nanoscale structures

Applied Surface Science 497 (2019) 143802

Contents lists available at ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Full length article

High strength and toughness epoxy nanocomposites reinforced with graphene oxide-nanocellulose micro/nanoscale structures ⁎

Meiling Yan, Weicheng Jiao , Guomin Ding, Zhenming Chu, Yifan Huang, Rongguo Wang

T ⁎

National Key Laboratory of Science and Technology for National Defence on Advanced Composites in Special Environments, Harbin Institute of Technology, Harbin 150080, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Graphene oxide Micro/nanoscale structure Chemically-assembly Epoxy nanocomposite

The prepared micro/nanoscale structure had a strong and flexible 2D graphene oxide (GO) component (elastic modulus 250 GPa) with a high aspect ratio of 1D nanocellulose (NCC), which possessed excellent mechanical properties (elastic modulus 150 GPa) and was a versatile material for enhancing epoxy (EP) nanocomposites. In this work, we had proposed a strategy for chemically assembling a micro/nanoscale structure comprising GO grafted with NCC (GO-NCC). The GO-NCC, combined with the dense, covalently and hydrogen bonded EP molecule, led to highly effective load transfer between the micro/nanoscale structure and the EP. The inclusion of NCC effectively increased the contact area between the GO and EP and enhanced the interfacial strength and contact surface area of the GO-NCC/EP composite, contributing to its excellent mechanical properties (high toughness and strength). The GO-NCC provided dual advantages over conventional GO in its prominent reinforcement and toughening of the EP composite. Only 0.3 wt% of the GO-NCC was needed to significantly increase the tensile strength and Young's modulus by 69% and 13%, respectively, compared to those of the pristine EP. Moreover, the nanocomposite showed a 244% increase in critical stress intensity factor (KIC) and an 801% increase in critical strain energy release rate (GIC).

1. Introduction In view of the cross-linked network structure of epoxies (EPs), these polymers are used in structural engineering applications owing to their low moisture absorption and high temperature performance [1,2]. However, epoxies also come with the undesirable properties of brittleness with low fracture resistance because of their structure [3]. In engineering structures, the strength and toughness of a material are two critical properties that determine the suitability and lifetime of the material. A wide range of particle reinforcements have been employed to enhance these two properties in EPs, but usually strength and toughness changes occur with opposite trends [4]. Nanoparticles have a substantial interfacial area in EPs, strongly influencing the mechanical response of the EPs. Hence, the incorporation of nanomaterials into a polymer matrix is considered to be a highly effective technique to improve the mechanical properties of EPs [5]. Reinforcement nanoparticles including carbon nanotubes, grapheme/graphene oxide (GO), nanoclay and nanosilica [6,7]. Extensive research studies were carried out using GO, with an extremely large surface area and flat structure [8], to improve the mechanical properties of Eps. Fig. 1 showed the relative results of mode I fracture

⁎

toughness and tensile strength of the GO/EPs compared with the neat EPs reported in previous literature. Yi Wei et al. concluded that the presence of GO enhanced the fracture toughness of EP by crack face separation and crack pinning [9]. Relative results showed that modified GO exhibited a higher improvement on toughness and strength [16–20]. Jaemin Chaa et al. utilized melamine and nondestructive ball milling to functionalize graphene nanoplatelets (M-GNPs). With 2 wt%, the M-GNP/EP nanocomposites exhibited enhanced fracture toughness values (50%) and ultimate tensile strengths (17%) compared with GNP/ EP. The addition of melamine prevented the agglomeration of GNPs. MGNPs composites exhibited improved nanofiller dispersion and enhanced interfacial adhesion, providing a large effective interfacial area [16]. Most of the studies on GO demonstrated a very good effect on tensile strength at very low GO loading (under 1 wt%, Fig. 1b). The tensile strength showed a maximum improvement of about 8.1% in the GO/EP composite with 0.5 wt% GO compared to the neat EP. At 1 wt% GO, the tensile strength was nearly equal to that of the neat EP, but much lower than GO/EP composites [22]. The highly dispersed GOs were more effective than the poorly dispersed one, due to transmitting the applied load by the agglomerate. Moreover, there was a synergistic effect when both GO and CNTs were utilized as fillers at the same time.

Corresponding authors. E-mail addresses: [email protected] (W. Jiao), [email protected] (R. Wang).

https://doi.org/10.1016/j.apsusc.2019.143802 Received 16 April 2019; Received in revised form 27 July 2019; Accepted 25 August 2019 Available online 29 August 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

Applied Surface Science 497 (2019) 143802

M. Yan, et al.

Fig. 1. (a) Map of the relative fracture toughness (KIC) of the GO/EP nanocomposites with respect to GO content [9–26], (b) map of the relative tensile strength (σ) of the GO/EP nanocomposites with respect to GO content [9–14,16–20,22–25,27]. Full names for Fig. 1a and b: functionalized graphite oxide (FGO), Nacre-like graphene nanosheets (GNS), Fe3O4-modified graphene oxide (Fe3O4 GO), graphite nanoplatelets (GNP), Amine functionalized expanded graphene nanoplatelets (EGNPs), melamine-functionalized GNP (M-GNP), graphite fluoride (GrF), poly(p-phenylenediamine) nanosized particles attached to graphene oxide (GO/PDA), polyamide 6 chains grafted to graphene oxide (PA6/GO), hyperbranched polyamide functionalized graphene oxide (HBPA-GO), silica nanoparticles attached to graphene oxide (ATGO), amino-functionalized GO (APTS-GO), graphene nanoplatelet (GPL), thermally reduced graphene oxide (TRGO).

