Surface Sizing Treated MWCNTs and Its Effect on the Wettability, Interfacial Interaction and Flexural Properties of MWCNT/Epoxy Nanocomposites

A surface-sizing technique was offered to take full advantage of multi-walled carbon nanotubes (MWCNTs) and epoxy resins. Two surface-sizing treated MWCNTs were obtained through a ball-milling treatment of amino-functionalized MWCNTs (MWCNT-NH2) with n-butyl glycidylether (BuGE) and benzyl glycidylether (BeGE). These were referred to as MWCNT-BuGE and MWCNT-BeGE. The results indicated that the surface sizing effectively enhanced wettability, dispersibility of MWCNTs in the epoxy resin. These ameliorating effects, along with improved interfacial interaction between MWCNT-BeGE containing benzene rings and the epoxy matrix, which can offer a more efficient local load-transfer from matrix to MWCNTs, as observed by a higher G-band shift in Raman spectrum under bending loads than that of MWCNT-BuGE reinforced ones. Correspondingly, MWCNT-BeGE/epoxy nanocomposites exhibited increasing flexural strength and modulus of 22.9% and 37.8% respectively compared with the neat epoxy, and 7.3% and 7.7% respectively compared with MWCNT-BuGE/epoxy nanocomposites with the same MWCNT content.


Introduction
Epoxy resin has been widely used as the polymer matrix of high performance composites in many engineering fields due to its three-dimensional cross-linked thermoset structures with outstanding mechanical and bonding properties [1][2][3]. Since the performance requirements of advanced composite materials have been continuously increased in some special industrial applications, such as aviation and aerospace industry, the mechanical properties of epoxy resin also need to be further improved. In addition to changing the chemical structure of the resin matrix, nano-enhancement is also an important technical mean to improve mechanical performances through the incorporation of nano-scale additives [4,5].
Because amino groups can provide strong interactions with epoxy groups via nucleophilic ring-opening, by far, amino functionalization has been known as the most effective way of tackling Two kinds of surface sizing treated MWCNTs were obtained by ball-milling (200 r/min, 4 h) MWCNT-NH2 and epoxy diluents BuGE and BeGE with a mass ratio of 1:2, and then let stand for 72 h at room temperature. After processing, the solid-liquid weight ratio of these surface sizing treated MWCNTs decreased to 1:1.5, which can be used as nano-fillers for the selected epoxy resin system. Besides, some surface sizing treated MWCNTs were rinsed for five times by using acetone to remove the adsorbed BuGE and BeGE, respectively, and then dried in an oven for later surface characterization. Because each amino group on the surface of MWCNT-NH2 had three reactive hydrogen atoms, ideally, there were three grafted molecule of diluents. The reaction mechanism of surface sizing treated MWCNTs is illustrated in Figure 2. MWCNT-NH2, MWCNT-BuGE and MWCNT-BeGE (2 mg) were dispersed in ethanol (10 mL) with ultrasonication for 30 min, respectively, and then MWCNT films were obtained by vacuum filtration of MWCNT dispersions via the cellulose filter with a pore size of 0.2 μm. The resulting MWCNT films were used in later wettability examinations. The 0.5 wt.% MWCNT-NH2, 1.25 wt.% MWCNT-BuGE and 1.25 wt.% MWCNT-BeGE (the contents of solid MWCNTs was 0.5 wt.%) were dispersed in DGEBA epoxy resin by mechanical stirring at room temperature for 10 h, respectively. After this, curing agents (mass ratio, DGEBA:DDM = 100: 25) were added to the MWCNT/epoxy system, and the homogeneous mixture was degassed for 30 min at 80 °C under vacuum before curing. Then the resin systems were poured into a steel mold, and cured at 90 °C for 1 h, 130 °C for 2 h, and 180 °C for 2 h in an oven. The MD simulation of interfacial interaction between the surface sizing treated MWCNTs and the resin matrix was performed using the Forcite module of Materials Studio (6.0, Accelrys Inc., San Diego, CA, USA). The condensed phase optimized molecular potential for atomistic simulation studies (COMPASS) force field was employed to analyze the intramolecular and intermolecular interactions with a constant number of molecules, constant volume, and constant temperature (NVT) ensemble at 300 K with no applied pressure [9]. A time step of 1 fs, an equilibration time of 0.5 ns, and a production run time of 30 ps were used in this simulation. The covalent interaction energy (Einter) between resin matrix and surface-sizing MWCNTs was calculated according to the Equation (1) [9]: where Etotal was the total potential energy between the surface-sizing MWCNTs and resin matrix pair via covalent interaction, and ECNTs was the potential energy of surface-sizing MWCNTs when the resin matrix was put at infinite distance, Eresin matrix can be defined as the potential energy of resin matrix when the surface-sizing MWCNTs were put at infinite distance. Two kinds of surface sizing treated MWCNTs were obtained by ball-milling (200 r/min, 4 h) MWCNT-NH 2 and epoxy diluents BuGE and BeGE with a mass ratio of 1:2, and then let stand for 72 h at room temperature. After processing, the solid-liquid weight ratio of these surface sizing treated MWCNTs decreased to 1:1.5, which can be used as nano-fillers for the