Fabrication and characterization of honeycombpatterned poly (N-Vinylcarbazole) carbon nanotube composite films based on non surface modified carbon nanotube

Nghiên cứu khoa học công nghệ

FABRICATION AND CHARACTERIZATION OF HONEYCOMBPATTERNED POLY (N-VINYLCARBAZOLE)/CARBON
NANOTUBE COMPOSITE FILMS BASED
ON NON-SURFACE MODIFIED CARBON NANOTUBE
Phung Xuan Thinh*
Abstract: A micro-scale honeycomb-patterned conducting film was fabricated by
using the Poly(N-vinylcarbazole)/ multiwalled carbon nanotube (PVK-MWCNT)
composite solution. These composite solutions with different MWCNT
concentrations were facially prepared by dispensing non-surface modified MWCNT
into PVK solution under vigorous sonication for a long time. Analysis for the
material characteristics and film morphology proved the interaction between PVK
and non-surface modified multiwalled carbon nanotubes. DC conductivity of the
patterned film is remarkable even at low non-surface modified MWCNT
concentration. In addition, calcination of the honeycomb-patterned films was
conducted at 150, 250, 400, and 490 oC to study the arrangement of MWCNTs in the
patterned films and to measure the DC conductivity depending on the calcination
temperature. DC conductivity of the patterned films was increased by increasing the
concentration of MWCNTs in the composites and in the increased calcination
temperature. This simple method is promising for the preparation of similar
honeycomb-patterned conducting films.
Keywords: Honeycomb pattern; Conducting film; Multiwalled carbon nanotubes; CNTs dispersion; Poly (Nvinylcarbazole).

1. INTRODUCTION
It is well known that carbon nanotubes (CNTs) exhibit interesting mechanical (high
tensile strength and stiffness), thermal, geometrical and electrical properties combined with
a good chemical stability [1-3]. Carbon nanotubes exhibit semiconducting and metallic
properties depending on their diameter and helicity [4]. With all possible applications of
CNTs in the industry, the interaction between CNTs with other organic/inorganic moieties
is not so appreciable [5, 6]. Several attempts have been made, one of the most often used
strategies is to functionalize or incorporate such materials into a polymer backbone [7].

However, practical application of CNTs still faces the problem of good dispersion, as the
substantial intertube van der Waals attraction makes them appear in bundles and affects
their extraordinary properties. To overcome this disadvantage, functionalization methods
have been adopted to chemically modify the surface properties of CNTs [8]. However, in
these methods, functional groups are covalently linked to the surface of CNTs, making their
mechanical and electrical properties change a lot, as compared with pristine tubes [9].
Therefore, the methods that can help disperse pristine CNTs in the composites are
particularly interested because it retains the structural integrity of CNTs and their properties
are hence not disrupted, which is important for the following applications.
Honeycomb-patterned thin films have attracted considerable attention because of their
potential use in microreactors, separation processes, electronics, photonics, and
biotechnology [10, 11]. These films are fabricated by various approaches, such as
lithography, use of colloidal crystals, self-assembly, and rod-coiled copolymers [11, 12].
In the self-assembly approach, the breath-figure method is the simple and useful technique
for preparing honeycomb-patterned thin films [12]. In this way, the substrate is completely
dispersed in organic solvents such as chloroform, and then the films were fabricated by
casting these solutions under humid conditions [13, 14].

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Recently, studies indicated that the application of honeycomb-patterned thin films of
conducting polymer/ polymer composites is great potential such as improving the
performances of sensing devices, or improving the performance of solar cells and solar cell
applications [13, 15, 16]. Therefore, with its enormous potential, the study for generating
well-defined CNT and CNT-based materials architectures are not an exception. The

