Fabrications and structural characterization of ultra-fine carbon fibres by electrospinning of polymer blends

Article abstract of DOI:10.1016/j.ssc.2007.01.030

Here we demonstrated the ability to fabricate ultra-fine carbon fibres through electrospinning of the polymer blends of polyacrylonitrile (PAN) and polyamic acid (PAA) and subsequent thermal treatment. The good spinnability of the polymer blends was achieved by reducing the solution's viscosity by the addition of PAN. There was a close correlation between the fibre diameter developed in the spinning process and the physical properties of the carbonized fibres; the smaller diameter fibre gave rise to the higher crystallinity of the single carbonized fibre and, furthermore, a higher electrical conductivity and mechanical strength of the carbonized fibre web. The high anisotropy in the mechanical strength of the carbonized fibre webs was ascribed to the partial alignment of the resulting fibres along the winding direction through the electrospinning process.

Full text of DOI:10.1016/j.ssc.2007.01.030

Solid State Communications 142 (2007) 20–23
www.elsevier.com/locate/ssc
Fabrications and structural characterization of ultra-fine carbon fibres by
electrospinning of polymer blends
C. Kima,∗, Y.J. Choa, W.Y. Yuna, B.T.N. Ngoca, K.S. Yanga, D.R. Changb, J.W. Leec, M. Kojimad,
Y.A. Kimd,∗, M. Endod
a
Center for Functional Nano Fine Chemicals, Chonnam National University 300 Yong-bond dong, Buk-gu, Gwangju 500-757, Republic of Korea
b
Korea Institute Industrial Technology (KITECH), Materials & Components R&D team, 971-35 Wolchul-dong, Bukgu, Gwangju, 500-460, Republic of Korea
c
Department of Chemical Engineering, Seonam University, Namwon 590-170, Republic of Korea
d
Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano-shi 380-8553, Japan
Received 4 July 2006; received in revised form 7 November 2006; accepted 30 January 2007 by B.-F. Zhu
Available online 2 February 2007
Abstract
Here we demonstrated the ability to fabricate ultra-fine carbon fibres through electrospinning of the polymer blends of polyacrylonitrile
(PAN) and polyamic acid (PAA) and subsequent thermal treatment. The good spinnability of the polymer blends was achieved by reducing the
solution’s viscosity by the addition of PAN. There was a close correlation between the fibre diameter developed in the spinning process and the
physical properties of the carbonized fibres; the smaller diameter fibre gave rise to the higher crystallinity of the single carbonized fibre and,
furthermore, a higher electrical conductivity and mechanical strength of the carbonized fibre web. The high anisotropy in the mechanical strength
of the carbonized fibre webs was ascribed to the partial alignment of the resulting fibres along the winding direction through the electrospinning
process.
c
ꢀ 2007 Elsevier Ltd. All rights reserved.
PACS: 81.07.-b
Keywords: A. Carbon fibres; B. Carbonization; D. Electrical properties; D. Mechanical properties
Carbon fibres with high specific strength, low weight and
high transport properties have been utilized as structural
reinforcements in a wide range of fields from space shuttles,
army to sports goods [1]. It is well known that the structure
and properties of the carbon fibres are highly depending
on the chemical nature of the precursors [2]. Thus, the
judicious selection of the precursors combined with the
optimized fabrication processes (i.e., spinning, stabilization
and carbonization) could be one of the ways to get high
performance carbon fibres. Unfortunately, the current spinning
systems have their intrinsic limitations for preparing ultra-
thin carbon fibres with diameters below 1 µm, and also have
problems with spinning polymer blends containing highly
different molecular structures.
To solve these problems, we have selected the recently
highlighted electrospinning system, because this technique
has proven to be a simple but powerful technique for the
preparation of fibres with sub-micrometer diameters [3,4].
In addition, the electrospun fibres hold promise in various
applications from tissue engineering, sensor, protective clothing
to high-performance filters etc. [5–8]. Up to now, there were
few reports on the synthesis of electrospun-derived carbon
nanofibres from neat polyacrylonitrile (PAN) as a flexible
polymer [9–15] and neat polyamic acid (PAA) as a rigid
polymer [16,17], respectively. Unfortunately, there were limited
studies on the electrospinning of polymer blends [18,19] even
though blending techniques enable us to tailor the diameter
and also the texture of the obtained fibrous carbons. In
this study, we demonstrated the ability to fabricate ultra-fine
carbon fibres from polymer blends (PAN/PAA) with highly
different molecular structure via an electrospinning process
and subsequent thermal treatment at 1000 ◦C in an inert
∗
Corresponding author.
E-mail addresses: ckim37@hanmail.net (C. Kim),
yak@endomoribu.shinshu-u.ac.jp (Y.A. Kim).
c
0038-1098/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2007.01.030

