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epoxy composites
Materials Science and Engineering A 517 (2009) 17–23
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Interfacial properties and microstructure of multiwalled carbon nanotubes/epoxy composites Peng Guo, Huaihe Song ∗ , Xiaohong Chen State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, 100029 Beijing, PR China
a r t i c l e
i n f o
Article history: Received 20 January 2009 Accepted 23 March 2009 Keywords: Microstructure Polymer–matrix composites (PMCs) Mechanical properties Interface
a b s t r a c t The present research highlighted the interfacial interaction between modified multiwalled carbon nanotubes (MWCNTs) and epoxy systemically. Stress–strain behaviors of MWCNTs/epoxy composites derived from tensile tests were discussed. The relationship among the morphology, microstructure of the composite interface and its mechanical properties were investigated by scanning electron microscope (SEM), high-resolution transmission electron microscope (HRTEM) and X-ray diffraction (XRD). It was found that MWCNTs with necking ends performed significant roles as reinforcing fillers through investigating stress–strain curves and their corresponding HRTEM images. Moreover, compared the microstructure of modified MWCNTs with the original, it was suggested that the modified MWCNTs with disordered outer and integrated inner layers structure are the main parts in absorbing the load and possess a good interfacial adhesion. The preliminary mechanism on performance of interface interaction was proposed in the form of a simplified schematic diagram describing the process of tensile test. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Epoxy resin is considered a proper and fine matrix for composite, due to its easy processing, excellent mechanical properties and low toxicity, and it has been utilized in many high-powered structural applications, where strength, stiffness, durability and lightness are required. But the inherent brittleness of epoxy resin limits its wider application fields to some extent. Fortunately, since the first report of carbon nanotubes (CNTs) in Nature by Iijima [1], the CNTs have been regarded as a significant choice for improving the performances of epoxy resin owing to their extraordinary properties, such as high tensile strength, Young’s modulus, high thermal conductivity as well as high flexibility [2–5]. Consequently, CNTsfilling epoxy composites have attracted great attention around the world [6–12]. Researchers put emphasis on the functionalization of nanotubes, processing of nanotubes-loaded composites and mechanical properties of composites, and some articles have reviewed the proceedings of CNTs–nanocomposites in recent years [13–17]. It is well known that in fiber reinforced composites, (1) the fiber is used to absorb the load, (2) matrix polymer is to support and protect the fiber and transmit the loading and (3) load transfer depends on the interfacial interaction between the filler and the matrix. It is maintained that the mechanical properties of composites are critically based on the microstructure and performance
∗ Corresponding author. Tel.: +86 10 6443 4916; fax: +86 10 6443 4916. E-mail address: [email protected] (H. Song). 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.03.064
of the interface between reinforcing filler and matrix. At the early stage of CNTs-filling composites research, Ajayan et al. [18] observed the weak interfacial bonding between the nanotubes and resin matrix by the transmission electron microscopy. Frankland et al. [19] used molecular dynamic simulation and found that accidental chemical bonding between nanotubes and matrix during processing may be partially responsible for the enhanced stress transfer. Barber et al. [20] developed an experiment to measure the interfacial adhesion in multiwalled nanotubes–polymer composites by ‘dragging-out’ a single nanotube from a polymer matrix using an atomic force microscope tip. It is perceptible that the interface between the carbon nanotubes and epoxy resin matrix plays a significant role in transferring the forces to the carbon nanotubes and determines the mechanical properties of the resulting composites. However, the slipping in the bundle and the weak interface bonding result in inefficient load transfer, which significantly decreases the mechanical properties of CNTs reinforced composite [21]. The deficiency in effective control of the interface makes many advanced polymer matrix composites fall short of their potential. Therefore, surface modification is an effective route to improve the interfacial properties. Eitan et al. [22] first modified MWCNTs by means of epoxide-based functionalization with carboxylate along their walls. Gojny et al. [23] obtained the surface modified nanotubes by refluxing the nanotubes with multi-functional amines, which alleviated the agglomeration and improved the interfacial interaction. Moreover, the improved mechanical properties of composites were reported by some researchers using different pretreatment and preparation methods. Zhu et al. [6] reported that composites with low addition of functionalized SWCNTs (1–4%) represented
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P. Guo et al. / Materials Science and Engineering A 517 (2009) 17–23
Fig. 1. Stress–strain curves of pure epoxy resin and MWCNTs/epoxy resin composites containing 1 wt.%, 2 wt.% and 3 wt.% MWCNTs.