combination of CNTs in selected polymer matrices is an effective strategy attracting the
attention of research group in the world [17].
Among the different polymeric materials currently available, poly(N-vinylcarbazole)
(PVK) has emerged as one of the more useful materials for electro-optically active
applications, including light-emitting diodes and xerography. PVK is very useful in
electronic devices because of its chemical and thermal stabilities and its excellent
electrical properties [18]. Moreover, PVK is considered an ideal model of a nonconjugated
photoconducting polymer with strong electron-donor properties. Reactions with
vinylcarbazole are easily undertaken and can be performed in bulk, solution, suspension,
or precipitation [19].
In our earlier study [20], the honeycomb-patterned films were fabricated from PVKMWCNT composite. This material was synthesized through the oxidative polymerization
of N-vinylcarbazole monomer with ferric chloride in the presence of surface modified
MWCNT. The film was shown optoelectronic properties remarkable that can be applied in
many potential areas. In an effort to attempt preserving the structure and properties of
pristine CNTs in the composite, in this study, MWCNT without any surface modification
was dispersed at different concentrations in the composite by ultrasonication techniques.
The resulting solution is then directly used to fabricate PVK-MWCNT honeycompatterned films using breath-figure technique. The dispersion and obtained linking
between MWCNT and PVK were characterized using Fourier transform infrared
spectroscopy (FTIR), ultraviolet-visible (UV-Vis) spectra, and thermogravimetric analysis
(TGA). The ordered structures of the PVK-MWCNT polymer films were obtained and
studied using scanning electron microscopy (SEM). Further to understand the selfassembly of MWCNTs in the ordered structures, calcination studies of the patterned films
were conducted at 150, 250, 400, and 490 oC. The conductivity of the film depending on
the concentration of MWCNT and calcinations temperature was also studied.
2. EXPERIMENTAL
2.1. Materials
All the other reagents of the Poly(N-vinylcarbazole) (average Mw ~1,100,000 d=1.2
g/mL, powder, Aldrich), pristine MWCNTs (>90%, diameter =110-170 nm, length = 5-9
nm, Aldrich), methanol (≥ 99.8%, Aldrich), and chloroform (≥ 99 %, Aldrich) were used
as-received without further purification. De-ionized (DI) water was used in this experiment.
2.2. Preparation of PVK-MWCNT composite and fabrication of the honeycomb

pattern in PVK-MWCNT films
In the preparation of PVK-MWCNT polymer composites, each predetermined
percentage of PVK (1g) and 30 mL of chloroform (CHCl3) was introduced in a beaker.
The mixture was sonicated at room temperature for 3 h to produce a homogenous
dispersion of PVK in solvent. Then, the different concentrations (in wt %) of pristine
MWCNTs were added in the mixture and further vigorously sonicated continuously for 6h
until obtaining homogenous state. The target mass loading of MWCNTs in the composites
varied from 1, 5, and 10wt %. The composites with 1, 5, and 10wt % of MWCNTs in PVK
are termed in this report as PC-1, PC-5, and PC-10, respectively.

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To characterize the dispersion of MWCNT into PVK, after sonication, a part of each
sonicated solution was centrifuged at 3000 rpm for 30 minutes to precipitate the free
MWCNTs. The upper solution (supernatnat) is then collected. An excess amount of
methanol was added to the supernatant to obtain a dark brown precipitate. After filtration,
this precipitate was washed several times with distilled methanol and water, and dried in a
vacuum oven for about 48 h at 60oC. The obtained products were used for
characterizations of FTIR, UV-Vis, and TGA.
To fabricate the honeycomb pattern in the obtained PVK-MWCNT polymer
composites, the constant volumes (5mL) of each PVK-MWCNT composite solution was
cast on a glass Petri dish. To obtain a highly ordered honeycomb-patterned structure,
evaporated water was applied on the solution surface through an air pump with a flow rate
of 0.6 L/min. The macroporous film was formed by the condensation and deposition of
water droplets on the solution surface due to evaporative cooling. The temperature and

relative humidity under which the ordered structures were conducted were 25oC and 60%,
respectively. After complete evaporation of the solution under humid conditions, a dark
gray film was obtained. The overall experimental scheme for formation of the honeycomb
structure for the PVK-MWCNT composite with the assistance of water-droplet is
introduced in figure 1 [20].

