Synthesis and Properties of the Amino-FunctionalizedMultiple-Walled Carbon Nanotubes/polyimideNanocomposites

Yizhe Hu, Jianfeng Shen, Chen Qin, Liping Wu, Binbin Zhang, Mingxin YeDepartment of Material Science, The Key Laboratory of Molecular Engineering of PolymersMinistry of Education, Fudan University, Shanghai, People’s Republic of China

The 1,6-hexanediamine-functionalized multi-walled car-bon nanotubes(a-MWNTs)/polyimide(PI) nanocompositefilms were prepared through in-situ polymerization fol-lowed by mixture casting, evaporation, and thermalimidization. To increase the compatibility of carbonnanotubes with the matrix polyimide, a-MWNTs wasused as the filler. According to the results, a-MWNTswere homogeneously dispersed in the nanocompositefilms. With the incorporation of a-MWNTs, the mechan-ical properties of the resultant films were improveddue to the strong chemical bonding and interfacialinteraction between a-MWNTs and 4,40-oxydiphthalicanhydride(ODPA)/4,40-Oxydianiline(ODA) polyimide ma-trix. The thermal stability of the a-MWNTs/polyimidenanocomposite was also improved by the addition ofa-MWNTs. The electrical tests showed a percolationthreshold at about 0.85 vol% and the electrical proper-ties were increased sharply. POLYM. COMPOS., 30:374–380, 2009. ª 2008 Society of Plastics Engineers

INTRODUCTION

The use of single-walled (SWNT) and multiple-walled

(MWNT) carbon nanotubes(CNTs) in polymeric compo-

sites to materialize their superior properties has been

attracting much attention recently since their discovery by

Iijima in 1991 [1–5]. These nanocomposites were investi-

gated to attain enhanced mechanical features and to

achieve certain levels of electric conductivity [2, 5–8].

There is a big challenge to have them homogeneously dis-

persed in desired polymer matrices since carbon nano-

tubes are generally insoluble and severely bundled. Four

prominent methods were investigated for desired solubili-

zation: mechanical methods [9, 10], functionalization of

the CNTs [11–16], using surfactants [17] and noncovalent

modification [18–20]. Each of them has their own advan-

tages and disadvantages. It is effective to disperse CNTs

via high powered sonication, but it can also decrease the

length and alter the properties of CNTs. Surfactants work

well in aqueous solutions but are ineffective in organic

solvents. Noncovalent modification does not alter the

properties of CNTs, but it is hard to maintain dispersion.

Functionalization will change the properties of CNTs.

However, the functional groups may be the most effective

method to increase the interactions between CNTs and

polymer matrix. Shen et al. [19] found that the amino

functionalized MWNTs showed a great dispersibility in

polar solvent. Shen et al. [16] also reported that amino-

functionalized MWNTs could obviously enhance the

interfacial adhesion between the nanotubes and the resin

epoxy.

Aromatic polyimides (PI) is one of the most important

candidate polymers for a variety of applications due to its

good dielectric properties, superior thermal stability, high

strength, low color, flexibility and radiation resistance

[20]. Polyimide/carbon nanotube nanocomposites are con-

sidered promising for many uses, such as aromatic materi-

als with light weight and high-performance [6, 21, 22].

For the dispersion of carbon nanotubes into the PI, nano-

tubes were usually pretreated with acid for the carboxy-

lated functionalization [17, 23]. Qu et al. reported the

synthesis of polyimide functionalized SWNTs and

MWNTs [22]. The amino groups functionalized on the

surface of MWNTs increased the possibility for the

MWNTs to react with the polyamic acid during the imid-

ization [24]. Herein we proposed an in-situ polymeriza-

tion process of polyimide with amino-functionalized

multi-walled carbon nanotubes(a-MWNTs) for fabricating

composite films. The results showed that in the beginning

the thermal stability of the nanocomposite films decreased

with the increasing a-MWNTs content because the amido

of the a-MWNTs destroyed equal moles of the dianhy-

dride and the diamine. When the a-MWNTs content

increased to 2 wt%, the thermal stability of the nanocom-

posite films began to increase due to the unique thermal

property of the MWNTs. The mechanical properties of

the a-MWNTs/PI nanocomposite were also improved with

Correspondence to: Mingxin Ye; e-mail: mxye@fudan.edu.cn

DOI 10.1002/pc.20563

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

VVC 2008 Society of Plastics Engineers

POLYMER COMPOSITES—-2009

addition of the a-MWNTs. The electrical tests showed a

percolation threshold at about 0.85 vol% and the electrical

properties were increased sharply.

EXPERIMENTAL

MWNTs used in this study were purchased from

Shenzhen Nanotech Port (Shenzhen, China). They were

produced using the chemical vapor deposition (CVD)

method and averagely 20 nm in diameter (D) and 20–

50 lm in length (L)(the L/D ratio ¼ 1–2.5 3 103). Diamine

4,40-Oxydianiline (ODA) and dianhydride 4,40-oxydiph-

thalic anhydride (ODPA) were predried at 100 and 1508C,

respectively under vacuum. Dimethylformamide (DMF)

was distilled over calcium hydride.

Synthesis of Amino-Functionalized MWNTs

The 1,6-hexanediamine-functionalized MWNTs(a-

MWNTs) were synthesized as reported by Shen et al. [16,

19]. The final products were dried under vacuum each

time before use. Corresponding chemical reactions are

illustrated in Scheme 1.

Fabrication of the Amino-FunctionalizedMWNTs/Polyimide Composite Films

A solution of a-MWNTs in anhydrous DMF was soni-

cated for 4 h in an ultrasonic bath (50 Hz, SCS1200

Shanghai Qinchao) and was then transferred to a three-

neck round bottom flask equipped with a mechanical stir-

rer, nitrogen gas inlet and drying tube outlet filled with

anhydrous calcium chloride (CaCl2). Predried diamine

(ODA) was added to the solution, and the mixture was

stirred for 30 min. And then the mixture was stirred over-

night under nitrogen after dried dianhydride (ODPA) was

added. Solid content of ODPA-ODA polyamic acid

(PAA)-MWNT solution was 10 wt% against DMF. The

contents of a-MWNTs against PAA were 0.1–10 wt%.

This mixture was casted onto a clean glass, and evapo-

rated at 608C for 4 h followed by stepwise curing at each

temperature of 100, 200, 250, and 3008C for 1 h, respec-

tively. Finally, the solvent-free a-MWNTs/PI composite

films were obtained [21, 23, 25]. Scheme 2 shows the

fabrication of the a-MWNTs/PI composite films. The

thickness of these films is about 40 lm. The density of

MWNTs is 1.9 g/cm3. The density of pure ODPA-ODA is

1.4 g/cm3. For the weight content of 0.2–10% a-MWNTs

against matrix, the corresponding volume ratio of a-

MWNTs was 0.15–7.57%.

Measurements

Functionalization of the nanotubes was confirmed with

Fourier transform infrared spectra (FTIR, NEXUS 670

spectrometer), Raman scattering spectra (Raman, Dilor

LABRAM-1B), X-ray diffraction (XRD, D/max-rB 12

KW) and elemental analyzesis (Elementar Analysensys-

teme GmbH Elemental Analyzer Vario EL III). Disper-

sion of the MWNTs into the polymer matrix was assessed

via Raman scattering spectra (Raman, Dilor LABRAM-

1B), X-ray diffraction (XRD, D/max-rB 12 KW) and

scanning electron microscopy (SEM, Philips XL30 FEG

FE-SEM). Tensile properties were determined by an Instron

model DXLL 1000–20000 testing machine (Shanghai,

China). More than five specimens of each sample were

tested, while the mean values were calculated. Thermal

stability was investigated by dynamic thermogravimetric

analysis (TGA, TA TGA Q50) at a heating rate of 108C/

min under nitrogen. The AC electrical properties were

tested by Novocontrol Concept 40.

SCHEME 1. The strategy for the synthesis of amino-functionalized

MWNTs.

SCHEME 2. The fabrication of MWNTs/polyimide composite films.

DOI 10.1002/pc POLYMER COMPOSITES—-2009 375

RESULTS AND DISCUSSION

Synthesis of the Amino-Functionalized MWNTs

Figure 1 shows the Raman spectra of raw and amino-

functionalized MWNTs. In spectrum a, the strong bands

at around 1,575 cm21 (G mode) correspond to the

Raman-allowed phonon high frequency mode and the dis-

ordered-induced peak at around 1,328 cm21 (D mode)

may originate from the defects in the curved graphene

sheets and tube ends [26]. The D band is activated in the

first order scattering process of sp2 carbons by the pres-

ence of in-plane substitutional hetero-atoms, vacancies,

grain boundary or other defects and by finite size effects,

all of which lower the crystalline symmetry of the quasi-

infinite lattice. Because of the strongest conjugated effect

of ethylenediamine, a-MWNTs have the lower frequency

(1,326 cm21) than the raw MWNTs [19]. In case of the

R��CO��NH��R structure in the amino-functionalized

MWNTs, a peak at around 3,300 cm21 can also be found

in spectrum b, which represents the N��H stretching

vibrations [27]. Compared with the untreated MWNTs, it

is confirmed that the surfaces of the modified MWNTs

process the amino groups.

Figure 2 shows the FTIR spectra of raw, acid-function-

alized MWNTs and amino-functionalized MWNTs. The

peaks at 1,216 and 1,714 cm21 in spectrum b correspond

to C¼¼O, C��O stretching vibration. Because of the for-

mation of amide linkages, the peaks of C¼¼O, C��O

stretching vibration in spectrum c shifted to 1,660 and

1,206 cm21. The peak of C��N stretching of amide

groups is at 1,184 cm21. Compared with the O��H

stretching vibration peak at 3,380 cm21 in spectrum b,

the peak at 3,410 cm21 in spectrum c may be assigned to

the N��H stretching vibrations [16].

The elemental analysis also confirmed the amino-func-

tionalization. After the functionalization, N wt% listed in

Table 1 was raised from 0.072 to 3.613%.

Figure 3 shows the FESEM photos of raw (a) and

amino-functionalized MWNTs (b). Compared with photo

a and b, MWNTs were cut into relatively short length

aggregation in a losser state and are seen with more open

ends after the functionalization [25, 26].

Structure of a-MWNTs/PI Nanocomposite Films

From the Raman spectra of a-MWNTs/polyimide nano-

composite films in Fig. 1, the characteristic peaks of the

a-MWNTs can be obviously found when the content of a-

MWNTs increased to 5 wt%.

We used XRD to further study the structure of the a-

MWNTs/PI nanocomposite films. The XRD characteriza-

tions of a-MWNTs (curve d), the pure polyimide (curve

a), and the a-MWNTs/PI nanocomposite containing 2 and

5 wt% a-MWNTs (curve b and c) were listed in Fig. 4.

The diffraction peaks at 2y ¼ 25.8 and 42.88 were

observed in curve d, corresponding to the interlayer spac-

ing of the nanotube (d002) and the (d100) reflection of the

carbon atoms, respectively. Traces of diffraction peaks at

2y ¼ 25.88 were observed as the ratio of amino-MWNTs

increased to 2 wt%, indicating that the part of a-MWNTs

were successfully introduced into ODPA-ODA matrix. In

addition, the intensity of the peak assigned to the a-

MWNTs at 2y ¼ 25.88 enhanced with the increase of a-

MWNTs [25, 26].

The microstructure of a fracture surface of the amino-

MWNTs/PI nanocomposites was investigated by FESEM.

Figure 3c shows that the amino-MWNTs (the white dots)

were uniformly dispersed throughout the bulk of the poly-

imide matrix. Figure 3d, the enlargement of Fig. 3c,

shows a clearer vision of the dispersion.

FIG. 1. Raman spectra of raw MWNTs (a), amino-functionalized

MWNTs (b), ODPA-ODA polyimide film (c), the MWNTs /polyimide

nanocomposite films containing 2% a-MWNTs (d) and 5% a-MWNTs (e).

FIG. 2. FTIR spectra of (a) raw MWNTs, (b) acid-functionalized

MWNTs, (c) amino-functionalized MWNTs.

TABLE 1. Element analysis of raw-MWNTs and a-MWNTs.

Sample N (wt%) C (wt%) H (wt%)

Raw-MWNTs 0.072 97.49 0.541

a-MWNTs 3.613 87.68 1.355

376 POLYMER COMPOSITES—-2009 DOI 10.1002/pc

Thermal Properties of the a-MWNTs/PI Nanocomposites

Figure 5 shows the thermogravimetric profiles of the

pure ODPA-ODA polyimide, amino-MWNTs and the a-

MWNTs/PI nanocomposite films. At first, the thermal sta-

bility of the nanocomposite films decreased with the

increasing a-MWNTs content because the amido of the a-

MWNTs destroyed equal moles of the dianhydride and

the diamine. As shown in Fig. 5, the decomoposition of

FIG. 3. FESEM images of raw (a), amino-functionalized MWNTs (b), and a fracture surface of the 1 wt%

a-MWNTS/PI nanocomposite film (c: 40,0003, d: 800003).

FIG. 4. XRD spectra of (a) pure polyimide, (b, c) the MWNTs/poly-

imide nanocomposite containing 2 and 5 wt% amino-functionalized

MWNTs, and (d) amino-functionalized MWNTs.

FIG. 5. TGA curves of the pure PI, the amino-MWNTs/PI nanocompo-

sites, and a-MWNTs.

DOI 10.1002/pc POLYMER COMPOSITES—-2009 377

the a-MWNTs must have some influences on the drop of

the thermostability of the nanocomposites. But we have

already cured the a-MWNTs/PI films at 3008C for an

hour. So this may not be the main reason. The 5 wt%

decomposition temperature (Td) measured under N2

decreased from 5368C (pure PI) to 5298C (1 wt% a-

MWNTs) and the 10 wt% decomposition temperature

(Td10) also decreased. When the a-MWNTs content

increased to 2 wt%, the thermal stability of the nanocom-

posite films began to enhance due to the unique thermal

property of the MWNTs. The 5 wt% decomposition tem-

perature (Td) is enhanced with the increasing a-MWNTs

content, and shifts from 5418C (2 wt% a-MWNTs) to

5508C (10 wt% a-MWNTs), as shown in Table 2. The

10 wt% decomposition temperatures (Td10) were also

listed in Table 2.

Mechanical Properties of thea-MWNTs/PI Nanocomposites

It is widely accepted that the mechanical properties of

nanocomposites increase with the nanoparticals [25, 27–

29]. As shown in Fig. 6 and Table 3, the elongation break

and the tensile strength increased in the beginning with

the addition of a-MWNTs when the content of a-MWNTs

was lower than 1 wt%. The maximum values of the elon-

gation break and the tensile strength appeared when 1

wt% a-MWNTs were loaded. Then they both decreased

with the enhancement of a-MWNTs. Yuen’s former work

[24] showed nearly the same results as ours. Zhu et al.

[25] did the similar researches in the acid-functionalized

MWNTs/PI nanocomposites. The elongation break de-

creased with the increase of acid-MWNTs content. The

tensile strength firstly rose and then decreased with the

loading of acid-MWNTs. Our study showed some differ-

ent results. The improvement in mechanical properties

may be caused by the strong interactions between poly-

imide matrix and a-MWNTs. These well-dispersed

MWNTs may act as crosslinking points due to the amino

groups on the a-MWNTs. The a-MWNTs may begin to

agglomerate to big clusters and cause the poor dispersion

of a-MWNTs in the polyimide matrix when the content is

higher. From Fig. 6, it was obvious that the mechanical

properties of the a-MWNTs/PI nanocomposites began to

fall when the content of a-MWNTs was over 1 wt%.

When the mass percentage of a-MWNTs is 5 or 10 wt%,

the mechanical properties of the a-MWNTs/PI nanocom-

posites were even worse than the pure PI.

Electrical and Dielectric Properties ofthe a-MWNTs/PI Nanocomposites

Figure 7 shows the AC volume resistivity as a function

of a-MWNTs concentration. These results showed an

electrical percolation behavior, which has been reported

in many previous studies [12, 25, 30, 31]. The percolation

theory predicts that there is a percolation threshold at

which a conductive path is formed in the composite due

to the interconnected fillers. The volume electrical resis-

tivity of the nanocomposite is linear with (F 2 Fth) in a

logarithmic scale and the relationship is described as fol-

lows [32]:

qc ¼ q0ðU� UthÞ�t

where qc and q0 are the volume electrical resistivity of

the composite and the filler, Fth is the threshold volume

fraction of the fillers, t is the critical exponent.

This equation is valid when F [ Fth and the value of

t relies on the dimension of the lattice and the aspect ratio

of the filler in the range of 1.1–3.1. [12, 25, 30, 31, 33].

TABLE 2. Thermogravimetric analysis of pure PI and a-MWNTs/PI

nanocomposites.

Pure PI

a-MWNTs/PI nanocomposites

(a-MWNTs wt%)

0.2 1 2 5 7 10

Td (8C) 536 530 529 541 545 549 550

Td10 (8C) 565 561 558 566 567 572 575

FIG. 6. Effect of a-MWNTs content on failure elongation and tensile

strength.

TABLE 3. Mechanical properties of pure PI and a-MWNTs/PI nanocomposites.

Pure PI

a-MWNTs/PI nanocomposites (a-MWNTs wt%)

0.2 0.5 1 2 5 10

Elongation break (%) 10.15 10.53 11.18 12.04 9.43 9.08 6.57

Tensile strength (MPa) 110.717 111.906 115.475 125.527 112.557 110.172 107.660

378 POLYMER COMPOSITES—-2009 DOI 10.1002/pc

On the basis of Fig. 8, Fth was determined as to be 0.85

vol%. The results of the least-square fit are t ¼ 3.48 and

q0 ¼ 39.1O cm. This percolation threshold value is higher

than the SWNT reinforced PI [30] and the MWNTs/PI

nanocomposites. The reason may be that the interactions

between a-MWNTs and PI are much stronger than those

between acid treated CNTs and PI. On the other hand, the

percolation threshold is lower than those MWNTs/PI

composites [25, 31]. This indicates that the a-MWNTs

can be homogeneously dispersed in the matrix through in-

situ polymerization. Figure 9 shows a great decrease in

electrical resistivity with the increase of the a-MWNTs.

CONCLUSION

The amino-functionalized multi-walled carbon nano-

tubes were used to prepare the a-MWNTs/ polyimide

nanocomposites, and they were successfully prepared

through in-situ polymerization. The results showed that

the in-situ polymerization is useful to homogeneously dis-

perse the a-MWNTs into the PI matrix. Furthermore, the

study demonstrated that there is a strong interaction

between the a-MWNTs and the PI matrix. The thermal

stability of the nanocomposite films decreased with the

increasing a-MWNTs content because the amido of the a-

MWNTs destroyed equal moles of the dianhydride and

the diamine. When the a-MWNTs content increased to 2

wt%, the thermal stability of the nanocomposite films

began to enhance due to the unique thermal property of

the MWNTs. The results also showed that the percolation

threshold for the electrical volume resistivity of the a-

MWNTs/PI nanocomposites was about 0.85 vol%. Although

the percolation value is higher than that reported previ-

ously, it is much lower than other MWNTs/PI nanocom-

posites. The electrical and mechanical properties were

both proved by the addition of a-MWNTs.

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