a promoted ultimate strength and Young’s modulus as well as a longer fracture elongation. Gojny et al. [24] utilized a mini-calendar to prepare MWCNTs/epoxy resin composites, and found that the significant enhancement in fracture toughness (+28% at 0.3 wt.% amino-functionalized MWCNTs) is related to the huge specific surface area of the modified MWCNTs and the integration of CNTs into the epoxy–network structure. Hernández-Pérez et al. [25] fabricated the non-functionalized MWCNTs-loaded epoxy composites, which possessed 3.5% ultimate tensile strain. Yeh et al. [26] observed that the incorporation of 5 wt.% of MWCNTs in the epoxy matrix resulted in an increase in Young’s modulus (+52%) but a decrease in fracture strain (−67%) with the low failure elongation (0.04%–0.06%). In the previous research, well-dispersed MWCNTs/epoxy composites can be produced by an acid-treated surface modification and high-energy sonication dispersion technique [27]. It was found that both the tensile strength and the fracture elongation enhanced evidently with the elevation of MWCNTs content. The mechanism on the improvement in fracture toughness with loading rate is still rough and ambiguous. In order to explore the potential application
Fig. 2. HRSEM images of the fracture surfaces of MWCNTs/epoxy composites (a) macroshape of the fracture surface, (b) raised part on the fracture surfaces, (c) drawn MWCNTs, (d) MWCNTs/epoxy axle–sleeve structure and (e) broken MWCNTs.
P. Guo et al. / Materials Science and Engineering A 517 (2009) 17–23
of multiwalled carbon nanotubes/epoxy composites, an exhaustive investigation on the interfacial properties of carbon nanotubes is necessary and significant. In this paper, we inquired into the interfacial interaction of MWCNTs/epoxy composites from two aspects. On the one hand, the connection between the fracture surface morphology and stress–strain behaviors was discussed for the purpose of finding which parts of MWCNTs play a significant role in improving mechanical properties. On the other hand, in order to fundamentally understand the formation kinetics of distinct morphology at the fracture surface, the microstructure of acid-treated MWCNTs was compared with that of the pristine MWCNTs in details. In addition, a schematic diagram for the tensile process was proposed. 2. Experimental procedures 2.1. Materials and processing The surface modified MWCNTs were prepared by nitro-sulfuric acid treatment according to the method of previous paper [27]. Before this treatment, purification of the crude MWCNTs was first performed with 20 wt.% dilute nitric acid at a constant temperature. The surface modification was followed by acid reflux in a 3:1 (volume ratio) mixture of concentrated sulfur acid (98%) and nitric acid (67%) to remove the metal residues and cut MWCNTs into short ones attached with functional groups. The resulting MWCNTs were washed with deionized water until neutrality, and dried at 353 K in a vacuum oven. In order to keep epoxy resin low viscosity during the process of cast moulding, the resin was heated at 333 K before blending. Afterwards, the modified MWCNTs were manually mixed with diglycidyl ether of biphenol-A (epoxy resin). In order to disperse MWCNTs in the resin homogeneously, the suspension was sonicated for 3 h utilizing a high-power ultrasonic machine periodically with the interval of 5 s. Five minutes before completion of ultrasonication, few drops of 2-ethylic-4-methyl imidazole (curing agent) were added into the mixture of epoxy and MWCNTs. After degasification in a vacuum oven for 3 h at room temperature, the blending were cast into a stainless steel mold and cured at 393 K in an oven under atmospheric pressure for 4 h, the desired MWCNTs/epoxy composites were obtained. The dimensions of specimens have been described in the former paper [27]. 2.2. Characterization of MWCNTs and their epoxy composites Transmission electron microscope observations for pristine and modified MWCNTs were performed on JEOL TEM-3010 FEG microscope at 300 kV and 10 A. X-ray diffraction measurements of raw and modified MWCNTs were performed on Rigaku D/max-2500 VB2+/PC system operation at 40 kv and 20 mA using Cu K␣ radiation ( = 1.5406 Å) with the scanning angle from 5◦ to 90◦ (2) at room temperature. The tensile strength, Young’s modulus and fracture elongation of composites were measured using an InstronTM 1185 universal tensile tester. Tensile tests were carried out at 298 K at a constant crosshead speed of 2 mm/min. A statistical evaluation of the results was derived from at least five individually tested specimens. The slice of composites using the superthin cutting technique (ULTROTOME® LKB-V) was also observed on the JEOL TEM-3010 FEG microscope to characterize the shape and dispersion of MWCNTs.
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Moreover, the fracture surface of MWCNTs/epoxy composites was also characterized by a high-resolution scanning electron microscope (HRSEM, Hitachi S4700) to determine the morphology and distribution of MWCNTs.
