the effects of triethylenetetramine grafting of multi-walled carbon nanotubes on its dispersion,...

The Effects of Triethylenetetramine Grafting ofMulti-Walled Carbon Nanotubes on Its Dispersion,Filler-Matrix Interfacial Interaction and theThermal Properties of Epoxy Nanocomposites

Kai Yang, Mingyuan GuState Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, 800 Dongchuan Road,Shanghai 200240, People’s Republic of China

This study describes the influence of triethylenetetr-amine (TETA) grafting of multi-walled carbon nanotubes(MWCNTs) on the dispersion state, interfacial interac-tion, and thermal properties of epoxy nanocomposites.MWCNTs were first treated by a 3:1 (v/v) mixture ofconcentrated H2SO4/HNO3, and then TETA grafting wasperformed. Chemically grafted MWCNT/bisphenol-Aglycidol ether epoxy resin/2-ethyl-4-methylimidazolenanocomposites were prepared. TETA grafting couldestablish the connection of MWCNTs to the epoxymatrix and transform the smooth and nonreactiveMWCNT surface into a hybrid material that possessesthe characteristics of both MWCNTs and TETA, whichfacilitates homogeneous dispersion of MWCNTs andimproves nanotube-epoxy interfacial interaction. There-fore, the impact property, glass transition temperature,thermal stability, and thermal conductivity of epoxynanocomposites are enhanced. POLYM. ENG. SCI.,49:2158–2167, 2009. ª 2009 Society of Plastics Engineers

INTRODUCTION

Since the discovery of carbon nanotubes (CNTs) [1],

nanocomposites filled with CNTs have been actively stud-

ied 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 real-

ize these exceptional properties of CNT/polymer compo-

sites, we have to resolve two fundamental and important

issues. One is how to facilitate the homogeneous disper-

sion 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 are 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 alterna-

tive 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 mechan-

ical strength of the nanocomposites [16]. Sidewall func-

tionalization 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

Correspondence to: K. Yang; e-mail: [email protected]

Contract grant sponsor: Instrumental Analysis Centre of Shanghai Jiao

Tong University.

DOI 10.1002/pen.21461

Published online in Wiley InterScience (www.interscience.wiley.com).

VVC 2009 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE—-2009

from the polymer matrix to the CNT network and conduc-

tion 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 com-

bines efficiently the design of MWCNT-epoxy interfacial

interaction with the need for high performance of

MWCNT/epoxy composites. In this functionalization pro-

cess, grafting triethylenetetramine (TETA) on the MWCNT

surface can bridge the connection of MWCNTs to the

epoxy matrix. Some investigations reported TETA modifi-

cation 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. Simultane-

ously, we also evaluate the effects of TETA grafting on the

dispersion of MWCNTs, MWCNT-epoxy interfacial inter-

action, and the thermal properties of epoxy nanocomposites.

We can have a comprehensive and in-depth understanding

of the role and design of chemical functionalization.

EXPERIMENTAL

Materials

MWCNTs were obtained from the Nanotech Port Com-

pany, Shenzhen, China. The MWCNTs were produced by

chemical vapor deposition. This kind of catalytic produc-

tion is simple and has a high productivity. The external

diameter and the length of MWCNTs are 60–100 nm and

5–15 lm, respectively. Epoxy resin used in this work was

a nominally difunctional epoxy resin, bisphenol-A glyci-

dol 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 pos-

sesses 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 deforma-

tion 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.

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 H2SO4/HNO3, with sonication at 408C for

10 h. After acid treatment, MWCNTs were washed using

deionized water, filtered until the pH value reached 7, and

dried at 808C for 24 h. The acid-treated MWCNT

(MWCNT-COOH) was reacted with excess SOCl2 for 24

h under reflux to yield acyl chloride-functionalized

MWCNT (MWCNT-COCl). Then the residual SOCl2 was

removed by the reduced pressure distillation. The

MWCNT-COCl was further reacted with TETA at 1208Cunder magnetic stirring for 96 h and subsequently washed

four times using excess chloroform to produce pure

TETA-functionalized MWCNTs.

Fabrication of MWCNT/Epoxy Nanocomposites

MWCNT/epoxy nanocomposites were prepared in so-

lution blended method, namely, using chloroform as the

FIG. 1. Molecular structures of epoxy resin (DGEBA) and curing agent (EMI-2,4).