C. Kim et al. / Solid State Communications 142 (2007) 20–23
21
atmosphere. This study reveals that the diameter and properties
of the carbonized fibres are strongly depending on the blend
ratio of two polymers.
PAN/PAA blend solutions were prepared by dissolution
of pyromellitic dianhydride (PMDA) previously dissolved in
PAN in N,N-dimethylformamide (DMF), and followed by
the additional dissolution of benzidine in a one to one molar
ratio, and then stirred with mechanical stirrer until the reaction
completed. The blend ratios of PAN/PAA were controlled
through varying the relative weight ratios of PAN/PAA as 10/0,
7/3, 5/5, 3/7, and 0/10 to be total polymer concentrations of
10 wt% in the DMF solvent. The blend solutions were spun
into fibre webs through a positively charged capillary, using an
electrospinning apparatus at dc 20 kV (NT-PS-35K, NTSEE
Co., Korea). The negative electrode was connected to a drum
winder collecting the fibre web. The electrospun fibres were
collected as a web on an aluminium foil wrapped on a metal
drum (diameter = 16 cm) with a rotation speed of ca. 300 rpm.
The webs were stabilized by heating the samples at a rate of
◦
◦
1 C/min up to 280 C and then holding for 1 h under an air
current. The stabilized fibre webs were carbonized at a rate of
5 ◦C/min up to 1000 ◦C in a tubular electrical furnace under an
argon atmosphere.
The fibre morphology and diameter distribution of the
PAN/PAA polymer blends-derived electrospun fibres are
studied using field emission-scanning electron microscope
(FE-SEM) (note that count number is over 100). Pure
PAN-derived electrospun ultra-thin fibres have a small-sized
and homogenous diameter distribution and long fibrous
morphology (Fig. 1(a)). On the other hand, pure PAA-derived
fibres contain (Fig. 1(e)) irregularly shaped and rather short
fibrous morphology in a wide distribution of diameters ranging
from few hundred nanometers to 3 µm, possibly due to the
partial phase separation of the PAA solution. It should be noted
that we are able to obtain long and straight fibrous shapes
from the PAN/PAA polymer blends. In addition, the diameter
distributions of electrospun ultra-thin fibres increase with an
increase in concentration of PAA in polymer blends (see Fig. 1).
In order to better understand the spinnability of blend polymers,
the viscosities of the polymer solutions in DMF were measured
by a cone and plate rheometer (DV II, Brookfield Co.) with a
spindle number of 3. All polymer solutions exhibit typical non-
Newtonian behaviours that are shear-thinning with an increase
in the shear rate (Fig. 2). The viscosity values decrease as the
concentration of the PAN increased. Therefore, it is established
that the reduced average diameter of the electrospun ultra-thin
fibres is the result of the improved spinnability of polymer
blends by reducing the solution viscosity by the addition of the
PAN polymer [15,16].
Fig. 1. FE-SEM images and the corresponding diameter distributions of the
pure PAN-, the pure PAA-, the PAN/PAA blend polymer-derived fibre webs at
different blending ratios: (a) PAN/PAA = 100/0, (b) PAN/PAA = 70/30, (c)
PAN/PAA = 50/50, (d) PAN/PAA = 30/70, (e) PAN/PAA = 0/100. Arrows
in all images indicate the winding direction of the drum.
550–650 ◦C), which ranges are likely to indicate solvent
evaporation, imidization or stabilization, and carbonization,
respectively (Fig. 3). On the other hand, the weight changes
in case of pure PAN-derived fibres occur largely around
◦
◦
300 C, and stabilize for temperature region above 400 C.
The thermal behaviours of the blend polymer-derived fibres
show a combination of pure PAN and PAA. Based on the FT-
IR and TGA analyses, it is clear that the PAN/PAA blend
fibre-derived electrospun ultra-thin fibres present the typical
nature of PAN and aromatic PI materials. When heated in
nitrogen to temperatures above 700 ◦C, the carbonization yield
of PAN/PAA blend fibres show ca. 40%, indicating a relatively
high thermal stability. In addition, when they were exposed
From the FT-IR (Nicolet 520p, USA) study on electrospun
fibres, no new peak appearance indicates that only the physical
blends occur instead of a chemical reaction between PAN and
PAA in electrospun fibres. Thermal behaviours of electrospun
ultra-thin fibres in a nitrogen atmosphere were evaluated
by thermo-gravimetric analysis (TGA, STA 1640, Stanton
Redcroft). Pure PAA-derived fibres in a nitrogen atmosphere
lose weight in three ranges (i.g., below 100, 100–200 and