selected epoxy resin system. Besides, some surface sizing treated MWCNTs were rinsed for five times by using acetone to remove the adsorbed BuGE and BeGE, respectively, and then dried in an oven for later surface characterization. Because each amino group on the surface of MWCNT-NH 2 had three reactive hydrogen atoms, ideally, there were three grafted molecule of diluents. The reaction mechanism of surface sizing treated MWCNTs is illustrated in Figure 2. MWCNT-NH 2 , MWCNT-BuGE and MWCNT-BeGE (2 mg) were dispersed in ethanol (10 mL) with ultrasonication for 30 min, respectively, and then MWCNT films were obtained by vacuum filtration of MWCNT dispersions via the cellulose filter with a pore size of 0.2 µm. The resulting MWCNT films were used in later wettability examinations. The 0.5 wt.% MWCNT-NH 2 , 1.25 wt.% MWCNT-BuGE and 1.25 wt.% MWCNT-BeGE (the contents of solid MWCNTs was 0.5 wt.%) were dispersed in DGEBA epoxy resin by mechanical stirring at room temperature for 10 h, respectively. After this, curing agents (mass ratio, DGEBA:DDM = 100:25) were added to the MWCNT/epoxy system, and the homogeneous mixture was degassed for 30 min at 80 • C under vacuum before curing. Then the resin systems were poured into a steel mold, and cured at 90 • C for 1 h, 130 • C for 2 h, and 180 • C for 2 h in an oven.  Two kinds of surface sizing treated MWCNTs were obtained by ball-milling (200 r/min, 4 h) MWCNT-NH2 and epoxy diluents BuGE and BeGE with a mass ratio of 1:2, and then let stand for 72 h at room temperature. After processing, the solid-liquid weight ratio of these surface sizing treated MWCNTs decreased to 1:1.5, which can be used as nano-fillers for the selected epoxy resin system. Besides, some surface sizing treated MWCNTs were rinsed for five times by using acetone to remove the adsorbed BuGE and BeGE, respectively, and then dried in an oven for later surface characterization. Because each amino group on the surface of MWCNT-NH2 had three reactive hydrogen atoms, ideally, there were three grafted molecule of diluents. The reaction mechanism of surface sizing treated MWCNTs is illustrated in Figure 2. MWCNT-NH2, MWCNT-BuGE and MWCNT-BeGE (2 mg) were dispersed in ethanol (10 mL) with ultrasonication for 30 min, respectively, and then MWCNT films were obtained by vacuum filtration of MWCNT dispersions via the cellulose filter with a pore size of 0.2 μm. The resulting MWCNT films were used in later wettability examinations. The 0.5 wt.% MWCNT-NH2, 1.25 wt.% MWCNT-BuGE and 1.25 wt.% MWCNT-BeGE (the contents of solid MWCNTs was 0.5 wt.%) were dispersed in DGEBA epoxy resin by mechanical stirring at room temperature for 10 h, respectively. After this, curing agents (mass ratio, DGEBA:DDM = 100:25) were added to the MWCNT/epoxy system, and the homogeneous mixture was degassed for 30 min at 80 °C under vacuum before curing. Then the resin systems were poured into a steel mold, and cured at 90 °C for 1 h, 130 °C for 2 h, and 180 °C for 2 h in an oven. The MD simulation of interfacial interaction between the surface sizing treated MWCNTs and the resin matrix was performed using the Forcite module of Materials Studio (6.0, Accelrys Inc., San Diego, CA, USA). The condensed phase optimized molecular potential for atomistic simulation studies (COMPASS) force field was employed to analyze the intramolecular and intermolecular interactions with a constant number of molecules, constant volume, and constant temperature (NVT) ensemble at 300 K with no applied pressure [9]. A time step of 1 fs, an equilibration time of 0.5 ns, and a production run time of 30 ps were used in this simulation. The covalent interaction energy (Einter) between resin matrix and surface-sizing MWCNTs was calculated according to the Equation (1) [9]: where Etotal was the total potential energy between the surface-sizing MWCNTs and resin matrix pair via covalent interaction, and ECNTs was the potential energy of surface-sizing MWCNTs when the resin matrix was put at infinite distance, Eresin matrix can be defined as the potential energy of resin matrix when the surface-sizing MWCNTs were put at infinite distance. The MD simulation of interfacial interaction between the surface sizing treated MWCNTs and the resin matrix was performed using the Forcite module of Materials Studio (6.0, Accelrys Inc., San Diego, CA, USA). The condensed phase optimized molecular potential for atomistic simulation studies (COMPASS) force field was employed to analyze the intramolecular and intermolecular interactions with a constant number of molecules, constant volume, and constant temperature (NVT) ensemble at 300 K with no applied pressure [9]. A time step of 1 fs, an equilibration time of 0.5 ns, and a production run time of 30 ps were used in this simulation. The covalent interaction energy (E inter ) between resin matrix and surface-sizing MWCNTs was calculated according to the Equation (1) [9]: where E total was the total potential energy between the surface-sizing MWCNTs and resin matrix pair via covalent interaction, and E CNTs was the potential energy of surface-sizing MWCNTs when the resin matrix was put at infinite distance, E resin matrix can be defined as the potential energy of resin matrix when the surface-sizing MWCNTs were put at infinite distance. The MWCNT-NH 2 , MWCNT-BuGE and MWCNT-BeGE materials were analyzed and determined through high resolution transmission electron microscopy (HRTEM, Tecnai G2 F30 S, FEI Co., Hillsboro, OR, USA), transmission electron microscopy (TEM, JEM 3010, JEOL, Tokyo, Japan), Fourier-transform infrared spectroscopy (FT-IR, Nicolet 670, Nicolai Instruments, Madison, WI, USA), X-ray photoelectron spectroscopy (XPS, PHI-5300, Ulvac-Phi Inc., Chigasaki, Japan), thermogravimetric analysis (TGA, TA Q50, TA Instruments, New Castle, DE, USA) and Raman spectroscopy (TY-HR 800, HORIBA, Paris, France). In the thermogravimetric analysis, the heating rate was 10 • C/min from room temperature to 800 • C under N 2 (FR = 40 mL/min). The Raman spectroscopy was obtained by using a near-infrared laser line (wavelength of 785 nm). Micro-mechanical behaviors of MWCNTs in resin matrix under bending loads were characterized by measuring a total of 676 points per image over a scan width and height of 5 µm × 5 µm. The contact angles of DGEBA on the MWCNT films were measured to investigate the changes in wettability on a goniometer (OCA20, DataPhysics, Stuttgart, Germany) at room temperature. The image analyzing software (Image J, 1.48, National Institutes of Health, Bethesda, MD, USA) was used to calculate the contact angle. Three key points were found first: left-end point, right-end point and vertex of the droplet. Then, the height (h) and width (2r) of droplet and the solid-liquid contact angle (θ) of the droplet can be obtained from the defined coordinates of these points. The dispersion performance of the MWCNTs in the resin matrix was observed using fractured, cured composite samples on a scanning electron microscope (SEM, JSM 7401F, JEOL, Tokyo, Japan) under 20 kV accelerating voltage. According to the standard test method ASTM D790, flexural tests of epoxy composites were performed on a material testing machine (WDS-10, Beijing Guance Jingdian Instrument Inc., Beijing, China) at the speed of 2 mm/min. Each data point was obtained from at least ten samples. Compared with MWCNT-NH 2 , the structural integrity of MWCNT-BuGE and MWCNT-BeGE maintained well, indicating negligible damages to graphite sheet layer structure during surface sizing treatment (Figure 3a-c, inset). At the same time, the surfaces of both MWCNT-NH 2 and surface sizing treated MWCNTs were covered by amorphous materials, which can be assigned to the chemical groups [28]. However, the difference between covering layers of these MWCNT samples did not distinguish well, due to the low molecular weight of chemical compounds. The MWCNT-NH2, MWCNT-BuGE and MWCNT-BeGE materials were analyzed and determined through high resolution transmission electron microscopy (HRTEM, Tecnai G2 F30 S, FEI Co., Hillsboro, OR, USA), transmission electron microscopy (TEM, JEM 3010, JEOL, Tokyo, Japan), Fourier-transform infrared spectroscopy (FT-IR, Nicolet 670, Nicolai Instruments, Madison, WI, USA), X-ray photoelectron spectroscopy (XPS, PHI-5300, Ulvac-Phi Inc., Chigasaki, Japan), thermogravimetric analysis (TGA, TA Q50, TA Instruments, New Castle, DE, USA) and Raman spectroscopy (TY-HR 800, HORIBA, Paris, France). In the thermogravimetric analysis, the heating rate was 10 °C/min from room temperature to 800 °C under N2 (FR = 40 mL/min). The Raman spectroscopy was obtained by using a near-infrared laser line (wavelength of 785 nm). Micro-mechanical behaviors of MWCNTs in resin matrix under bending loads were characterized by measuring a total of 676 points per image over a scan width and height of 5 μm × 5 μm. The contact angles of DGEBA on the MWCNT films were measured to investigate the changes in wettability on a goniometer (OCA20, DataPhysics, Stuttgart, Germany) at room temperature. The image analyzing software (Image J, 1.48, National Institutes of Health, Bethesda, MD, USA) was used to calculate the contact angle. Three key points were found first: left-end point, right-end point and vertex of the droplet. Then, the height (h) and width (2r) of droplet and the solid-liquid contact angle (θ) of the droplet can be obtained from the defined coordinates of these points. The dispersion performance of the MWCNTs in the resin matrix was observed using fractured, cured composite samples on a scanning electron microscope (SEM, JSM 7401F, JEOL, Tokyo, Japan) under 20 kV accelerating voltage. According to the standard test method ASTM D790, flexural tests of epoxy composites were performed on a material testing machine (WDS-10, Beijing Guance Jingdian Instrument Inc., Beijing, China) at the speed of 2 mm/min. Each data point was obtained from at least ten samples. . At the same time, the surfaces of both MWCNT-NH2 and surface sizing treated MWCNTs were covered by amorphous materials, which can be assigned to the chemical groups [28]. However, the difference between covering layers of these MWCNT samples did not distinguish well, due to the low molecular weight of chemical compounds.   Observing the dispersion stability of CNTs in an organic solvent has been considered a semi-qualitative method to judge success or failure of the surface modification of CNTs [29]. 