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Carbonization of oriented polyacrylonitrile and multiwalled carbon nanotube composite films.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2008; 3: 521–526
Published online in Wiley InterScience
(www.interscience.wiley.com) DOI:10.1002/apj.167
Research Article
Carbonization of oriented polyacrylonitrile and multiwalled
carbon nanotube composite films
Ai Koganemaru,1 Yuezhen Bin,1 * Hidekazu Tohora,2 Fujio Okino,2 Shingo Komiyama,2 John Zhu3 and
Masaru Matsuo1
1
Graduate School of Humanities and Science, Nara Women’s University, Nara 630-8263, Japan
Department of Chemistry, Faculty of Textile Science and Technology, Shinshu University, Ueda 386-8567, Japan
3
Department of Chemical Engineering, University of Queensland, Australia
2
Received 17 September 2007; Accepted 1 January 2008
ABSTRACT: Composite films of polyacrylonitrile (PAN)/multiwalled carbon nanotubes (MWNTs) were fabricated
by gelation/crystallization from solution. The composite films were elongated twofold and heat-treated at 2800 ◦ C to
produce oriented carbon films. The orientation and morphology of the resultant carbon films were investigated by
scanning electron microscopy, wide-angle X-ray diffraction and Raman spectrometers. The results indicate that the
degree of graphitization of PAN was enhanced by the addition of MWNTs, at the same time, the elongation induced
the orientation of MWNTs along elongation direction and the co-orientation of MWNTs and the molecular chains of
the matrix PAN greatly promoted the carbonization of composite precursor. It is suggested that carbon nanotubes acted
as nucleation side of graphitic crystal during the carbonization of PAN-MWNTs composites.  2008 Curtin University
of Technology and John Wiley & Sons, Ltd.
KEYWORDS: polyacrylonitrile; carbon nanotubes; composites; carbon films; carbonization
INTRODUCTION
Polyacrylonitrile (PAN)-based carbon fiber has been
commercially available for over 30 years, and now
dominates nearly 90% of all the carbon fiber consumptions in the world, because of its high tensile strength
and low volume fraction of voids.[1,2] Carbon nanotubes
with high mechanical properties are expected to play an
important role in developing functional composites.[3 – 7]
Depending on their atomic structure, carbon nanotubes
behave electrically as metal or as semiconductors.[8,9]
As is well known, carbon nanotubes possess one of
the highest thermal conductivities which suggest their
use in composites for thermal management. The thermal conductivities of carbon nanotubes are highly
anisotropic and diamond-like over the length of the
tube and insulating in the transverse direction. Theory predicts a room-temperature thermal conductivity of
6000 W/mK for an isolated single-walled nanotube.[10]
Thermal transport measurements of individual carbon
nanotubes using a microfabricated suspended device
yield thermal conductivity of 3000 W/mK at room
temperature[11] The nanotubes can delay the onset of
*Correspondence to: Yuezhen Bin, Graduate School of Humanities
and Science, Nara Women’s University, Nara 630-8263, Japan.
E-mail: yuezhen@cc.nara-wu.ac.jp
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
thermal degradation of the polymer[12,13] and promote
the thermal stability,[14,15] and enable the composite to
be used for high-temperature applications.[16]
Even though the high thermal conductivity of carbon
nanotubes has been widely investigated for its application in the composite, the researches focused on its
effect on the thermal stability of the materials. In our
research, we first focused on its effect on thermal transformation during the heat-treatment of the composite at
high temperature, and investigated the role in producing
uniform high-performance films.[17,18]
In our previous article,[17] we reported fabrication of
the composite films of PAN and multiwalled carbon
nanotubes (MWNTs) by gelation/crystallization from
solution, and investigated the mechanical and electrical
properties in relation to its morphology. The influence
of carbon nanotubes on the transformation from precursor to carbon film was also studied under the stabilization and carbonization process. PAN homopolymer
prepared by the gelation/crystallization method became
flakes during carbonization processes at 1000 ◦ C. In
contrast, the composites containing carbon nanotubes
showed a good form of stability during the heattreatment at high temperature. It is also suggested that
the MWNTs within PAN matrix promoted the formation
of a condensed aromatic ladder structure during stabilization process as a catalyst and played an important
522
A. KOGANEMARU ET AL.
role in producing PAN-based graphite material with a
high degree of graphitization and high mechanical properties.
On the basis of the above results, this article mainly
dealt with the heat-treatment further at high temperature
(2800 ◦ C) for the PAN/MWNT composites to prepare
oriented graphite films. The graphitization mechanism
of PAN by heat-treatment up to 2800 ◦ C was investigated in relation to the co-orientation of MWNTs and
PAN matrix.
