Assessment of carbon nanotube dispersion and mechanical property of epoxy nanocomposites by curing reaction heat measurement

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1 Original Article Assessment of carbon nanotube dispersion and mechanical property of epoxy nanocomposites by curing reaction heat measurement Journal of Reinforced Plastics and Composites 2016, Vol. 35(1) 71 80! The Author(s) 2015 Reprints and permissions: sagepub.co.uk/journalspermissions.nav DOI: / jrp.sagepub.com Sihwan Kim 1,2, Woo I Lee 2 and Chung H Park 1 Abstract The dispersion state of carbon nanotubes (CNTs) is a key parameter to the mechanical properties of nanocomposites. In the previous work, we proposed a method to assess the dispersion state of CNTs in a thermoset polymer matrix by measuring the curing reaction heat. In the present paper, we propose new parameters to represent the degree of CNT dispersion which can be defined from dynamic scanning calorimetry (DSC) measurement data. Then, we investigate a relationship among the degree of CNT dispersion, the curing reaction heat and the mechanical property of nanocomposites. A micromechanics model is proposed to predict the tensile modulus of nanocomposites considering not only the content but also the degree of dispersion of CNTs which is represented in terms of the curing reaction heat. The model predictions and experimental measurement data for the tensile modulus of nanocomposites show that the exothermic reaction heat during the curing process is a good quantitative measure to estimate the mechanical property of nanocomposites as well as to evaluate the degree of CNT dispersion. Keyword Carbon nanotube, nanocomposites, mechanical property, differential scanning calorimetry, degree of dispersion Introduction By virtue of the excellent mechanical, thermal, and electrical properties, carbon nanotubes (CNTs) have been attracting great attention from the engineers and researchers in nanotechnology, electronics, and other fields of materials science. Especially, the use of CNTs as reinforcement of composites has been extensively examined to develop high performance materials. Since the first report of Ajayan et al. published in 1994, 1 the number of research papers on CNT-reinforced polymer materials, either thermoset or thermoplastic, has been dramatically increasing. Among the thermoset matrix polymers, epoxy is the most commonly used one by dint of its good thermal and mechanical properties. Thus, a lot of research on CNT-reinforced thermoset polymer composites has focused on epoxy nanocomposites. In general, CNT reinforced epoxy composites have better mechanical properties such as modulus and strength, than neat polymer matrix. It has been reported, however, that the improvement of mechanical properties by the CNT reinforcement is not consistent even for a same concentration of CNTs in the nanocomposites. 2 5 According to Allaoui et al., 2 Young s modulus was improved by 100% by an addition of 1 wt% CNTs. In other references, 3,4 however, the improvement was not substantial even with the same amount of CNTs. Therefore, additional factors other than CNT 1 Mines Douai, Department of Polymers and Composites Technology & Mechanical Engineering, Douai, France 2 Department of Mechanical and Aerospace Engineering, Seoul National University, Gwanak-gu, Seoul, Korea Corresponding author: Chung H Park, Mines Douai, Department of Polymers and Composites Technology & Mechanical Engineering, 941 rue Charles Bourseul, Douai, France. chung-hae.park@mines-douai.fr

2 72 Journal of Reinforced Plastics and Composites 35(1) concentration should be considered to obtain good reinforcement effects. It has been reported that there are a number of parameters affecting the mechanical properties of CNT nanocomposites, such as the dispersion state, 6 alignment and aspect ratio of CNTs, and the interfacial adhesion between CNTs and matrix. 7 In particular, the CNT dispersion plays an important role because homogeneous dispersion not only guarantees uniform load-sharing by the reinforcing particles but also increases the interfacial surface area between the reinforcement and the matrix where stress transfer takes place. The detrimental effect of poor dispersion on the mechanical properties such as elastic modulus and strength can be seen in a number of CNT composite systems if CNT loading level is increased beyond the point where CNT aggregation begins. 