Indian Journal of Engineering & Materials Sciences
Vol. 21, October 2014, pp. 567-572
Preparation of MWNTs/polyaniline composite membranes by filtration and
flash welding method
Sisi Caia,b, Xiaoyan Lia*, Xia Wanga, Dengguang Yua, Jie Dingb & Yaozu Liaoc
a
School of Material Science and Technology, University of Shanghai for Science and Technology, Shanghai, 200093, China
b
School of Environment and Architecture, University of Shanghai for Science and Technology, Shanghai, 200093, China
c
School of Chemistry, University of Bristol, Bristol, England BS8 1TS, UK
Received 31 October 2013; accepted 30 May2014
This paper reports the fabrication of multiwalled carbon nanotubes (MWNTs)/polyaniline (PANi) composite
membranes by a filtration and flash welding method. MWNTs are acid treated to acquire better distribution in the composite
membranes. PANi-nanofibers with a unique photothermal property are obtained by chemical oxidative polymerization of
aniline. Dense MWNTs/PANi composite membranes are produced by filtration and a subsequent flash welding method. For
the strong shear forces during the suction process, partly aligned MWNTs could be found in MWNTs/PANi composite
membranes. Owing to the cross-linking structure of PANi nanofibers resulted by flash welding, composite membranes with
an improved wettability could be obtained by intensity flash irradiation. Furthermore, the wettability improved with the
increasing of flash times.
Keywords: Multiwalled carbon nanotubes, Polyaniline, Composite membrane, Flash welding
With the rapid increase of carbon dioxide in the
environment, gas separation membranes have been
actively researched to address this issue1. The inner
core of carbon nanotubes2 (CNTs) possess
exceptionally high transport rates, which can be
attributed to their inherent molecular smoothness3,4.
Furthermore, CNTs have extraordinary high
strength and modulus, so they are good candidates
to serve as excellent separation membranes5,6. In
recent years, CNTs/polymer composite membranes
have been studied by many researchers7-9. Polymers
having an impermeable property infiltrate the
spaces between CNTs to form a dense and
continuous matrix, such that gases will only pass
through the inner core of the CNTs, a process that
can take full advantage of their high flux and
selectivity. At the same time, oriented CNTs can
further develop this advantage.
Several routes have been developed to achieve
aligned CNTs. Hinds et al.10 grew well-aligned
multiwalled carbon nanotubes (MWNTs) in
polystyrene by chemical vapor deposition (CVD), but
the method is of difficult operations, high cost and
time-consuming. Park et al.11 fabricated aligned
single-walled carbon nanotubes (SWNTs) using an
_______________
*Corresponding author (lixiaoyan@usst.edu.cn)
electric field with a complex build, and the individual
CNTs always intend to form bundles. Kim et al.3
produced partly-aligned SWNTs membrane by
low-cost and less time-consuming filtration method,
and then polysulfone was spin-coated into the SWNTs
membrane. With the two-step method, the gas
transport properties are limited.
Polyaniline (PANi) stands out with its low-cost,
facile
synthesis,
electrical
property
and
environmental
stability.
MWNTs/PANi
nanocomposite has been extensively studied12-14.
Furthermore, PANi-nanofibers have demonstrated a
unique photothermal property when exposed to flash
irradiation15. Kaner and Liao et al.16 fabricated
CNTs/PANi composite membranes via a nonsolvent
induced phase separation (NIPS) and acquired
denser and less defective composite membranes
surfaces with a cross-linking structure after flash
welding, however, the orientated CNTs orientation
was not obtained.
In this paper, dense MWNTs/PANi composite
membranes were fabricated by filtration and a
subsequent flash welding method. With the simple,
versatile and inexpensive filtration method,
MWNTs can be partly aligned in PANi-nanofibers
during the suction process; after that, smoother and
denser MWNTs/PANi composite membranes can
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INDIAN J. ENG. MATER. SCI., OCTOBER 2014
be welded under flash irradiation using illumination
from a simple camera flash.
Experimental Procedure
Materials
MWNTs (30-50 nm in diameter and 10-20 µm in
length, ˃95 wt%) were purchased from Chengdu
Organic Chemicals Co., Ltd. Chinese Academy of
Sciences. N-phenyl-p-phenylenediamine, aniline,
ammonium peroxidisulfate (APS), sulfuric acid
(H2SO4), nitric acid (HNO3) and hydrochloric acid
(HCl) (all are analytical reagents) were purchased
from Sinopharm Chemical Reagent Co., Ltd.
Acid-treatment of MWNTs
To obtain short MWNTs, 400 mg of the raw
MWNTs was sonicated in the mixture of HNO3 and
H2SO4 (1/3, v/v, 50 mL) at 40°C for 24 h17,18, then the
suspension was poured into deionized water (1 L) to
dilute the strong acids. The acids were removed by
centrifuging until the pH value of the suspension was
about 7. Finally, 100 mL of MWNTs as a deionized
water suspension (2.5 mg/mL) was obtained.