3. Results and discussion 3.1. Relationship between strain/stress diagram and morphology of the composites interface In order to obtain the intensive information about interfacial properties, the strain–stress curves were investigated. The composites loaded with different amount of MWCNTs (1 wt.%, 2 wt.%, and 3 wt.%) exhibited the similar curve with a yield point elongation. In comparison with the curve of neat epoxy resin (black line) in Fig. 1, it was distinctly interesting to find that the strain/stress diagram of composites can be divided into two parts (elastics and plastic regions), which is similar to that of the low-carbon (mild) steel style and different from pure epoxy resin style. The investigation on relationship between strain/stress diagram and morphology was put emphasis on the composites containing 3 wt.% MWCNTs (red line) in this work. In the elastic region, each increase in stress produced a proportionate increase in the strain until point A. Then an increase in strain occurred with a smaller increase in stress beyond the yield point A. Obviously, there was a large yield point elongation from point A to point B. In order to seek the reason for the formation of platform, the images of fracture surfaces were observed. The morphology showed typical fracture characteristics of well-dispersed MWCNTs/epoxy composites (Fig. 2(a)). Some short MWCNTs ends existed at the protruding part of the fracture surface (Fig. 2(b) (as rough arrow denoted) and Fig. 2(c)). On the one hand, the characteristic flexibility of MWCNTs is a potential reason for the enhancement of toughness of composites. On the other hand, the relative gliding occurred between a small fraction of nanotubes and polymer matrix, caused by the interfacial structural relaxation which led to poor interfacial adhesion. It is estimated that relatively large yield point elongation is obtained as a result of formation of MWCNTs/epoxy axle–sleeve structure in the process of tensile test. Through the observation of the fracture surface by heat treatment at 1273 K under an inert atmosphere in Fig. 2(d), the MWCNTs were covered by an epoxy layer through the covalent bonding. Subsequently, the strain continued rising with the increase of stress relevantly until the fracture of composite (point C). It was noticed that the composite had a much higher ultimate strength compared with pure resin. It was estimated that MWCNTs that gained a favorable interfacial bonding and turned parallel to the tensile loading direction through further stretching could exert a positive strengthened performance drastically in the matrix. Some round cross-section of MWCNTs (Fig. 2(b) (as thin arrow denoted) and Fig. 2(e)) exhibited that they were snapped by the tensile stress and reached their ultimate tensile strength, which explained why the strain still elevated with the increase of stress after the platform. Ci and Bai [28] prepared a kind of soft and ductile MWCNTs/epoxy composites, and also obtained the stress–strain diagram with a platform showing a significant reinforcement without fracture strain decrease because of the possible expedited curing process
Table 1 Tensile properties of MWCNTs/epoxy resin composites. The numbers in parentheses represent the standard deviation. Epoxy composites formulation
Young’s modulus (MPa)
Ultimate tensile strength (MPa)
Modulus of toughness (J/m3 )
Fracture elongation (%)
Neat epoxy 1% modified MWCNTs 2% modified MWCNTs 3% modified MWCNTs
1006 (24) 1096 (57) 1115 (49) 1110 (18)
45 (1.6) 41 (2.3) 43 (1.8) 61 (1.1)
153 (8) 422 (37) 495 (31) 1004 (23)
4.8 (0.31) 13.1 (0.76) 13.9 (0.55) 23.9 (0.34)
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P. Guo et al. / Materials Science and Engineering A 517 (2009) 17–23
Fig. 3. XRD patterns of the original (a), purified (b) and mixed-acid modified (c) MWCNTs. (The small illustration is local amplification of C (20–30◦ ) and peak A (25.5◦ ), peak B (26.5◦ ).)
with injection method. Diversely, the composites produced by castmolding had improved mechanical properties with hard and stiff characteristics in this work. The results of the tensile tests in terms of Young’s modulus, ultimate tensile strength and fracture elongation and respective standard deviations, corresponding to an average of at least five samples tested for each MWCNTs content, are reported in Table 1. Totally, tensile strength and fracture strain presented a relatively augment with the enhancement of MWCNTs content, while Young’s modulus is nearly basically unchanged and only 8.9%–10.8% larger than the neat epoxy resin. The standard deviation fell with the increase of MWCNTs content, which indicated nanotubes are scattered homogeneously in higher loading MWCNTs samples. Besides, through calculating the modulus of toughness (equal to the total area the stress–strain curve up to the point of rupture) by integral, we received the useful information to represent the energy per unit volume of composites required to produce facture under static conditions [29]. When 3 wt.% MWCNTs is added, it is found that the modulus of toughness and fracture elongation increased by 556.2% and 397.9% respectively as compared to the neat epoxy. Relative to composites with lower MWCNTs content, the more nanotubes owned a wonderful interfacial adhesion with matrix in composites containing 3 wt.% MWCNTs, thus, higher modulus of toughness, tensile strength and fracture elongation were obtained. It is a proof
Fig. 4. HRTEM images of the original (a), modified (b) MWCNTs and (c) end of modified MWCNTs.
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Fig. 5. The schematic of the tensile test processing.