1 mg/mL of MWCNT-NH2, MWCNT-BuGE and MWCNT-BeGE in absolute ethanol were ultrasonicated for 10 min, respectively, and their optical images were taken after standing for 1 h ( Figure 3d) and 24 h (Figure 3e). MWCNT-NH2 was precipitated gradually, while MWCNT-BuGE and MWCNT-BeGE still retained dispersion stability after standing for 24 h, because the sizing processing changed the surface polarity of MWCNTs, preventing MWCNTs from re-aggregation. Observing the dispersion stability of CNTs in an organic solvent has been considered a semi-qualitative method to judge success or failure of the surface modification of CNTs [29]. 1 mg/mL of MWCNT-NH 2 , MWCNT-BuGE and MWCNT-BeGE in absolute ethanol were ultrasonicated for 10 min, respectively, and their optical images were taken after standing for 1 h (Figure 3d) and 24 h (Figure 3e). MWCNT-NH 2 was precipitated gradually, while MWCNT-BuGE and MWCNT-BeGE still retained dispersion stability after standing for 24 h, because the sizing processing changed the surface polarity of MWCNTs, preventing MWCNTs from re-aggregation.   Observing the dispersion stability of CNTs in an organic solvent has been considered a semi-qualitative method to judge success or failure of the surface modification of CNTs [29]. 1 mg/mL of MWCNT-NH2, MWCNT-BuGE and MWCNT-BeGE in absolute ethanol were ultrasonicated for 10 min, respectively, and their optical images were taken after standing for 1 h (Figure 3d) and 24 h (Figure 3e). MWCNT-NH2 was precipitated gradually, while MWCNT-BuGE and MWCNT-BeGE still retained dispersion stability after standing for 24 h, because the sizing processing changed the surface polarity of MWCNTs, preventing MWCNTs from re-aggregation.    Figure 5 shows the FT-IR spectra of MWCNT-NH2, MWCNT-BuGE and MWCNT-BeGE. The 3428 cm −1 peak was originated from the O-H stretching vibration of water molecules which were present in the KBr as well as in carbon nanotubes [30]. The peaks of MWCNT-NH2 at 1630 cm −1 and 1092 cm −1 were ascribed to the stretching vibration of N-H, and C-N deriving from primary amino groups [25], respectively. Peaks at 2850-2926 cm −1 region were characteristics of symmetric and asymmetric stretching vibration of -CH2-groups, and their intensities were greatly enhanced in MWCNT-BuGE and MWCNT-BeGE materials. The introduction of BuGE and BeGE can be observed from the peak at 1109 cm −1 , which was assigned to C-O-C stretching frequencies. Moreover, strong absorption peaks at 739 cm −1 and 696 cm −1 resulting from out of plane bending of the benzene ring were also observed in the MWCNT-BeGE. To distinguish the adsorbed or chemically bonded surface molecules on the surface of MWCNTs, the identification of surface functionalities was performed through calculating the N1s peak area of MWCNT-NH2, MWCNT-BuGE and MWCNT-BeGE. Figure 6 shows XPS (N1s) curve fitting for three kinds of MWCNTs, and corresponding N1s peak area of each type of MWCNTs was shown in Table 1 Figure 5 shows the FT-IR spectra of MWCNT-NH 2 , MWCNT-BuGE and MWCNT-BeGE. The 3428 cm −1 peak was originated from the O-H stretching vibration of water molecules which were present in the KBr as well as in carbon nanotubes [30]. The peaks of MWCNT-NH 2 at 1630 cm −1 and 1092 cm −1 were ascribed to the stretching vibration of N-H, and C-N deriving from primary amino groups [25], respectively. Peaks at 2850-2926 cm −1 region were characteristics of symmetric and asymmetric stretching vibration of -CH 2 -groups, and their intensities were greatly enhanced in MWCNT-BuGE and MWCNT-BeGE materials. The introduction of BuGE and BeGE can be observed from the peak at 1109 cm −1 , which was assigned to C-O-C stretching frequencies. Moreover, strong absorption peaks at 739 cm −1 and 696 cm −1 resulting from out of plane bending of the benzene ring were also observed in the MWCNT-BeGE.   Figure 5 shows the FT-IR spectra of MWCNT-NH2, MWCNT-BuGE and MWCNT-BeGE. The 3428 cm −1 peak was originated from the O-H stretching vibration of water molecules which were present in the KBr as well as in carbon nanotubes [30]. The peaks of MWCNT-NH2 at 1630 cm −1 and 1092 cm −1 were ascribed to the stretching vibration of N-H, and C-N deriving from primary amino groups [25], respectively. Peaks at 2850-2926 cm −1 region were characteristics of symmetric and asymmetric stretching vibration of -CH2-groups, and their intensities were greatly enhanced in MWCNT-BuGE and MWCNT-BeGE materials. The introduction of BuGE and BeGE can be observed from the peak at 1109 cm −1 , which was assigned to C-O-C stretching frequencies. Moreover, strong absorption peaks at 739 cm −1 and 696 cm −1 resulting from out of plane bending of the benzene ring were also observed in the MWCNT-BeGE. To distinguish the adsorbed or chemically bonded surface molecules on the surface of MWCNTs, the identification of surface functionalities was performed through calculating the N1s peak area of MWCNT-NH2, MWCNT-BuGE and MWCNT-BeGE. Figure 6 shows XPS (N1s) curve fitting for three kinds of MWCNTs, and corresponding N1s peak area of each type of MWCNTs was shown in Table 1  To distinguish the adsorbed or chemically bonded surface molecules on the surface of MWCNTs, the identification of surface functionalities was performed through calculating the N1s peak area of MWCNT-NH 2 , MWCNT-BuGE and MWCNT-BeGE. Figure 6 shows XPS (N1s) curve fitting for three kinds of MWCNTs, and corresponding N1s peak area of each type of MWCNTs was shown in Table 1.