2. Experimental methods

Zhou et al. reported a strategy for improving the strength and toughness of an epoxy using GO/CNTs as the filler. The toughness was increased by 58%, and the strength was increased by 42% at an extremely low filler content (0.2 wt%) [21]. However, inhomogeneous dispersion of GO and weak interfacial interactions between GO and EPs resulted in limited strength and toughness [28,29]. Despite the progress in designing GO/EP composites, developing a nanocomposite with a balance of high strength and high toughness remains a challenge. In this work, the fracture toughness and tensile strength in this work were higher than most of the reported values, especially for the fracture toughness (Fig. 1a and b). We suggest chemically assembling a strong (elastic modulus of 200–250 GPa) and flexible 2D GO with a high aspect ratio of 1D nanocellulose (NCC, elastic modulus of 150 GPa) [30]. The GO-NCC had the advantages of a rough surface, lamellar structure and a multifunctional group on the surface, and it could be applied to reinforce the strength and toughness of an EP. GO-NCC reinforced EP composites were prepared by the epoxy glue exchange of acetone in a GO-NCC micro/nano structure dispersion, improving the problems of nanoparticle redispersion and the recycling of acetone. NCC was grafted chemically onto GO to form an outstandingly stable micro/nanoscale structure, and the NCC effectively increased the contact area between the GO and EP. The micro/nanoscale structure could effectively prevent crack initiation and propagation in the EP and improve its toughness. The surface of the GO-NCC composite had amide groups, which substantially facilitates the chemical binding between the GO-NCC and EP against high tensile forces.

2.1. Materials Graphite powder (300 mesh) was purchased from Nanjing XFNANO Materials Tech Co., Ltd. (China). Additionally, 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO), EDA (> 99.0%), 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC·HCl), and N-hydroxysuccinimide (NHS) were purchased from Aldrich Biochemical Technology Co., Ltd. Sodium bromide, sodium hypochlorite, and other chemicals and solvents were obtained from Sinopharm Chemical Reagent Co., Ltd. (China). The epoxy resin (TDE-85) was supplied by Tianjin Jindong Chemical Factory (China). The curing agent, m-phenylenediamine (mPD), was supplied by Beijing Chemical Reagent Co., Ltd. (China). 2.2. Fabrication of GO-NCC The preparation details for NCC and GO are described in the supporting information (Section S2). To introduce covalent bonds between GO and NCC, EDA, EDC, and NHS were incorporated for a coupling reaction (Fig. 2a and b). GO (150 mg) and NCC (100 mg) were dissolved in HCl and distilled water (100 ml) at pH 5. Then, EDA (56 g), EDC·HCl (0.6 g, 3.14 mmol), and NHS (0.36 g, 3.14 mmol) were added to the GO/NCC solution. The suspension was stirred at room temperature for 3 days. 2.3. Fabrication of the GO-NCC/EP nanocomposites Fig. 2 and Fig. S2a–d show the detailed preparation procedures for

Fig. 2. Schematic illustration of the process for preparing the GO-NCC/EP composites: (a) GO and NCC, (b) GO-NCC micro/nanoscale structure and (c) GO-NCC/EP composite. 2

Applied Surface Science 497 (2019) 143802

M. Yan, et al.

the GO-NCC/EP nanocomposites. To prepare the EP nanocomposites reinforced with different loadings of nanofillers in accordance with Table 1, the nanofillers were dispersed in acetone by an ultrasonic cleaner. TED-85 was added to the GO-NCC/acetone suspension, magnetically stirred for 12 h, heated to 60 °C for 5 h for solvent evaporation, and then heated to 70 °C for 12 h under vacuum to remove acetone completely. The acetone was recycled in this step to reduce environmental damage and resource waste. TDE-85 and the curing agent (mDP) were mixed in a ratio of 100:18 by weight and cured at 80 °C for 2 h and 120 for 4 h. In contrast, 0.1, 0.3, and 0.5 wt% GO/EP were also prepared under the same conditions.

Table 1 Material contents of EP and nanofillers in the samples. Sample

EP 0.1 wt% 0.3 wt% 0.5 wt% 0.1 wt% 0.3 wt% 0.5 wt%

EP (wt%)

GO/EP GO/EP GO/EP GO-NCC/EP GO-NCC/EP GO-NCC/EP

Nanofillers (wt%)

TDE-85

mPD

84.74 84.66 84.49 84.32 84.66 84.49 84.32

15.26 15.24 15.21 15.18 15.24 15.21 15.18

GO 0 0.1 0.3 0.5 0 0 0

GO-NCC 0 0 0 0 0.1 0.3 0.5

Fig. 3. The micro/nanoscale structure of chemically assembled GO grafted with the NCC. (a) Schematic diagram of GO grafted with NCC. Digital pictures of the aqueous dispersion: (b) GO, (c) NCC, and (d) GO-NCC. (e) and (f) represent the 2D and 3D AFM images of GO, (g) and (h) show the 2D and 3D AFM images of NCC, (i) and (g) show the 2D and 3D AFM images of GO-NCC. 3

