 2917 cm -1 and 2850 cm -1 (C-H stretch of alkyl chain) together with 721 cm -1 represent the formation of a chemical bond between GO and ODA [29-31]. All results, verified the ability of Figure 1. Schematic of nanocomposites preparation.

4 1998 Fibers and Polymers 2017, Vol.18, No.10 Sara Jahandideh et al. according to their surface functionalization [32]. Also, an increase in the D band shift increases the stress transfer efficiency from the epoxy matrix to GO sheets [22]. The ratio of the intensities of the D and G peaks (ID/IG) in GO- ODA (0.655) is a slightly greater than that of GO (0.561). The remarkable thing is that the ID/IG almost remains the same after functionalization, which shows that formed covalent bonds between ODA chains and GO sheets occur without significant destruction of the carbon lattice. The Raman results are in good agreement with previous reports on GO and functionalized GO [17]. Figure 2. FTIR spectra of (a) GO, (b) ODA, and (c) GO-ODA. amino groups of ODA to react with epoxy and carboxyl groups of GO. Therefore, the functionalization of GO with ODA was successfully confirmed. Raman Spectroscopy Figure 3 shows Raman spectra of the GO and GO-ODA powders. It is well known that the D-band comes from the breathing mode of A 1G symmetry, is associated with disordered, sp 3 hybridized carbon that shows impurities and defects in the graphene structure, while the G band is related to the first order scattering of the E 2g phonon of sp 2 hybridized carbon atoms [17]. The G peak in GO is at about 1597 cm -1 and D peak is at about 1340 cm -1. The Raman spectrum of GO- ODA has G and D peaks at 1607 cm -1 and 1356 cm -1, respectively. The G band shifts to a higher wavenumber and higher intensity that reflects the separation of graphene layers means a larger interlayer space for GO-ODA [18]. On the other hand, the transmission of D band to the higher frequency and intensity is more related to an increase in the number of defects in the structure of the GO-ODA, Microstructure Characterization XRD analysis is an important technique for studying the dispersion state of GO and GO-ODA in the composites [22]. Figure 4 shows the XRD pattern of ER, ER/GO and ER/GO- ODA nanocomposites. Graphene oxide powder exhibits a diffraction peak at 11.5 o, corresponding to an interlayer spacing of ~0.93 nm. After the amine functionalization of GO, the XRD spectra of GO-ODA shows a peak at 3.7 o, which represents the increase in distance between the layers of GO sheets to ~2.39 nm, agreeing with the reported results [29]. It means that the ODA chains are successfully grafted on the surface of GO sheets, which agrees well with the FTIR analysis. The neat epoxy shows a broad peak from 12 to 28 o due to the amorphous nature of epoxy matrix. It is noteworthy that the diffraction patterns of all the epoxy resin nanocomposites filled with GO and GO-ODA are similar to the neat epoxy. Therefore, it is probable that GO and GO- ODA sheets are highly intercalated and dispersed in the epoxy resin [8]. Notably, for the further evaluation the degree of filler dispersion and exfoliation, the SEM and TEM observations were investigated, respectively. Figure 5 shows SEM images of the surface of ER, ER/GO and ER/GO-ODA nanocomposites. As can be seen, samples exhibit smooth surface without any flaws and voids. This Figure 3. Raman spectra of (a) GO and (b) GO-ODA. Figure 4. XRD patterns of (a) GO, (b) GO-ODA, (c) ER, (d) ER/ GO, and (e) ER/GO-ODA.

5 Functionalized Graphene Oxide/Epoxy Nanocomposites Fibers and Polymers 2017, Vol.18, No Figure 5. SEM images from surfaces of (a) ER, (b) ER/GO, and (c) ER/GO-ODA. result could indicate the homogenous dispersion and lack of aggregation of GO and ODA-GO in the matrix near the surface of nanocomposites, which prevent the formation of voids. As can be seen in Figure 5(c), nanocomposites containing GO-ODA showing smoother surface in comparison to the other samples. Functionalization of GO by ODA molecules not only can improve dispersion of GO, but also because of the presence of functional groups can enhance and improve the structural quality and modified the surface of the nanocomposite. Also, the fracture surfaces of epoxy composites were investigated by SEM. As can be seen in Figure 6(a), the fracture surface of unreinforced epoxy due to brittle fracture, poor resistance to crack initiation and propagation shows a relatively smooth surface with long crack propagation [7,33,34]. Figure 6(b) shows that the epoxy containing GO has a rougher fracture surface. The rough fracture surface indicates the crack deflection mechanism, which the particles divert the crack propagation path. GO particles prevent the propagation of cracks and thereby increase the strain energy required for fracture [18]. The fractured surface of ER/GO- ODA nanocomposite is rougher and rougher than the other samples (Figure 6(c)), indicating the GO-ODA is more effective in reinforcing the epoxy resin, which agrees well with the mechanical analysis in the next section. Typically, the surface roughness is improved associated with matrix plastic deformation, and thus much fracture energy is dissipated. Better dispersion of GO-ODA in the polymer matrix could increase the energy dissipation during the fracture and improve fracture toughness [16]. The underlying mechanism for this behavior will be explored in the following discussion (mechanical). In addition, TEM was used to further investigation of GO- ODA dispersion and exfoliation in the composite. As can be seen in Figure 6(d), the degree of GO-ODA exfoliation shows a good level in the epoxy matrix. Thermal Properties The thermogravimetric analysis (TGA) and differential thermogravimetric (DTG) curves of the ER, ER/GO and ER/ GO-ODA composites are illustrated in Figure 7. The TGA curves show two step thermal degradation. The first mass loss stage occurs below 200 o C, which may be related to the destruction of the pendant chain of the polymer matrix and the destruction of the main chain of the epoxy matrix creates a second stage of weight loss [35]. The initial degradation temperature (T onset ), the temperature that about 20 % of the destruction of the composites occures in it, for the ER/GO