Surface Characterization of MWCNTs
N1s peak positions at 399.87 eV, 401.03 eV and 402.30 eV were attributed to the chemical bonds C-N-C, C-NH 2 and -NH-C=O [25]. In the samples of the MWCNT-BuGE and MWCNT-BeGE, the peak area of C-N-C bond increased, while the corresponding peak area of C-NH2 bond decreased compared with the results for the MWCNT-NH2 (Table 1). These results suggested that new C-N-C chemical bonds were formed, which directly validated the chemical reaction between the epoxy diluent and the amino group of the MWCNT-NH2. The results of XPS analysis demonstrated that BuGE and BeGE have been successfully bonded to the surface of the MWCNT-NH2 through covalent bonds.    In the samples of the MWCNT-BuGE and MWCNT-BeGE, the peak area of C-N-C bond increased, while the corresponding peak area of C-NH 2 bond decreased compared with the results for the MWCNT-NH 2 ( Table 1). These results suggested that new C-N-C chemical bonds were formed, which directly validated the chemical reaction between the epoxy diluent and the amino group of the MWCNT-NH 2 . The results of XPS analysis demonstrated that BuGE and BeGE have been successfully bonded to the surface of the MWCNT-NH 2 through covalent bonds. Figure 7a shows the TGA analysis for MWCNT-NH 2 , MWCNT-BuGE and MWCNT-BeGE. The weight loss of MWCNT-NH 2 was about 10% at 800 • C, due to the decomposition of the amino groups. The weight loss of the MWCNT-BuGE and MWCNT-BeGE was 32% and 28%, respectively. This increase in weight loss was attributed to the grafting of mono-functional epoxy molecules on the surface of MWCNT-NH 2 .  In addition, the weight loss of MWCNT-BuGE at 800 °C was higher than that of MWCNT-BeGE. This difference in weight loss between the MWCNT-BuGE and MWCNT-BeGE was attributed to the increased reactivity of the amino group (NH2-(CH2)2-NH-CO-) in the BuGE and BeGE molecules on the surface of MWCNTs. The ratio (n) of epoxy diluents to amino groups is roughly calculated using the Equation (2) [31]: where Xe and Xa is the weight loss of the MWCNT surface sizing and MWCNT-NH2; Me and Ma is the molecular mass of BuGE (or BeGE) and the amino groups. The n of MWCNT-BuGE and MWCNT-BeGE was 1.5 and 1.0, indicating that BuGE reacted easier with the amine groups on the surface of MWCNT-NH2 than that of BeGE. Moreover, the remaining activated amine groups in the surface sizing still had the ability to participate into the crosslinking network via ring-opening reaction with epoxy groups. Raman spectroscopy is one of the most important techniques for the analysis of the vibrational modes and the structure of carbon nanotubes [32,33]. Figure 7b shows Raman analysis results of the functionalized MWCNTs, where the peak positions at 1311 cm −1 , 1601 cm −1 and 2608 cm −1 were attributed to the disorder-induced D band (in-plane vibrations of sp 3 carbon), the G band (the crystalline graphitic and in-plane vibrations of sp 2 carbon), and the G' band (the second overtone of the D-band) [34]. The relative intensity ratio, ID/IG, was sensitive to physical and chemical processing of the CNTs, and it was an indication of the degree of amorphous carbon atoms to graphitic carbon atoms on the surface of the CNTs, which reflected the structural integrity of the graphite [33]. The ID/IG of MWCNT-BuGE (2.4) and MWCNT-BeGE (2.5) was a little higher than that of the MWCNT-NH2 (2.3), suggesting that the surface sizing treatment had little negative effect on the structural integrity of the graphite, consistent with the observation of HRTEM analysis (Figure 3a-c).

Interfacial Analysis of MWCNT/Epoxy Nanocomposite
For inorganic filler-reinforced polymeric composites, a good wettability is a primary prerequisite to rapidly construct interfaces between the fillers and the resin matrix [35,36]. Figure 8 shows contact angles changing with time of DGEBA on the MWCNT film at room temperature, which is used to evaluate the effect of surface sizing treatment on the MWCNT wettability with the resin matrix. As shown, the initial contact angle of DGEBA on the MWCNT-NH2 film was 129°. After the surface sizing treatment, the initial contact angles of DGEBA on the MWCNT-BuGE and MWCNT-BeGE films were down to around 110°, because the polarity of alkyl chain was smaller than that of the amino group. The liquid DGEBA spread over the MWCNT film due to the surface tension effect and the interfacial energy of the droplet, resulting in the decrease of contact angles In addition, the weight loss of MWCNT-BuGE at 800 • C was higher than that of MWCNT-BeGE. This difference in weight loss between the MWCNT-BuGE and MWCNT-BeGE was attributed to the increased reactivity of the amino group (NH 2 -(CH 2 ) 2 -NH-CO-) in the BuGE and BeGE molecules on the surface of MWCNTs. The ratio (n) of epoxy diluents to amino groups is roughly calculated using the Equation (2) [31]: where X e and X a is the weight loss of the MWCNT surface sizing and MWCNT-NH 2 ; M e and M a is the molecular mass of BuGE (or BeGE) and the amino groups. The n of MWCNT-BuGE and MWCNT-BeGE was 1.5 and 1.0, indicating that BuGE reacted easier with the amine groups on the surface of MWCNT-NH 2 than that of BeGE. Moreover, the remaining activated amine groups in the surface sizing still had the ability to participate into the crosslinking network via ring-opening reaction with epoxy groups. Raman spectroscopy is one of the most important techniques for the analysis of the vibrational modes and the structure of carbon nanotubes [32,33]. Figure 7b shows Raman analysis results of the functionalized MWCNTs, where the peak positions at 1311 cm −1 , 1601 cm −1 and 2608 cm −1 were attributed to the disorder-induced D band (in-plane vibrations of sp 3 carbon), the G band (the crystalline graphitic and in-plane vibrations of sp 2 carbon), and the G' band (the second overtone of the D-band) [34]. The relative intensity ratio, I D /I G , was sensitive to physical and chemical processing of the CNTs, and it was an indication of the degree of amorphous carbon atoms to graphitic carbon atoms on the surface of the CNTs, which reflected the structural integrity of the graphite [33]. The I D /I G of MWCNT-BuGE (2.4) and MWCNT-BeGE (2.5) was a little higher than that of the MWCNT-NH 2 (2.3), suggesting that the surface sizing treatment had little negative effect on the structural integrity of the graphite, consistent with the observation of HRTEM analysis (Figure 3a-c).