Applied Surface Science 497 (2019) 143802

M. Yan, et al.

Graphite has hardly any functional groups. The FTIR spectrum of GO displays peaks at 3400, 1728 and 1620 cm−1, corresponding to the OH, C]O and C]C characteristic peaks, in agreement with the corresponding GO structure [34]. The FTIR spectrum of GO-NCC was quite similar to the FTIR spectrum of NCC. The NH2 and NH stretching vibrations generally peaked at 3500 and 3400 cm−1, overlapping with the OH stretching vibration [35]. The NH2 bending vibration often occurs at 1560–1650 cm−1, which may overlap with the C]O peaks. Therefore, it is difficult to use FTIR to characterize the reaction of GO and NCC, and further analytical evaluation should be adopted for characterizing GO grafted with NCC. To further verify the reaction of GO and NCC in this work, XPS was applied to characterize the surface chemical composition (C/N/O) of NCC, GO, and GO-NCC (shown in Fig. 4b). In the XPS spectra of NCC and GO, two obvious peaks were observed at 285.0 eV and 533.0 eV, which could be assigned to C 1s and O 1s, respectively [34,36]. For GONCC, a strong signal for the N 1s peak was found at 399.9 eV. The C 1s peaks in the obtained XPS spectra are shown in Fig. 4c. The peak at 284.6 eV originated from graphitic sp2 carbon atoms (CeC bonds), whereas the peak at 286.6 eV was attributed to the CeO bonds in hydroxyl and epoxide groups and possibly to the C]C bonds in defect structures. The peak located at 288.4 eV was due to the carbon atoms in the C]O bonds [37]. Compared with the spectrum of NCC and GO, additional peaks at 285.4 eV and 287.2 eV in the spectrum of GO-NCC, which correspond to the CeN and O=C-N bonds, provided evidence of the amidation between GO and NCC [38,39]. Moreover, the N 1s XPS spectrum of GO-NCC is shown in Fig. 4d. The peak at 400.4 eV was attributed to the NeC group, and the peak at 398.2 eV was assigned to the NeH bond, which suggested that amine was present on the surface of GO-NCC. The component peak areas of the C and N elements in the various materials are listed in Table S1 in the Supporting Information. The amine groups of GO-NCC had a N 1s content of 7.2% (Fig. 4b), in which the active hydrogen content was 71.43% (Table S2). GO-NCC and mDP synergistically enhanced the curing reaction of the epoxy resin. The Raman spectra showed a D-band at 1346 cm−1 and a G-band at 1583 cm−1, confirming the GO lattice distortion (Fig. 4e) [40]. The intensity ratio of the D-band relative to the G-band, ID/IG, provides information about the degree of disorder in the sample due to the presence of defects associated with edges and basal planes. The ID/IG ratio increased from 1.13 for GO to 1.17 for GO-NCC. The increased ID/ IG ratio was attributed to the formation of new defects as well as sp3 hybridization, which might have occurred during amidation of the GO with NCC [41]. The X-ray diffraction (XRD) patterns of NCC, GO, and GO-NCC are displayed in Fig. 4f. A sharp diffraction peak corresponding to the (100) plane appears for the pristine GO sheets at 8.8°, which indicated that the interlayer spacing of the GO was approximately 0.97 nm [42,43]. This result indicated that the GO retained a layered structure. Intense peaks appeared at 15° and 22.5°, which were due to the (101) and (002) planes [44]. NCC was present in the form of cellulose I, and the crystallinity index was Ic = 72.45%. The Ic can be calculated by Eq. (S1) in the Supporting Information. The XRD pattern of GO-NCC retained the (100) peak of GO and the (101) and (002) peaks of NCC, and the type I crystal structure of the NCC was not destroyed by chemical grafting. There are several types of cellulose (I, II, III, IV and V), with type I exhibiting the best mechanical properties [44]. AFM images of the GO-NCC (Fig. 3g) and SEM images of the freezedried particles (Fig. 4g) support GO and NCC being closely associated. The FTIR, XPS, and Raman results together led to the conclusion that NCC was grafted onto the surface of GO successfully. The oxygencontaining and ammonia functional groups in GO-NCC can be effective for improving its dispersion in the EP and triggering an etherification ring-opening reaction between the active hydrogen and EP. GO and NCC were fixed in the micro/nanoscale structure, preventing the aggregation of the nanoparticles, providing a large surface area and

2.4. Characterization Transmission electron microscopy (TEM) analysis was performed with a G2 F30 electron microscope (Tecnai) at an acceleration voltage of 20 kV. Atomic force microscopy (AFM, Dimension Fastscan, Bruker, USA) was performed in tapping mode. Fourier transform infrared (FTIR) spectroscopy was carried out with KBr plates (Nicolet 8700, Thermo Fisher Scientific, USA) over the range of 400–4000 cm−1. X-ray photoelectron spectroscopy (XPS) was performed with a Sigma Probe system (ESCALAB 250Xi, Thermo Fisher, USA) with an Al Kα radiation source. The structure of GO-NCC was examined with X-ray diffraction (XRD) (Empyrean (2.2 kW) with Cu Kα radiation at a scan rate of 5° min−1 in the 2-theta range of 5–50°. Raman spectra were recorded by using a Raman microspectrometer (Renishaw inVia) with a 633 nm excitation laser. Mechanical properties were measured using a universal testing machine (Instron 5882, USA). The fracture surfaces of mechanically tested specimens were analyzed by scanning electron microscopy (SEM). The tensile properties of the EP were tested at a speed of 2 mm/min according to ASTM-D 638 (shown in Fig. S2e and f). Single-edge-notch bending (SENB) was used to test the plane-strain fracture toughness of the EP according to ASTM-D 5045 (shown in Fig. S2g). The value of the critical stress intensity factor (KIC) was calculated as follows:

KIC =

SPQ f (x ) BW 3/2

f (x ) =

3x 1/2 [1.99 − x (1 − x )(2.15 − 3.93x + 2.7x 2)] 2(1 + 2x )(1 − x )3/2

x = a/W

(1)

(2) (3)

where S is the tested span (mm), PQ is the critical load for crack propagation (N), B is the specimen thickness (mm), W is the specimen width (mm), and a is the crack length (mm). The critical strain energy release rate (GIC) was calculated using KIC and the following equation:

GIC =

2 (1 − υ2) KIC E

(4)

where υ is the Poisson's ratio of the EP, and E is the tensile modulus. 3. Results and discussion 3.1. GO-NCC micro/nanoscale structure A schematic illustration of the GO-NCC fabrication process is shown in Fig. 3a. Fig. 3b–d shows the aqueous dispersions of GO, NCC and GONCC. The NCC and GO suspensions were light white and brown, respectively. The GO-NCC suspension blackened after the amide reaction had occurred. The AFM investigation indicated an almost completely individualized GO with thicknesses of 0.84–1.14 nm (Fig. 3e and f). Fig. 3g and h showed the 2D and 3D AFM images of the NCC, in which the needle-like structures with diameters and lengths of 19–40 nm and 196 nm, respectively, are similar to those of nanocellulose reported in the literature [31]. Further observation of the GO-NCC micro/nanoscale structure can be seen in the AFM images in Fig. 3g, in which GO-NCC has a homogenous and stable structure in solution. The edge thickness of GO-NCC was 2.73 nm due to two or three layers of GO being grafted with the NCC. No residues were seen around the GO-NCC, demonstrating the strong interaction between GO and NCC. Fig. 4a showed the FTIR spectra of NCC, GO and GO-NCC. The FTIR spectra of the NCC samples prepared with TEMPO oxidation were similar, and a strong absorption band at ~1600 cm−1 appeared, which was attributed to the C]O stretching vibration of the carboxyl group compared with that of microcrystalline cellulose (MCC) (Fig. S3). The presence of the –COOH functional group in the NCC structure was observed [32,33]. The FTIR spectrum of graphite is shown in Fig. S3. 4

Applied Surface Science 497 (2019) 143802

M. Yan, et al.

Fig. 4. (a) FTIR spectra of NCC, GO and GO-NCC. (b) XPS spectra of GO and GO-NCC. (c) C 1s XPS spectra of GO and GO-NCC. (d) N 1s XPS spectra of GO and GONCC. (e) Raman spectra of GO and GO-NCC. (f) XRD patterns of NCC, GO and GO-NCC. (g) SEM image showing the binding of GO with NCC. (f) Schematic diagram showing EDA interactions between GO and NCC.

