The effects of triethylenetetramine grafting of multi-walled carbon nanotubes on its dispersion, filler-matrix interfacial interaction and the thermal properties of epoxy nanocomposites.
Since the discovery of carbon nanotubes (CNTs) (1), nanocomposites filled with CNTs have been actively studied to achieve superior mechanical, electrical, and thermal properties. The development of polymer composites with nanoscaled modifiers has become an attractive subject in materials science (2), (3). CNTs are considered to be a type of very attractive filler for high-strength structural and high-performance functional polymer composites because of their tremendous mechanical strength, nanometer-scale diameter, high aspect ratio, and extremely high electrical and thermal conductivities (4-7). However, to fully realize these exceptional properties of CNT/polymer composites, we have to resolve two fundamental and important issues. One is how to facilitate the homogeneous dispersion of CNTs in polymeric matrix; the other is how to improve the polymer-CNT interfacial interaction.
Owing to large surface areas and van der waals forces, CNTs are rather difficult to be effectively dispersed in a polymeric matrix (8). In addition, CNTs are long slender fullerenes where the walls of the tubes are hexagonal carbon (graphite structure) and often capped at each end. Basically, due to the seamless arrangement of hexagon rings without any dangling bonds, CNT walls are rather unreactive (9). This is the cause why CNTs arc very difficult to be introduced functional groups onto their surface. So it is quite significant to investigate how to combine uniform distribution of CNTs in the polymer matrix and improved interfacial interaction between CNTs and the polymer with the demand for high performance of CNT/polymer composites. While this work has been still deficient.
Efforts to obtain good dispersion of CNTs and to enhance their compatibility with the polymer matrix have been previously undertaken. The introduction of defects by oxidation routes using strong acids may be an alternative to fabricate composites where the tube surface is strongly bonded to the polymer matrix (10), (11). It has been proved that treating CNTs with concentrated nitric acid generates acidic sites on CNTs, such as carboxylic, carbonyl, and hydroxyl groups (12-15). These reactive groups on CNTs greatly improve the combination of CNTs with polymer matrix and thus enhance the mechanical strength of the nanocomposites (16). Sidewall funclionalization of CNTs with organic chains or functional groups is another effective way to improve the dispersion and reinforce the combination of CNTs with the polymer matrix (17-19). The construction of a polymer-CNT covalent bond constitutes the strongest type of interfacial interaction, and it is superior to physical contact (20), (21). It can further improve the compatibility of CNTs to the polymer matrix. Dispersibility and filler-matrix interfacial interaction are very important for efficient stress transfer from the polymer matrix to the CNT network and conduction improvement of polymer composites.
In this study, we provide an effective route to prepare chemically modified multi-walled CNT (MWCNT)/epoxy nanocomposites. This method not only realizes the uniform dispersion of MWCNTs in the epoxy matrix but also combines efficiently the design of MWCNT-epoxy interfacial interaction with the need for high performance of MWCNT/epoxy composites. In this functionalization process, grafting triethylenetetramine (TETA) on the MWCNT surface can bridge the connection of MWCNTs to the epoxy matrix. Some investigations reported TETA modification of MWCNTs (22-24). However, acid-oxidized MWCNTs are severely entangled and more easily form big agglomeration than raw MWCNTs. These researchers did not notice the phenomenon. We find that TETA grafting could vary the forces among acid-treated MWCNTs from polar to nonpolar and recover the loose state of MWCNTs. In addition, the studies on analytical characterization of TETA-grafted MWCNTs and the influence of this kind of functionalization method on the physical properties of epoxy nanocomposites are still deficient. In this article, TETA functionalization transforms the smooth and non-reactive surface of MWCNTs into a hybrid material that has the characteristics of both MWCNTs and TETA. We characterize the morphologies of TETA-grafted MWCNTs and the corresponding epoxy nanocomposites. Simultaneously, we also evaluate the effects of TETA grafting on the dispersion of MWCNTs, MWCNT-epoxy interfacial interaction, and the thermal properties of epoxy nanocomposites. We can have a comprehensive and in-depth understanding of the role and design of chemical functionalization.