8 After all, the CNT dispersion state and CNT/matrix interfacial bonding are the most important factors to influence the mechanical performance of CNT nanocomposites. Nevertheless, no reliable experimental method is available to quantitatively evaluate the overall dispersion state of a whole specimen. In most research efforts, image analysis techniques by direct microscopic observation with an optical microscope, a scanning electron microscope or a transmission electron microscope have been employed to assess the state of dispersion. 2 4,9 12 In these direct image analysis methods, individual particles are observed to examine the degree of dispersion. Hence, it is difficult to assess the overall dispersion state in a quantitative way. Moreover, the CNT dispersion in thermoset polymer composites should be investigated in a cured specimen. Consequently, the dispersion state can be evaluated only after the composites fabrication and no information is available during the manufacturing process to remedy a poor dispersion state. Meanwhile, it has been observed that resin cure kinetics can be changed if CNTs are added to thermoset resin. 10,13 For instance, the use of different solvents to disperse CNTs resulted in different degrees of epoxy resin cure even after the solvents were completely removed. 10 From this observation, it has been speculated that CNTs can affect the cure kinetics by acting as obstacles to thermoset polymer cross-linking. If this argument is valid, the effect of CNTs on the curing reaction may be related to the number of individual CNTs which are well dispersed in the epoxy. Therefore, a relationship between the cure kinetics and the number of welldispersed CNTs can provide a good quantitative measure of the CNT dispersion state. 13 In this work, we investigate a relation among the curing reaction heat, the degree of CNT dispersion, and the elastic modulus of CNT-reinforced epoxy (see Figure 1). We fabricated CNT epoxy composite specimens with different contents and degrees of dispersion of CNTs. Each specimen was submitted to tensile test to investigate the relation between the elastic modulus and the degree of dispersion. Based on the experimental results, we propose new parameters defined in terms of total reaction heat during the curing process to represent the degree of CNT dispersion. The new parameter representing the degree of dispersion is incorporated into a micromechanics model to evaluate the elastic modulus of nanocomposites considering the effect of CNT dispersion. Materials and experimental procedures Materials Multi-walled CNTs (by Hanhwa Nanotech, Korea) were used to fabricate nanocomposites. Surface treatment of CNTs was not performed to rule out any effect other than dispersion. The diameter of CNTs ranged from 10 to 15 nm and the aspect ratio of CNT was around A wide range of values for the mechanical properties of CNTs have been reported in the literature In the current work, the elastic modulus of the CNT was assumed to be 1 TPa. 15 The density of CNT was supposed to be 1.8 g/cm 3. As the matrix material, bisphenol-a type epoxy resin (YD114 by Kukdo Chemical, Korea) was used with a curing agent (KBH1089 by Kukdo Chemical, Korea). The density and Young s modulus of the fully cured epoxy resin obtained by independent tensile tests were 1.2 g/cm 3 and 3.03 GPa, respectively. Sample preparation In order to facilitate dispersion, the epoxy resin was diluted using a solvent. First, the epoxy resin of 400 g was dissolved in acetone of 600 ml and CNTs were dispersed in the solution using a conical ultrasonicator (CV 505 power supply and CV 33 convertor from Sonics & Materials, Inc.) with a prone tip whose diameter was 25 mm. To obtain different states of dispersion, the duration of ultrasonication was varied from 12 to 24 h viz., 12 h, 18 h, and 24 h. Throughout the ultrasonication, ultrasonic energy of 150 W was applied for 1 s at every 3 s. After the dispersion process, the solvent was removed by low power sonication of 20 W, using another ultrasonicator (SD-D300H). The ultrasonic bath was filled with water of 10 l and the water was circulated to cool down. The beaker that contained the solution was not in contact with the bottom of the sonic bath. The residual solvent was further eliminated in a vacuum oven at 80 Cfor1h. The CNT/epoxy suspension was mixed with the curing agent by a mechanical stirrer for 10 min. Then, the mixture was degassed at 70 C for 30 min under the vacuum condition.