(Fig. 1a). As expected, the addition of GO-NCC as a filler effectively improved the KIC (3.55 MPa·m1/2) and the GIC values (2799 J·m−2). The inclusion of only 0.3 wt% of GO-NCC enhanced both the KIC and GIC by 244% and 801% compared to that of the pristine EP. ε represents the elongation at break, υ is the Poisson ratio, u is the strain energy density, and δ represents the crack tip opening displacement. The performance of a nanocomposite is determined by its good nanoparticle dispersion. It has been proven that the particles dispersed in solution were nanosized, and the nanoparticles were stably dispersed in the solution by Brownian motion to form a Tyndall effect [45–48]. The dispersion of GO and GO-NCC was confirmed by red light scattering (the dispersion of GO-NCC is shown in the Supplementary Information in Fig. S6). The Tyndall effect was clearly observed in the 0.1 wt% GO/ EP and 0.1 wt% GO-NCC/EP colloidal suspensions. The GO-NCC particles maintained their high dispersibility and nanoscale size in one dimension. The good dispersibility and stability of GO-NCC in acetone and the EP were useful in the solvent exchange process for preparing the EP nanocomposites. As a comparison, distilled water, acetone and the EP had no Tyndall effect. Both the tensile test and SENB test results signified that adding GO-NCC into the EP could effectively produce reinforced and toughened EP nanocomposites. The mechanical properties of the GO-NCC/EP and previously reported values are compared and shown in Fig. 1a and b. The fracture toughness and tensile strength in this work are higher than most of the reported values, especially for the fracture toughness. This was mainly due to GO being grafted with the stable NCC structure, which markedly improved the mechanical properties of the EP. This paper provides a detailed analysis of the strengthening and toughening mechanisms of the GO-NCC/EP, which are described in the following section.

exposing more functional groups, synergistically enhancing the strength and toughness of the EP. 3.2. Mechanical properties of the GO-NCC/EP nanocomposite The actual working temperature of the EP is much lower than its glass transition temperature (Tg = 150–160 °C, shown in Fig. S4). The molecule chain of the EP is stable at the working temperature. The pristine EP typically exhibits brittle fracturing under tensile force, and its stress-strain curve is a typical brittle fracture curve (Fig. S5). To identify the superiority of the GO-NCC/EP nanocomposites, the EP, GO/EP and GO-NCC/EP samples were compared in terms of their tensile performance (Fig. 5a and b). Increasing the amount of GO and GONCC resulted in higher tensile strengths until saturation occurred at 0.3 wt%. Compared to the pristine EP, adding 0.3 wt% of GO to the EP caused 48.51% and 38.10% increases in the tensile strength and elongation at break, respectively. In addition, the 0.3 wt% GO-NCC/EP nanocomposite exhibited 90.33% and 101.36% increases in the tensile strength and elongation at break, respectively. Apart from the significant increase in the elongation at break, the tensile toughness (strain energy density), which can be calculated by integrating the area under the stress-strain curve (Fig. S5), can also be employed to compare the toughening effects of different fillers [4]. As expected, the 0.3 wt% GO/ EP and 0.3 wt% GO-NCC/EP provided 105% and 240% increases in the tensile toughness in comparison to that of the pristine EP (Table 2). The GO-NCC filler is therefore far superior to GO for fabricating strong and tough functional polymer nanocomposites. The exceptional toughening reinforcement of the GO-NCC filler was further confirmed by the fracture toughness of its EP nanocomposites. Fig. 5c and d shows the KIC and GIC values. KIC and GIC of the neat EP were 1.04 MPa·m1/2 and 309 J·m−2, respectively, which are typical values for brittle EP materials [10]. The GO/EP nanocomposite showed a fracture toughness of 2.09 MPa·m1/2 (103% increase compared with the pristine EP) when a loading of 0.3 wt% GO was used, which was consistent with the reported values for GO/EP nanocomposites

3.3. Strengthening and toughening mechanisms in the GO-NCC/EP The characteristic brittleness and low fracture toughness of the pristine EP is a product of its high cross-linking density, which results in 5

Applied Surface Science 497 (2019) 143802

M. Yan, et al.

Fig. 5. Mechanical properties of both the pristine EP and EP nanocomposites containing various weight fractions (wt%) of GO and GO-NCC: (a) tensile strength (σ), (b) Young's modulus (E), (c) critical stress intensity factor (KIC) and (d) critical strain energy release rate (GIC).

GO-NCC particles but remains pinned at the nanofillers. The dimensions of GO grafted with NCC sheets are understood to be large enough to explain the observation of pinning (Fig. 6f). Amide groups appear in domains on the surfaces of the GO-NCC particles, with the nanofiller being chemically bonded to the epoxy resin (Fig. 7a). Pullout of the GO-NCC particles is considered a primary source of fracture toughness given the strong interfacial adhesion between GO-NCC and the EP. Images of the tensile surfaces of the 0.3 wt% GO-NCC/EP nanocomposites showed pullout was a part of the NCC and GO particles because of the stronger interfacial interactions between the GO-NCC and the EP (Fig. 7b-e). The pullout energy of GO (Epull−out) can be denoted as follows [50]:

poor absorption of energy during fracture [24]. These factors frequently lead to mirror-like fracture surfaces, as shown in Fig. 6a. Evidence of the toughening caused by GO or GO-NCC pullout, rupture, and crack bridging are often visible on the fractured GO or GO-NCC-reinforced EP composite surfaces. Of the two samples, the fractured surface of GONCC/EP was rougher, and jagged multiplanar patterns appeared more apparently so that more energy was required (Fig. 6b and c). When a crack encounters a nanofiller, it is immobilized, and i propagation is locally interrupted. To proceed, the crack front bends itself, increasing the energy required for its propagation. If the crack interacts with GO or GO-NCC, the crack must tilt or twist at greater angles to pass the nanofillers (shown in Fig. 6b and c). The crack tip opening displacement (CTOD) will be much larger than that of the pristine EP, shown in Fig. 6d-f. The CTOD, δ, can be estimated according to [49]:

δ=

2 KIC G (1 − υ2) = IC Eσ σ

w

Epull − out =

∫ 2(w + t )(w − xGO ) KGO dxGO

(6)

0

(5)

where xGO is the displacement of GO. The pullout energy of GO-NCC (Epull−out′) can be denoted as follows:

The CTOD values for GO/EP and GO-NCC/EP were higher than that of pristine EP in all cases, while the CTOD value of 0.3 wt% GO-NCC/EP (30.98 μm) was higher than that of 0.3 GO/EP (13.06 μm), as shown in Table 2. Crack pinning occurs when a propagating crack encounters a series of impenetrable obstacles; the crack front bows out between the

Table 2 Mechanical properties of the EP with additions of GO and GO-NCC between 0.1 wt% and 0.5 wt%. Material

EP 0.1 wt% 0.3 wt% 0.5 wt% 0.1 wt% 0.3 wt% 0.5 wt%

GO/EP GO/EP GO/EP GO-NCC/EP GO-NCC/EP GO-NCC/EP

σ (MPa)

E (GPa)

ε (%)

υ

53.45 60.26 79.38 58.32 81.56 90.33 85.42

3.21 3.39 3.56 3.63 3.53 3.65 3.73

1.47 1.71 2.03 1.62 2.26 2.96 2.18

0.28 0.28 0.29 0.29 0.29 0.28 0.28

KIC (MPa m1/2)

6

1.04 1.52 2.09 1.42 2.32 3.55 2.18

GIC (J/m2) 310.53 601.49 1036.46 508.77 1214.22 2798.67 1174.21

u (10−6 J/m2)

δ (um)

0.39 0.52 0.81 0.47 0.92 1.34 0.93

5.81 9.98 13.06 8.72 14.89 30.98 13.75

Applied Surface Science 497 (2019) 143802

M. Yan, et al.

Fig. 6. SEM images of the SENB fracture surfaces of (a) pristine EP, (b) 0.3 wt% GO/EP and (c) 0.3 wt% GO-NCC/EP. In the images, the red arrows represent crack pinning, and the red dashed lines represent crack branching. Schematic diagram of CTOD (d) EP, (e) GO/EP and (f) GO-NCC/EP.

nanocomposite strength relies on the dispersion and stability of the nanoparticles and the effectiveness of stress transfer between the EP and GO-NCC particles. A model was established according to the characteristics of the tensile force. Both the top and bottom of the EP are subjected a tensile stress (σ). According to the principle of force balance, the σ of the GO/EP and GO-NCC/EP composites is as follows:

′ − out Epull l

=

w′

′ ) K GO ′ dxGO ′ ∫ πd (l − xNCC ) KNCC dxNCC + ∫ 2(w′ + t′)(w′ − xGO 0

0

(7) where xNCC is the displacement of the NCC in GO-NCC, and xGO′ is the displacement of the GO in GO-NCC. During the pullout process, the NCC molecules embedded in the GO cut through (penetrate) the EP, indicating that NCC is a critical factor that should be taken into account when designing the GO-NCC structure. The GO-NCC provided a larger contact area with the EP, which leads to more resistance during pullout, resulting in a higher pullout energy than that of the GO/EP composites. The increase in the interfacial bond strength greatly improved the pullout energy of the GO-NCC particles in the EP, so the fracture toughness of the EP composite with the GO-NCC filler was improved significantly. Young's modulus was markedly improved by adding nanoparticles to the polymer since GO or NCC have much higher stiffness values than that of EP [51]. The strength of a material is defined as the maximum stress that the material can sustain under tensile loading. The

σc S = σm Sm + σGO SGO

(8)

′ SGO ′ + σNCC SNCC σc′ S′ = σm Sm′ + σGO

(9)

where σc and σc′ are the tensile stresses of the GO/EP and GO-NCC/EP composites, respectively, σm is the tensile stress of the pristine EP, σGO and σGO′ are the interfacial stresses between GO and EP, σNCC is the interfacial stress between NCC and EP, S and S′ are the cross-sectional areas of GO/EP and GO-NCC/EP, respectively, Sm and Sm′ are the crosssectional areas of pristine EP, and SGO and SNCC are the surface areas of GO and NCC. A mechanical model for predicting the tensile stress of plate [52] and fiber [53] reinforced polymer composites as follows:

Fig. 7. (a) Schematic diagram of the chemical bonds between the GO-NCC and EP molecules. (b) SEM images of the tensile fracture surface of the 0.3 wt% GO-NCC/ EP composite. (c) GO pullout, (d) NCC pullout, and (e) GO-NCC incomplete pullout from the EP. 7

Applied Surface Science 497 (2019) 143802

M. Yan, et al. 2/3

2 ⎛ t VGO ⎞ σc = σm ⎡ 2 ⎢1 − ⎝ w ⎠ ⎣ ⎜

⎟

2/3

⎤ + 2σ ⎛ tVGO ⎞ GO ⎥ ⎝ w ⎠ ⎦

4. Conclusions

t ⎛1 + ⎞ w⎠ ⎝

(10)

In this study, covalently conjugated micro/nanoscale structures of GO and NCC were generating by using EDA as a linker. The GO-NCC showed good dispersion in acetone and the EP. The EP was loaded to form GO-NCC/EP composites to study their tensile properties and fracture toughness by using tensile and SENB tests, respectively. The optimum tensile strength and fracture toughness were achieved when the content of GO-NCC was 0.3 wt%. The GO-NCC/EP composites showed toughness and tensile strength characteristics higher than those of conventional GO/EP composites. Moreover, the strengthening and toughening mechanisms in the GO-NCC/EP composite were introduced in this paper. GO grafted with the stable NCC structure could prevent crack initiation and propagation effectively. The addition of NCC increased the contact area between the GO and EP. The amide groups on the surface of GO-NCC substantially improved the chemical binding between GO-NCC and EP, which could enhance the tensile stress and pullout energy. The GO-NCC particles have a strong and flexible micro/ nanoscale structure and multiple functional groups on the surface that could result in the high toughness and tensile strength enhancement of EP nanocomposites.

