nanofibrilar polyaniline: direct route to carbon nanotube water dispersions of high concentration
TRANSCRIPT
Communication
418
Nanofibrilar Polyaniline: Direct Route toCarbon Nanotube Water Dispersions of HighConcentrationa
Pablo Jimenez, Wolfgang K. Maser, Pere Castell, M. Teresa Martınez,Ana M. Benito*
Water dispersible nanofibrilar polyaniline (NF-PANI) provides a novel and direct route towardscarbon nanotubewater dispersions of high concentration. Carrying out the chemical synthesisof NF-PANI in the presence of carbon nanotubes (CNTs) results in an entirely nanostructurednanofibrilar polyaniline/carbon nanotube (NF-PANI/CNT) composite material that contains wellsegregated CNTs partially coated by NF-PANI. Thisnew approach is simple, fast, and inexpensive, andenables the direct preparation of stable and homo-geneous dispersions of the composites in water atconcentrations up to 10 mg �mL�1, even for thehighest CNT loadings of 50 wt.-% without theparticipation of surfactants or stabilizers.
Introduction
Polyaniline (PANI) is one of themost prominentmembers of
the family of intrinsically conducting polymers.[1] Because of
its ease of synthesis, environmental stability, and reversibly
tunable redox characteristics that allow the control of
electrical and optical properties, PANI is of great interest for
the development of novel applications in various fields.[2] A
major drawback for this polymer has always been its
problematic processing behaviour since it does notmelt and
only presents very low solubility. Many research studies
P. Jimenez, W. K. Maser, P. Castell, M. T. Martınez, A. M. BenitoInstituto de Carboquımica (CSIC), Department of Nanotechno-logy, C/Miguel Luesma Castan4, 50018 Zaragoza, SpainFax: þ34 976 73 33 18; E-mail: [email protected]
a : Supporting information for this article is available at the bottomof the article’s abstract page, which can be accessed from thejournal’s homepage at http://www.mrc-journal.de, or from theauthor.
Macromol. Rapid Commun. 2009, 30, 418–422
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have focused on the production of soluble forms of PANI
with special emphasis on water as the preferred solvent for
environmental reasons. Typical strategies towards water-
soluble PANIs comprise the use of polyelectrolitic counter-
ions,[3] hydrophylic polymer mixtures,[4] chemical modifica-
tion of PANI,[5] or copolymerization.[6] Nevertheless, a more
simple route involves the synthesis of PANI nanostruc-
tures. Recently, it has been shown that PANI nanofibres
form spontaneously during the early stages of the
chemical oxidative polymerization of aniline.[7] They can
be isolated following diverse known approaches like
interfacial polymerization,[8] unstirred mixing of reac-
tants,[9] or ultrasonic irradiation.[10] These nanofibers can
form homogeneous and stable colloidal dispersions in
water as a result of electrostatic repulsion between the
nanostructures.[11] Nanofibers not only allow an easy
solvent processing of PANI but have also been successfully
probed in applications like chemical sensors, actuators,
light weight battery electrodes, or supercapacitors.[12]
On the other hand, carbon nanotubes (CNTs) –
cylindrical nanometer-scale objects of unique mechanical,
DOI: 10.1002/marc.200800707
Nanofibrilar Polyaniline: Direct Route to Carbon Nanotube Water Dispersions . . .
thermal, electrical, electronic, and optical properties – are
currently a major research topic in the field nanoscience
and are of great interest for the development of novel
materials and devices.[13] Nonetheless, straightforward
dispersion of CNTs and their subsequent processing are
critical issues to be overcome on the way towards
technological applications. The production of water
dispersions of CNTs is even more challenging because of
their intrinsic hydrophobic nature. To circumvent this
problem addition of surfactants,[14] stabilizers,[15] and
chemical modification of the CNTs[16] have proven
successful. However, one of the most elegant approaches
is the combination of CNTs with polymers that may attach
to the CNT surface by non-covalent interactions, which
thus makes them stable in water dispersion.[17] This
strategy avoids the degradation of properties that a
chemical modification of CNTs usually entails and, more-
over, enables higher concentrations of CNTs in dispersion,
which in the case of water typically are in the range of
about 1 mg �mL�1.[17,18]
During the last years several syntheses that combine
neat PANI and CNTs have been presented, which result in
corresponding PANI/CNT composites with a wide range of
interesting properties,[19] although in none of the cases
have the reported materials shown any dispersability in
water. To prepare these materials the in-situ oxidative
PANI polymerization method in the presence of CNTs has
been proven to enhance the interaction between the
composite components.[20] Following this strategy, com-
posites with high CNT loadings were produced in our
group, which yield homogeneous dispersions in N-
methylpyrrolidone (NMP) in the emeraldine base (EB)
state.[21] In this article, for the first time we combine the
two abovementioned approaches, namely the synthesis of
PANI nanofibers (NF-PANI) and CNT polymer association
through non-covalent interactions by in-situ polymeriza-
tion techniques. Our new process has the advantage of
being simple, fast, and inexpensive, and the resulting
compositematerials can be directly dispersed intowater or
aqueous solutions. Without the use of surfactants or
stabilizers this method allows the preparation of stable
and homogeneous water dispersions at concentrations as
high as 10 mg �mL�1 even for the highest CNT loadings of
50 wt.-%.
