Mechanical and Thermal Properties ofFunctionalized Multiwalled CarbonNanotubes/Cyanate Ester Composite

Jinhe Wang, Guozheng Liang, Hongxia Yan, Shaobo HeDepartment of Applied Chemistry, School of Science, Northwestern Polytechnical University,Xi’an Shaanxi 710072, China

Multiwalled carbon nanotubes/cyanate ester compositeusing epoxy group as a bridge was prepared. After atwo-step functionalization, the epoxy groups wereintroduced on the surface of multiwalled carbon nano-tubes (MWNTs) successfully, which was confirmed byX-ray diffraction and Fourier transform infrared spec-tra. The mechanical and thermal properties of thecured cyanate ester resin and MWNTs/cyanate estercomposites were examined by mechanical tests, scan-ning electron microscopy, and dynamic mechanicalthermal analysis. The results showed that 2 wt% func-tionalized MWNTs can enhance the flexural strengthand impact strength significantly and increase theglass transition temperature (Tg) slightly. POLYM. ENG.SCI., 49:680–684, 2009. ª 2009 Society of Plastics Engineers

INTRODUCTION

Cyanate ester resins (CEs) are among the most impor-

tant engineering thermosetting polymers and have

received attentions because of their outstanding physical

properties such as low water absorptivity and outgassing,

excellent mechanical properties, dimensional and thermal

stability, and flame resistance [1–4]. Unfortunately, highly

crosslinked thermosets, such as CEs, tend to be brittle and

have reduced impact resistance. To improve the toughness

of a cured CE, a number of different modifiers have been

used. These additives include reactive and nonreactive

rubbers [5, 6], a variety of engineering thermoplastics [7–

9] and some kind of thermosets such as epoxy and bisma-

leimide [10–15]. All of these modifiers have proven use-

ful to some extent. However, toughening usually occurs at

the cost of other characteristics of CEs. For example, rub-

bers can improve the toughness of CEs largely, whereas

the thermal property is generally reduced [16]. Although

a few examples of utilization of thermoplastic modifiers

to improve mechanical properties without sacrificing ther-

mal stability, most of them need special solvents in fabri-

cation processes, which will unavoidably affect the final

properties of the cured resin and add difficulty to the

fabrication operation [17].

Since the first observation of the carbon nanotubes

(CNTs) in 1991 by Iijima, the awareness of their unique

mechanical properties, such as extremely high strength

and stiffness and enormous aspect ratio, make them

potentially excellent reinforcing fillers of polymer materi-

als [18, 19]. A small addition of CNTs can improve the

mechanical property of polymer significantly with little

side-effect on other properties. However, there are two

problems which have to be overcome in order to achieve

the goal. One is the lack of interfacial adhesion, which is

critical for load transfer in composites. Indeed, CNTs’

surfaces are atomically smooth, which may limit the

transfer of load from the matrix to nanotubes reinforce-

ment. Another is the poor dispersion of nanotubes in the

polymer matrix, which is also significant for the fabrica-

tion of reinforced nanocomposite.

Functionalization of CNTs by covalent molecules is

the most popular method to improve the compatibility

and the interaction with the polymer matrixes. Up to now,

various functional groups have been attached onto the

convex surfaces of CNTs via covalent bonding [20]. In

general, major approaches include: (i) amidation or esteri-

fication of carboxylated CNTs, (ii) side-wall covalent

attachment of functional groups directly to the pristine

CNTs. The chemical group that should be attached to the

CNTs depends on the nature of the polymer to be rein-

forced, and the CNTs functionalized with epoxy groups

are very important to cyanate ester modification, as epoxy

groups can react with cyanate ester to form oxazlalidinone

structure with high mechanical and thermal properties

[21]. In this article, we are mainly concerned with the

functionalization of the multiwalled carbon nanotubes

(MWNTs) with 3-glycidyloxypropyltrimethoxy silane

(coupling agent, KH-560) and the properties of the func-

tionalized MWNTs/CE composites.

Correspondence to: Guozheng Liang; e-mail: lgzheng@nwpu.edu.cn

Contract grant sponsor: Natural Science Foundation of Shaanxi Province

of China; contract grant number: 2007B10.

DOI 10.1002/pen.21277

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

VVC 2009 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE—-2009

EXPERIMENTAL

Materials

The pristine MWNTs (p-MWNTs) were obtained from

a commercial source (Shenzhen Nanotech Port, China).

They were produced by chemical vapor deposition (CVD)

and contained about 5% impurities, consisting primarily

of amorphous carbon. The nanotubes were 5-15 lm long

and 10–20 nm in diameter. KH560, analytical grades, was

purchased from Jingzhou Jianghan fine chemical.

(Jingzhou, China) and used without further purification.

Bisphenol A dicyanate (BADCy) was supplied by Shang-

hai Huifen Kemao. (Shanghai, China). Other reagents

were all provided by Xi’an Chemical Reagent Company

and used as received without any further treatment.