σc′ 2/3

1/3 ′ ⎞ tV ′ 2/3 t ′2VGO πV 2 ⎡ ⎤ ′ ⎛ GO ⎞ = σm ⎢1 − ⎛ − ⎛ NCC ⎞ ⎥ + 2σGO 2 ⎝ 4 ⎠ ⎦ ⎝ w ⎠ ⎝ w′ ⎠ ⎣ t l ⎛1 + ⎞ + σNCC (16πVNCC )1/3 w⎠ d ⎝ ⎜

⎟

⎜

⎟

⎜

⎟

(11)

where VGO, VGO′ and VNCC are the volume fractions. The volume fraction calculations are shown in Table S3 in the Supplementary Information. Supposing the GO plate is a square, w = 1.44 μm and t = 1.05 nm are the width and thickness, respectively, of the GO based on the AFM image statistics, and l = 0.65 μm and d = 23.35 nm are the length and diameter, respectively, of the NCC based on the AFM image statistics. σ′ σ If defining relative tensile strength as σR = σ c and σR′ = σ c , then Eqs. m m (10) and (11) may be rewritten as follows: 1/3

1 ⎞ σR = 1 + ⎜⎛ ⎟ ζ ⎝ GO ⎠

[2K GO (ζGO + 1) − 1] VGO 2/3

(12)

Acknowledgements

σR′ 1/3

1 ⎞ = 1 + ⎜⎛ ⎟ ′ ⎝ ζGO ⎠ VNCC 2/3

This work is supported by National Natural Science Foundation of China (Grant No. U1837203, Grant No. 51872065). The works are also was supported by the National Key Laboratory of Science and Technology on Advanced Composite in Special Environments, KL.PYJH.2017.003 and the Aeronautical Science Foundation of China (Grant No. 2017ZF77010), the Natural Science Foundation of Heilongjiang Province of China (Grant No. E2018030). We are grateful for the constructive comments and valuable advices from all the reviewers for further improvement of our work.

1 ′ (ζGO ′ + 1) − 1] VGO [2K GO ′ 2/3 + (16π )2/3 ⎛KNCC ζNCC − ⎞ 4⎠ ⎝ (13)

where ζGO = w/t and ζGO′ = w′/t′ are the aspect ratios of GO, and ζNCC = l/d is the length and diameter ratio, and KGO′ = σGO′/σm, KGO = σGO/σm and KNCC = σNCC/σm are the interfacial strength factors. Eqs. (10) and (11) describe the relationship between the σc of GONCC reinforced polymer composites, the σm of the EP, the interfacial stress between GO-NCC and EP, and the GO-NCC volume fraction parameters. To investigate the influence of these parameters on the σc, the σR of the GO or GO-NCC reinforced EP composites was estimated by applying Eq. (9) and Eq. (10), and the results are shown in Fig. S7. The σR of GO-NCC/EP increased with increasing GO-NCC content. Taking KGO = 0.5, KGO′ = 0.7 and KNCC = 0.7, the estimates of σR closely match the experimentally measured data. These findings provided clear evidence of the strong interaction between the GO-NCC particles and EP. GO and NCC were grafted by an amide reaction and consequently amine functionalized to achieve better affinity to the EP. With the unique micro/nanoscale structure of the GO-NCC particles and strong bonding of the GO-NCC/EP interfaces, the GO-NCC reinforcement generally had a higher enhancement in the strength and modulus than those of conventional GO fillers. Fig. S8a and b presents SEM images of the pristine EP with clean and smooth fractured surfaces after the tensile tests. The fracture surfaces of the GO/EP and GO-NCC/EP nanocomposites appeared much coarser compared with that of the pristine EP shown in Fig. S8c–j. The fracture surfaces of the nanocomposites with 0.1 wt% and 0.3 wt% of GO and GO-NCC possessed strong interfacial bonding with the EP, and the GO or GO-NCC were homogeneously dispersed in the EP. However, when the contents of GO and GO-NCC increased to 0.5 wt%, vacuum defoaming was difficult, resulting in pore defects on the inside of the EP (Fig. S8 l and n), which caused the concentration of stress and decreased the tensile strength of the nanocomposite. GOs connected with NCCs made it easy to use NCC for force transfer from the EP to GO, and the two-dimensional flexible structure of the GO was utilized effectively. In particular, the GO-NCC particles can prevent the stress concentration of the EP composite caused by GO or NCC agglomeration. The addition of GO-NCC can reinforce both the strength and toughness by stress transfer, energy consumption and strong interfacial bonding between the GO-NCC particles and EP.

Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.143802. References [1] B. Zhao, T. Bai, Improving the tribological performance of epoxy coatings by the synergistic effect between dehydrated ethylenediamine modified graphene and polytetrafluoroethylene, Carbon 144 (2019) 481–491. [2] M. Gholipour-Mahmoudalilou, H. Roghani-Mamaqani, R. Azimi, A. Abdollahi, Preparation of hyperbranched poly (amidoamine)-grafted graphene nanolayers as a composite and curing agent for epoxy resin, Appl. Surf. Sci. 428 (2018) 1061–1069. [3] N. Domun, H. Hadavinia, T. Zhang, T. Sainsbury, G.H. Liaghat, S. Vahid, Improving the fracture toughness and the strength of epoxy using nanomaterials – a review of the current status, Nanoscale 7 (2015) 10294–10329. [4] Y. Chen, H.B. Zhang, Y. Yang, M. Wang, A. Cao, Z.Z. Yu, High-performance epoxy nanocomposites reinforced with three-dimensional carbon nanotube sponge for electromagnetic interference shielding, Adv. Funct. Mater. 26 (2016) 447–455. [5] P. Vijayan P, D. Puglia, M.A.S.A. Al-Maadeed, J.M. Kenny, S. Thomas, Elastomer/ thermoplastic modified epoxy nanocomposites: the hybrid effect of ‘micro’ and ‘nano’ scale, Mater. Sci. Eng. R 116 (2017) 1–29. [6] Y. Zhou, F. Pervin, V.K. Rangari, S. Jeelani, Fabrication and evaluation of carbon nano fiber filled carbon/epoxy composite, Mat Sci Eng A-Struct 426 (2006) 221–228. [7] R. Rafiee, R. Shahzadi, Mechanical properties of nanoclay and nanoclay reinforced polymers: a review, Polym. Compos. 40 (2019) 431–445. [8] F. Jiang, W. Zhao, Y. Wu, Y. Wu, G. Liu, J. Dong, K. Zhou, A polyethyleneiminegrafted graphene oxide hybrid nanomaterial: synthesis and anti-corrosion applications, Appl. Surf. Sci. 479 (2019) 963–973. [9] Y. Wei, X. Hu, Q. Jiang, Z. Sun, P. Wang, Y. Qiu, W. Liu, Influence of graphene oxide with different oxidation levels on the properties of epoxy composites, Compos. Sci. Technol. 161 (2018) 74–84. [10] Y.J. Wan, L.C. Tang, L.X. Gong, D. Yan, Y.B. Li, L.B. Wu, J.X. Jiang, G.Q. Lai, Grafting of epoxy chains onto graphene oxide for epoxy composites with improved mechanical and thermal properties, Carbon 69 (2014) 467–480. [11] Z. Xu, P. Song, J. Zhang, Q. Guo, Y.W. Mai, Epoxy nanocomposites simultaneously strengthened and toughened by hybridization with graphene oxide and block ionomer, Compos. Sci. Technol. 168 (2018) 363–370.