MWCNTs were obtained from the Nanotech Port Company, Shenzhen, China. The MWCNTs were produced by chemical vapor deposition. This kind of catalytic production is simple and has a high productivity. The external diameter and the length of MWCNTs are 60-100 nm and 5-15 [micro]m, respectively. Epoxy resin used in this work was a nominally difunctional epoxy resin, bisphenol-A glycidol ether epoxy resin (DGEBA) with the epoxy value of 0.48-0.52 mol/100 g, supplied by Shanghai Resin Co. The curing agent, 2-ethyl-4-methylimidazole (EMI-2,4), was provided by Beijing Chemical Reagent Co. Epoxy resin was selected as the polymer matrix because it is known that CNTs are dispersed well in the epoxy resin compared with other polymer resins. Imidazole is a type of nucleophilic hardener. The net epoxy cured by it possesses better heat endurance, higher modulus, and wider curing temperature range than that cured by amine curing agents. In this work, EMI-2,4 has excellent compatibility with DGEBA. The cure temperature of DGEBA/EMI-2,4 system is relatively low and its gel time is long. In the meantime, the final cured product has high heat deformation temperature, good chemical stability, and superior mechanical properties. Therefore, we used EMI-2,4 to perform the cure of epoxy. The molecular structures of the epoxy and curing agent are shown in Fig. 1.
[FIGURE 1 OMITTED]
Chemical Functionalization of MWCNTs
The synthetic procedure is exhibited in Fig. 2. MWCNTs were first treated by a 3:1 (v/v) mixture of concentrated [H.sub.2][SO.sub.4]/[HNO.sub.3], with sonication at 40[degrees]C for 10 h. After acid treatment, MWCNTs were washed using deionized water, filtered until the pH value reached 7, and dried at 80[degrees]C for 24 h. The acid-treated MWCNT (MWCNT-COOH) was reacted with excess [SOCl.sub.2] for 24 h under reflux to yield acyl chloride-functionalized MWCNT (MWCNT-COC1). Then the residual [SOCl.sub.2] was removed by the reduced pressure distillation. The MWCNT-COC1 was further reacted with TETA at 120[degrees]C under magnetic stirring for 96 h and subsequently washed four times using excess chloroform to produce pure TETA-functionalized MWCNTs.
[FIGURE 2 OMITTED]
Fabrication of MWCNT/Epoxy Nanocomposites
MWCNT/epoxy nanocomposites were prepared in solution blended method, namely, using chloroform as the co-solvent. Unfunctionalized or functionalized MWCNTs were dispersed in chloroform, with sonication for 1 h. The dispersion was mixed with DGEBA, and the MWCNT/DGEBA ratio was adjusted for the different content of MWCNTs. The mixture was sonicated and stirred at 60[degrees]C. This treatment increased the viscosity of the dispersion, which in turn limited the MWCNT aggregation. Subsequently, the mixture was treated by a postheating at 90[degrees]C to completely remove the solvent. Then the mixture was kept in a vacuum oven for 24 h to get rid of air bubbles. After adding the curing agent, the mixture was stirred by using a magnetic bar for 30 min under sonication. Finally, the mixture was poured into a mold, and the whole system was placed in an oven. The MWCNT/epoxy compound was precured at 70[degrees]C for 1 h, cured at 110[degrees]C for 1.5 h, and postcured at 140[degrees]C for 1.5 h.