3 Kim et al. 73 Figure 1. Schematic of the assessment method for CNT dispersion and mechanical properties of nanocomposites by means of curing reaction heat measurement. The mixture of epoxy and CNT was then cast and cured at 130 C for 60 mins in a mold to fabricate tensile test coupons with the dimensions defined by ASTM D 638 type 1 standard. The specimens were cooled down to around 50 C in the mold. It has been reported that CNTs may form clusters in CNT/epoxy suspension during curing as high curing temperature lowers the resin viscosity by increasing the mobility of CNTs in the matrix. 18 The same curing condition was applied for the fabrication of all the specimens to rule out the effect of the viscosity change on the CNT mobility during the curing process. Consequently, the CNT dispersion state of the initial CNT/epoxy suspension should haveastrongcorrelationwith that of the final cured specimen under the same curing condition, even though the aggregation of CNTs may occur during the resin curing. Samples with different CNT contents, viz., 0.1 wt%, 0.2 wt%, and 0.3 wt% were prepared. For each CNT concentration, three types of samples with different dispersion states of CNTs were prepared by varying the duration of ultrasonication (i.e. 12, 18 and 24 h). For each condition, five specimens were fabricated. Experimental measurements (DSC, TA-Q1000 from TA instrument, Inc.). During the dynamic scanning, the sample temperature was raised from the ambient temperature to the maximum temperature (i.e. 250 C) at a rate of 10 C/min. Three dynamic scans were performed for each sample to obtain an average value of total reaction heat. 19 Tensile modulus was measured using a universal testing machine (UTM, MTS alliance RT/10 from MTS systems, Inc.) with a crosshead speed of 50 mm/ min. As stated previously, the tensile test of the specimens was conducted according to ASTM D 638 type 1 standard. Five measurements were performed and an average value was obtained for each condition. Results and discussion Degree of dispersion Definition of degree of dispersion. We propose two parameters to represent the degree of dispersion. Above all, the degree of dispersion can be defined as the ratio of the well-dispersed CNT concentration to the total CNT concentration of a sample. In order to measure the total heat of reaction, dynamic scanning was done using a differential scanning calorimeter D 1 ¼ C S C I ð1þ

4 74 Journal of Reinforced Plastics and Composites 35(1) where C S is the well-dispersed CNT concentration in the sample and C I is the total concentration of CNTs in the sample. To determine this parameter, however, the well-dispersed CNT concentration C S should be found for each sample by independent measurements. In the literature, it has been observed from differential scanning calorimetry (DSC) analysis that an increase of CNT concentration results in a decrease of reaction heat because CNTs can act as a physical hindrance to the mobility of monomers. 13 This reduction of monomers mobility will be proportional to the number of disaggregated individual CNTs in the solution. Thus, a lower mobility of monomers may result in a lower level of cross-linking and the total heat of reaction will be decreased. On the contrary, aggregated CNTs are much less efficient in blocking chemical reactions due to their bigger size than individual monomer and have insignificant influence on the change of the total heat of reaction. According to this speculation, a method to assess the dispersion state of CNTs in epoxy resin by means of DSC analysis was proposed in the previous work. 19 This method is illustrated in Figure 2. The total reaction heat for neat epoxy resin can be obtained by DSC measurement (see the point A in Figure 2). Then, the reaction heat for CNT/epoxy mixture is measured by DSC for different CNT concentrations. In order to estimate the mass fractions of aggregated and of well-dispersed CNTs prior to DSC analysis, dynamic light scattering (DLS) analysis is employed. DLS analysis is a method to measure the size distribution of particles from 1 nm to 10 mm. From the DLS analysis and DSC measurement data, we can obtain the data of total reaction heat for well-dispersed CNT mass for different samples (see the points B and C), assuming that aggregated CNTs have negligible influence on the resin curing reaction. From this procedure, we can obtain the curve I in Figure It should be kept in mind that the CNT concentration values for the points B and C are not the total concentrations of CNTs, but the concentrations