8

Applied Surface Science 497 (2019) 143802

M. Yan, et al.

[32] M. Yan, S. Li, M. Zhang, C. Li, F. Dong, W. Li, Characterization of surface acetylated nanocrystalline cellulose by single-step method, Bioresources 8 (2013) 6330–6341. [33] J. Hao, S. Xu, N. Xu, D. Li, R.J. Linhardt, Z. Zhang, Impact of degree of oxidation on the physicochemical properties of microcrystalline cellulose, Carbohydr. Polym. 155 (2017) 483–490. [34] Y. Feng, B. Wang, X. Li, Y. Ye, J. Ma, C. Liu, X. Zhou, X. Xie, Enhancing thermal oxidation and fire resistance for reduced graphene oxide by phosphorus and nitrogen co-doping: mechanism and kinetic analysis, Carbon 146 (2019) 650–659. [35] J. Che, L. Shen, Y. Xiao, A new approach to fabricate graphene nanosheets in organic medium: combination of reduction and dispersion, J. Mater. Chem. 20 (2010) 1722–1727. [36] S. Tang, S. Jin, R. Zhang, Y. Liu, J. Wang, Z. Hu, W. Lu, S. Yang, W. Qiao, L. Ling, M. Jin, Effective reduction of graphene oxide via a hybrid microwave heating method by using mildly reduced graphene oxide as a susceptor, Appl. Surf. Sci. 473 (2019) 222–229. [37] C. Xia, Z. Xu, J. Yu, Y. Sun, W. Jing, Fabrication of microporous GO-TiO2 membrane via an improved weak alkaline sol–gel method, J. Membr. Sci. 561 (2018) 10–18. [38] Y. Ma, C. Yan, H. Xu, D. Liu, P. Shi, Y. Zhu, J. Liu, Enhanced interfacial properties of carbon fiber reinforced polyamide 6 composites by grafting graphene oxide onto fiber surface, Appl. Surf. Sci. 452 (2018) 286–298. [39] J. Jia, Y. Gai, W. Wang, Y. Zhao, Green synthesis of biocompatiable chitosan–graphene oxide hybrid nanosheet by ultrasonication method, Ultrason. Sonochem. 32 (2016) 300–306. [40] Q. Zhang, Z. Du, X. Huang, Z. Zhao, T. Guo, G. Zeng, Y. Yu, Tunable microwave absorptivity in reduced graphene oxide functionalized with Fe3O4 nanorods, Appl. Surf. Sci. 473 (2019) 706–714. [41] P. Dhar, S.S. Gaur, A. Kumar, V. Katiyar, Cellulose nanocrystal templated graphene nanoscrolls for high performance supercapacitors and hydrogen storage: an experimental and molecular simulation study, Sci Rep-Uk 8 (2018) 3886. [42] S.W. Kim, H.K. Kim, K. Lee, K.C. Roh, J.T. Han, K.B. Kim, S. Lee, M.H. Jung, Studying the reduction of graphene oxide with magnetic measurements, Carbon 142 (2019) 373–378. [43] T.M.D. Alharbi, D. Harvey, I.K. Alsulami, N. Dehbari, X. Duan, R.N. Lamb, W.D. Lawrance, C.L. Raston, Shear stress mediated scrolling of graphene oxide, Carbon 137 (2018) 419–424. [44] J.I. Morán, V.A. Alvarez, V.P. Cyras, A. Vázquez, Extraction of cellulose and preparation of nanocellulose from sisal fibers, Cellulose 15 (2008) 149–159. [45] P. Shandilya, D. Mittal, A. Sudhaik, M. Soni, P. Raizada, A.K. Saini, P. Singh, GdVO4 modified fluorine doped graphene nanosheets as dispersed photocatalyst for mitigation of phenolic compounds in aqueous environment and bacterial disinfection, Sep. Purif. Technol. 210 (2019) 804–816. [46] K.H. Tseng, C.J. Chou, S.H. Shih, D.C. Tien, C.Y. Chang, L. Stobinski, Preparation of graphene through EDM interfered with CO2, J. Clust. Sci. 29 (2018) 555–559. [47] S. Acharya, B. Das, U. Thupakula, K. Ariga, D.D. Sarma, J. Israelachvili, Y. Golan, A bottom-up approach toward fabrication of ultrathin PbS sheets, Nano Lett. 13 (2013) 409–415. [48] D. Nuvoli, L. Valentini, V. Alzari, S. Scognamillo, S.B. Bon, M. Piccinini, J. Illescas, A. Mariani, High concentration few-layer graphene sheets obtained by liquid phase exfoliation of graphite in ionic liquid, J. Mater. Chem. 21 (2011) 3428–3431. [49] S.Y. Fu, X.Q. Feng, B. Lauke, Y.W. Mai, Effects of particle size, particle/matrix interface adhesion and particle loading on mechanical properties of particulate–polymer composites, Compos Part B-Eng 39 (2008) 933–961. [50] J.Z. Liang, Estimation of tensile strength of inorganic plate-like particulate reinforced polymer composites, Polym. Eng. Sci. 53 (2013) 1823–1827. [51] J.Z. Liang, Predictions of tensile strength of short inorganic fibre reinforced polymer composites, Polym. Test. 30 (2011) 749–752. [52] T. Gómez-del Río, A. Salazar, R.A. Pearson, J. Rodríguez, Fracture behaviour of epoxy nanocomposites modified with triblock copolymers and carbon nanotubes, Compos Part B-Eng 87 (2016) 343–349. [53] J. Zhang, D. Jiang, F. Scarpa, H.-X. Peng, Enhancement of pullout energy in a singlewalled carbon nanotube-polyethylene composite system via auxetic effect, Compos. A: Appl. Sci. Manuf. 55 (2013) 188–194.

