Functionalization of Multi-Walled Carbon Nanotubes by Electrografting of Polyacrylonitrile

Functionalization of Multi-Walled Carbon Nanotubes

by Electrografting of Polyacrylonitrile

Petar Petrov,*a Xudong Lou, Christophe Pagnoulle, Christine Jerome, Cedric Calberg, Robert Jerome*

Center for Education and Research on Macromolecules (CERM), University of Liege, Sart-Tilman, B6, 4000 Liege, BelgiumFax: (þ32) 4-3663497; E-mail: [email protected]

Received: February 9, 2004; Revised: March 8, 2004; Accepted: March 8, 2004; DOI: 10.1002/marc.200400055

Keywords: carbon nanotubes; dispersions; electrochemistry; electropolymerization; surfaces

Introduction

As a result of their unique architecture and remarkable

mechanical and electrical properties,[1,2] carbon nanotubes

(CNTs)[3] have great potential that remains, however, un-

exploited because of poor dispersibility in liquids and

problems of processability. Major efforts have, therefore,

been devoted towards any modification of the CNTs that

could improve their handling.[4,5]

Typically, chemical modification of CNTs is based on

severe oxidation processes that often damage the tubes.[5]

Mild and single-step electrochemicalmodification of CNTs

is a valuable alternative to the oxidative treatment. There

are several reports in the scientific literature on the elec-

trochemical functionalization of CNTs by electroreduction

of diazonium salts,[6–8] and on the halogenation ofMWNTs

by electrolysis.[9]However, the grafting of polymers instead

of low-molecular-weight compounds onto CNTs by an

electrochemical process is a possible strategy for dispersing

CNTs in polymer matrices and improving the mechanical

properties of CNT-based nanocomposites.

For the past few years, we have been interested in the

electrografting of insulating polymer films onto cathodic

surfaces, for example, nickel and glassy carbon, provided

that the potential and solvent are properly selected.[10–15]

These films are designated as ‘‘grafted films’’, because they

have the unique property of remaining attached to the

electrode surface even when they are prepared and kept in a

solvent in which the polymer is highly soluble. The

extension of this technique to CNTs has been explored

with acrylonitrile as monomer.

This communication aims at reporting the functionaliza-

tion of multi-walled carbon nanotubes (MWNTs) by the

electropolymerization of acrylonitrile.

Experimental Part

Commercially available carbon vapour deposition (CVD)MWNTs (supplied by ‘‘Nanocyl S.A.’’ Belgium; up to 50 mm

Summary: Multi-walled carbon nanotubes (MWNTs) havebeen successfully modified with polyacrylonitrile (PAN) bya cathodic electrochemical process. The surface-modifiedMWNTs afforded are then dispersible in good solvents forPAN, such as N,N-dimethylformamide (DMF). Collectedfrom a dilute dispersion, theseMWNTs are essentially disen-

tangled, as confirmed by transmission electron microscopy(TEM) analysis. From the differential scanning calorimetry(DSC) traces for polyacrylonitrile and polyacrylonitrile-grafted MWNTs, the maximum grafting ratio is estimatedat 0.28.

Electrochemical grafting of polyacrylonitriles onto the surface of multi-walled carbon nanotubes.

Macromol. Rapid Commun. 2004, 25, 987–990 DOI: 10.1002/marc.200400055 � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication 987

a Permanent address: Institute of Polymers, Bulgarian Academyof Sciences, ‘‘Akad. G. Bonchev’’ 103A, 1113 Sofia, Bulgaria;E-mail: [email protected].

longwith an inner diameter of 2–7 nm, an outer diameter of 3–15 nm, and a purity higher than 60 wt.-%) were used in thisstudy without further purification. The as-received MWNTswere previously flame dried under reduced pressure and usedeither as native entangled CNT-bundles, or after dispersion inN,N-dimethylformamide (DMF) by ultrasonication. Acryloni-trile was dried with CaH2 and distilled just before use. DMFwas dried for 5 d over P2O5, distilled at 60 8C under reducedpressure, and stored under dried nitrogen in a glove-box.Tetraethylammonium perchlorate (TEAP) was dried by over-night under vacuum at 80 8C.