EXPERIMENTAL
PAN/MWNT composites were prepared by gelation/
crystallization from solutions. Solvent used was dimethyl sulfoxide (DMSO). PAN with an average molecular weight of M w = 500, 000, furnished by Mitsubishi
Rayon Co., Ltd was used as matrix, and MWNTs
named Hyperion Graphite Fibrils, the diameter being
ca 17 nm and the aspect ratio being 1–2 × 103 , were
used as fillers. The weight proportions of MWNTs
in the PAN/MWNT composites were 20 wt%, which
corresponds to the volume concentration of 11 vol%.
DMSO solutions containing PAN and MWNTs were
prepared by heating the well blended mixture at 110 ◦ C
for 30 min under nitrogen. The concentration of PAN
against solvent was fixed to 1.5 g/100 ml. The elongation was carried out at 160 ◦ C with a manual stretching device. The details were described in our previous
article.[17]
The undrawn and drawn PAN/MWNT composite
films were first heat-treated for oxidization in an atmosphere under a constant applied stress to assure the thermal stabilization.[18] The heating rate was 4 ◦ C/min up
to 150 ◦ C. Beyond 150 ◦ C, it was switched to 1 ◦ C/min
up to 330 ◦ C and kept for 60 min at 330 ◦ C, and then
cooled down to room temperature in 30 min. The stabilized films were then heat-treated in argon gas to
promote carbonization and graphitization, during which
the heating rate from room temperature to 500 ◦ C was
4 ◦ C/min and from 500 to 1000 ◦ C it was 8 ◦ C/min. The
film was maintained at 1000 ◦ C for 10 min and then
left to cool down to room temperature. As the second
step, the carbonized films were further heat-treated from
room temperature to 2200 ◦ C at 25 ◦ C/min and then
from 2200 to 2800 ◦ C at 10 ◦ C/min. The film was maintained at 2200 ◦ C for 5 min and 2800 ◦ C for 15 min,
and then cooled down to room temperature. During the
heat-treatment, the film was sandwiched between two
graphite plates.
The morphology of heat-treated specimens was
observed by field emission scanning electron microscope (FE-SEM, JSM 6700) with an energy dispersive
X-ray spectrometer (EDS). The observation was carried
out at an accelerating voltage of 5 kV. The microscopic
structure of MWNTs was observed with a transmission
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pacific Journal of Chemical Engineering
electron microscope (TEM) (JEOL, JEM-2010) operated at 200 kV.
Wide-angle X-ray diffraction (WAXD) measurements
were carried out by a 12-kW rotating-anode X-ray
generator (Rigaku RAD–rA) with a point focusing,
and the monochromatic CuKα radiation (wave-length
of 0.154 nm) was used. In this condition, an incident
beam was collimated by a collimator of 2 mm in
diameter and the diffraction beam was detected by a slit
of 0.9 mm × 0.9 mm. Corrections of X-ray diffraction
intensity were made for air-scattering, polarization and
absorption.
The Raman counts were taken with Raman scope
of JRS-System 1000 from Renishaw with He–Ne gas
laser as a light source and the standard spectrum
is 520 cm−1 (Silicon). Spectra were measured in the
1200–1700 cm−1 range. Five-second accumulation was
taken with a laser power of 100 mW.
RESULTS AND DISCUSSION
Figure 1 shows the WAXD photos (through view) for
the undrawn (λ = 1) and drawn (λ = 2) PAN/MWNT
films in the as-prepared state [patterns (a) and (b)] and
after heat-treatment at 2800 ◦ C [patterns (c) and (d)].
For the as-prepared specimens, the weak diffraction
from the (002) plane of MWNTs could be observed,
although it was very weak because of its low graphitization degree.[5] Besides the (002) reflection of MWNTs,
the strong overlapped diffraction peak from the (200)
and (110) planes of PAN crystallite appeared at 17.0◦ ,
and also the week overlapped diffraction peak from
the (310) and (020) planes appeared at 29.5◦ .[19] For
the as-prepared samples with λ = 2, the diffraction pattern shows arcs indicating the preferential orientation
of PAN crystallites along the stretching direction. At
the same time, the indistinct diffraction arcs from the
(002) planes of MWNTs could be observed, denoting
the alignment of MWNTs along the elongation direction with the orientation of PAN crystallites. By heattreatment at 2800 ◦ C, the overlapped (002) diffraction
ring from the graphite crystallites transformed from
PAN and the diffraction from MWNTs can be observed
clearly, as shown in pattern (c). For the elongated composite film carbonized at 2800 ◦ C (λ = 2) as shown in
pattern (d), the diffraction from (002) planes indicates
the evident orientation of the resultant film. The strong
(002) reflection means the progressive graphitization at
the oriented state. Comparison of pattern (c) with pattern (d) indicates that the oriented graphitic film could
be produced from the oriented film precursor.