of well-dispersed CNTs in the samples. By DSC analysis, we obtain experimental data for the total CNT concentration and the reaction heat of real CNT/epoxy composite system where some CNTs are poorly dispersed or aggregated while making insignificant influence on the curing reaction (see point D in Figure 2). As an alternative to the degree of dispersion defined by equation (1), we propose another parameter defined in terms of total reaction heat that can be measured by DSC analysis. D 2 ¼ H S H I where H S is the difference in the total reaction heat between the neat resin and the current sample, and H I is the difference in the total reaction heat between the neat resin and the well-dispersed CNT resin mixture. 19 It should be noted that the degree of dispersion can be represented by the parameter D 2 which can be easily obtained by DSC measurement once Curve I has been prepared, even if the real concentration of well-dispersed CNT is not directly measured. Moreover, the value of D 1 corresponding to D 2 can be estimated by Curve I as shown in Figure 2 without independent measurement of well-dispersed CNT concentration for each sample. ð2þ Experimental results of DSC analysis. Table 1 presents the total reaction heat of the well-dispersed CNT/epoxy mixtures for different CNT concentrations which are taken from the Curve I. 19 These values were used to evaluate the degree of dispersion according to the definitions of D 1 and D 2 (see equations (1) and (2)). The experimental results obtained by the DSC analysis are presented in Table 2. For different cases of Table 1. Total heat of reaction of well-dispersed CNT/ epoxy mixture for different CNT concentrations. Well dispersed CNT concentration (wt %) Total heat of reaction (J/g) Figure 2. Definitions of degree of CNT dispersion in epoxy suspension. (D 1 is defined by the heat of reaction and D 2 is defined by the CNT concentration of the sample. Curve I represents the total heat of reaction for well-dispersed mixture.). 0 (Neat resin)

5 Kim et al. 75 CNT concentration and of ultrasonication duration, the total reaction heat was measured by the DSC (see the third column in Table 2). The mass of well-dispersed CNTs obtained from the Curve I (which is shown at the fourth column in Table 2) was used to calculate the parameter D 1 (which is presented at the fifth column in Table 2) by equation (1). It should be noted that the parameter D 2 obtained from the DSC analysis result can be used to estimate the parameter D 1 without direct measurement of well-dispersed CNT mass once the curve I in Figure 2 is prepared as preliminary data. As can be seen from Table 2, the duration of ultrasonication is an important factor to determine the dispersion state of CNTs. As the duration of ultrasonication is increased, the degree of dispersion is improved. It becomes more difficult to obtain good dispersion even with the same ultrasonication duration, however, as the CNT concentration increases. When the long duration of ultrasonication (viz., 24 h) was applied, for example, some CNTs remained to be aggregated in the case of high concentration (namely 0.3 wt%), whereas most of CNTs were well dispersed in the case of low CNT concentrations (namely, 0.1 and 0.2 wt%). It has been reported that extended exposure to high ultrasonic energy may damage the CNTs 20 and thus adversely affect the mechanical properties. We do not consider the adverse effect of the defects of CNTs on the mechanical property, but only the influence of the dispersion state. As shown in Figure 2, the relationship between the number of well-dispersed CNTs and the decrease in the heat of reaction is not linear. As a matter of fact, D 2 has a limitation to directly indicate the state of dispersion, even though it is easy to implement. Meanwhile, D 1 explicitly represents the amount of well-dispersed CNTs and thus can be used to figure out the effect of dispersion state of CNTs in the epoxy resin. Thus, D 1 which can be obtained by the Curve I and D 2, is used to evaluate the influence of CNT dispersion on the mechanical property as presented in a subsequent section. Mechanical property Tensile test results. The experimental results of the tensile test are shown in Figure 3. In order to better represent the improvement of Young s modulus, the modulus of the composites is divided by that of the neat epoxy resin. As can be expected, the elastic modulus of nanocomposites was increased as the degree of dispersion was improved despite the same loading of CNTs. In general, the elastic modulus of