Synthesis of PANi-nanofibers
PANi-nanofibers had been previously synthesized
by chemical oxidation polymerization19. Briefly, a
monomer solution composed of aniline monomer
(100 mg, 1.073×10−3 mol L−1) dissolved in HCl
(20 mL, 0.1 mol L−1) and an initiator of N-phenyl-pphenylenediamine (5 mg, 2.7×10−5 mol L−1) was
produced. An oxidant solution composed of APS
(61.3 mg, 2.69×10−4 mol L−1) dissolved in HCl
(20 mL, 0.1 mol L−1) was poured into the monomer
solution rapidly and shaken vigorously for 15 s. The
reaction was subsequently left undisturbed for 24 h.
The product was centrifuged and washed with
deionized water, aqueous ammonia (1×10−1 mol L−1)
and deionized water. Finally, 100 mL of PANinanofibers as a deionized water suspension
(0.62 mg/mL) was obtained.
Preparation of MWNTs/PANi composite membranes
Acid-treated MWNTs (14.4 mL) and PANinanofibers suspension (38.7 mL) were mixed and then
diluted to 300 mL using deionized water. Then the
solution was sonicated for 4 h and subsequently
filtered through a cellulose filter (0.45 µm pore size)
using a circulating water multi-purpose vacuum pump
(180 W, Gongyi City Yuhua Instrument Co., Ltd,
China). The membranes with a MWNTs content of
about 60 wt% were dried at ambient temperature and
peeled from the filter carefully. By numerous
pre-experiments, it could be found that unwelded
composite membranes with 60wt% MWNTs
contents are less defective.
The obtained unwelded composite membranes
were flash welded (Studio flash light, A8-600 Oubao
Co., Ltd) for one, two and three times, respectively,
on the surface at a distance of 4 cm by 600 W
flashlight to form welded MWNTs/PANi composite
membranes.
Characterization
The structures of the raw MWNTs, acid-treated
MWNTs, PANi-nanofibers and unwelded composite
membranes were characterized by Fourier transform
infrared spectroscopy (FTIR, Spectrum 100 Perkin
Elmer Co., Ltd, USA), using pellets of potassium
bromide in a frequency range of 450 to 4000 cm−1.
Raw and acid-treated MWNTs in deionized water
and PANi-nanofibers in deionized water (1mg/mL)
were dried and then imaged using a scanning
electron microscopy (SEM, Quanta FEG450 FEI
Co., Ltd, USA) with a field emission at 30 kV and
10 kV respectively. The cross-section (brittle
fracture produced in liquid nitrogen) of unwelded
composite membranes and surfaces of welded
composite membranes were also imaged by SEM at
20 kV. All samples scanned by SEM were coated
with a thin layer of gold. Water contact angle images
measured after the drop becoming stable for 5 s in a
static regime (contact angle measuring instrument,
DSA30 KRUSS GMBH, Germany) have been taken
of the surface of composite membranes with
different flash welding times.
Results and Discussion
FTIR and morphology of acid-treated MWNTs
Typically, carboxylic acid groups were produced
by attacking the sidewall defects of MWNTs in strong
acids, and it would cut the nanotubes into smaller
lengths. Figure 1(a) displays the obvious differences
between the acid-treated MWNTs and the raw
MWNTs. After being acid treated, MWNTs exhibit
substantial improvement of dispersibility. The FTIR
spectrum of raw MWNTs in Fig. 1(b) shows two
bands near 1640 and 1400 cm−1 owing to
C=C stretching. And acid-treated MWNTs display an
obvious band near 1211 and 1057 cm−1 that
can be attributed to C–OH stretching20, indicating the
CAI et al.: MULTIWALLED CARBON NANOTUBES
569
Fig. 1 – (a) Images of dispersion of raw (left) and acid-treated (right) MWNTs from suspensions being held one week, (b) FTIR spectrum
of the raw MWNTs and acid-treated MWNTs, (c) SEM images of the raw MWNTs and (d) acid-treated MWNTs
formation of carboxylic acid groups, which response
the improved dispersibility of MWNTs.
The SEM images in Figs. 1(c) and (d) indicate an
obvious decrease in the lengths of MWNTs after
strong acid treatment.
Morphology and FTIR spectra of PANi-nanofibers
Figure 2(a) shows the morphology of as prepared
PANi-nanofibers. The lengths of PANi-nanofibers are
approximately 300 nm, and the widths are 50 nm. As
shown in the FTIR spectrum of Fig. 2(b),
PANi-nanofibers exhibit six typical characteristic
bands at 1590, 1494, 1377, 1296, 1151 and 829 cm-1,
which can be attributed to quinoid, benzenoid, C–N
aromatic amine, –N=quinoid=N– (electron-like band)
stretching modes and aromatic C–H out-plane
bending modes21,22, respectively. The SEM and FTIR
spectrum imply that PANi-nanofibers have been
suitably
produced
by
chemical
oxidation
polymerization.
