A novel nanocomposite from multiwalled carbon nanotubes functionalized with a conducting

polymer

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INSTITUTE OF PHYSICS PUBLISHING SMART MATERIALS AND STRUCTURES

Smart Mater. Struct. 13 (2004) 295–298 PII: S0964-1726(04)74805-3

A novel nanocomposite from multiwalledcarbon nanotubes functionalized with aconducting polymerBiju Philip, Jining Xie, Anupama Chandrasekhar, Jose Abrahamand Vijay K Varadan

Center for Engineering of Electronics and Acoustics Materials and Devices,The Pennsylvania State University, University Park, PA 16802, USA

Received 15 December 2003Published 18 February 2004Online at stacks.iop.org/SMS/13/295 (DOI: 10.1088/0964-1726/13/2/007)

AbstractA nanocomposite of a multiwalled carbon nanotube and polythiophene wasprepared by functionalizing the nanotube surface with a polythiophene,poly[3-(2-hydroxyethyl)-2,5-thienylene], containing pendant hydroxylgroups. The composite was characterized by IR, high resolution TEM andconductivity measurements. The poly[3-(2-hydroxyethyl)-2,5-thienylene](PHET) was synthesized by the oxidation polymerization of2-(3-thienylethanol) using FeCl3 and CHCl3. Multiwalled carbon nanotubeswere synthesized by a microwave CVD method and oxidized withpotassium permanganate using a phase transfer catalyst in mild conditions.The COOH groups formed on the nanotube surface were converted to COClusing thionyl chloride and it was then condensed with the polythiophene athigh temperature in anhydrous DMF. High resolution TEM images showedthat the functionalization provided a firm coating of the conducting polymeron nanotube walls. This nanocomposite with PHET grafted to CNT showedhigher conductivity than a nanocomposite of PHET and CNT in the samepercentage weight composition prepared by ultrasonic mixing of the two.Such a material was designed and synthesized with a view to electronic andsensor applications.

1. Introduction

Carbon nanotubes (CNTs) have unique electrical andmechanical properties combined with chemical stability whichmakes them potential candidates to serve as building blocksfor several device architectures in chemical sensors, hydrogenenergy storage, field emission materials, catalyst support,electronic devices, etc [1–4]. Chemical modification and theconstruction of complex new structures from carbon nanotubeswill allow a much broader applicability of these materialswith tailor-made properties. Chemical connection of otherfunctional molecules will ultimately lead to new propertyprofiles [5, 6]. The chemical modification and the covalentfunctionalization of carbon nanotubes with organic species likelong chain alcohols and amines, dendrimers and polymers havebeen recently reported [7–9]. The exohedral functionalizationof carbon nanotubes can be done in many ways. One of themethods is the generation and functionalization of defect sites

at the tube ends and side walls by oxidation and subsequentconversion into derivatives such as amides and esters. Theoxidation of CNTs will lead to the formation of the COOHfunctional group. The carboxylic acid groups can be convertedinto acylchloride groups by treatment with thionylchloride.The acid chloride-functionalized CNTs are then susceptibleto reaction with NH2 or OH groups to give amides and esters,respectively.

The embedding of carbon nanotubes in conductingpolymeric matrices such as polythiophene, polyaniline, ansPPV, for various nanocomposites material is now a popularsubject in nanotube research. The research is fuelledby the hope of delivering the properties of CNTs to thecomposites [10–12]. Among the various conducting polymers,polythiophene (PT) has always been one of the morelikely candidates because of its high stability of doped andundoped states, ease of structural modification and controllableelectrochemical behaviour. The highly processable substituted

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B Philip et al

thiophenes, in particular, have been the subject of intenseresearch and development as candidate materials in the areasof microelectronics, electrode materials, optoelectronics andsensors [13–15]. Recently the composites of polythiophenewith CNTs were reported in the application of organic lightemitting diodes [12].

The general methods used to prepare polymer–carbonnanotube composites are:

(a) ultrasonification of carbon nanotubes or surface function-alized carbon nanotubes in the presence of matrix poly-mers;

(b) the in situ polymerization of monomers of matrix polymerin the presence of carbon nanotubes;

(c) polymerization of the matrix polymer from the surface ofthe nanotube.

Carbon nanotubes functionalized with polymers were foundto give a better dispersion of carbon nanotubes in thepolymer matrices of nanocomposites [16, 17]. Lin and co-workers reported that the use of polymers that are structurallyclose to the matrix polymer for functionalization of CNTsis a good approach for the development of polymericnanocomposites [18]. In this paper we report for the firsttime the preparation of a nanocomposite from CNTs and aconducting polymer by the grafting of a polythiophene, poly[3-(2-hydroxyethyl)-2,5-thienylene], on to nanotubes.

2. Experimental details

2.1. Materials and methods

2-(3-thienyl ethanol), DMF, chloroform and ferric chloride(all anhydrous) were purchased from Sigma Aldrich and usedas such. CNTs were synthesized in our laboratory by thedecomposition of acetylene in a microwave CVD at 600 ◦Cusing fine iron (III) nitrate as catalyst. Fourier transforminfrared (FTIR) spectra were recorded with a Digilab modelFTS60 spectrometer using KBr pellets. The conductivitywas measured using a Magnetron, Model M-700 four-probeinstrument. The high resolution TEM images were obtainedusing a JEOL 2010 F model instrument.