Interfacial Analysis of MWCNT/Epoxy Nanocomposite
For inorganic filler-reinforced polymeric composites, a good wettability is a primary prerequisite to rapidly construct interfaces between the fillers and the resin matrix [35,36]. Figure 8 shows contact angles changing with time of DGEBA on the MWCNT film at room temperature, which is used to evaluate the effect of surface sizing treatment on the MWCNT wettability with the resin matrix. As shown, the initial contact angle of DGEBA on the MWCNT-NH 2 film was 129 • . After the surface sizing treatment, the initial contact angles of DGEBA on the MWCNT-BuGE and MWCNT-BeGE films were down to around 110 • , because the polarity of alkyl chain was smaller than that of the amino group. The liquid DGEBA spread over the MWCNT film due to the surface tension effect and the interfacial energy of the droplet, resulting in the decrease of contact angles with time. Dynamic contact angles of DGEBA on the MWCNT-BuGE and MWCNT-BeGE film were similar, but always smaller than that on the MWCNT-NH 2 film. The results indicated that a better wettability with the epoxy resin can be obtained via surface-sizing treatment of MWCNT-NH 2 . An adequate interfacial bonding strength can guarantee efficient load-transfer between the MWCNTs and resin matrix, and promote the comprehensive performance of the final nanocomposites [22,37]. When a strain occurred in a material system under the action of external force, the inter-atomic distances were changed, resulting in some changes of vibration frequency of some normal modes, which would be shown as a slight shift of Raman peak. The Raman G band of a better interfacial interaction between the MWCNTs and the epoxy matrix would shift to a higher wave number when the MWCNTs embedded in the epoxy resin were subjected to the strain [37]. Therefore, Raman G band shift can be used to assess the interfacial interaction between the MWCNTs and epoxy matrix. Figure 9 shows the G band intensity distribution over the scanned area of cured DGEBA/DDM nanocomposites reinforced by MWCNT-NH2, MWCNT-BuGE and MWCNT-BeGE without or with 1% bending load at room temperature. There was no significant difference in the G band frequency (~1582.0) among the MWCNT-NH2, MWCNT-BuGE and MWCNT-BeGE before applying the external stress. However, when the 1% bending load was applied, the G band frequency of MWCNT-BuGE (1596.2) and MWCNT-BeGE (1598.4) shifted to a higher wave number than that of MWCNT-NH2 (1594.6), indicating the effective stress transfer from the resin matrix to the surface sizing treated MWCNTs at their interfaces. The G band frequency of MWCNT-BeGE/epoxy nanocomposites was higher than that of MWCNT-BuGE/epoxy nanocomposites owing to the stronger interfacial interaction among the MWCNT-BeGE and the epoxy matrix. An adequate interfacial bonding strength can guarantee efficient load-transfer between the MWCNTs and resin matrix, and promote the comprehensive performance of the final nanocomposites [22,37]. When a strain occurred in a material system under the action of external force, the inter-atomic distances were changed, resulting in some changes of vibration frequency of some normal modes, which would be shown as a slight shift of Raman peak. The Raman G band of a better interfacial interaction between the MWCNTs and the epoxy matrix would shift to a higher wave number when the MWCNTs embedded in the epoxy resin were subjected to the strain [37]. Therefore, Raman G band shift can be used to assess the interfacial interaction between the MWCNTs and epoxy matrix. Figure 9 shows the G band intensity distribution over the scanned area of cured DGEBA/DDM nanocomposites reinforced by MWCNT-NH 2 , MWCNT-BuGE and MWCNT-BeGE without or with 1% bending load at room temperature. There was no significant difference in the G band frequency (~1582.0) among the MWCNT-NH 2 , MWCNT-BuGE and MWCNT-BeGE before applying the external stress. However, when the 1% bending load was applied, the G band frequency of MWCNT-BuGE (1596.2) and MWCNT-BeGE (1598.4) shifted to a higher wave number than that of MWCNT-NH 2 (1594.6), indicating the effective stress transfer from the resin matrix to the surface sizing treated MWCNTs at their interfaces. The G band frequency of MWCNT-BeGE/epoxy nanocomposites was higher than that of MWCNT-BuGE/epoxy nanocomposites owing to the stronger interfacial interaction among the MWCNT-BeGE and the epoxy matrix. Figure 10 shows relaxed conformations of the modeled MWCNT-BuGE/DGEBA/DDM (Figure 10a) and MWCNT-BeGE/DGEBA/DDM (Figure 10b) at 300 K using the MD simulation, and the evolution of interaction energy between each model chain and surface sizing treated MWCNTs during the relaxation process (Figure 10c). MWCNT-BeGE/DGEBA/DDM exhibited a higher average interaction energy (−124.6 kcal/mol) than that of the MWCNT-BuGE/DGEBA/DDM (−159.3 kcal/mol), which was in accordance with the results of the Raman spectral analysis above. The improved interfacial interaction can be contributed to the formation π-π stacking between the aromatic rings in the MWCNT-BeGE backbone and the resin matrix [9,38]. www.mdpi.com/journal/nanomaterials external stress. However, when the 1% bending load was applied, the G band frequency of MWCNT-BuGE (1596.2) and MWCNT-BeGE (1598.4) shifted to a higher wave number than that of MWCNT-NH2 (1594.6), indicating the effective stress transfer from the resin matrix to the surface sizing treated MWCNTs at their interfaces. The G band frequency of MWCNT-BeGE/epoxy nanocomposites was higher than that of MWCNT-BuGE/epoxy nanocomposites owing to the stronger interfacial interaction among the MWCNT-BeGE and the epoxy matrix.   (Figure 10b) at 300 K using the MD simulation, and the evolution of interaction energy between each model chain and surface sizing treated MWCNTs during the relaxation process (Figure 10c). MWCNT-BeGE/DGEBA/DDM exhibited a higher average interaction energy (−124.6 kcal/mol) than that of the MWCNT-BuGE/DGEBA/DDM (−159.3 kcal/mol), which was in accordance with the results of the Raman spectral analysis above. The improved interfacial interaction can be contributed to the formation π-π stacking between the aromatic rings in the MWCNT-BeGE backbone and the resin matrix [9,38].  (Figure 10c). MWCNT-BeGE/DGEBA/DDM exhibited a higher average interaction energy (−124.6 kcal/mol) than that of the MWCNT-BuGE/DGEBA/DDM (−159.3 kcal/mol), which was in accordance with the results of the Raman spectral analysis above. The improved interfacial interaction can be contributed to the formation π-π stacking between the aromatic rings in the MWCNT-BeGE backbone and the resin matrix [9,38].