22
C. Kim et al. / Solid State Communications 142 (2007) 20–23
Table 1
◦
Structural and physical properties of the carbonized ultra-thin fibres (HTT = 1000 C)
Sample I.D.
b
2
f
Average fibre
diameter (nm)
υ(2θ)
R value
Electrical
conductivity (S/cm)
Carbon yield
(%)
Specific surface area (m /g)
a
c
d
e
(I /I
)
D
G
Pure PAN
300
350
500
700
24.46
20.72
20.31
20.04
19.91
1.74
2.11
2.21
2.52
2.55
12.5
10.3
8.2
5.8
2.5
39
42
44
47
62
700
680
1000
1046
–
PAN/PAA = 7/3
PAN/PAA = 5/5
PAN : PAA = 3/7
Pure PAA
>800
a
Average fibre diameter was obtained by detailed FE-SEM observations.
υ indicates the position of (002) X-ray diffraction lines.
R value is the intensity of the D band divided by the intensity of the G band from the Raman spectra.
b
c
d
Electrical conductivities (σ) are obtained by the four-point probe using this equation, σ = L/(AR), where R is the electrical resistance (Ω), A is the cross-section
2
area (cm ) and L is the distance between the electrodes (cm).
e
f
◦
Carbon yield (%) is the weight of thermally treated fibres at 1000 C divided by the weight of air oxidized fibre web.
◦
Specific surface area was measured by N adsorption (All samples were prepared under the same activation conditions (e.g., steam at 800 C for 30 min)).
2
Fig. 4. Stress–strain curves of the carbonized electrospun fibre webs in
different directions (a) Parallel to the winding direction (b) Perpendicular to
the winding direction.
Fig. 2. Variations of solution viscosities as a function of sthe hear rate for three
polymers blends of PAN/PAA = 70/30, 50/50 and 30/70, respectively.
to winding direction (Fig. 4 (b)) by using micro-strain tester
(MST-I, Shimazu). The tensile strength of the pure PAN- and
PAN/PAA (=7/3)-derived carbon fibre webs is twice as high as
that of pure PAA-derived carbon fibres, and in addition, no large
changes in the strain-at-break (ca. 2.2) are observed. Possible
explanations for this data are: (1) the decreased diameter of
a single carbon fibre, and (2) in addition, increased fibre-
to-fibre contact points in the carbonized fibre web (see athe
verage diameter of the carbonized nanofibres in Table 1). In
addition, the high anisotropy of the mechanical properties in
two different directions is ascribed to the partial alignment of
carbon fibre along the winding direction.
In order to evaluate the structural changes by thermal
treatment, we carried out Raman, XRD study and electrical
conductivity measurements for the carbonized electrospun
fibre webs. The data representing structural factors and
physical properties are summarized in Table 1. The electrical
conductivity and crystallinity (higher (002) diffraction line and
R value) increase linearly with an increasing concentration of
PAN. On the contrary, the carbon yields of the PAN/PAA blend
polymer-derived carbonized fibre increase with an increase in
PAA concentration. The improved spinnability derived from
the reduced viscosity of the polymer blend solution by adding
PAN leads to a smaller fibre diameter, due to the stronger
deformation during the electrospinning, which might give rise
to a higher alignment of molecular chains along the fibre
direction. From these experimental results, it is proved that
Fig. 3. Thermo-gravimetric analyses of the pure PAN-, the pure PAA-,
PAN/PAA blend polymer-derived fibre webs in a nitrogen atmosphere.
◦
to steam at 800 C for 30 min, blend-derived carbon fibres
exhibited the highest specific surface area of ca. 1046 m2/g
(pure PAN = 700 m2/g), possibly due to their heterogeneous
textures.
For the carbonized electrospun ultra-thin fibre webs at
1000 ◦C, we carried out a systematic analysis in terms of
their microstructures and physical properties. Fig. 4 shows
stress–strain curves for the carbonized electrospun fibre web
in parallel to winding direction (Fig. 4 (a)) and perpendicular

