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: abenito@icb.csic.es

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

� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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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DOI: 10.1002/marc.200800707