of the strong toughening effect by the mixed-acid modified MWCNTs, considering its high flexibility and high aspect ratio different from traditional fiber-reinforced composites. The curved and entangled MWCNTs are gradually stretched in the process of tensile tests. The firm interfacial covalent bonding linked with epoxy polymer chains could absorb energy continually, so an increased elongation was achieved [6]. 3.2. Mechanism of interfacial interaction Recently, Zhou et al. [30] investigated the microstructure and interfacial interaction between MWCNTs and epoxy resin matrix mainly by Fourier-transfer infrared spectroscopy and positron annihilation technology. Dissimilarly, the XRD and HRTEM were used to study the interfacial microstructure in this work. By making a careful comparison between the original and modified MWCNTs in the HRTEM images and their XRD patterns, some significant and remarkable differences in nanostructure were observed [31]. Being consistent with the XRD results (Fig. 3), the edge-on HRTEM image of raw nanotubes showed a wider interplanar distance of 0.342 nm compared with the theoretical value (0.3354 nm) of the interlayer spacing of graphite [32]. These MWCNTs made from
chemical vapor deposition usually had some defects, and it was obviously found that there were 2 nm thick amorphous carbon shells at sidewall of MWCNTs and some graphene sheets twisted (Fig. 4(a)). The purified MWCNTs in Fig. 3 showed similar XRD patterns with the original MWCNTs except for the relatively high intensity of (0 0 2) peak. We learnt from Fig. 3 that after nitro-sulfuric acid treatment, the characteristic (0 0 2) peak of MWCNTs became weakened and diffused, and was separated into two peaks. Using the Bragg equation, the peak at 25.5◦ indicated that the interplanar spacing of the sample broadened into 3.488 Å. Moreover, the other sharp peak that emerged at 26.5◦ (interlayer distance = 3.358 Å) suggested that the inside microstructure of modified MWCNTs is close to graphite. The (1 1 0) and (1 0 1) peaks represented an asymmetrical shape owing to the curvature of the nanotubes [33]. The decrease in height and sharpness of (1 0 1) and (1 1 0) peaks illuminated that the microstructures of MWCNTs were broken partially through a high-intensity mixed acid treatment. The result of HRTEM (Fig. 4(b)) was in concordance with the XRD analysis, i.e., after purification and acid-treatment, the modified MWCNTs kept the well-formed structure in the inner layers while the outer layers became relaxed, which is convenient for well-
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Fig. 6. HRTEM images of (a) MWCNTs with cone-shaped ends in composites and (b) end of MWCNTs in composite.
bonding with the resin matrix. Likewise, in the arrow denoted areas of Fig. 4(b), the stacks of graphene sheets arranged in a random structure and some sections of graphene layer were incontinuous through mixed acid treatment. In the mean time, fine framework in the inner layers indicated that MWCNTs kept the same cylinder wall structure as raw MWCNTs. It is hold that the sidewalls and openends of nanotubes with etched microstructure are the active areas that attached with functional group like –OH and –COOH during the acid-treatment. Therefore, there was favorable interaction and conjunction between those regions and resin matrix. However, the fine and typical microstructure of MWCNTs was deformed according to the XRD pattern and HRTEM images, which possibly led to the decrease in the mechanical properties of composites compared with the expected results based on the theoretical calculation. Especially, Fig. 4(c) indicated that the end structure of MWCNTs was damaged substantially and sp2 –sp3 hybridized carbon sheet turned disordered.
A simplified schematic illustration of the tensile test and a potential mechanism of interface interaction are proposed in Fig. 5. The first stage illustrated that the ends of disarrayed MWCNTs were stretched along the direction of external force under tensile stress. Gradually, some gaps were left between the ends of MWCNTs and the matrix because of relative sliding that was not propitious for further increase of Young’s modulus. The rest of MWCNTs had a distortion at the top and necking ends (as black arrows denoted Fig. 6(a)). The reasons for the necking phenomenon were that smaller curvature radius and better modification of the end of nanotubes result in stress concentration (Fig. 4(c)). The second stage presented the interface structural relaxation appeared around partial MWCNTs during tensile loading, so some nanotubes was separated from matrix entirely after composites rupture. At the same time, other nanotubes that have a firmer interface bonding with matrix are snapped. The circumstances are favorable for enhancement of fracture elongation and tensile
P. Guo et al. / Materials Science and Engineering A 517 (2009) 17–23
strength. The illustration matched with Fig. 6 approximately and gave us an effective explanation on the kinetics of interfacial interaction. Fig. 6(b) showed that the nanotubes with a cone-shaped end owned a discontinuous graphene sheet in composites similar to the morphology of acid-treated MWCNTs (Fig. 4(b)), and remarkable structural failure occurred at the end of nanotubes illustrating more defects existing there, which proved that MWCNTs with the properly disordered structure would provide a relatively nice interfacial bonding and play significant roles in absorbing the loading in the composites. Intermediate-strength interfacial interaction provided a relatively high tensile strength as well as long fracture elongation. 4. Conclusions This paper focused on the structural characterization and the interfacial mechanism of MWCNTs/epoxy composites to determine which part of MWCNTs in composites playing a key role to improve the mechanical properties. In contrast to pure resin, the toughness and ductility of materials were enhanced remarkably with the improvement of interfacial properties by mixed acid surface modification. Meanwhile, stress–strain behaviors in combination with electron microscope provided a fundamental understanding on the process of interfacial interaction. The moderate acid-treated MWCNTs owning relaxed outer layers had a positive effect on interfacial interaction. The identical thickness and complete structure of the basal inner layers retained the originally excellent feature of MWCNTs and performed a positive action on improvement of mechanical properties of composites. It is supposed that well-dispersed MWCNTs with necking ends offered the matrix efficient enforcement and absorbed the loading mainly by observation of composite slice. A mechanism of interfacial interaction was proposed in the form of a simplified schematic illustration describing the process of tensile test. The higher tensile strength and longer fracture elongation were obtained from the moderate interfacial interaction. Acknowledgement This work was supported by the Program for New Century Excellent Talents in the University (NCET-04-0122).