C. Kim et al. / Solid State Communications 142 (2007) 20–23
23
the fibre diameter controlled by the electrospinning conditions
strongly affects the physical and electrical properties of the
carbonized ultra-thin carbon fibres. Therefore, the combination
of the increased crystallinity of the single carbon fibre and
the increased fibre-to-fibre contact density of the carbonized
fibre, intrinsically due to the small-sized fibre diameter, leads
to the higher tensile strength and higher electrical conductivity
of the carbonized fibre web. Further studies have to be
done for understanding the expected two-phased textures in
a single carbon fibre because two polymeric components
might experience highly different structural changes during air
stabilization and as well carbonization.
to the higher electrical conductivity and as well higher tensile
strength of the carbonized fibre web.
Acknowledgments
This work was supported by the Next Korea Institute
of Industrial Technology (KITECH). One of us (Lee)
acknowledges the support by grant No R01-2005-000-10742-0
from the Korea Science & Engineering Foundation. This
work was supported by a Grant-in-Aid from the Ministry of
Education, Culture, Sports, Science and Technology of Japan
(Nos 16201024 and 18710084).
In summary, we successfully fabricated ultra-thin carbon
fibre webs by electrospinning PAN and PAA polymer
blends and by subsequent air-oxidation and thermal treatment
processes, and then evaluated their structural and physical
properties by using various analytic techniques. The ability
to form straight and long fibres from the polymer blend is
achieved by decreasing the solution viscosity with the addition
of PAN. This leads to the formation of the smaller fibre
diameter. The smaller in fibre diameter indicates the higher
stretching rate experienced during the electrospinning, because
it is well known that the electrically charged jet elongates in
accordance with the evaporation of solvent until its attachment
on the winding drum. More specifically, the smaller the fibre
diameter, the higher the degree of molecular chain orientations
along the fibre length within a single electrospun fibre due to
the stronger deformation during the whipping mode. The two
structural factors (e.g., fibre diameter and chain orientation)
developed in the spinning process are believe to determine
the physical properties of the carbonized fibres. The expected
higher degree of the polymer chain orientation for smaller fibre
diameters leads to the higher crystallinity (lower R value) of
the carbonized fibres. Consequently, the smaller fibre diameter
and the higher crystallinity in a single fibre combined with an
increased fibre-to-fibre contact ratio in a fibre web gives rise
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