Functionalization of MWNTs

The p-MWNTs were functionalized in a two-step pro-

cess. First, put p-MWNTs (1 g) to 50 ml acid-K2Cr2O7

solution (K2Cr2O7: H2O: concentrated H2SO4 ¼ 7:12:150,

by weight), 1 h sonication at 408C, and 2 h stirring at

1008C. The solution was extensively washed again with

deionized water until pH value reached 5–6. The oxida-

tion-treated nanotubes (o-MWNTs) were collected on a

0.45 lm PTFE membrane by vacuum filtration and dried

overnight in a vacuum oven at 608C, subsequently, theseo-MWNTs (0.5 g) were then dispersed into 25 ml anhy-

drous ethanol. KH-560 (10 g), diluted in 25 ml anhydrous

ethanol, was added into the mentioned solution. The

resulting mixture was stirred for 24 h at the temperature

of 608C, and the chemical modifications of MWNTs were

formed. The functionalized MWNTs (denoted f-MWNTs)

were extensively washed with anhydrous ethanol and col-

lected on a 0.45 lm PTFE membrane by vacuum filtration

and dried overnight in a vacuum oven at 608C.

Preparation of the MWNTs/BADCy Composites

The MWNTs/BADCy composites containing 2 wt% of

p-MWNTs and f-MWNTs were prepared. MWNTs were

dispersed in BADCy via an ultrasonicator for 30 min at

1008C, and then dibutyltin laurate (200 ppm) used as cur-

ing catalyst was added to the suspension, subsequently,

the system was moved immediately to an oil-bath and

reacted for 40 min at 1408C. The resultant mixture was

poured into a preheated mold (1508C) with the release

agent on the inner walls, and then, the mold was degassed

under vacuum for 20 min at 1408C, followed by the cur-

ing cycle: 1808C/2 h þ 2208C/2 h þ 2408C/3 h.

Characterization

X-ray diffraction (XRD) measurements were conducted

using D/Max-3C instrument (Anode Material: Cu; Gener-

ator Settings: 40 kV, 35 mA). Fourier transform infrared

spectroscopy (FTIR) (WQF-300IR, Beijing Optical Instru-

ments Factory) was used to assess the presence of the

organic groups in f-MWNTs. The MWNT samples were

measured by the method of KBr pellet. Flexural and

impact tests were performed on a universal testing

machine according to GB2570-81 standard and GB2571-

81 standard of China, respectively. Scanning electron mi-

croscopy (SEM) images were obtained with Quanta 200-

FEI to examine surfaces of the fractured specimens.

Dynamic mechanical analysis (DMA 2980, TA Instru-

ments) was carried out to determine the glass transition

temperature (Tg) of the composites, defined as the temper-

ature where the loss tangent reaches a peak. Measure-

ments were performed at a heating rate of 108C/min and

a load frequency of 3 Hz.

RESULTS AND DISCUSSION

Functionalization of MWNTs

The concentrated 3:1 H2SO4/HNO3 solution is the

most common oxidation reagent for MWNTs treatment

[22]. However, there are some disadvantages for this kind

oxidation reagent when compared with the acid-K2Cr2O7

solution. One is that the concentrated 3:1 H2SO4/HNO3

solution is easy to cut the MWNTs short during the treat-

ment, which is not expected in reinforcement modifica-

tion, and another is that its treatment time is usually long

[23–25]. Figure 1 illustrates the reactions in the whole

treatment processing.

Figure 2 shows the XRD patterns measured from pow-

der samples of p-MWNTs, o-MWNTs, and f-MWNTs.

The pattern for the p-MWNTs (Fig. 2a) exhibites three

peaks at 25.848, 42.878, and 53.638. The pattern of o-

MWNTs (Fig. 2b) is the same as that of p-MWNTs,

which indicates that the oxidation procedure did not cut

or damage the MWNTs structure badly and the remained

oxidation reagent had been washed clean. When com-

pared with the peaks of o-MWNTs, the peaks of f-

MWNTs (Fig. 2c) are lower and there are some new

small peaks. These differences indicate that KH560 has

been bonded to MWNTs as the bonded KH560 on the

surface of MWNTs weakens the strength of X-ray diffrac-

tion and forms some new small diffractive peaks.

FIG. 1. Illustration of preparation process of f-MWNTs.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2009 681

FTIR spectra also corroborate the attachment of

KH560 to the surface of MWNTs, as shown in Fig. 3.