Characterizations of the Materials
X-ray pholoelectron spectroscopy (XPS) analysis was carried out in an ultra high vacuum system equipped with a Kratos AXIS Ultra hemispherical electron analyzer, using a monochromated Al K[alpha] source (1486.6 eV), at a base pressure of 2 x [10.sup.-10] mbar. Thermogravimetric analysis (TGA) was performed using a Perkin-Elmer TGA7 instrument. Transmission electron microscopy (TEM) and High-resolution TEM (HRTEM) images were taken on JEM 2100F to characterize the microstructures of MWCNTs. The morphological observations were executed using a held emission scanning electron microscope (FESEM), FEI SIRION 200. Raman spectra were measured with a Renishaw RM3000 Raman microscope. A 785 nm [Ar.sup.+] laser beam, with an incident power of ~ 10 mW at the sample, served as the excitation source. The glass transition temperature ([T.sub.g]) of samples was measured by dynamic differential scanning calorimetry (DSC) (Perkin-Elmer DSC-7 system). Test data were measured from room temperature to 300[degrees]C at a heating rate of 10[degrees]C/min. The fracture toughness property of net cured epoxy and MWCNT/epoxy composites was evaluated from the impact strength. Charpy impact tests were performed on an impact tester (Charpy XCJ-L) with an impactor energy of 5J at room temperature in accordance with ISO 179-2. The specimen dimensions were 64 x 10 X 4 [mm.sup.3]. Five specimens were tested for each set of conditions, and the mean values and their standard deviations were calculated. The thermal conductivity was measured on a TCT416 thermal analyser (NETZSCH Co., Germany) in accordance with ISO 8894. A minimum of four individual measurements was performed on bulk specimens (5 X 5 X 35 [mm.sup.3]).
RESULTS AND DISCUSSION
Morphology and Dispersibility Studies
The XPS survey spectra (revealed in Fig. 3) of as-received MWCNTs and TETA-grafted MWCNTs were obtained to identify the chemical composition of the surfaces. All the spectra exhibit C 1s and O 1s peaks, and furthermore, the XPS spectrum of TETA-grafted MWCNTs shows the remarkable intensification of O 1s peak. The reason for this phenomenon could be assigned to the effect of [[H.sub.2][SO.sub.4]/[HNO.sub.3]] treatment on the MWCNT surface. Likewise, the reason for the intensity of C 1s peak following the sequence as-received MWCNTs > TETA-grafted MWCNTs may be directly due to a series of chemical treatments. Additionally, the appearance of an N 1s peak indicates that TETA has been successfully grafted onto the MWCNT surface.
[FIGURE 3 OMITTED]
The extent of surface functionalization can be further evaluated by the percentage of weight loss in the TGA measurements. As-received MWCNTs exhibit a minor weight loss of about 4 wt%, when the temperature is up to 850[degrees]C, as shown in Fig. 4. However, with respect to TETA-grafted MWCNTs, obvious weight loss occurs primarily in the temperature range of 150-550[degrees]C due to the degradation of grafted TETA on the MWCNT surface. When the temperature is higher than 600[degrees]C, the weight loss is attributed to the decomposition of grapheme layers of MWCNTs. TETA-grafted MWCNTs show a weight loss of about 14 wt% (the temperature range is from 50 to 850[degrees]C). The comparative analysis proves the existence of TETA grafted on the surface of MWCNTs.
[FIGURE 4 OMITTED]
The microstructures of as-received and chemically functionalized MWCNTs are shown in Fig. 5. In the TEM image of as-received MWCNTs (Fig. 5a), there are many black spots in the structure of nanotubes, which indicates that raw MWCNTs contain some impurities. In addition, it can be observed from Fig. 5b that the tube wall of the as-received MWCNT is relatively smooth and clean. No extra phase appears on the nanotube surface. After chemical modifications, there are few black spots in the structure of TETA-grafted MWCNTs (presented in Fig. 5c), which denotes that most of the impurities have been removed. Moreover, this structure is a little indistinct. In Fig. 5d, an extra phase appears on the MWCNT wall. This indicates that the grafting reactions have taken place on the MWCNT surface, and, thus, a TETA layer has been formed on the tube wall. Accordingly, HRTEM analysis supplies the direct evidence that TETA is effectively grafted onto the MWCNT wall to form a thin layer.