Morphology of unwelded MWNTs/PANi membranes
Fig. 2 – (a) SEM image and (b) FTIR spectrum of as prepared
PANi-nanofibers
Unwelded MWNTs/PANi composite membranes
were prepared by filtration. The diameters of the
membranes were approximately 1.4 inch (3.5 cm) as
shown in Fig. 3(a). Surprisingly, the obtained
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INDIAN J. ENG. MATER. SCI., OCTOBER 2014
unwelded MWNTs/PANi composite membrane.
Well disturbed and partly aligned MWNTs can be
found in the MWNTs/PANi composite membrane,
which can be attributed to the shear forces effect
from the solvent stream during the filteration23
together with the repulsive forces acting between the
MWNTs and the filter. While the shear forces acting
on the MWNTs are too week to get a well-aligned
MWNTs arrays. In Fig. 3(c), unwelded
MWNTs/PANi composite membranes show several
typical characteristic bands in 1641, 1491, 1400,
1290, 1211, 1124 and 1091 cm–1, which represents a
combination of the FTIR bands of acid-treated
MWNTs and those of PANi-nanofibers.
Morphology and contact angle of flash welded MWNTs/PANi
membranes
Fig. 3 – (a) Close-up view, (b) SEM cross-sectional image (white
arrow represents the orientation of MWNTs) and (c) FTIR spectra
of an unwelded MWNTs/PANi composite membrane
membranes can be bended to about 30° and exhibit
good mechanical properties. It can be imagined that
the mechanical properties are able to be further
improved when supporting matrix is introduced.
Figure 3(b) exhibits the cross-section of the
Figure 4(a) shows unwelded composite
membranes consisting of a clearly visible mixture
of MWNTs and PANi-nanofibers. After flash
welding, the surfaces of the composite membranes
become blurry and take on an obvious welded
appearance. With increasing times of flash welding,
the surfaces of the membranes are gradually welded
and display smoother and denser surfaces as shown
in Figs 4(b)-(d). It is possible that more welded
sites can be produced on the composite membranes
via increasing flash times. The process of flash
welding will cause the benzene ring to
dehydrogenate, producing a cross-linking structure,
as previously testified in membranes made by NIPS
method16,24.
PANi-nanofibers are a type of polymer with
hydrophobic property15, as is also the case with
MWNTs25.
The
unwelded
MWNTs/PANi
composite membranes have a more porous structure
that absorbs water droplets ignoring its
hydrophobic property, similar to fibrous filter
paper. As shown in Fig. 4(a)-(d), after flash
welding, the values of the water contact angles of
unwelded composite membranes increase from 6.8°
to 27.5° after a single flash and increase to 39.5°
and 41.5°, after welding with two and three flashes,
respectively. The improved contact angle can be
attributed to a less porous surface19 and the lower
polarity of membranes owing to the cross-linking
structure after flash welding. Increased PANinanofibers cross-linking upon increasing flash
welding times makes the surfaces of membranes
smoother and denser, thereby changing the
wettability of the membranes15.
CAI et al.: MULTIWALLED CARBON NANOTUBES
571
Fig. 4 – SEM and water contact angle images (θ1 = 6.8°, θ2 = 27.5°, θ3 = 39.5° and θ4 = 41.5°) of MWNTs/PANi composite membrane
surfaces flash welded for different times: (a) 0, (b) 1, (c) 2 and (d) 3 flashes at full power intensity
Conclusions
MWNTs/PANi composite membranes made by
filtration and flash welding method offer a simple,
versatile and inexpensive way to fabricate unique
membranes with denser, smoother and hydrophobic
surfaces. To improve the dispersibility and facilitate
nanotubes to align, short and functional MWNTs are
obtained by strong acids treatment. PANi-nanofibers
with a unique photothermal property have been
synthesized by adding an oxidant to an aniline monomer
with an N-phenyl-p-phenylenediamine initiator. The
acid-treated MWNTs and PANi-nanofibers suspensions
are mixed, and then the mixture are filtrated in a
vacuum condition. Upon the shear forces, the
composite membranes with partly aligned MWNTs
along the flow direction are fabricated. Because the
benzene ring in PANi nanofibers intends to
dehydrogenate, cross-linking structure can be
produced by flash welding under high intensity flash
irradiation. With fewer pores and lower polarity, the
surfaces of MWNTs/PANi composite membranes
have better wettability after flash welding.
Acknowledgements
The authors thank the National Natural Science
Foundation of China (51373100, 5140030478) and
Innovation project of the Shanghai Municipal
Education Commission (13YZ074).
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