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2009 2159

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 608C. This treatment increased the viscosity of

the dispersion, which in turn limited the MWCNT

aggregation. Subsequently, the mixture was treated by a

postheating at 908C 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 708Cfor 1 h, cured at 1108C for 1.5 h, and postcured at 1408Cfor 1.5 h.

Characterizations of the Materials

X-ray photoelectron 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 Ka source (1486.6 eV), at a

base pressure of 2 3 10210 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 exe-

cuted using a field emission scanning electron microscope

(FESEM), FEI SIRION 200. Raman spectra were meas-

ured with a Renishaw RM3000 Raman microscope. A

785 nm Arþ laser beam, with an incident power of �10

mW at the sample, served as the excitation source. The

glass transition temperature (Tg) of samples was measured

by dynamic differential scanning calorimetry (DSC)

(Perkin-Elmer DSC-7 system). Test data were measured

from room temperature to 3008C at a heating rate of

108C/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 3 10

3 4 mm3. Five specimens were tested for each set of

conditions, and the mean values and their standard devia-

tions 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 3 5 3 35 mm3).

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 sur-

faces. 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 H2SO4/HNO3 treatment on the MWCNT surface.

Likewise, the reason for the intensity of C 1s peak follow-

ing the sequence as-received MWCNTs [ TETA-grafted

FIG. 3. XPS survey spectra of as-received MWCNTs and TETA-

grafted MWCNTs.

FIG. 2. Scheme for the process of grafting TETA onto the MWCNT

surface.

2160 POLYMER ENGINEERING AND SCIENCE—-2009 DOI 10.1002/pen

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.

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 8508C, as shown in Fig. 4. However, with respect to

TETA-grafted MWCNTs, obvious weight loss occurs

primarily in the temperature range of 150–5508C due to

the degradation of grafted TETA on the MWCNT sur-

face. When the temperature is higher than 6008C, the

weight loss is attributed to the decomposition of graph-

eme layers of MWCNTs. TETA-grafted MWCNTs show

a weight loss of about 14 wt% (the temperature range

is from 50 to 8508C). The comparative analysis proves

the existence of TETA grafted on the surface of

MWCNTs.

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

FIG. 4. TGA curves of as-received MWCNTs and TETA-grafted

MWCNTs.

FIG. 5. (a) TEM image of as-received MWCNTs; (b) HRTEM image of an as-received MWCNT; (c) TEM

image of TETA-grafted MWCNTs; (d) HRTEM image of a TETA-grafted MWCNT.


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. Accord-

ingly, HRTEM analysis supplies the direct evidence that

TETA is effectively grafted onto the MWCNT wall to

form a thin layer.

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 agglom-

erates in them. After H2SO4/HNO3 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).

In Fig. 7, H2SO4/HNO3 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

FIG. 6. FESEM images of (a) as-received MWCNTs, (b) acid-treated

MWCNTs and (c) TETA-grafted MWCNTs.

FIG. 7. Reasons for varying sized interspaces formed in untreated and

chemically treated MWCNTs.


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 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; more-

over, 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 mean-

time, 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).

FIG. 8. Photograph of (a) as-received MWCNTs and (b) TETA-grafted

MWCNTs dispersed in chloroform and wait for 72 h.

FIG. 9. (a) FESEM image of the fracture surface of net epoxy cured by EMI-2,4; (b) Local zoomed image

from image (a); (c) FESEM image of the fracture surface of as-received MWCNT/epoxy composite (the con-

tent of the MWCNTs was 0.6 wt%); (d) Magnification of the position marked with the right white arrow in

(c); (e) FESEM image of the fracture surface of TETA-grafted MWCNT/epoxy composite (the content of the

MWCNTs was 0.6wt%); (f) FESEM image of the dispersion of TETA-grafted MWCNTs in epoxy matrix.


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 cm21 (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 cm21), which indicates very weak interfacial inter-

action between nanotubes and epoxy matrix. TETA graft-

ing establishes the connection of MWCNTs to the epoxy

matrix. It can be seen from Fig. 10c that the peak position

of 2618 cm21 shifts to a lower wavenumber (namely,

2613 cm21). This kind of shift is correlated with transfer-

ring of stress (this stress is generated in the curing pro-

cess) from the epoxy matrix to the MWCNTs, which

implies an existence of the interfacial adhesion.