Here, we have to emphasize that the form stability
of specimens was improved greatly by the addition of
MWNTs. The gel film of PAN homopolymer became
flakes under the heat-treating process at a temperature
higher than 1000 ◦ C in an inert gas, even though the
Asia-Pac. J. Chem. Eng. 2008; 3: 521–526
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
CARBONIZATION OF ORIENTED PAN/MWNT COMPOSITE FILMS
Figure 1. WAXD patterns of PAN/MWNT composites in the
as-prepared state and carbonized state: (a), (b) as-prepared
films; (c), (d) carbonized at 2800 ◦ C.
flakes have a smooth surface as shown in image (c).
The PAN/MWNT composite films, however, could keep
a stable form while being treated at any temperature in
argon atmosphere. It was suggested that the networks of
carbon nanotubes behave as the frameworks of the precursor and are helpful in maintaining the form stability
during carbonization and graphitization process.
To confirm the promotion effect of graphitization
degree by the addition of MWNTs as well as the orientations of MWNTs and PAN matrixes quantitatively,
the intensity distributions of WAXD were measured for
the PAN homopolymer and PAN/MWNT composites.
Figure 2 compares the results for the undrawn (λ = 1)
and drawn (λ = 2) specimens heat-treated at 2800 ◦ C.
Curve (b) of PAN homopolymer treated at 2800 ◦ C
shows a similar profile as curve (a) of MWNTs themselves, indicating a low graphitization. Curve (d) of
the undrawn composites film (PAN/MWNT, λ = 1)
shows the magnitude of the diffraction peaks of the
(110) + (101) and (110) + (112) peaks higher than the
peaks of curve (b), and the half-width of peak (002)
decreased slightly from 2.4◦ to 2.3◦ . This indicates an
effective progression of the graphitization by the introduction of MWNTs. For the elongated sample (λ = 2),
the diffraction from the (002) planes becomes extremely
sharper indicating the drastic promotion of the graphitization degree. The influence of orientation by the introduction of MWNTs, in comparison with the (002) plane
of curve (c) is absolutely significant. The half-width of
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
Figure 2. WAXD intensity distributions measured for the
original MWNTs, and heat-treated PAN and PAN/MWNT
composites films.
the diffraction peak from (002) plane of curves (e) and
(c) became 1.8◦ and 2.1◦ , respectively. They are much
narrower than that for the undrawn samples. This suggests the formation of bigger graphitic crystallites and it
is attributed to the great improvement in graphitization
of PAN because of the orientation.
The morphology of the carbonized PAN and PAN/
MWNT films was observed by FE-SEM. Figure 3
shows morphology of the MWNTs and the surface of
the undrawn films. Photo (b) shows the MWNTs heattreated at 2800 ◦ C, in which no obvious change can be
observed in comparison with the original one of photo
(a). Photo (c) shows the carbonized PAN homopolymer
with a smooth surface. Photo (d) is the carbonized PANMWNTs composites and photo (e) is the enlargement of
the white frame of photo (d). As observed in photo (e),
MWNTs kept the network structure in the carbonized
composite. Interestingly, from the detailed observation
on the pattern (e), we can find that MWNTs in heattreated composite films were thicker and rougher than
the original state as shown in image (a), where each
MWNT in the composite seems to be covered by a
layer of carbon.
The corresponding cross-section microscopic images
of PAN and PAN/MWNT films heat-treated at 2800 ◦ C
are shown in Fig. 4. A number of carbon grains
are observed in photo (a), but such noncontinuous
Asia-Pac. J. Chem. Eng. 2008; 3: 521–526
DOI: 10.1002/apj
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A. KOGANEMARU ET AL.
Asia-Pacific Journal of Chemical Engineering
Figure 5. SEM images of the cross-sections and surfaces of
the elongated composite films heat-treated at 2800 ◦ C: (a),
(b) cross-sections; (c), (d) surfaces.
Figure 3. SEM images for MWNT and the surfaces of the
undrawn films heat-treated at 2800 ◦ C. Photo (a) original
MWNTs, (b) heat-treated MWNT, (c) heat-treated PAN,
(d) heat-treated PAN/MWNT composite, (e) magnification of
(d).
Figure 4. SEM images of the cross-sections of the undrawn
films heat-treated at 2800 ◦ C: (a) PAN and (b) PAN/MWNT.
structure could not maintain the monolithic shape as
a bulk specimen. The addition of MWNTs provided a
continuous structure as shown in photo (b), and this is
the origin for the great improvement of form stability
of the carbonized composites.