2.2. Synthesis of poly[3-(2-hydroxyethyl)-2,5-thienylene](PHET)

The polythiophene was synthesized by oxidation polymeriza-tion of 2-(3-thienylethanol) using ferric chloride and chloro-form [19]. The polymerization was carried out in a three-neckflask under nitrogen atmosphere. The typical synthesis proce-dures utilized can be described as follows: 3.3 g (20 mmol) ofanhydrous FeCl3 was quickly put into the flask. Then, 5 ml ofanhydrous CHCl3 was added into the reaction vessel. It is thenstirred well to form a good slurry. To this was added 0.64 g(5 mmol) monomer in 3 ml CHCl3 dropwise over for 2 h. Afterall the monomer was added into the flask, the reaction mixturewas stirred at room temperature for 8 h. The reaction mixturewas then added to methanol. The precipitated product was thenwashed repeatedly with water and methanol, and dedoped inaqueous ammonia to get a red polymer.

2.3. Oxidation of nanotubes

The nanotubes were oxidized using potassium permanganatewith the help of a phase transfer catalyst [20]. Purifiednanotubes (0.12 g) and dichloromethane (25 ml) were added toa 100 ml flask and the suspension was vibrated ultrasonicallyfor 0.5 h. Phase transfer agent (1.0 g, Aliquat 336,from Aldrich) was added, followed by powdered potassiumpermanganate (5 g) in small portions over a period of 2 h.Acetic acid (5 ml) was also added. The mixture was thenstirred vigorously overnight at room temperature. It was thenfiltered, washed with concentrated HCl and water and dried.

2.4. Preparation of nanocomposites

2.4.1. By functionalization. The oxidized nanotubes(100 mg) were refluxed with thionyl chloride (25 ml) for about15 h. After the reaction the excess thionyl chloride was distilledoff. The residue was flushed with nitrogen. To this was addedPHET (200 mg) and anhydrous dimethylformamide (25 ml).The mixture was heated with stirring at a high temperature(∼120 ◦C) for 15 h in a nitrogen atmosphere. The reactionmixture was filtered, washed with DMF and acetone anddried. To this material was added methanol, and it was thensonicated for 5 h. The methanol was evaporated off to get thenanocomposite in which the carbon nanotube is functionalizedwith the PHET, (CNT- f -PHET).

2.4.2. Without functionalization. The nanotubes (100 mg)were added to methanol and sonicated for 5 h to dispersethe nanotubes. To this was added a suspension of thePHET (100 mg) in methanol and sonicated for another 5 h.Then methanol was evaporated off to get the nanocomposite.(CNT/PHET)

3. Results and discussion

The polythiophene derivative was prepared by dehydrogena-tion condensation, i.e. chemical oxidation, of the monomer inthe presence of FeCl3 in chloroform solution. This methodis advantageous because unstable intermediates or multistepreaction processes are not required. Furthermore, the yieldof polymers is quite high [19]. The oxidation of CNTs wasdone as reported earlier. To introduce sufficient COOH grouponto the nanotube surface, they were treated with potassiumpermanganate in the presence of a phase transfer catalyst [20].The higher yield of this functionalization process with less de-struction of nanotubes is the major advantage. The COOHgroups in the nanotubes were converted to COCl by reflux-ing with thionyl chloride. The poly[3-(2-hydroxyethyl)-2,5-thienylene] was attached to this by a high temperatures conden-sation reaction in anhydrous DMF in the presence of pyridineas acid acceptor (see the scheme 1). The final weight of theproduct after washing the unreacted polymer is 200 mg. So inthe final product the approximate weight ratio of the polymerand CNT can be taken as 1:1.

The chemical bonding between nanotube and polymer wasconfirmed by FT-IR. The peaks at 1717 cm−1 and 1260 cm−1

correspond to the C=O stretch and C–O stretch of the estergroup, respectively. The functionalization was done by taking

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A novel nanocomposite from multiwalled carbon nanotubes functionalized with a conducting polymer

CO

OHCO S

OCO

Cl

S

OH

S

OH

n

FeCl3

Chloroform

SOCl2

pyridine, DMF, 120 oCrefluxCNT functionalized with polythiophene

poly[3-(2-hydroxyethyl)-2,5-thienylene] (PHET)

PHET

CNT-f-PHETSurface oxidizedCNT

Scheme 1.

a b

c d

Figure 1. High resolution TEM images of (a) MWCT,(b)–(d) CNT- f -PHET.

100 mg of CNT and 200 mg of the PHET. The TEM images(figure 1) clearly showed the covering of nanotube surface bythe polythiophene in CNT- f -PHET.

The room temperature electrical conductivity of the poly-thiophene, CNT- f -PHET and CNT/PHET was determinedfrom pellets by a four-probe method. The room temperatureconductivity of the PHET was less than 10−8 S cm, of CNT-f -PHET was 38.46 S cm and of CNT/PHET was 1.38 S cm.The conductivity of the CNT- f -PHET composite was foundto be 28 times higher than that of CNT/PHET. The higher con-ductivity of CNT- f -PHET can be explained as follows.

(a) The potential interaction between CNTs and conjugatedpolymers is still intriguing, even though it can beconsidered that a physical doping of the polymer by CNTsis possible by charge transfer interactions [11, 21]. Suchinteractions can increase the electron delocalization whichenhances the conductivity of the polymer. There may bea higher such charge transfer interaction between CNTand polythiophene in CNT- f -PHET, induced by chemicalbonding.

(b) There is a greater extent of homogeneity in CNT- f -PHET composite than in CNT/PHET. Since in CNT- f -PHET composite the nanotubes were functionalized bythe matrix polymer itself, this ensures a compatibility ofCNTs with the polymer matrix and there was only a lowmicroscopic phase separation of the components, whichled to high conductivity.

4. Conclusion

Nanofunctional materials with tailor-made properties can besynthesized from preselected constituent molecular buildingblocks. In this communication we described a synthetic routefor the preparation of such a nanocomposite material in whichcarbon nanotubes were grafted with a polythiophene; this hasthe advantages over a composite prepared by simple mixing ofcomponents of homogeneity and enhanced conductivity of thecomposites.

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