[FIGURE 5 OMITTED]
The morphologies of as-received MWCNTs, acid-treated MWCNTs, and TETA-grafted MWCNTs are exhibited in Fig. 6. In Fig. 6a, as-received MWCNTs are curled and entangled; however, there are few big agglomerates in them. After [[H.sub.2] [SO.sub.4]/[HNO.sub.3]] treatment, nanotubes are severely entangled and form big agglomeration (shown in Fig. 6b). The interspaces formed in acid-treated MWCNTs are rather small. However, further TETA grafting breaks those big agglomerates and recovers the loose state of MWCNTs, as shown in Fig. 6c. These phenomena may be due to the different interactions among nanotubes (displayed in Fig. 7).
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
In Fig. 7, [[H.sub.2] [SO.sub.4]/[HNO.sub.3]] treatment could generate a number of carboxyl and hydroxyl groups on the MWCNT surface. Acid-treated MWCNTs possess large polar forces derived from the strong interactions among these polar groups on their surface. Accordingly, the interspaces formed in nanotubes get extremely small. TETA grafting varies the forces among MWCNTs from polar to nonpolar; in the meantime, TETA groups may increase the tube to tube distance. Thus, the interspaces formed in TETA-grafted MWCNTs become large again. Moreover, TETA-grafted MWCNTs are looser than as-received MWCNTs, which is conducive to their better dispersion in epoxy matrix.
Chemically modified MWCNTs show usually better dispersion in solvents. Equal amounts of as-received MWCNTs and TETA-grafted MWCNTs were added respectively to the same volume of chloroform, and subsequently, the mixtures were mechanically stirred. TETA-grafted MWCNTs could be dispersed stably in chloroform (revealed in Fig. 8b). With respect to raw MWCNTs, all of nanotubes sank, as shown in Fig. 8a. This indicates that TETA-grafted MWCNTs possess a higher degree of miscibility than as-received MWCNTs due to the appearance of TETA functional groups on the MWCNT surface.
[FIGURE 8 OMITTED]
Figure 9 shows the fractured morphologies of net epoxy cured by EMI-2,4, as-received MWCNT/epoxy composite and TETA-grafted MWCNT/epoxy composite. It can be obviously observed from Fig. 9a and b that regular river patterns occur on the fracture surface; moreover, the fracture surface is smooth. This indicates that the fracture pattern of the net epoxy cured by EMI-2.4 exhibits a brittle fracture. Due to the filling of as-received MWCNTs, the fracture surface of the composite becomes a little rough and some cracks appear on it. In the meantime, big MWCNT agglomerates exist in epoxy matrix (revealed in Fig. 9c and d). However, the fracture surface of TETA-grafted MWCNT/epoxy composite is rather rough and crack propagation is irregular, which denotes that its fracture is the result of a ductile deformation (shown in Fig. 9e). Furthermore, TETA-grafted MWCNTs are dispersed homogeneously in epoxy matrix (exhibited in Fig. 9f).
[FIGURE 9 OMITTED]
Filler-Matrix Interfacial Interaction Studies
Raman spectroscopy is sensitive to the strength of the interface between the individual nanotubes and the matrix (25-27). Thus, we can supply a qualitative analysis of MWCNT-epoxy interfacial strength by studying the Raman shift of the D* mode. Figure 10 exhibits Raman spectra of three samples: as-received MWCNTs, 0.6 wt% content as-received MWCNT/epoxy composite, and 0.6 wt% TETA-grafted MWCNT/epoxy composite. The band at 2618 [cm.sup.-1] (D* mode) appears in the spectrum of as-received MWCNTs, as shown in Fig. 10a. In Fig. 10b, there is a negligible position change of D* mode (namely, 2617 [cm.sup.-1]), which indicates very weak interfacial interaction between nanotubes and epoxy matrix. TETA grafting establishes the connection of MWCNTs to the epoxy matrix. It can be seen from Fig. 10c that the peak position of 2618 [cm.sup.-1] shifts to a lower wavenumber (namely, 2613 [cm.sup.-1]). This kind of shift is correlated with transferring of stress (this stress is generated in the curing process) from the epoxy matrix to the MWCNTs, which implies an existence of the interfacial adhesion.