Figure 1. Schematic diagram showing the fabrication of honeycomb-patterned PVKMWCNTs films, and treatment of the films by calcination.
2.3. Characterization
The infrared (IR) spectra of the samples were obtained with an FTIR spectrometer
(Perkin-Elmer Model 1600). The samples were prepared by cryogenically grinding the
synthesized polymer with KBr (polymer: KBr = 1:20) and compressing the mixture on a
disk. About 60 scans were signal-averaged at a resolution of 2 cm−1 from 4000 to 400
cm−1. The UV-Vis spectra of obtained PVK-MWCNT solution were further recorded using
a Shimadzu UV-Vis-NIR spectrophotometer (UV-3101PC). The thermal properties of the
samples were obtained by TGA (Perkin Elmer model TGA 7) in the range of 20 - 800 oC
at 10 oC /min in a nitrogen atmosphere. The DC electric measurements of the obtained
composite films were performed at room temperature using the four-probe technique with
a Keithly 224 constant current source and a Keithly 617 digital electrometer.
For the calcinations study, the honeycomb-patterned films of the PVK-MWCNT
composites were placed for about 1 h at a predetermined temperature in an electric muffle
furnace (Model C-FMD, Chang Shin Science Co.) in which the temperature is controlled
by 1oC precision. The calcination temperatures were chosen as 150, 250, 400 and 490 oC.
The films were then cooled in the oven to room temperature and removed carefully. The
structures of the films before and after calcination were analyzed using SEM (COXEM
CX-100) at room temperature. The DC electrical conductivity of the films after calcination
was also measured at room temperature.

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3. RESULTS AND DISCUSSIONS
3.1. Characterization of PVK-MWCNT polymer composites
Figure 2a shows the FTIR spectra of PVK and the PC-1, PC-5, and PC-10 composites
in the regions between 400 and 4000 cm-1. The spectra obtained for PVK and the PVKMWCNT polymer blends are consistent with the infrared spectrum of PVK that has been
reported in the literature [2023]. Ideally, pristine MWCNT does not show obvious
absorption peaks in FTIR analysis. The presence of PVK in the PVK-MWCNT
composites was supported by the appearance of FTIR peaks at 720, 745, 1223, 1327,
1450, 1625, and 3053 cm-1 in the region between 500 and 4000 cm-1. In addition, the
presence of the peaks at 2960 cm-1, and 2850 cm-1 in the composite spectra can be
assigned to asymmetric methyl stretching and asymmetric/ symmetric methylene
stretching bands for MWCNTs, respectively. It is usually assumed that these groups are
located at defect sites on the MWCNTs sidewall surface that may be caused during longterm sonication.

Figure 2. FTIR (a), UV-vis (b) spectra, and TGA curves (c) of PVK and PVK-MWCNT.
Figure 2b reports the UV-visible spectra of PVK and the PC-1, PC-5 and PC-10
composites. Absorbance peaks were observed at about 240, 262, 295, 320, and 340 nm, all
of which are similar to the PVK peak reported in the literature [24]. Interestingly, the UVvisible spectra of PC-1, PC-5 and PC-10 showed only a slight blue shift of about 5 - 6 nm
for the peaks between 260 and 295 nm. This shift in the composite absorbance bands may
be attributed to the interactions between MWCNT and the PVK polymer.
Figure 2c shows the thermogravimetric curves for PVK, PC-1, PC-5 and PC-10. PVK
shows three different stages of thermal degradation: The first one from room temperature
to 120oC probably the loss of moisture, the second from 120450oC probably due to the
loss of dopant ions from the polymer matrix; the third step is a complete degradation of
PVK which proceeds at about 450 oC. The thermal degradation process for PC-1, PC-5
and PC-10 is same as PVK, but with an increase in MWCNTs in PVK the thermal stability
increases. The weight loss of PC-1, PC-5 and PC-10 greater than that of PVK recorded in

120-450 oC range may be attributed to the decomposition of the unstable-thermal
components such as defects/dopant ions in the MWCNTs. Since MWCNTs are highly

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thermal stable, as shown in the figure, the remaining weights after 450 oC are
corresponding to the initial MWCNT weights in the composites. This indicates that
MWCNT was dispersed in the polymer network; and furthermore, the inclusion of
MWCNTs in PVK has a positive influence on its thermal property.
3.2. Pattern formation at room temperature