Fracture Morphology and Mechanical Analysis of MWCNT/Epoxy Nanocomposite
The fracture surfaces of neat resin and nanocomposites containing MWCNT-NH2, MWCNT-BuGE and MWCNT-BeGE were compared using SEM, as shown in Figure 11. Small MWCNT agglomerates were clearly seen from the sample containing MWCNT-NH2 (Figure 11b, inset), while these agglomerates were almost absent in the sample containing MWCNT-BuGE (Figure 11c, inset) and MWCNT-BeGE (Figure 11d, inset). This result indicated that the surface sizing treated MWCNTs further improved the dispersion effect of MWCNT-NH2 in the resin matrix. The surface sizing weakened the van der Waals' (vdW) force between the MWCNTs and enhanced the chemical compatibility with epoxy matrix, and thus led to the efficient debundling and satisfactory dispersion of MWCNTs in the nanocomposites.
The neat epoxy exhibited a smooth fracture surface, revealing the typical brittle fracture with poor resistance towards cracking or rupturing. By contrast, the fracture surfaces of the nanocomposites exhibited ridges, rivers and plastic deformation features. This was primarily because the presence of MWCNTs prevented cracks from propagating, increased the area of the fracture surface and provided a high resistance to fracture [39]. Compared with the MWCNT-NH2, surface sizing treated MWCNTs showed a more evident reinforcement role in the resulting epoxy nanocomposites. Especially, the MWCNT-BeGE reinforced nanocomposites showed rather intricate and tortuous crack propagation, owing to the improved MWCNT dispersion, and interfacial energy between the MWCNT-BeGE and resin matrix promoted the load transfer effectively.

Fracture Morphology and Mechanical Analysis of MWCNT/Epoxy Nanocomposite
The fracture surfaces of neat resin and nanocomposites containing MWCNT-NH 2 , MWCNT-BuGE and MWCNT-BeGE were compared using SEM, as shown in Figure 11. Small MWCNT agglomerates were clearly seen from the sample containing MWCNT-NH 2 (Figure 11b, inset), while these agglomerates were almost absent in the sample containing MWCNT-BuGE (Figure 11c, inset) and MWCNT-BeGE (Figure 11d, inset). This result indicated that the surface sizing treated MWCNTs further improved the dispersion effect of MWCNT-NH 2 in the resin matrix. The surface sizing weakened the van der Waals' (vdW) force between the MWCNTs and enhanced the chemical compatibility with epoxy matrix, and thus led to the efficient debundling and satisfactory dispersion of MWCNTs in the nanocomposites.