The electrografting experiments were carried out in a one-compartment cell with a platinum counter-electrode in a glove-box under a dry inert atmosphere. Because of the drasticanhydrous conditions needed for electrografting, the use ofconventional reference electrodes is precluded and the poten-tials have to be measured against a Pt pseudo-referenceelectrode. Therefore, it is difficult to compare the absolutevalues of the electrochemical potentials for processes occur-ring in different media. A PAR potentiostat (EG&G, model273A) was used. Zn was the working electrode, either spoon-shaped and containing the MWNTs or used as a plate, and wasdipped in the stirred dispersion of MWNTs that come inintermittent contact with it.

Differential scanning calorimetry (DSC) was carried outwith a TA Instruments DSC Q100 thermal analyzer in the 25–350 8C range, at a 10 8C �min�1 rate. A drop ofMWNT disper-sion was deposited on a transmission electron microscopy(TEM) copper grid (3.05 mm, 200 mesh) coated with aFormvar film, and the solvent was allowed to evaporate. APhilips CM 100 apparatus, equipped with a Gatan 675 CCDcamera for digital imaging,was used, at an accelerating voltageof 100 kV.

Results and Discussion

Commercially available CVD MWNTs were used in this

study without further purification. Purification by acids is a

possible way to remove metal residues. However, car-

boxylic and hydroxyl groups can be formed at the surface,

which might be detrimental to the electrografting of the

polymer from the MWNTs surface.

The electropolymerization of acrylonitrile (5.0 M) in

N,N-dimethylformamide, that is, a good solvent for poly-

acrylonitrile, with tetraethylammonium perchlorate

(0.05 M) added as a conducting salt, was carried out over

the �0.8 to �2.3 V potential range under a nitrogen

atmosphere at room temperature. In Figure 1 the voltam-

metric curves for acrylonitrile with or without MWNTs are

shown. It is known that acrylonitrile cannot be grafted onto

zinc under these reaction conditions.[11] In absence of

MWNTs, the current is very low in the potential range from

�0.8 to�1.8 V, which indicates that no electroreduction of

acrylonitrile occurs (Figure 1, curve C). At more cathodic

potentials the current intensity increases, and acrylonitrile

is polymerized in solution. When the electrolytic solution

contains dispersed MWNTs, a current peak is observed in

the �0.8 to �1.8 V potential range, which is the electro-

chemical signature for the grafting of an insulating polymer

at the surface ofMWNTs (Figure 1, CurveA). If the scan of

the cathodic potentials is repeated, the current intensity is

lower, consistent with the partial passivation of theMWNTs

surface during the first scan.

The polyacrylonitrile-grafted MWNTs were separated

from the reaction medium (solution of conducting salt and

non-grafted polyacrylonitrile in DMF) by centrifugation

and extensively washed with DMF, that is, a good solvent

for polyacrylonitrile. They were then dispersed in DMF by

ultrasonication for 3 min. Dispersions of non-modified

MWNTs were used as references. In Figure 2A the stability

of the dispersions 7 d after ultrasonication is compared.

The visual aspect of the dispersions is drastically diffe-

rent depending on whether theMWNTs are surface modifi-

ed or not. The reference dispersions are totally unstablewith

a solid precipitate and a nearly transparent liquid super-

natant. In sharp contrast, the polymer modified MWNTs

form a stable dispersion, which makes the solvent deeply

black. The solution colour typically serves as a visual indi-

cator for the stability of the carbon nanotubes dispersion, as

a result of the surface modification. As an additional piece

of and more direct evidence for the polymer electrografting

onto carbon nanotubes, the surface-modified tubes were

analyzed byTEM(Sample 2, Figure 2A).A fewdrops of the

MWNT dispersion were deposited onto a carbon-coated

copper grid and the solvent was allowed to evaporate.

Figure 2B shows individual nanotubes, in sharp contrast to

densely entangled pristine MWNTs dispersed in DMF

(Sample 1, Figure 2A). Undoubtedly, polyacrylonitrile

chains immobilized at the surface of the nanotubes form an

effective steric barrier against reagglomeration. In order to

confirm that the dispersion stability does not merely result

from interaction of the carbon nanotubes with polyacrylo-

nitrile, dispersions of pristineMWNTswere prepared under

Figure 1. Voltammetry of acrylonitrile, with and withoutMWNTs, in a 0.05 M TEAP solution in DMF, n¼ 20 mV � s�1:(A) first scan with MWNTs; (B) second scan with MWNTs; (C)scan without MWNTs.