[FIGURE 10 OMITTED]
The impact property test for the epoxy nanocomposites was performed (shown in Fig. 11). When the content of as-received MWCNTs is 0.6 wt%, the corresponding impact strength of epoxy nanocomposite reaches 15.21 kJ/[m.sup.2]. The impact strength of net epoxy cured by EMI-2,4 is 11.29 kJ/[m.sup.2]. Accordingly, the impact property of the as-received MWCNT/epoxy composite increases by 35%. The ductility can be increased further when TETA-grafted MWCNTs are filled into the epoxy matrix. The impact strength of the 0.6 wt% content TETA-grafted MWCNT/epoxy composite is 20.85 kJ/[m.sup.2], and its impact property increases by 85%. The phenomenon may be attributed to the fact that TETA-grafted MWCNTs are to be dispersed easier in epoxy matrix and more compatible with matrix material. These factors would be beneficial to better toughening effect on the epoxy matrix.
[FIGURE 11 OMITTED]
Pure MWCNT-TETA hybrid (namely, TETA-grafted MWCNT) possesses a composite structure. This type of structure not only overcomes the shortage of raw MWCNTs, but also endows MWCNTs with the special mechanism of reinforcing and toughening. TETA grafting could form a soft layer on the MWCNT wall. If this soft layer establishes good connection of the MWCNT to the epoxy matrix, it can efficiently transfer loading between the nanotube and the matrix and absorb impact energy. Consequently, the strength and toughness of the epoxy matrix could be enhanced. A TETA-grafted MWCNT can be considered to be a "core-shell" structure (MWCNT is the core, and TETA is the shell). The MWCNT has very high Young's modulus and tensile strength and plays the role of load bearing. The soft TETA layer has the strong connections with the MWCNT and the epoxy matrix. This kind of the structure realizes effective load transfer between the epoxy matrix and the MWCNT (revealed in Fig. 12). Furthermore, the soft layer could provide the units that are smaller than the chain segments in epoxy matrix with larger mobility, and, thus, more impact energy may be taken in. This combined nanotube structure supplies the possibility of simultaneously reinforcing and toughening the epoxy matrix.
[FIGURE 12 OMITTED]
Thermal Properties of Epoxy Nanocomposites
DSC was used to evaluate the effect of MWCNTs on the phase transition behavior of epoxy (exhibited in Fig. 13). The glass transition temperature of the hybrid composites is correlated with a cooperative motion of long-chain segments that may be hindered by the MWCNTs (28), (29). In addition, the glass transition of polymer does not take place at a definite temperature, but completes in a certain temperature range. In this work, we define the temperature at which the glass transition reaches to [DELTA][c.sub.p]/2 ([c.sub.p] is the specific heat capacity) as [T.sub.g]. Accordingly, both as-received MWCNT/epoxy composite and TETA-grafted MWCNT/epoxy composite recorded higher [T.sub.g] than pure cured epoxy ([T.sub.g] of net cured epoxy, as-received MWCNT/epoxy composite and TETA-grafted MWCNT/epoxy composite is 144.50, 147.65, and 150.08[degrees]C, respectively). It is also found that the enhancement in [T.sub.g] for TETA-grafted MWCNT/epoxy composite is more significant than that for as-received MWCNT/ epoxy composite. This may be attributed to the strong interaction between TETA-grafted MWCNTs and the epoxy chains (TETA grafting could bridge good connection of MWCNTs to the epoxy matrix). This kind of good interfacial interaction is able to hinder further the motion of the polymer chains and decrease the free volume of the nanocomposite. Therefore, [T.sub.g] of TETA-grafted MWCNT/epoxy composite is the highest. To better display the influence of chemical functionalization on physical properties of epoxy nanocomposites, we summarize the results of DSC and impact tests in Table 1.