[12] S. Chhetri, N.C. Adak, P. Samanta, N.C. Murmu, D. Hui, T. Kuila, J.H. Lee, Investigation of the mechanical and thermal properties of l-glutathione modified graphene/epoxy composites, Compos Part B-Eng 143 (2018) 105–112. [13] Y. He, Q. Chen, S. Yang, C. Lu, M. Feng, Y. Jiang, G. Cao, J. Zhang, C. Liu, Microcrack behavior of carbon fiber reinforced Fe3O4/graphene oxide modified epoxy composites for cryogenic application, Compos. A: Appl. Sci. Manuf. 108 (2018) 12–22. [14] D. Quan, D. Carolan, C. Rouge, N. Murphy, A. Ivankovic, Mechanical and fracture properties of epoxy adhesives modified with graphene nanoplatelets and rubber particles, Int. J. Adhes. Adhes. 81 (2018) 21–29. [15] S. Chatterjee, J.W. Wang, W.S. Kuo, N.H. Tai, C. Salzmann, W.L. Li, R. Hollertz, F.A. Nüesch, B.T.T. Chu, Mechanical reinforcement and thermal conductivity in expanded graphene nanoplatelets reinforced epoxy composites, Chem. Phys. Lett. 531 (2012) 6–10. [16] J. Cha, J. Kim, S. Ryu, S.H. Hong, Comparison to mechanical properties of epoxy nanocomposites reinforced by functionalized carbon nanotubes and graphene nanoplatelets, Compos Part B-Eng 162 (2019) 283–288. [17] F. Lei, C. Zhang, Z. Cai, J. Yang, H. Sun, D. Sun, Epoxy toughening with graphite fluoride: toward high toughness and strength, Polymer 150 (2018) 44–51. [18] A. Hussein, S. Sarkar, K. Lee, B. Kim, Cryogenic fracture behavior of epoxy reinforced by a novel graphene oxide/poly(p-phenylenediamine) hybrid, Compos Part B-Eng 129 (2017) 133–142. [19] X. Zhao, Y. Li, W. Chen, S. Li, Y. Zhao, S. Du, Improved fracture toughness of epoxy resin reinforced with polyamide 6/graphene oxide nanocomposites prepared via in situ polymerization, Compos. Sci. Technol. 171 (2019) 180–189. [20] Z. Qi, Y. Tan, L. Gao, C. Zhang, L. Wang, C. Xiao, Effects of hyperbranched polyamide functionalized graphene oxide on curing behaviour and mechanical properties of epoxy composites, Polym. Test. 71 (2018) 145–155. [21] B. Ahmadi-Moghadam, F. Taheri, Fracture and toughening mechanisms of GNPbased nanocomposites in modes I and II fracture, Eng. Fract. Mech. 131 (2014) 329–339. [22] T. Jiang, T. Kuila, N.H. Kim, B.C. Ku, J.H. Lee, Enhanced mechanical properties of silanized silica nanoparticle attached graphene oxide/epoxy composites, Compos. Sci. Technol. 79 (2013) 115–125. [23] Z. Li, R. Wang, R.J. Young, L. Deng, F. Yang, L. Hao, W. Jiao, W. Liu, Control of the functionality of graphene oxide for its application in epoxy nanocomposites, Polymer 54 (2013) 6437–6446. [24] D.R. Bortz, E.G. Heras, I. Martin-Gullon, Impressive fatigue life and fracture toughness improvements in graphene oxide/epoxy composites, Macromolecules 45 (2012) 238–245. [25] M.M. Shokrieh, S.M. Ghoreishi, M. Esmkhani, Z. Zhao, Effects of graphene nanoplatelets and graphene nanosheets on fracture toughness of epoxy nanocomposites, Fatigue Fract Eng M 37 (2014) 1116–1123. [26] S. Chandrasekaran, N. Sato, F. Tölle, R. Mülhaupt, B. Fiedler, K. Schulte, Fracture toughness and failure mechanism of graphene based epoxy composites, Compos. Sci. Technol. 97 (2014) 90–99. [27] J. Long, S. Li, B. Liang, Z. Wang, Investigation of thermal behaviour and mechanical property of the functionalised graphene oxide/epoxy resin nanocomposites, Plast Rubber Compos 48 (2019) 127–136. [28] X. Gong, Y. Liu, Y. Wang, Z. Xie, Q. Dong, M. Dong, H. Liu, Q. Shao, N. Lu, V. Murugadoss, T. Ding, Z. Guo, Amino graphene oxide/dopamine modified aramid fibers: preparation, epoxy nanocomposites and property analysis, Polymer 168 (2019) 131–137. [29] J. Lin, P. Zhang, C. Zheng, X. Wu, T. Mao, M. Zhu, H. Wang, D. Feng, S. Qian, X. Cai, Reduced silanized graphene oxide/epoxy-polyurethane composites with enhanced thermal and mechanical properties, Appl. Surf. Sci. 316 (2014) 114–123. [30] J.M. Kim, V. Guccini, K.D. Seong, J. Oh, G. Salazar-Alvarez, Y. Piao, Extensively interconnected silicon nanoparticles via carbon network derived from ultrathin cellulose nanofibers as high performance lithium ion battery anodes, Carbon 118 (2017) 8–17. [31] F.V. Ferreira, M. Mariano, S.C. Rabelo, R.F. Gouveia, L.M.F. Lona, Isolation and surface modification of cellulose nanocrystals from sugarcane bagasse waste: from a micro- to a nano-scale view, Appl. Surf. Sci. 436 (2018) 1113–1122.

9