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Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Interfacial properties and microstructure of multiwalled carbon nanotubes/epoxy composites Peng Guo, Huaihe Song ∗ , Xiaohong Chen State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, 100029 Beijing, PR China
a r t i c l e
i n f o
Article history: Received 20 January 2009 Accepted 23 March 2009 Keywords: Microstructure Polymer–matrix composites (PMCs) Mechanical properties Interface
a b s t r a c t The present research highlighted the interfacial interaction between modified multiwalled carbon nanotubes (MWCNTs) and epoxy systemically. Stress–strain behaviors of MWCNTs/epoxy composites derived from tensile tests were discussed. The relationship among the morphology, microstructure of the composite interface and its mechanical properties were investigated by scanning electron microscope (SEM), high-resolution transmission electron microscope (HRTEM) and X-ray diffraction (XRD). It was found that MWCNTs with necking ends performed significant roles as reinforcing fillers through investigating stress–strain curves and their corresponding HRTEM images. Moreover, compared the microstructure of modified MWCNTs with the original, it was suggested that the modified MWCNTs with disordered outer and integrated inner layers structure are the main parts in absorbing the load and possess a good interfacial adhesion. The preliminary mechanism on performance of interface interaction was proposed in the form of a simplified schematic diagram describing the process of tensile test. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Epoxy resin is considered a proper and fine matrix for composite, due to its easy processing, excellent mechanical properties and low toxicity, and it has been utilized in many high-powered structural applications, where strength, stiffness, durability and lightness are required. But the inherent brittleness of epoxy resin limits its wider application fields to some extent. Fortunately, since the first report of carbon nanotubes (CNTs) in Nature by Iijima [1], the CNTs have been regarded as a significant choice for improving the performances of epoxy resin owing to their extraordinary properties, such as high tensile strength, Young’s modulus, high thermal conductivity as well as high flexibility [2–5]. Consequently, CNTsfilling epoxy composites have attracted great attention around the world [6–12]. Researchers put emphasis on the functionalization of nanotubes, processing of nanotubes-loaded composites and mechanical properties of composites, and some articles have reviewed the proceedings of CNTs–nanocomposites in recent years [13–17]. It is well known that in fiber reinforced composites, (1) the fiber is used to absorb the load, (2) matrix polymer is to support and protect the fiber and transmit the loading and (3) load transfer depends on the interfacial interaction between the filler and the matrix. It is maintained that the mechanical properties of composites are critically based on the microstructure and performance
∗ Corresponding author. Tel.: +86 10 6443 4916; fax: +86 10 6443 4916. E-mail address: [email protected] (H. Song). 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.03.064
of the interface between reinforcing filler and matrix. At the early stage of CNTs-filling composites research, Ajayan et al. [18] observed the weak interfacial bonding between the nanotubes and resin matrix by the transmission electron microscopy. Frankland et al. [19] used molecular dynamic simulation and found that accidental chemical bonding between nanotubes and matrix during processing may be partially responsible for the enhanced stress transfer. Barber et al. [20] developed an experiment to measure the interfacial adhesion in multiwalled nanotubes–polymer composites by ‘dragging-out’ a single nanotube from a polymer matrix using an atomic force microscope tip. It is perceptible that the interface between the carbon nanotubes and epoxy resin matrix plays a significant role in transferring the forces to the carbon nanotubes and determines the mechanical properties of the resulting composites. However, the slipping in the bundle and the weak interface bonding result in inefficient load transfer, which significantly decreases the mechanical properties of CNTs reinforced composite [21]. The deficiency in effective control of the interface makes many advanced polymer matrix composites fall short of their potential. Therefore, surface modification is an effective route to improve the interfacial properties. Eitan et al. [22] first modified MWCNTs by means of epoxide-based functionalization with carboxylate along their walls. Gojny et al. [23] obtained the surface modified nanotubes by refluxing the nanotubes with multi-functional amines, which alleviated the agglomeration and improved the interfacial interaction. Moreover, the improved mechanical properties of composites were reported by some researchers using different pretreatment and preparation methods. Zhu et al. [6] reported that composites with low addition of functionalized SWCNTs (1–4%) represented
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P. Guo et al. / Materials Science and Engineering A 517 (2009) 17–23
Fig. 1. Stress–strain curves of pure epoxy resin and MWCNTs/epoxy resin composites containing 1 wt.%, 2 wt.% and 3 wt.% MWCNTs.