[FIGURE 13 OMITTED]
TABLE 1. A summarization of the results of DSC and impact tests (the content of the MWCNTs was 0.6% by weight). Samples Impact strength [T.sub.g] ([degrees]C) (kJ/[m.sup.2]) Net epoxy cured 11.29 144.50 As-received MWCNT/epoxy 15.21 147.65 TETA-grafted MWCNT/epoxy 20.85 150.08
Figure 14 presents the weight loss curves, and the corresponding differential curves (DTG) of pure cured epoxy and TETA-grafted MWCNT/epoxy composite. It can be seen clearly that all samples undergo the decomposition mainly as a two-stage process. In the first stage process, the onset decomposition temperatures ([T.sub.onset1]) of pure cured epoxy and TETA-grafted MWCNT/epoxy composite are 391.3 and 397.1[degrees]C, respectively. In the second stage process, their onset decomposition temperatures ([T.sub.onset2]) are separately 561.7 and 606.2[degrees]C. The filling of TETA-grafted MWCNTs enhances the values of [T.sub.onset1] and [T.sub.onset2], which indicates the thermal stability improvement of the nanocomposite. The reason may be due to the fact that TETA-grafted MWCNTs possess a good affinity for the epoxy matrix.
[FIGURE 14 OMITTED]
Previous researches have reported that the thermal conductivities of CNT/polymer composites would be much lower than the values estimated from the intrinsic thermal conductivity of CNTs and their volume fraction (30). Some investigations indicate that the resistance to the heat flow caused by polymer-CNT interface is responsible for the low thermal conductivities of polymer-based CNT composites (31). The interface thermal resistance, also called the Kapitza resistance ([R.sub.K]), is expected to be more important in the composites filled with nano-structured fillers because these inclusions are small in size and their surface to volume ratios are high. The case of CNT/polymer composite falls in the category. Accordingly, optimization of the MWCNT-epoxy interfacial interaction is significant for the improvement of thermal transport property of epoxy nanocomposites. Figure 15 exhibits the thermal conductivity of MWCNT/epoxy composites. TETA grafting would make tube carbon atoms be covalently attached to matrix molecules. Therefore, these carbon atoms will act as scattering centers for the heat carrying wave packages (phonons) and reduce tube thermal conductivity. In addition, acid treatment could shorten the length of MWCNTs, so the aspect ratio would decrease. These factors make against the enhancement of thermal conductivity of MWCNT/epoxy composites. However, it can be clearly seen that with regard to different nanotube volume fraction, the thermal conductivities of TETA-grafted MWCNT/epoxy composites are larger than those of as-received MWCNT/epoxy composites. The phenomenon means that TETA grafting improves the interfacial heat transport between the MWCNTs and the epoxy matrix and facilitates better distribution of MWCNTs in the matrix, which is very conducive to the enhancement of the thermal conductivity of MWCNT/epoxy composites.
[FIGURE 15 OMITTED]
An effective method to fabricate chemically functionalized MWCNT/epoxy nanocomposites was supplied. TETA-grafted MWCNTs were successfully obtained when the amine groups in TETA reacted with the MWCNTs with--COC1 groups that were prepared by treating the raw MWCNTs with [[H.sub.2][SO.sub.4]/[HNO.sub.3]] followed by [SOCl.sub.2]. TETA grafting makes MWCNTs become a hybrid material that possesses the features of both MWCNTs and TETA. The resulting MWCNT-TETA is uniformly dispersed in epoxy matrix. Simultaneously, the soft TETA layer could bridge the connection of MWCNTs to the epoxy matrix, which improves efficiently the MWCNT-epoxy interfacial interaction. The combination of more homogenous nanotube dispersion and stronger interfacial adhesion between the nanotubes and the epoxy matrix would contribute to the enhancements of impact property, glass transition temperature, thermal stability, and thermal conductivity of epoxy nanocomposites.
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Kai Yang, Mingyuan Gu
State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People's Republic of China
Correspondence to: K. Yang; e-mail: email@example.com
Contract grant sponsor: Instrumental Analysis Centre of Shanghai Jiao Tong University.
Published online in Wiley InterScience (www.interscience.wiley.com).