composites is improved as the reinforcement concentration is increased. We can see this tendency in the experimental data for high degree of dispersion (i.e. D 1 > 0.4 and D 2 > 0.6) in Figure 3. The reinforcement effect by CNTs is almost negligible, however, even for high CNT loading (i.e. 0.3 wt%), in the case of low degree of dispersion. Consequently, these results imply that the mechanical property of CNT-reinforced epoxy depends on the degree of dispersion as well as the concentration of CNTs. Agglomerations of CNTs were observed in SEM images. As shown in Figure 4(a), there were many CNT agglomerations in the epoxy nanocomposites with 0.2 wt% CNT concentration which had been submitted to 12 h ultrasonication. On the contrary, fewer agglomerations were observed in the 24 h ultrasonicated sample and their sizes were smaller than those in the 12 h ultrasonicated sample as shown in Figure 4(b). Model development. The mechanical properties of nanocomposites have been estimated by micromechanics models for discontinuous fiber reinforced composites, Table 2. Heat of reaction, well-dispersed CNT concentration, and degree of dispersion for various concentrations (definitions of D 1 and D 2 are given in equations (1) and (2)). Total CNT concentration (wt%) Ultrasonication time (h) Total heat of reaction (J/g) Well-dispersed CNT concentration (wt%) D 1 D

6 76 Journal of Reinforced Plastics and Composites 35(1) Figure 3. Experimental results of elastic modulus of CNT/epoxy composites for different degrees of dispersion (D 1 and D 2 ) and for three different CNT concentrations: (a) D 1 and (b) D 2. such as Halpin Tsai equation and Cox s shear lag model, in the literature. 21,22 According to Halpin Tsai equation, the tensile modulus of aligned short fiber-reinforced composites can be obtained by a following relation E C E M ¼ 1 þ V F 1 V F ð3þ ð ¼ E F=E M Þ 1 ðe F =E M Þþ where E C, E M and E F are the elastic moduli of composites, of matrix and of fiber, respectively and V F is the volume fraction of reinforcement. In equation (3), the geometric parameter x is introduced to take into account the reinforcement packing arrangement and ð4þ

7 Kim et al. 77 CNTs were oriented in planar directions in an extensional flow condition. We adopted the parameter ¼ 1/ 3 for the planar random orientation in equation (6). As a matter of fact, the principal advantage of nanocomposites is a large specific surface area between the nano-sized reinforcement and the matrix by virtue of the great aspect ratio of CNT which can enhance the load transferring capacity. If CNTs are aggregated, however, the surface area between the reinforcement and the matrix is decreased and the load transfer efficiency is reduced. Consequently, the elastic modulus of poorly dispersed CNT composites would be overestimated by the conventional Halpin Tsai equations (i.e. equations (3) to (6)). The influence of CNT dispersion on the specific surface area and the mechanical property can be taken into account by adjusting the aspect ratio of reinforcement in Halpin Tsai equation. If the CNTs are aggregated, the effective aspect ratio of the reinforcement which is in contact with the matrix is smaller than the actual aspect ratio of the individual CNT (see Figure 5). Hence, a knock-down factor is introduced to evaluate the effective geometric parameter eff by taking into account the degree of dispersion. eff ¼ fd ð 1 Þ ¼ 2 a fd ð 1 Þ ð7þ b Figure 4. SEM images of CNT/epoxy composites with 0.2 wt% CNT concentration: (a) Duration of ultrasonication: 12 h and (b) duration of ultrasonication: 24 h. geometry. In general, is defined as a function of the aspect ratio of reinforcement. ¼ 2 a ð5þ b where a denotes the largest dimension of the reinforcement (e.g. the length of fiber) and b represents the smallest dimension (e.g. the diameter of fiber). Halpin Tsai equation is used for unidirectional composites. Cox introduced a parameter to take into account the orientation state of discontinuous fibers. 