Figure 5 shows the FE-SEM patterns observed for
the oriented PAN/MWNT composite films treated at
2800 ◦ C (λ = 2). The micrograph of the cross-section
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
of the carbonized composite film is shown in photo
(a), and its magnification in photo (b). It can be seen
that, as a whole, the carbonization occurred uniformly
and bulky carbon grains were formed during the heattreatment. Photos (c) and (d) show the micrographs
of the surface. The alignment of MWNTs covered
by carbon layer could be observed clearly along the
elongation direction. Even though the orientation of
MWNTs with respect to the stretching direction is not
so significant because of the low draw ratio (λ = 2),
image (c) shows that the orientation of MWNTs led to
the formation of oriented carbon layers, i.e. the carbon
layers grew along the elongation direction under the
heat-treatment process. The networks of MWNTs were
retained and they were covered by carbon layers.
The above results observed by SEM suggest that the
MWNTs may form cross-linking structures with preferential orientation with respect to the stretching direction
during the elongation of composites, and the MWNTs
with high thermal conductivity acted as nucleation side
of graphitic crystal during the carbonization processes
of the PAN, and promoted the transformation from PAN
to turbostratic carbon or graphitic carbon.
Figure 6 shows the TEM patterns of random MWNT
networks in the original state (a) and that of the
composite film heat-treated at 2800 ◦ C (b). The figure
clearly shows that the networks of MWNTs were
random in the original state, and those in the oriented
Asia-Pac. J. Chem. Eng. 2008; 3: 521–526
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
CARBONIZATION OF ORIENTED PAN/MWNT COMPOSITE FILMS
Figure 6. TEM micrographs of MWNTs and the elongated
PAN/MWNT composite film heat-treated at 2800 ◦ C. (λ = 2).
carbon films disentangled partly and aligned along the
elongation direction.
To obtain more detailed results for the heat-treatment
effect of MWNTs and PAN/MWNT films, Raman spectra were studied. Figure 7 shows the Raman spectra
curves (a) and (b), for the original and heat-treated
MWNTs at 2800 ◦ C, respectively. Two peaks at around
1582 cm−1 (G-band) and around 1320 cm−1 (D-band)
are observed for both the samples. Comparing spectrum (a) with spectrum (b), the peak of G-band of curve
(b) is much sharper and its half-width is much narrower
indicating a much more perfect graphitic structure.
Figure 8 shows the Raman spectra for (a) the undrawn
(λ = 1) and (b) the drawn (λ = 2) PAN/MWNT composite films heat-treated at 2800 ◦ C. For the undrawn
sample (λ = 1), the intensity ratio of the G-band at
around 1580 cm−1 to D-band at around 1320 cm−1 is
much higher than that of the original MWNTs shown in
spectrum (a) in Fig. 7, but is slightly lower than that of
the heat-treated MWNTs as shown in spectrum (b) in
Fig. 7. On the other hand, for the elongated sample
(λ = 2), the intensity ratio of G-band to D-band
increased further and became higher than that for
the heat-treated MWNTs indicating that the elongation
caused a high graphitization degree of PAN. Besides
this, a shoulder in the vicinity of G-band at 1610 cm−1
could be plainly observed in both the undrawn and
drawn samples. As suggested by Bacsa et al .[20] the
shoulder is attributed to the structural defects. The
shoulder for the drawn sample is smaller than that
observed from the undrawn one. It also proved that
the structure became more perfect through orientation
before heat-treatment. These results are in agreement
with the WAXD results; shown in Figs 1 and 2.
CONCLUSIONS
Figure 7. Raman spectra for the original MWNTs
and MWNTs heat-treated at 2800 ◦ C.
Figure 8. Raman spectra for the undrawn and
drawn PAN/MWNT composite films heat-treated
at 2800 ◦ C.
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
PAN/MWNT composite films were fabricated by gelation/crystallization from solution in this study. The composite films were elongated twofold and heat-treated at
2800 ◦ C to form oriented graphitic films. The observation of morphology of carbonized composite film from
SEM and X-ray diffraction indicated that the MWNTs
may form cross-linking structures with the preferential orientation along the stretching direction, and the
networks of MWNT in composites are very helpful to
improve the form stability of carbon films.
The oriented graphitic film could be produced from
the oriented film precursor. The degree of graphitization
of PAN was enhanced by the addition of carbon
nanotubes, and greatly promoted by the orientation of
the carbon nanotubes and the molecular chains of the
PAN matrix. Under SEM observation, the MWNTs were
found to be covered by the carbon layers and oriented
along the elongation direction. It may be suggested
that MWNTs acted as nucleation side of graphitic
crystal during the carbonization of the PAN/MWNT
composites, and thus promoted the transformation from
PAN to turbostratic carbon or graphitic carbon.
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DOI: 10.1002/apj
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