For the p-MWNTs (Fig. 3a), the bands at 3430 cm21 and

1398 cm21 is attributed to the presence of hydroxyl

groups (��OH) on the surface of the MWNTs, which are

believed to result from oxidation during purification of

the raw material [26]. Another band at 1620 cm21 is

assigned to the C¼¼O stretching of quinone groups on the

surface of MWNTs. For the oxidized MWNTs (Fig. 3b),

a new band appears at 1720 cm21, which is attributed to

the C¼¼O stretching vibrations of the carboxylic groups

(��COOH). The increase in the relative intensities of the

bands at 3430 and 1398 cm21 suggests that there are

more ��OH groups on the MWNTs surface after the oxi-

dation. In the f-MWNTs (Fig. 3c), the band at 3430 cm21

becames weak, and the band at 1398 cm21 disappears.

On the other hand, the bands at 2940 and 2840 cm21, the

bands at 1000–1200 cm21 and the band at 826 cm21 are

attributed to the methylene group, the Si��O group and

the Si��C group, respectively. The weak band at 910

cm21 confirms the presence of the epoxy group on the

f-MWNTs.

The Properties of MWNTs/BADCy Composites

Figure 4 compares the impact strength of MWNTs/

BADCy composites with that of pure BADCy resin. The

impact strength decreased a little (9%) for the samples

containing 2 wt% p-MWNTs, but an significant (41%)

improvement was observed for the 2 wt% f-MWNTs

based BADCy composites. The same trend appears in the

flexural strength (see Fig. 5). The flexural strength of the

samples containing 2 wt% p-MWNTs decreased by 28%.

However, the flexural strength of the samples containing

2 wt% f-MWNTs increased by 50%. Fracture surfaces

morphology of the cured BADCy and of MWNTs/

BADCy composites with 2 wt% MWNTs were examined

FIG. 2. XRD of (a) p-MWNTs, (b) o-MWNTs, and (c) f-MWNTs.

FIG. 3. FTIR spectra of (a) p-MWNTs, (b) o-MWNTs, and (c) f-

MWNTs.

FIG. 4. Impact strength of MWNTs/BADCy composites.

FIG. 5. Flexural strength of MWNTs/BADCy composites.

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

by SEM (see Fig. 6). Composites prepared with p-MWNTs

(Fig. 6b, 31000) exhibited isolated regions of agglomer-

ated MWNTs. The agglomerates cause cracks to initiate

and propagate easily. The generated cracks reduce the

strength of the composite. The dispersion of the f-

MWNTs/BADCy composite is much better than that of the

p-MWNTs/BADCy composite. It can be observed that the

ridges, dimpled patterns, and crevices on the fracture surfa-

ces are much more prominent for the f-MWNTs/BADCy

composite (Fig. 6c, 310,000). More energy dissipation

features are displayed. The well dispersed f-MWNTs are

more efficient than the aggregated p-MWNTs in transfer-

ring applied load. What is more, the interfacial adhesion

between the f-MWNTs and the BADCy matrix is not only

van der Waals forces but also chemical bonds which can

bear more applied load. It is possible that these fracture

features are the manifestation of the increased strength of

the f-MWNTs/BADCy composite.

Figure 7 represents the storage modulus of the cured

BADCy and MWNTs/BADCy composites. It shows that

the p-MWNTs/BADCy composite and the f-MWNTs/

BADCy composite have almost the same storage modulus

when temperature is low than Tg and both of them are

FIG. 6. The morphology of the fracture surface: (a) Pure BADCy, (b)

p-MWNTs/BADCy composite, and (c) f-MWNTs/BADCy composite.

FIG. 7. Storage modulus of the cured BADCy and MWNTs/BADCy

composites.

FIG. 8. Tg of the cured BADCy and MWNTs/BADCy composites.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2009 683

higher than that of the cured BADCy resin. This result is

easy to be understood because of the high storage modu-

lus and aspect ratio of MWNTs. The f-MWNTs/BADCy

composite presents higher storage modulus at higher tem-

perature (2508C–3008C). Tg of the p-MWNTs/BADCy

composite is the same as that of the cured BADCy resin

and both of them are little lower than that of the

f-MWNTs/BADCy composite (see Fig. 8). It appears that

the functional groups of epoxy on f-MWNTs play a role

in increasing the interfacial adhesion between the nano-

tubes and the matrix. The chemical bonding between the

matrix and the f-MWNTs restricts the movement of the

polymer chains at high temperature.

CONCLUSION

A MWNTs/BADCy composite using epoxy group as a

bridge was prepared. The acid-K2Cr2O7 solution is an

effective treatment agent for introducing ��OH groups on

the MWNTs without cutting the MWNTs short or damag-

ing the structure of the MWNTs. The two-step functional-

ization of MWNTs is effective in achieving uniform dis-

persions of MWNTs in BADCy resin. The presence of

epoxy group on the surface of MWNTs increases the

interfacial adhesion between MWNTs and BADCy and

improves the mechanical properties of the f-MWNTs/

BADCy composite greatly. The f-MWNTs also increase

the Tg and the storage modulus of the f-MWNTs/BADCy

composite slightly.

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