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/m2. The impact strength of net epoxy cured by EMI-

2,4 is 11.29 kJ/m2. 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/m2, 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.

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

FIG. 10. Raman spectra of (a) as-received MWCNTs, (b) 0.6 wt% con-

tent as-received MWCNT/epoxy composite and (c) 0.6 wt% content

TETA-grafted MWCNT/epoxy composite.

FIG. 11. Impact strength vs. MWCNT content for as-received

MWCNT/epoxy and TETA-grafted MWCNT/epoxy composites.

FIG. 12. Model structure of TETA-grafted MWCNT/epoxy composites.


structure supplies the possibility of simultaneously rein-

forcing and toughening the epoxy matrix.

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 Dcp/2 (cp is the specific heat capacity) as Tg.Accordingly, both as-received MWCNT/epoxy composite

and TETA-grafted MWCNT/epoxy composite recorded

higher Tg than pure cured epoxy (Tg of net cured epoxy,

as-received MWCNT/epoxy composite and TETA-grafted

MWCNT/epoxy composite is 144.50, 147.65, and

150.088C, respectively). It is also found that the enhance-

ment in Tg 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 connec-

tion 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, Tg of TETA-grafted

MWCNT/epoxy composite is the highest. To better

display the influence of chemical functionalization on

physical properties of epoxy nanocomposites, we summa-

rize the results of DSC and impact tests in Table 1.

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 decompo-

sition mainly as a two-stage process. In the first stage pro-

cess, the onset decomposition temperatures (Tonset1) of

pure cured epoxy and TETA-grafted MWCNT/epoxy

composite are 391.3 and 397.18C, respectively. In the sec-

ond stage process, their onset decomposition temperatures

(Tonset2) are separately 561.7 and 606.28C. The filling of

TETA-grafted MWCNTs enhances the values of Tonset1and Tonset2, which indicates the thermal stability improve-

ment of the nanocomposite. The reason may be due to the

fact that TETA-grafted MWCNTs possess a good affinity

for the epoxy matrix.

Previous researches have reported that the thermal con-

ductivities 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

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 (kJ/m2) Tg (8C)

Net epoxy cured 11.29 144.50

As-received MWCNT/epoxy 15.21 147.65

TETA-grafted MWCNT/epoxy 20.85 150.08

FIG. 14. TGA and DTG curves of net cured epoxy and TETA-grafted

MWCNT/epoxy composite (the content of the MWCNTs was 0.6% by

weight).

FIG. 13. DSC glass transition curves of net epoxy cured by EMI-2,4,

as-received MWCNT/epoxy composite and TETA-grafted MWCNT/

epoxy composite. The content of the MWCNTs was 0.6% by weight.


the low thermal conductivities of polymer-based CNT

composites [31]. The interface thermal resistance, also

called the Kapitza resistance (RK), is expected to be

more important in the composites filled with nano-struc-

tured 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. Accord-

ingly, 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. There-

fore, 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 enhance-

ment of thermal conductivity of MWCNT/epoxy compo-

sites. However, it can be clearly seen that with regard to

different nanotube volume fraction, the thermal conduc-

tivities of TETA-grafted MWCNT/epoxy composites are

larger than those of as-received MWCNT/epoxy compo-

sites. The phenomenon means that TETA grafting

improves the interfacial heat transport between the

MWCNTs and the epoxy matrix and facilitates better dis-

tribution of MWCNTs in the matrix, which is very con-

ducive to the enhancement of the thermal conductivity of

MWCNT/epoxy composites.

CONCLUSIONS

An effective method to fabricate chemically function-

alized MWCNT/epoxy nanocomposites was supplied.

TETA-grafted MWCNTs were successfully obtained

when the amine groups in TETA reacted with the

MWCNTs with ��COCl groups that were prepared by

treating the raw MWCNTs with H2SO4/HNO3 followed

by SOCl2. 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 effi-

ciently 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 enhance-

ments of impact property, glass transition temperature,

thermal stability, and thermal conductivity of epoxy

nanocomposites.

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