a promoted ultimate strength and Young’s modulus as well as a longer fracture elongation. Gojny et al. [24] utilized a mini-calendar to prepare MWCNTs/epoxy resin composites, and found that the significant enhancement in fracture toughness (+28% at 0.3 wt.% amino-functionalized MWCNTs) is related to the huge specific surface area of the modified MWCNTs and the integration of CNTs into the epoxy–network structure. Hernández-Pérez et al. [25] fabricated the non-functionalized MWCNTs-loaded epoxy composites, which possessed 3.5% ultimate tensile strain. Yeh et al. [26] observed that the incorporation of 5 wt.% of MWCNTs in the epoxy matrix resulted in an increase in Young’s modulus (+52%) but a decrease in fracture strain (−67%) with the low failure elongation (0.04%–0.06%). In the previous research, well-dispersed MWCNTs/epoxy composites can be produced by an acid-treated surface modification and high-energy sonication dispersion technique [27]. It was found that both the tensile strength and the fracture elongation enhanced evidently with the elevation of MWCNTs content. The mechanism on the improvement in fracture toughness with loading rate is still rough and ambiguous. In order to explore the potential application
Fig. 2. HRSEM images of the fracture surfaces of MWCNTs/epoxy composites (a) macroshape of the fracture surface, (b) raised part on the fracture surfaces, (c) drawn MWCNTs, (d) MWCNTs/epoxy axle–sleeve structure and (e) broken MWCNTs.
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of multiwalled carbon nanotubes/epoxy composites, an exhaustive investigation on the interfacial properties of carbon nanotubes is necessary and significant. In this paper, we inquired into the interfacial interaction of MWCNTs/epoxy composites from two aspects. On the one hand, the connection between the fracture surface morphology and stress–strain behaviors was discussed for the purpose of finding which parts of MWCNTs play a significant role in improving mechanical properties. On the other hand, in order to fundamentally understand the formation kinetics of distinct morphology at the fracture surface, the microstructure of acid-treated MWCNTs was compared with that of the pristine MWCNTs in details. In addition, a schematic diagram for the tensile process was proposed. 2. Experimental procedures 2.1. Materials and processing The surface modified MWCNTs were prepared by nitro-sulfuric acid treatment according to the method of previous paper [27]. Before this treatment, purification of the crude MWCNTs was first performed with 20 wt.% dilute nitric acid at a constant temperature. The surface modification was followed by acid reflux in a 3:1 (volume ratio) mixture of concentrated sulfur acid (98%) and nitric acid (67%) to remove the metal residues and cut MWCNTs into short ones attached with functional groups. The resulting MWCNTs were washed with deionized water until neutrality, and dried at 353 K in a vacuum oven. In order to keep epoxy resin low viscosity during the process of cast moulding, the resin was heated at 333 K before blending. Afterwards, the modified MWCNTs were manually mixed with diglycidyl ether of biphenol-A (epoxy resin). In order to disperse MWCNTs in the resin homogeneously, the suspension was sonicated for 3 h utilizing a high-power ultrasonic machine periodically with the interval of 5 s. Five minutes before completion of ultrasonication, few drops of 2-ethylic-4-methyl imidazole (curing agent) were added into the mixture of epoxy and MWCNTs. After degasification in a vacuum oven for 3 h at room temperature, the blending were cast into a stainless steel mold and cured at 393 K in an oven under atmospheric pressure for 4 h, the desired MWCNTs/epoxy composites were obtained. The dimensions of specimens have been described in the former paper [27]. 2.2. Characterization of MWCNTs and their epoxy composites Transmission electron microscope observations for pristine and modified MWCNTs were performed on JEOL TEM-3010 FEG microscope at 300 kV and 10 A. X-ray diffraction measurements of raw and modified MWCNTs were performed on Rigaku D/max-2500 VB2+/PC system operation at 40 kv and 20 mA using Cu K␣ radiation ( = 1.5406 Å) with the scanning angle from 5◦ to 90◦ (2) at room temperature. The tensile strength, Young’s modulus and fracture elongation of composites were measured using an InstronTM 1185 universal tensile tester. Tensile tests were carried out at 298 K at a constant crosshead speed of 2 mm/min. A statistical evaluation of the results was derived from at least five individually tested specimens. The slice of composites using the superthin cutting technique (ULTROTOME® LKB-V) was also observed on the JEOL TEM-3010 FEG microscope to characterize the shape and dispersion of MWCNTs.
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Moreover, the fracture surface of MWCNTs/epoxy composites was also characterized by a high-resolution scanning electron microscope (HRSEM, Hitachi S4700) to determine the morphology and distribution of MWCNTs.