Experimental Part
In a typical procedure, a 0.3 M solution of distilled aniline (99.5%,
Scharlau Chemie, Spain) in 1 M aqueous HCl was prepared. Multi-
walled carbon nanotubes (MWNTs) were added to the aqueous
solution in various percentages with respect to aniline. MWNTs
were straight, highly graphitized, of micrometer length, and 20 to
30 nm in diameter prepared in a home-built electric arc-discharge
apparatus under standard conditions as described elsewhere.[22]
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After sonicating the mixtures for 10 min a solution of ammonium
peroxidisulfate (APS, 98%, Sigma Aldrich) in distilled water was
added at once. Sonication was maintained for a further 2 h over a
temperature range of 15 to 20 8C. The molar ratio of aniline to APS
was 3/1 and percentages of 0, 5, 10, and 30 wt.-% of MWNTs (NF-
PANI, NF-PANI/5M, NF-PANI/10M, NF-PANI/30M) with respect to
aniline were used. The NF-PANI labeled sample that did not
contain MWNTs served as a reference material. The deep green
reaction mixtures were filtered and washed thoroughly, first
with 1 M aqueous HCl and then with acetone.
Results and Discussion
The obtained composite materials that correspond to NF-
PANI/5M, NF-PANI/10M, and NF-PANI/30M contain final
CNT loadings of 15, 22, and 49 wt.-%, respectively, as
revealed by elemental and thermogravimetric analysis
(see the Supporting Information). Interestingly, these
studies also showed that higher initial CNT percentages
improve the polymerization yield (see the Supporting
Information), an observation that has been previously
reported for PANI/MWNT composites.[23] Furthermore,
lower oxidation temperatures of the MWNTs in the
composites compared to pure MWNTs can be observed,
which is indicative of a good degree of nanotube
segregation.[24] Raman and infrared spectroscopy (see
Supporting Information) show that the obtained compo-
site materials contain PANI in its emeraldine salt (ES)
state.[25] Here, the presence of MWNTs does not lead to
appreciable shifts or modification of bands typical of HCl-
doped ES.
On the other hand, the NF-PANI/MWNT composites
revealed an entirely nanostructured morphology as can be
seen by scanning electron microscopy (SEM, Hitachi
S3400N) and transmission electron microscopy (TEM, JEOL
JSM-6400). In both SEM and TEM images of the composites
two types of structures can be appreciated: Small
elongated structures with a typical size of 300 nm length
and 100 nm in diameter that correspond to NF-PANI
obtained without MWNTs (Figure 1a,b) and a new kind of
nanostructures that consists of MWNTs partially covered
by blocks of PANI (Figure 1c,d). These additional PANI
blocks usually wrap around more than one MWNT and
have diameters between 200 and 300 nm. Their lengths
typically are shorter than the ones of MWNTs, which
usually protrude from these blocks.
The entirely nanostructured morphology confers these
NF-PANI/MWNT composite materials its most prominent
property, namely its water dispersability. All of the
composites up to a 50 wt.-% nanotube loading are readily
and completely dispersible in water by sonication of
powder materials within less than a minute. Homoge-
neous dispersions, at concentrations as high as 10 mg �mL�1, can be easily prepared. They exhibit a high stability
www.mrc-journal.de 419
P. Jimenez, W. K. Maser, P. Castell, M. T. Martınez, A. M. Benito
Figure 1. Electronic microscopy images of NF-PANI (a: SEM, b: TEM) and NF-PANI/30M(c: SEM, d: TEM).
Figure 2. UV-vis absorption of fully protonated (a) and deproto-nated (b) water dispersions of NF-PANI, NF-PANI/5M, and NF-PANI/30M.
420
for a period of weeks (no significant decay of absorbance
intensity was observed), and allowed neat UV-vis absorp-
tion spectra to be recorded over a wavelength range of 210
to 900 nm for water dispersions of the obtained
composites, in the ES state (Figure 2a) and EB state
(Figure 2b). Diluted dispersions of nanostructured compo-
sites in a doped state showed absorption spectra more
characteristic of the undoped EB state (Figure 2b), with a
blue colour instead of the green colour characteristic of the
ES state. This dedoping reaction occurs through a
deprotonation of PANI-ES carried out by water itself, a
well-known effect.[26] In order to avoid this phenomenon,
and allow the characterization of composites in the doped
state, dispersions were made in HCl aqueous solutions
(30� 10�3M). Absorption spectra of PANI-ES composite
dispersions (Figure 2a) showed the same features as HCl-
doped PANI, with absorption maxima wavelengths at 830
and 355 nm, with a shoulder at 430 nm.[27] Here, the
presence of CNTs only causes an unspecific increase of the
absorption intensity in the measured wavelength range
(because of the featureless absorbance of CNTs) and a
broadening of absorption peaks. No shift of the above-
mentioned bands is observed.