6 2000 Fibers and Polymers 2017, Vol.18, No.10 Sara Jahandideh et al. Figure 6. SEM images from fracture surfaces of (a) ER, (b) ER/GO, (c) ER/GO-ODA, and (d) TEM image of ER/GO-ODA. and ER/GO-ODA composites is ~337 o C and ~344 o C, respectively and is more than pure resin (~333 o C). The peak of the DTG curves represents the temperature which the maximum weight loss occurs. The major weight loss related to the degradation of the epoxy network occurs about o C [36]. In addition, the thermal stability of ER/GO- ODA composite (~365 o C) is higher than the ER/GO (~360 o C) and neat epoxy (~356 o C). The strong interfacial interaction between GO and GO-ODA with epoxy matrix and good dispersion of them, thereby increasing the degradation temperature of the composites [36], which is consistent with the results obtained by XRD and SEM. Besides, the layered structure of GO can prevent the diffusion of small gas molecules that produced by thermal degradation to nanocomposites [24]. Strong covalent bonding of GO-ODA reduces chain mobility of the polymer matrix and provides a better inhibitory effect of GO-ODA sheets in delaying the production of polymer decomposition [10]. Also, the removal of oxygen heat-sensitive functional groups by grafting ODA, leading to greatly increased the thermal stability of ER/GO- ODA nanocomposite [15]. The char yield of the epoxy composites studied at about 550 o C. High-magnified image of TGA curves shows that the ER/GO-ODA composite has a higher char yield (6.1 %) when compared to the neat epoxy (4.8 %) and ER/GO (5.8 %) composites. The presence of the large amounts of organic and inorganic molecules, which form a layer of char on the burning polymer that prevents the release of flammable gases and prevents heat transfer to the polymer, ultimately, leads to increased degradation temperature or in other words to an increase in thermal stability of the ER/GO-ODA composite [10]. Mechanical Properties Tensile properties of epoxy nanocomposites filled with GO and GO-ODA were investigated at the fixed overall filler concentration (0.5 wt%). Tensile toughness, tensile strength, Young s modulus and elongation at break were studied from the strain-stress curve. Figure 8(a) shows stress versus strain curves for the neat epoxy and its nanocomposites. Toughness or absorbed energy during fracture, which is equivalent to the total area under the stress-strain curve [7, 18], is more for nanocomposite containing GO-ODA than the neat epoxy, while a smaller amount of toughness can be seen for the ER/GO nanocomposite. The average of numerical values of tensile strength, Young s modulus and elongation at break are shown in Figure 8(b) to (d). According to the results, significant improvement in tensile properties of reinforced samples compared with neat epoxy was observed. Figure 8(b) and (c) show that the tensile

7 Functionalized Graphene Oxide/Epoxy Nanocomposites Fibers and Polymers 2017, Vol.18, No Figure 7. (a) TGA (b) DTG curves of neat epoxy and its nanocomposites. Figure 8. Tensile properties of neat epoxy and its nanocomposites; (a) stress-strain curves, (b) tensile strength, (c) Young s modulus, and (d) elongation at break. strength and Young s modulus of ER/GO nanocomposite increase from 2.2 to ~2.7 MPa (~22 %) and from 12.6 to 14 MPa (~11 %), respectively, due to the better dispersion of GO and strong surface interaction between the GO and epoxy matrix, which results in good transfer applied stress from the matrix to the GO without reducing elongation at break of the composite [36]. The nanocomposite filled with GO-ODA has the most improvement. This composite shows ~104 % (from 2.2 to 4.5 MPa) in the tensile strength and ~97 % (from 12.6 to 24.9 MPa) remarkable increase in the Young s modulus in comparison with neat epoxy. The dispersion of nano-fillers in the polymer matrix is greatly influenced by compatibility between the polymer matrix and filler [7]. The large surface area and the presence of amino groups in the GO-ODA increase the surface adhesion and the compatibility between matrix and filler, which leads to a significant improvement in mechanical properties of the composite [3]. Amine functional groups of GO-ODA form chemical bonds with epoxy resin, leading to better compatibility