988 P. Petrov, X. Lou, C. Pagnoulle, C. Jerome, C. Calberg, R. Jerome

Macromol. Rapid Commun. 2004, 25, 987–990 www.mrc-journal.de � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

the same experimental conditions, except that solutions of

preformed polyacrylonitrile in DMF were used instead of

pure DMF. The stability of the dispersions was not impro-

ved at all by the presence of polyacrylonitrile. Furthermore,

no polymer was attached to the nanotubes, after recovery

by centrifugation and/or filtration and washing. Clearly,

polyacrylonitrile has no affinity for the MWNTs under the

experimental conditions used. This statement relies on the

analysis of the nanotubes by differential scanning calori-

metry (DSC). It must be noted that polyacrylonitrile is not

completely decomposed and eliminated under the thermal

conditions needed to maintain the carbon substrate. There-

fore, thermogravimetric analysis (TGA) is not suited to the

quantitative analysis of the composition of polyacryloni-

trile-graftedMWNTs. In contrast, DSC is a reliable method

to determine the grafting ratio, defined as theweight ratio of

grafted polymer with respect to nanotube. Indeed, poly-

acrylonitrile undergoes exothermic cyclization at high tem-

peratures, which can be monitored by DSC, as illustrated in

Figure 3 (line 1). The DSC curve for polyacrylonitrile-

grafted MWNTs (trace 2) shows an exothermic peak at the

same temperature as neat polyacrylonitrile (trace 1), which

confirms the coating of the nanotubes by this polymer. From

the DSC traces for pure polyacrylonitrile and polyacryloni-

trile-graftedMWNTs, themaximum grafting ratio has been

estimated at 0.28,whichmeans that extensivegrafting is not

necessary for stabilizing MWNT dispersions in a liquid

phase. No thermal signal is detected for non-modified

MWNTs that have been dispersed in a solution of pre-

formed polyacrylonitrile and recovered by centrifugation

and washing. Finally, the electrografting of polyacryloni-

trile onto pristine MWNTs bundles rather than onto nano-

tubes predispersed by ultrasonification, does not impart

high stability to dispersions in DMF, which strongly

suggests that the electrografting occurs preferably on the

outer surface of the bundles.

Figure 2. (A) Visual observation of MWNT dispersions of: 1)PAN-grafted MWNTs in DMF; 2) pristine MWNTs in DMF; (B)Transmission electron microscopy of predispersed MWNTs afterpolyacrylonitrile electrografting; (C) TEM of pristine MWNTs(bundles).

Figure 3. DSC traces (heating rate 10 8C �min�1) for neatpolyacrylonitrile (1), PAN-electrograftedMWNTs, after puri-fication by centrifugation (2) MWNTs mixed with preformedpolyacrylonitrile in DMF, followed by centrifugation (3).

Functionalization of Multi-Walled Carbon Nanotubes by Electrografting of Polyacrylonitrile 989


Conclusion

Although Raman spectroscopy has the potential to detect

the chemical bonding of organic molecules to graphitic

carbon, the MWNTs available to us contain too many

defects for the technique to be sensitive enough. Effort will

be devoted to the search for direct proof of covalent bonding

of polymer chains to carbon nanotubes in the future.

To the best of our knowledge, this is the first report on the

contribution of electropolymerization of acrylonitrile to

the efficient surface modification of carbon nanotubes.

Because this electrografting technique is applicable to

(meth)acrylates,[15] a large variety of electrocoatings can be

envisioned (from hydrophilic to hydrophobic), which is a

substantial advantage of the method.

Acknowledgements: X.L., C.P. and R.J. are indebted to the‘‘Region Wallonne’’ for support in the framework of the‘‘Nanotechnologies’’ programme: ENABLE project. P.P. is verygrateful to the ‘‘Belgian Science Policy’’ for a postdoc fellowship.CERM also thanks the ‘‘Belgian Science Policy’’ for financialsupport in the framework of the ‘‘Interuniversity Attraction PolesProgramme (PAIV/03)-Supramolecular chemistry andsupramolecular catalysis’’.

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Functionalization of Multi-Walled Carbon Nanotubes by Electrografting of Polyacrylonitrile

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