Figure 3. Photographs of pristine MWCNTs and PVK-MWCNT composites dispersion in
chloroform at concentration of 5 mg mL-1 after 7 days sonication.
The ordered films are formed by casting the homogeneous PVK-MWCNT composite
solution under humid conditions. The homogeneous dispersion of the composite in
chloroform may be due to the interaction of two factors: the formation of hydrogen
bonding between the functional groups on MWCNTs surface and the ‘N’ atoms of PVK
moieties [25], and the interactions between the polymer main chain and MWCNTs
sidewalls through noncovalent electrostatic and van der Waals attraction after sonication
for long times [8]. In the case of treated MWCNT, the presence of oxygen-containing
functional groups helped modified MWCNT become more active, and therefore to be
easier to engage in the bondings with PVK; Moreover, MWCNTs truncated during the
chemical treatment will be dispersed more easily into the polymer solution. For the
pristine case, there are not these factors leading to lower homogeneity. However, the
dispersion state of CNTs into composite was stable even though after a long time of

sonication treatment as shown in figure 3.

Figure 4. SEM images: top-view (left)
and cross-sectional view (right) of (a)
PC-1, (b) PC-5, and (d) PC-10 films.

Figure 5. SEM images of the films after
calcination, (a) at 150 oC, (b) 250 oC , (c) 400
o
C, and (d) 490 oC for PC-1 (left), PC-5
(center), and PC-10 (right).

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Figure 4 shows SEM images obtained at 20 μm of the ordered patterns for PC-1, PC-5
and PC-10 polymer films at room temperature. The ordered structures formed in PC-1 are
circular in nature with almost uniform pore-size. The increase of MWCNTs concentration
in the composites not only increases the pore size but also increases the irregular in the
pore arrangement as observed in PC-5 and PC-10 films. This may be due to the change of
surface tension of the composite solution caused by the MWCNT presence in the polymer
matrix. Compared with the films obtained in earlier results that using surface modified
MWCNT [20], the regularity of pores in PC-5, PC-10 is lower, and the effect of MWCNTs
concentration on the pore distribution is significantly more (considered in the same
concentration range).
3.3. Calcination on the ordered structures in PVK-MWCNT films

Figures 5 is the SEM images obtained at 20 μm for PC-1, PC-5 and PC-10 films
calcinated at 150 and 250 oC; and for PC-5 and PC-10 films calcinated at 400 oC
respectively (at low MWCNT concentration such as PC-1, it is impossible to prepare SEM
samples after calcination at temperature more than 400 oC). At the ordered structures of
the films do not seem to be affected and therefore the structures obtained in PC-1, PC-5
and PC-10 are same as that of the structures obtained at room temperature. The PVK is
responsible and contributes to the formation of ordered structures of composite that
containing MWCNT. The temperature has an effect only on the PVK since PVK is less
thermally stable as compared to MWCNTs. At higher temperature as at 490oC, according
to the thermogravimetric analysis, PVK will be disappeared completely, and skeleton
frame of the ordered structures by MWCNT will be exposed visually and therefore the
nature of the films were analyzed. Clearly, MWCNTs were distributed uniformly in the
film even though without any chemical modification. This demonstrated to the effective
dispersion of pristine MWCNTs in the composite under long-term sonication condition.
3.4. DC conductivity of the ordered structures in PVK-MWCNT films
The DC conductivity of the PC-1, PC-5 and PC-10 films was measured after obtaining
the ordered structures both at room temperature and also after calcination. The results are
indicated in table 1.
Table 1. DC conductivities of PC-1, PC-5, and PC-10 films
before and after calcination at different temperatures.
Materials At room temperature
After calcination at different temperatures (S.cm-1)
(S.cm-1)
150 oC
250 oC
400 oC
PC-1
PC-5
PC-10

2.15x10-4
0.65x10-2
3.22

2.54x10-4
0.71x10-2
3.68

0.036
1.25
8.62

xxx
xxx
325.5

xxx- film has broken and not possible to obtain conductivity
It is similar to earlier study [20], it was not possible to obtain the conductivity data at
400 and 490oC after calcination of the films, because these films were very sensitive to
handle and are easily breakable. Obviously, even at room temperature the conductivity
values of the PC-5, PC-10 films were thousands times greater than the corresponding
values of the films used modified MWCNT that noted in previous reports [20]. After
calcination, the difference was becoming increasingly clear (it was increasing
exponentially). This can be explained by the preservation of the structure and the unique
properties of pristine MWCNT compared to modified MWCNT. At the higher calcination
temperature, the structure of PVK gradually degraded, thus, the conductivity of the film