8 2002 Fibers and Polymers 2017, Vol.18, No.10 Sara Jahandideh et al. Figure 9. Flexural properties of neat epoxy and its nanocomposites; (a) flexural strength and (b) flexural modulus. between the GO-ODA and the matrix and stronger interactions between the components, which improve dispersion of GO- ODA and that increase the stress transferring efficiency [17]. Also, elongation at break increases nearly 33 % and 10 % by the addition of GO and GO-ODA to the epoxy resin, respectively, probably due to the plasticizing effect of GO, which makes the polymer chains to move easily (Figure 8(d)). Graphene sheets can slip by each other when forces are applied to the composites [16]. It should be noted that the elongation at break of ER/GO-ODA reduced compared to ER/GO nanocomposite due to the limited motion of the polymer chain and restricted slippage of GO sheets by a strong interaction between GO-ODA and epoxy resin, though this value is still higher than that of neat epoxy [7,17,37]. Figure 9 presents the flexural properties of the ER, ER/GO and ER/GO-ODA composites. The flexural strength and modulus show a similar trend with tensile properties. The flexural strength of the ER/GO-ODA nanocomposite increases to ~75 % and ~62 % when compared with neat epoxy and ER/GO. In addition, flexural modulus of the ER/GO-ODA and ER/GO nanocomposites significantly increases to ~120 % and ~27 % in comparison with the neat epoxy. The highly dispersed GO-ODA can cause a strong bond between the nanoparticle and the matrix, which increases the stress transfer efficiency and results in increased modulus and strength [32]. The impact strengths of pure epoxy and its nanocomposites with GO and GO-ODA are shown in Figure 10. The composites filled with 0.5 wt.% GO showed about 143 % improvement in impact strength as compared to the pure epoxy matrix. This improvement can be attributed to the strong interface interaction between oxygen groups of GO and the epoxide group of resin [6]. Functionalization of GO by ODA molecules cause a remarkable increase the impact strength of nanocomposite about 251 % due to the more uniform dispersion and stronger surface interaction between GO-ODA and epoxy resin. These interactions are related to Figure 10. Impact strength of neat epoxy and its nanocomposites. the reaction between amino groups of GO-ODA and epoxide groups of epoxy resin, which facilitate the chemical bonds between the GO-ODA and the epoxy network, consistent with the FTIR observations. Homogeneous dispersion of GO-ODA in the matrix, effectively strengthen and toughen the epoxy matrix by increasing the stress that required to break the interfacial interaction between the sheet/sheet or sheet/matrix, which significantly increase the dissipation of fracture energy and thereby improve the impact strength and fracture toughness of the composite [17,24]. Well dispersed GO-ODA nanosheets covered by a thin layer of soft interphase and mobility of them in the matrix can be increased, which increase the energy dissipation during deformation. Such a soft interphase has a major role in reducing load transferred through the interface and improves the fracture toughness of nanocomposites. In addition, better dispersion of GO-ODA sheets mean further modified surfaces with soft interphase, resulting in more fracture toughness [33].

9 Functionalized Graphene Oxide/Epoxy Nanocomposites Fibers and Polymers 2017, Vol.18, No Also, GO-ODA significantly increases the energy absorbed during crack propagation, which is due to toughening mechanisms such as pinning, crack tip branching and crack deflection and increases the toughness of the nanocomposite [17,38]. The fracture surface of the nanocomposite analyzed through SEM, shown the rough surface confirming the enhanced impact strength of the ER/GO-ODA nanocomposite. In these mechanisms, the emergence of many new cracks caused an increase in the total fracture surface area [22]. These micro-cracks can prevent crack propagation by releasing stress and thereby increase the mechanical properties of nanocomposites [17]. The absorbed energy by samples during the impact fracture, could be calculated by the area under the stress-strain curves [7]. In general, the impact results show that the effect of GO- ODA to improve the impact strength of epoxy nanocomposite is more than GO. Conclusion Epoxy nanocomposites containing GO and GO-ODA were fabricated via vacuum shock technique and the effect of this preparation to reduce voids and improve homogeneity and loading of GO (with and without functionalization) on epoxy resin has been investigated by morphological, thermal and mechanical analyses. The thermal degradation temperature of nanocomposites shifted toward higher temperatures by addition of GO-ODA to epoxy resin. The results showed that both GO and GO-ODA are suitable to reinforce the mechanical properties of epoxy resins. For example, with the addition of 0.5 wt.% GO-ODA, 104 %, 75 %, 97 %, 120 % and 251 % considerable increases in tensile and flexural strength, tensile and flexural modulus and impact strength of the nanocomposite were observed, respectively, which indicate that the nanocomposite is strong and ductile. 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