A noteworthy observation is the fast protonation–
deprotonation reaction kinetics of composite aqueous
dispersions. In UV-vis absorbance studies we observed an
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instantaneous response of the diluted
materials to changes in the pH value,
which results in a shift of the wave-
lengths of the absorption maxima. Dis-
persions at basic pH are blue and show
the characteristic absorption features of
PANI in its EB state (maxima at 350 and
690 nm). The reversibility of these
doping/undoping reactions is also con-
firmed through successive addition of
dilute acid (HCl) and base (NaOH) solu-
tions to dispersions of composites. A thin
film of the ES composites, easily obtained
by drop casting the water dispersions,
also show a fully reversible protonation
and deprotonation behaviour. On the
other hand, the NF-PANI/MWNT compo-
sites once dedoped into the EB state
water dispersions become unstable and
tend to precipitate. However, stable
acetone dispersions of EB composites
can be prepared by sonication. The NF-
PANI/MWNT composites in the EB state
also showed good solubility in solvents
like NMP, N,N-dimethylformamide,
dimethyl sulfoxide, and o-cresol, hence
behaving like PANI/MWNT composites
previously reported.[21,28] Nevertheless,
DOI: 10.1002/marc.200800707
Nanofibrilar Polyaniline: Direct Route to Carbon Nanotube Water Dispersions . . .
Figure 3. Water dispersions of NF-PANI and NF-PANI/30M, andschematic representation of the composite nanostructures insuspension.
the use of these types of solvents results in the loss of the
original nanostructured morphology and processability
from aqueous dispersions.
The main reason for the water dispersability of these
new NF-PANI/MWNT composites is the entirely nanos-
tructured morphology of the whole composite material.
Both components, the NF-PANI and theMWNTs covered by
PANI nanostructures, become stable in suspension because
of their nanometric size and the hydrophilic nature of
the charged ES state of PANI (Figure 3). It has been shown
that PANI nanofiber water dispersions are stable as
electrostatic colloids,[11] because of the repulsion between
the charged nanostructures, so we can assume that the
mechanism of stabilization for our composites is the same
and the CNTs here behave as a mere filler material. On
the other hand, rapid formation of homogeneous and
stable dispersions by sonication is also suggestive of the
good segregation of the bulk material into individual
nanostructures. The straightforward preparation and
stability of water dispersions represent a significant
improvement in the processing of PANI/nanotube com-
posites. Moreover the overall nanostructured morphology,
and the concomitant high surface/volume ratio, are
responsible for the fast protonation and deprotonation
reaction kinetics. Therefore, it can be expected that these
NF-PANI/MWNT composites would be excellent candi-
dates for application in chemical sensing devices.
Conclusion
In summary, the concept of water dispersible nanofibrilar-
polyaniline (NF-PANI) was successfully combined with
CNT–polymer wrapping by an in-situ polymerization
process. A straightforward direct synthesis results in a
novel type of entirely nanostructured NF-PANI/CNT
composite material composed of well segregated CNTs
partially coated by intrinsically hydrophilic PANI. Stable
and homogeneous dispersions of these composites in
water at concentrations up to 10 mg �mL�1, even for the
highest CNT loadings of 50 wt.-%, are readily available by a
short sonication process. The new approach is simple, fast,
Macromol. Rapid Commun. 2009, 30, 418–422
� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
and inexpensive, and results in CNT–water dispersions
whose concentrations significantly go beyond those
typically offered by employing surfactants and stabilizers,
while the final composite material itself preserves the
unique properties of both CNTs and NF-PANI. Our findings
entail enhanced processing possibilities of special interest
for the development of organic electronic devices.
Acknowledgements: Funding from the Spanish Ministry ofEducation and Science (MEC) and the European Regional Devel-opment Fund (ERDF) under projects NANOPOLICOND (MAT 2006-13167-C02-02) and NANO-TPE (MAT2007-66927-C02-01), as wellas from the Regional Government of Aragon (DGA) underConsolidated Group Programme (DGA-T66 CNN) is gratefullyacknowledged. P.J. gratefully acknowledges Fundacion RamonAreces for his Ph.D. grant.
Received: November 13, 2008; Accepted: December 1, 2008; DOI:10.1002/marc.200800707
Keywords: carbon nanotubes; dispersions; nanocomposites;polyaniline
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