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will tend to rise and reach of MWCNT conductivity value. After calcination at 250oC, the
value recorded for the film containing 10% wt/wt of non-modified MWCNT is 8.62 S.cm1
[20], while this value is only 0.065 S.cm-1 for modified MWCNT case. After calcination
at 400oC, these values are 325.5 S.cm-1 [20], and 0.255 S.cm-1 respectively for the nonmodified- and modified MWCNT composite.
The increase in DC conductivity of the patterned PVK-MWCNT composite films in
room temperature and after calcination supports the uniform distribution of MWCNT in
the PVK polymer composites. The honeycomb-patterned structures formed in the PVKMWCNT composite films are due to PVK. During the formation of PVK-MWCNT
polymer films, the polymer PVK wraps MWCNT. The study on the remained honeycomb
structures after calcination provides the valuable information how MWCNTs are arranged
during the formation of ordered structures in the PVK-MWCNT composite films.
4. CONCLUSION
PVK-MWCNT composite solutions were prepared by simply dispensing non-surface
modified MWCNTs into PVK solution under long-term vigorous sonication. The
honeycomb-patterned films were obtained at room temperature under humid conditions by
assistance of water droplet and the obtained ordered structures were calcinated. After
calcination, the ordered structures remained nearly the same before the calcination. DC
conductivity of the patterned films was also greatly increased by the increase in the
MWCNT concentration in the films compared to the case of modified MWCNT. The
research results gave useful information to help choose how to fabricate the films based on
specific requirements. The film based on modified MWCNT is suitable for highly ordered
porous and uniform structure requirements, while the fabrication method using pure CNTs is
a good idea for preserving unique properties of CNT material. The inclusion of pristine
MWCNT in PVK-MWCNT composites not only increased thermal stability but also
increased the conductivity of PVK. Due to the specific photo-physical properties of PVK
which is widely used as an electro-optical active polymer, the honeycomb patterned films of
the PVK-MWCNT composite are good candidates for applications in electronics and optics.

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TÓM TẮT

NGHIÊN CỨU CHẾ TẠO VÀ ĐẶC TRƯNG TÍNH CHẤT MÀNG MỎNG DẪN ĐIỆN
CẤU TRÚC XỐP TỔ ONG POLY (N-VINYLCARBAZOLE)/CARBON NANOTUBE
TRÊN CƠ SỞ CARBON NANOTUBE KHÔNG BIẾN TÍNH
Màng mỏng dẫn điện Poly(N-vinylcarbazole)/ multiwalled carbon nanotube
(PVK-MWCNT) cấu trúc tổ ong được chế tạo từ MWCNTs chưa biến tính phân tán
trực tiếp vào dung dịch PVK thông qua kỹ thuật khuấy siêu âm đơn giản. Các phân

tích về đặc trưng vật liệu, cấu trúc và hình thái học của màng mỏng xác nhận sự
phân bố về liên kết của MWCNTs vào trong cấu trúc mạng polymer PVK. Độ dẫn
điện của màng mỏng cấu trúc tổ ong là đáng chú ý ngay cả ở nồng độ MWCNT
thấp. Kết quả khảo sát cấu trúc và độ dẫn điện của màng mỏng ở các nhiệt độ 150,
250, 400, và 490 oC cho thấy rõ sự phân bố của MWCNTs trong màng mỏng cũng
như quy luật phụ thuộc của độ dẫn điện màng mỏng vào nhiệt độ. Theo đó, độ dẫn
điện của màng cấu trúc tổ ong tăng theo sự gia tăng của nồng độ MWCNTs không
biến tính và sự gia tăng của nhiệt độ. Phương pháp đơn giản này có nhiều triển
vọng để chuẩn bị chế tạo các màng mỏng dẫn điện tương tự.
Từ khóa: Tạo hình tổ ong; Màng dẫn điện; MWCNT; Phân tán CNT; Poly (N-vinylcarbazole).

Received 24th July 2020
Revised 5th October 2020
Published 5th October 2020
Author affiliations:
Training Department, Academy of Military Science and Technology.
*Email:

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