The neat epoxy exhibited a smooth fracture surface, revealing the typical brittle fracture with poor resistance towards cracking or rupturing. By contrast, the fracture surfaces of the nanocomposites exhibited ridges, rivers and plastic deformation features. This was primarily because the presence of MWCNTs prevented cracks from propagating, increased the area of the fracture surface and provided a high resistance to fracture [39]. Compared with the MWCNT-NH 2 , surface sizing treated MWCNTs showed a more evident reinforcement role in the resulting epoxy nanocomposites. Especially, the MWCNT-BeGE reinforced nanocomposites showed rather intricate and tortuous crack propagation, owing to the improved MWCNT dispersion, and interfacial energy between the MWCNT-BeGE and resin matrix promoted the load transfer effectively. because the presence of MWCNTs prevented cracks from propagating, increased the area of the fracture surface and provided a high resistance to fracture [39]. Compared with the MWCNT-NH2, surface sizing treated MWCNTs showed a more evident reinforcement role in the resulting epoxy nanocomposites. Especially, the MWCNT-BeGE reinforced nanocomposites showed rather intricate and tortuous crack propagation, owing to the improved MWCNT dispersion, and interfacial energy between the MWCNT-BeGE and resin matrix promoted the load transfer effectively. Flexural properties of MWCNT/epoxy resin composites were closely related to the MWCNT dispersion and interfacial interactions between the MWCNTs and resin matrix [25,[39][40][41]. Figure  12a shows typical flexural stress-displacement curves of DGEBA/DDM reinforced by MWCNT-NH2, MWCNT-BuGE and MWCNT-BeGE, and the average flexural strength and modulus were summarized in Figure 12b,c. Flexural strength and modulus of the neat resin were found to be 122 MPa and 2.6 GPa, while these properties of MWCNT-NH2 reinforced nanocomposites reached 135 MPa and 3.2 GPa, with an increase of 11.0% and 23.1%. By contrast, the surface sizing treated MWCNT/epoxy nanocomposites showed better flexural properties owing to the improved dispersion and wettability performance of epoxy-sizing MWCNTs compared with the MWCNT-NH2. The flexural strength and modulus of MWCNT-BeGE/epoxy nanocomposites can reach 150 MPa and 3.6 GPa, with an increase of 22.9% and 37.8% compared with the neat epoxy, and 7.3% and 7.7% compared with MWCNT-BuGE/epoxy nanocomposites with the same MWCNT content, indicating that the surface sizing containing benzene rings offered a more efficient local load-transfer between MWCNTs and the epoxy matrix. Flexural properties of MWCNT/epoxy resin composites were closely related to the MWCNT dispersion and interfacial interactions between the MWCNTs and resin matrix [25,[39][40][41]. Figure 12a shows typical flexural stress-displacement curves of DGEBA/DDM reinforced by MWCNT-NH 2 , MWCNT-BuGE and MWCNT-BeGE, and the average flexural strength and modulus were summarized in Figure 12b,c. Flexural strength and modulus of the neat resin were found to be 122 MPa and 2.6 GPa, while these properties of MWCNT-NH 2 reinforced nanocomposites reached 135 MPa and 3.2 GPa, with an increase of 11.0% and 23.1%. By contrast, the surface sizing treated MWCNT/epoxy nanocomposites showed better flexural properties owing to the improved dispersion and wettability performance of epoxy-sizing MWCNTs compared with the MWCNT-NH 2 . The flexural strength and modulus of MWCNT-BeGE/epoxy nanocomposites can reach 150 MPa and 3.6 GPa, with an increase of 22.9% and 37.8% compared with the neat epoxy, and 7.3% and 7.7% compared with MWCNT-BuGE/epoxy nanocomposites with the same MWCNT content, indicating that the surface sizing containing benzene rings offered a more efficient local load-transfer between MWCNTs and the epoxy matrix.
The comparison of flexural properties and their increases between this study and other researches for MWCNT/DGEBA systems is summarized in Table 2. Compared with control samples, increasing flexural strength from 12.1% to 26.7% and modulus from 17.6% to 21.7% of MWCNT/epoxy nanocomposites was observed in previous articles, which were inferior to synthetically reinforcing effect of surface sizing treated MWCNTs. dispersion and wettability performance of epoxy-sizing MWCNTs compared with the MWCNT-NH2. The flexural strength and modulus of MWCNT-BeGE/epoxy nanocomposites can reach 150 MPa and 3.6 GPa, with an increase of 22.9% and 37.8% compared with the neat epoxy, and 7.3% and 7.7% compared with MWCNT-BuGE/epoxy nanocomposites with the same MWCNT content, indicating that the surface sizing containing benzene rings offered a more efficient local load-transfer between MWCNTs and the epoxy matrix. The comparison of flexural properties and their increases between this study and other researches for MWCNT/DGEBA systems is summarized in Table 2. Compared with control samples, increasing flexural strength from 12.1% to 26.7% and modulus from 17.6% to 21.7% of MWCNT/epoxy nanocomposites was observed in previous articles, which were inferior to synthetically reinforcing effect of surface sizing treated MWCNTs.
From the analysis above, the surface sizing treatment can effectively improve the wettability and dispersion of MWCNTs in the epoxy matrix. Moreover, a higher interfacial energy between the MWCNTs and epoxy matrix can improve the local load-transfer from weak resin matrix to the strong MWCNTs through the structural design of surface sizing. The surface sizing treatment is an effective process method to take full advantage of MWCNTs and epoxy resins, and has shown a good application prospect.

Conclusions
In this study, through a ball-milling treatment with BuGE and BeGE, the MWCNT-NH2 material was converted into two surface sizing treated MWCNTs: MWCNT-BuGE and  From the analysis above, the surface sizing treatment can effectively improve the wettability and dispersion of MWCNTs in the epoxy matrix. Moreover, a higher interfacial energy between the MWCNTs and epoxy matrix can improve the local load-transfer from weak resin matrix to the strong MWCNTs through the structural design of surface sizing. The surface sizing treatment is an effective process method to take full advantage of MWCNTs and epoxy resins, and has shown a good application prospect.

Conclusions
In this study, through a ball-milling treatment with BuGE and BeGE, the MWCNT-NH 2 material was converted into two surface sizing treated MWCNTs: MWCNT-BuGE and MWCNT-BeGE. The surface sizing treatment effectively enhanced wettability, dispersibility and mechanical reinforcement of MWCNTs in the epoxy resin compared with MWCNT-NH 2 . The results of MD simulation and Raman spectroscopy indicated that the surface sizing containing benzene rings (MWCNT-BeGE) improved the interaction energy, and offered a more efficient local load-transfer between MWCNTs and the epoxy matrix, as observed by a higher G-band shift in Raman spectrum under bending loads than that of MWCNT-BuGE reinforced ones. As a consequence, the flexural strength and modulus of MWCNT-BeGE/epoxy nanocomposites reached 150 MPa and 3.6 GPa, with an increase of 22.9% and 37.8% compared with the neat epoxy, and 7.3% and 7.7% compared with MWCNT-BuGE/epoxy nanocomposites with the same MWCNT content.