22 By introducing the orientation factor, the parameter in Halpin Tsai equation can be modified as a following relation. ð ¼ E F=E M Þ 1 ðe F =E M Þþ In this work, the specimens of CNT composites were fabricated by pouring a CNT/epoxy mixture into an open mold. Therefore, it was assumed that most of ð6þ where eff is the effective geometric parameter considering a knock-down factor f (D 1 ) which is defined as a function of degree of dispersion D 1. This knock-down factor should be smaller than the unity. Replacing the geometric parameter by the effective geometric parameter eff in equations (3) and (6), the effective geometric parameter eff can be deduced as a following relation. ððe F =E M Þ ðe F =E M ÞV F þ V F Þ ðe C =E M Þ ðe F =E M Þ eff ¼ ððe F =E M Þ 1ÞV F ððe C =E M Þ 1Þ The volume fraction of CNTs can be obtained from the densities of CNT and of epoxy, and the mass fraction of CNTs in the nanocomposites. M M F V F ¼ F F M F þ M M F where F and M are the densities of the CNT and the epoxy matrix, and M F is the mass fraction of the CNTs. From the experimental data of tensile test, the effective geometric parameter eff was inversely estimated using equation (8). In this estimation, we excluded the data showing lower elastic modulus of nanocomposites ð8þ ð9þ

8 78 Journal of Reinforced Plastics and Composites 35(1) Figure 5. Schematics for dispersion states of CNTs and their effective aspect ratio. Figure 6. Plot of effective geometric parameter against degree of CNT dispersion. than that of neat epoxy. The calculation results are shown in Figure 6 where the effective geometric parameter eff in logarithmic scale is plotted against the degree of dispersion D 1 in linear scale. We can see that the effective geometric parameter can be represented by the degree of dispersion, irrespective of the CNT concentration. From this graph, we can assume that the knock-down factor is an exponential function of the degree of dispersion D 1. where m and n are model constants which can be obtained by curve-fitting of the results in Figure 6. Subsequently, the effective geometric parameter eff is represented in terms of degree of dispersion D 1 in the following equation. eff ¼ 2 a e 6:34D1 6:72, 0:19 D 1 1, b a b ¼ 1000 ð11þ eff ¼ 2 a b fd ð 1 Þ ¼ 2 a e md 1þn b ð10þ From this relation, it is noticeable that the effective geometric parameter eff is 1496 if all the CNTs are well

9 Kim et al. 79 Figure 7. Comparison between experimental data and model prediction of the elastic modulus of CNT/epoxy composites. dispersed or the degree of dispersion D 1 is the unity. If the geometric parameter is computed by equation (5), the aspect ratio of CNT (i.e. a/b in equation (5)), should be 748 which is smaller than the actual aspect ratio (i.e in this case). In fact, the basic assumption in Halpin Tsai equation is that all the fibers in composites are perfectly straight. As can be seen in the SEM images (see Figure 4), CNTs in the nanocomposites are not necessarily straight but curved. Therefore, the effective aspect ratio of the CNTs in the nanocomposites is smaller than the actual geometric aspect ratio of the individual CNT. The model predictions obtained by equations (3), (6) and (11) are compared with the experimental data in Figure 7. We can observe that nanocomposites with lower CNT concentration can exhibit greater modulus than nanocomposites with higher CNT concentration depending on the state of dispersion. Poor dispersion state can result in lower mechanical properties as the interfacial interaction between CNTs and epoxy matrix is reduced. 23 Therefore, the state of dispersion may become more crucial to the strengthening effect of CNT particles for higher concentration. Conclusions We introduced the new parameters to represent the degree of CNT dispersion in thermoset matrix nanocomposites by means of total reaction heat measurement during the curing process. These new parameters can be obtained by DSC analysis to estimate the overall dispersion state of the CNT/resin mixture before final sample fabrication. We also proposed the micromechanics model to estimate the elastic modulus of CNT nanocomposites in terms of the degree of dispersion as well as the concentration of CNTs. In particular, the reduction of specific surface area between the CNTs and the matrix in the case of poor dispersion was considered by introducing the knock-down factor which was defined by the degree of dispersion. The method and model developed in this work can be useful to optimize the nanocomposites preparation process. During the ultrasonication process, for example, a sample of mixture of CNTs and thermoset resin can be submitted to the DSC analysis in order to evaluate the degree of dispersion. Using the value for the degree of dispersion obtained by the DSC analysis, the final mechanical property of nanocomposites can be predicted by the micromechanics model. If the mechanical property estimated by the model is not sufficiently high, the ultrasonication duration should be increased to improve the degree of dispersion or the CNT loading should be increased. Thus, the DSC analysis and micromechanics model can be practical tools for the material design and nanocomposites processing optimization. In this work, it was assumed that the CNTs were randomly oriented in planar directions. However, the CNTs can exhibit a complex orientation state in three-dimensional directions according to the flow pattern during nanocomposites fabrication process. Moreover, the CNTs can be damaged during the ultrasonication process. This issue can be especially problematic if the

