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: [email protected]
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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