3. Results and discussion 3.1. Relationship between strain/stress diagram and morphology of the composites interface In order to obtain the intensive information about interfacial properties, the strain–stress curves were investigated. The composites loaded with different amount of MWCNTs (1 wt.%, 2 wt.%, and 3 wt.%) exhibited the similar curve with a yield point elongation. In comparison with the curve of neat epoxy resin (black line) in Fig. 1, it was distinctly interesting to find that the strain/stress diagram of composites can be divided into two parts (elastics and plastic regions), which is similar to that of the low-carbon (mild) steel style and different from pure epoxy resin style. The investigation on relationship between strain/stress diagram and morphology was put emphasis on the composites containing 3 wt.% MWCNTs (red line) in this work. In the elastic region, each increase in stress produced a proportionate increase in the strain until point A. Then an increase in strain occurred with a smaller increase in stress beyond the yield point A. Obviously, there was a large yield point elongation from point A to point B. In order to seek the reason for the formation of platform, the images of fracture surfaces were observed. The morphology showed typical fracture characteristics of well-dispersed MWCNTs/epoxy composites (Fig. 2(a)). Some short MWCNTs ends existed at the protruding part of the fracture surface (Fig. 2(b) (as rough arrow denoted) and Fig. 2(c)). On the one hand, the characteristic flexibility of MWCNTs is a potential reason for the enhancement of toughness of composites. On the other hand, the relative gliding occurred between a small fraction of nanotubes and polymer matrix, caused by the interfacial structural relaxation which led to poor interfacial adhesion. It is estimated that relatively large yield point elongation is obtained as a result of formation of MWCNTs/epoxy axle–sleeve structure in the process of tensile test. Through the observation of the fracture surface by heat treatment at 1273 K under an inert atmosphere in Fig. 2(d), the MWCNTs were covered by an epoxy layer through the covalent bonding. Subsequently, the strain continued rising with the increase of stress relevantly until the fracture of composite (point C). It was noticed that the composite had a much higher ultimate strength compared with pure resin. It was estimated that MWCNTs that gained a favorable interfacial bonding and turned parallel to the tensile loading direction through further stretching could exert a positive strengthened performance drastically in the matrix. Some round cross-section of MWCNTs (Fig. 2(b) (as thin arrow denoted) and Fig. 2(e)) exhibited that they were snapped by the tensile stress and reached their ultimate tensile strength, which explained why the strain still elevated with the increase of stress after the platform. Ci and Bai [28] prepared a kind of soft and ductile MWCNTs/epoxy composites, and also obtained the stress–strain diagram with a platform showing a significant reinforcement without fracture strain decrease because of the possible expedited curing process
Table 1 Tensile properties of MWCNTs/epoxy resin composites. The numbers in parentheses represent the standard deviation. Epoxy composites formulation
Young’s modulus (MPa)
Ultimate tensile strength (MPa)
Modulus of toughness (J/m3 )
Fracture elongation (%)
Neat epoxy 1% modified MWCNTs 2% modified MWCNTs 3% modified MWCNTs
1006 (24) 1096 (57) 1115 (49) 1110 (18)
45 (1.6) 41 (2.3) 43 (1.8) 61 (1.1)
153 (8) 422 (37) 495 (31) 1004 (23)
4.8 (0.31) 13.1 (0.76) 13.9 (0.55) 23.9 (0.34)
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Fig. 3. XRD patterns of the original (a), purified (b) and mixed-acid modified (c) MWCNTs. (The small illustration is local amplification of C (20–30◦ ) and peak A (25.5◦ ), peak B (26.5◦ ).)
with injection method. Diversely, the composites produced by castmolding had improved mechanical properties with hard and stiff characteristics in this work. The results of the tensile tests in terms of Young’s modulus, ultimate tensile strength and fracture elongation and respective standard deviations, corresponding to an average of at least five samples tested for each MWCNTs content, are reported in Table 1. Totally, tensile strength and fracture strain presented a relatively augment with the enhancement of MWCNTs content, while Young’s modulus is nearly basically unchanged and only 8.9%–10.8% larger than the neat epoxy resin. The standard deviation fell with the increase of MWCNTs content, which indicated nanotubes are scattered homogeneously in higher loading MWCNTs samples. Besides, through calculating the modulus of toughness (equal to the total area the stress–strain curve up to the point of rupture) by integral, we received the useful information to represent the energy per unit volume of composites required to produce facture under static conditions [29]. When 3 wt.% MWCNTs is added, it is found that the modulus of toughness and fracture elongation increased by 556.2% and 397.9% respectively as compared to the neat epoxy. Relative to composites with lower MWCNTs content, the more nanotubes owned a wonderful interfacial adhesion with matrix in composites containing 3 wt.% MWCNTs, thus, higher modulus of toughness, tensile strength and fracture elongation were obtained. It is a proof
Fig. 4. HRTEM images of the original (a), modified (b) MWCNTs and (c) end of modified MWCNTs.
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Fig. 5. The schematic of the tensile test processing.
of the strong toughening effect by the mixed-acid modified MWCNTs, considering its high flexibility and high aspect ratio different from traditional fiber-reinforced composites. The curved and entangled MWCNTs are gradually stretched in the process of tensile tests. The firm interfacial covalent bonding linked with epoxy polymer chains could absorb energy continually, so an increased elongation was achieved [6]. 3.2. Mechanism of interfacial interaction Recently, Zhou et al. [30] investigated the microstructure and interfacial interaction between MWCNTs and epoxy resin matrix mainly by Fourier-transfer infrared spectroscopy and positron annihilation technology. Dissimilarly, the XRD and HRTEM were used to study the interfacial microstructure in this work. By making a careful comparison between the original and modified MWCNTs in the HRTEM images and their XRD patterns, some significant and remarkable differences in nanostructure were observed [31]. Being consistent with the XRD results (Fig. 3), the edge-on HRTEM image of raw nanotubes showed a wider interplanar distance of 0.342 nm compared with the theoretical value (0.3354 nm) of the interlayer spacing of graphite [32]. These MWCNTs made from
chemical vapor deposition usually had some defects, and it was obviously found that there were 2 nm thick amorphous carbon shells at sidewall of MWCNTs and some graphene sheets twisted (Fig. 4(a)). The purified MWCNTs in Fig. 3 showed similar XRD patterns with the original MWCNTs except for the relatively high intensity of (0 0 2) peak. We learnt from Fig. 3 that after nitro-sulfuric acid treatment, the characteristic (0 0 2) peak of MWCNTs became weakened and diffused, and was separated into two peaks. Using the Bragg equation, the peak at 25.5◦ indicated that the interplanar spacing of the sample broadened into 3.488 Å. Moreover, the other sharp peak that emerged at 26.5◦ (interlayer distance = 3.358 Å) suggested that the inside microstructure of modified MWCNTs is close to graphite. The (1 1 0) and (1 0 1) peaks represented an asymmetrical shape owing to the curvature of the nanotubes [33]. The decrease in height and sharpness of (1 0 1) and (1 1 0) peaks illuminated that the microstructures of MWCNTs were broken partially through a high-intensity mixed acid treatment. The result of HRTEM (Fig. 4(b)) was in concordance with the XRD analysis, i.e., after purification and acid-treatment, the modified MWCNTs kept the well-formed structure in the inner layers while the outer layers became relaxed, which is convenient for well-
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Fig. 6. HRTEM images of (a) MWCNTs with cone-shaped ends in composites and (b) end of MWCNTs in composite.
bonding with the resin matrix. Likewise, in the arrow denoted areas of Fig. 4(b), the stacks of graphene sheets arranged in a random structure and some sections of graphene layer were incontinuous through mixed acid treatment. In the mean time, fine framework in the inner layers indicated that MWCNTs kept the same cylinder wall structure as raw MWCNTs. It is hold that the sidewalls and openends of nanotubes with etched microstructure are the active areas that attached with functional group like –OH and –COOH during the acid-treatment. Therefore, there was favorable interaction and conjunction between those regions and resin matrix. However, the fine and typical microstructure of MWCNTs was deformed according to the XRD pattern and HRTEM images, which possibly led to the decrease in the mechanical properties of composites compared with the expected results based on the theoretical calculation. Especially, Fig. 4(c) indicated that the end structure of MWCNTs was damaged substantially and sp2 –sp3 hybridized carbon sheet turned disordered.
A simplified schematic illustration of the tensile test and a potential mechanism of interface interaction are proposed in Fig. 5. The first stage illustrated that the ends of disarrayed MWCNTs were stretched along the direction of external force under tensile stress. Gradually, some gaps were left between the ends of MWCNTs and the matrix because of relative sliding that was not propitious for further increase of Young’s modulus. The rest of MWCNTs had a distortion at the top and necking ends (as black arrows denoted Fig. 6(a)). The reasons for the necking phenomenon were that smaller curvature radius and better modification of the end of nanotubes result in stress concentration (Fig. 4(c)). The second stage presented the interface structural relaxation appeared around partial MWCNTs during tensile loading, so some nanotubes was separated from matrix entirely after composites rupture. At the same time, other nanotubes that have a firmer interface bonding with matrix are snapped. The circumstances are favorable for enhancement of fracture elongation and tensile
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strength. The illustration matched with Fig. 6 approximately and gave us an effective explanation on the kinetics of interfacial interaction. Fig. 6(b) showed that the nanotubes with a cone-shaped end owned a discontinuous graphene sheet in composites similar to the morphology of acid-treated MWCNTs (Fig. 4(b)), and remarkable structural failure occurred at the end of nanotubes illustrating more defects existing there, which proved that MWCNTs with the properly disordered structure would provide a relatively nice interfacial bonding and play significant roles in absorbing the loading in the composites. Intermediate-strength interfacial interaction provided a relatively high tensile strength as well as long fracture elongation. 4. Conclusions This paper focused on the structural characterization and the interfacial mechanism of MWCNTs/epoxy composites to determine which part of MWCNTs in composites playing a key role to improve the mechanical properties. In contrast to pure resin, the toughness and ductility of materials were enhanced remarkably with the improvement of interfacial properties by mixed acid surface modification. Meanwhile, stress–strain behaviors in combination with electron microscope provided a fundamental understanding on the process of interfacial interaction. The moderate acid-treated MWCNTs owning relaxed outer layers had a positive effect on interfacial interaction. The identical thickness and complete structure of the basal inner layers retained the originally excellent feature of MWCNTs and performed a positive action on improvement of mechanical properties of composites. It is supposed that well-dispersed MWCNTs with necking ends offered the matrix efficient enforcement and absorbed the loading mainly by observation of composite slice. A mechanism of interfacial interaction was proposed in the form of a simplified schematic illustration describing the process of tensile test. The higher tensile strength and longer fracture elongation were obtained from the moderate interfacial interaction. Acknowledgement This work was supported by the Program for New Century Excellent Talents in the University (NCET-04-0122).
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