10 80 Journal of Reinforced Plastics and Composites 35(1) CNT loading is increased and the ultrasonication duration becomes longer. These subjects will be extensively investigated to further improve the dispersion state assessment method and the micromechanics model of nanocomposites in a future work. Declaration of Conflicting Interests The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article. Funding The author(s) received no financial support for the research, authorship, and/or publication of this article. References 1. Ajayan PM, Stephan O, Colliex C, et al. Aligned carbon nanotube arrays formed by cutting a polymer resinnanotube composite. Science 1994; 265: Allaoui A, Bai S, Cheng HM, et al. Mechanical and electrical properties of a MWNT/epoxy composite. Compos Sci Technol 2002; 62: Gojny FH, Wichmann MHG, Ko pke U, et al. Carbon nanotube-reinforced epoxy-composites: enhanced stiffness and fracture toughness at low nanotube content. Compos Sci Technol 2004; 64: Zhu J, Kim J, Peng H, et al. Improving the dispersion and integration of single-walled carbon nanotubes in epoxy composites through functionalization. Nano Lett 2003; 3: Visco A, Calabrese L and Milone C. Cure rate and mechanical properties of a DGEBF epoxy resin modified with carbon nanotubes. J Reinf Plast Compos 2009; 28: Gkikas G, Barkoula NM and Paipetis AS. Effect of dispersion conditions on the thermo-mechanical and toughness properties of multi-walled carbon nanotubes-reinforced epoxy. Compos Part B: Eng 2012; 43: Coleman JN, Khan U, Blau WJ, et al. Small but strong: A review of the mechanical properties of carbon nanotubepolymer composites. Carbon 2006; 44: Manchado MAL, Valentini L, Biagiotti J, et al. Thermal and mechanical properties of single-walled carbon nano tubes-polypropylene composites prepared by melt processing. Carbon 2005; 43: Bhattacharyya S, Sinturel C, Salvetat JP, et al. Proteinfunctionalized carbon nanotube-polymer composites. Appl Phys Lett 2005; 86: Lau KT, Lu M, Lam CK, et al. Thermal and mechanical properties of single-walled carbon nanotube bundlereinforced epoxy nanocomposites: The role of solvent for nanotube dispersion. Compos Sci Technol 2005; 65: Gupta ML, Sydlik SA, Schnorr JM, et al. The effect of mixing methods on the dispersion of carbon nanotubes during the solvent-free processing of multiwalled carbon nanotube/epoxy composites. J Polym Sci Part B: Polym Phys 2013; 51: Tang LC, Wan YJ, Peng K, et al. Fracture toughness and electrical conductivity of epoxy composites filled with carbon nanotubes and spherical particles. Compos Part A: Appl Sci Manuf 2013; 45: Tao K, Yang S, Grunlan JC, et al. Effects of carbon nanotube fillers on the curing processes of epoxy resinbased composites. J Appl Polym Sci 2006; 102: Treacy MMJ, Ebbesen TW and Gibson JM. Exceptionally high Young s modulus observed for individual carbon nanotubes. Nature 1996; 381: Poncharal P, Wang ZL, Ugarte D, et al. Electrostatic deflections and electromechanical resonances of carbon nanotubes. Science 1999; 283: Yu MF, Lourie O, Dyer MJ, et al. Strength and breaking mechanism of multiwalled carbon nanotubes under tensile load. Science 2000; 287: Xie S, Li W, Pan Z, et al. Mechanical and physical properties on carbon nanotube. J Phys Chem Solids 2000; 61: Martin CA, Sandler JKW, Shaffer MSP, et al. Formation of percolating networks in multi-wall carbon-nanotubeepoxy composites. Compos Sci Technol 2004; 64: Kim SH, Lee WI and Park JM. Assessment of dispersion in carbon nanotube reinforced composites using differential scanning calorimetry. Carbon 2009; 47: Lu KL, Lago RM, Chen YK, et al. Mechanical damage of carbon nanotubes by ultrasound. Carbon 1996; 34: Halpin JC and Tsai SW. Effects of environmental factors on composite materials. AFML-TR Yeha MK, Taib NH and Liua JH. Mechanical behavior of phenolic-based composites reinforced with multiwalled carbon nanotubes. Carbon 2006; 44: Wang B, Qi N, Gong W, et al. Study on the microstructure and mechanical properties for epoxy resin/montmorillonite nanocomposites by positron. Radiat Phys Chem 2007; 76: