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Thin Solid Films 468 (2004) 17–22

Morphology and conductivity in poly(ortho-anisidine)/

carbon nanotubes nanocomposite films

Valter Bavastrelloa,*, Victor Erokhina, Sandro Carraraa, Francesca Sbranab,Davide Riccib, Claudio Nicolinia,c

aDepartment of Biophysical M&O Science and Technologies, University of Genoa, Corso Europa 30, 16132 Genoa, ItalybDIBE, University of Genoa, Via Opera Pia 11a, 16145 Genoa, ItalycFondazione EL.B.A, Via delle Testuggini 181, 00143 Rome, Italy

Received 2 October 2003; received in revised form 20 January 2004; accepted 26 March 2004

Available online 8 May 2004

Abstract

A nanocomposite of multi-walled carbon nanotubes (MWNTs) embedded in poly(ortho-anisidine) (POAS) and pure POAS were

synthesised by oxidative polymerisation. Langmuir–Schaefer (LS) films were fabricated at air– liquid interface. The in situ doping process

was performed during the film deposition itself by spreading chloroform solutions of the materials onto 1 M HCl. The specific conductivity

of both the nanocomposite (POAS-MWNTs) and pure POAS was investigated to verify the effective enhancement provided by the insertion

of MWNTs inside the polymeric matrix. The morphology of POAS-MWNTs nanocomposite films was investigated by atomic force

microscopy. Evident relationship between the film morphology and its conducting properties was revealed.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Surface morphology; Atomic force microscopy; Conductivity; Polymers

1. Introduction

In recent years, two classes of organic materials like

conducting polymers and carbon nanotubes have gained

enormous interest for their attractive chemical–physical

properties [1–4]. Among conducting polymers, polyaniline

and its derivatives have been deeply studied in the last

decades for their good electrical properties, easy methods of

synthesis and high environmental stability [5–8]. The

chemistry of polyanilines is generally more complex with

respect to other conducting polymers. This fact is due to

their dependence on both the pH value and the oxidation

states, described by three different forms known as leucoe-

meraldine base (fully reduced form), emeraldine base (EB)

(50% oxidised form), and pernigraniline base (fully oxidised

form). The most important is the EB form and its proton-

ation by means of H+ ions generated from protic acids gives

the emeraldine salt form, responsible of the strong increment

of the conducting properties [9]. This process is reversible

and it is possible for the presence of imine groups basic sites

0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.tsf.2004.03.036

* Corresponding author. Tel.: +39-10-3538145; fax: +39-10-3538541.

E-mail address: valter@ibf.unige.it (V. Bavastrello).

located along the conducting polymer backbone [10,11].

The doping process of polyanilines is always associated to

conformational modifications of the polymeric chains, due

to the local distortions created by the addition of H+ ions to

the basic sites [12]. These distortions affect the morphology

of the deposited films by varying their organisation and play

an important role for the mere electrical properties of the

conducting polymer. An interesting application of both the

materials can be the embedding of a little quantity of carbon

nanotubes, either single walled carbon nanotubes or multi-

walled carbon nanotubes (MWNTs), inside the polymeric

matrix of conducting polymers for the fabrication of nano-

composites [13,14]. This simple method of synthesis is able

to improve the chemical–physical properties of the con-

ducting polymer. In other words, the presence of carbon

nanotubes inside the polymeric matrix can provide a me-

chanical support to the polymeric chains conformational

rearrangement. Taking into account these considerations,

the aim of the present study was to investigate the conduc-

tivity of poly(ortho-anisidine) (POAS)-MWNTs nanocompo-

site Langmuir–Shaefer (LS) films by comparing with POAS

pure conducting polymer to verify the effective enhance-

ment given by the presence of MWNTs inside the polymeric

V. Bavastrello et al. / Thin Solid18

matrix. Furthermore, a study of the morphology of the

nanocomposite thin films was performed for both the

undoped and doped forms by the acquisition of atomic force

microscopy (AFM) images.

Fig. 1. V/I characteristics for POAS pure polymer in both the undoped and

doped forms [(a) = 1 monolayer; (b) = 10 monolayers; (c) = 30 monolayers].

2. Experimental details

2.1. Materials

Monomers of ortho-anisidine, ammonium persulfate

[(NH4)2S2O8] as oxidising agent and other reagents were

obtained from Sigma while the MWNTs were purchased

from MER, Tucson, Arizona. The MWNTs employed in

the synthesis had diameters ranging between 2 and 15 nm

and lengths between 1 and 10 Am, with 5–20 graphitic

layers.

2.2. Syntheses of the nanocomposite and pure polymer

The synthesis of the nanocomposite material was per-

formed by oxidative polymerisation by dispersing 100 mg

of MWNTs in 1 M HCl (medium of reaction) in order to

have a monomer/MWNTs weight ratio of 100/1, and using a

monomer/oxidant molar ratio of 1/1 [14]. The pure polymer

was synthesised by following the same steps carried out for

the nanocomposite save for the presence of MWNTs in the

medium of reaction. The final products obtained from the

syntheses were the EB forms of POAS-MWNTs nanocom-

posite and POAS pure polymer. The EB forms were

completely soluble in chloroform.

2.3. Fabrication of LS films

The spreading solutions of POAS-MWNTs nanocompo-

site and POAS pure polymer were prepared by dissolving

5 mg of materials in 10 ml of chloroform, then filtering with

a solvent resistant filter (0.5 Am). The LS films were

deposited similar to our earlier investigation [14]. Distilled

water and 1 M HCl aqueous solution were used as sub-

phases for the deposition in order to obtained the films of

materials in the undoped and in situ doped forms, respec-

tively. Different numbers of monolayers were transferred

onto pure glass to carry out specific conductivity measure-

ments and glass hydrophobised by silanisation process for

AFM acquisitions.

2.4. Specific resistance LS films measurements

Specific resistance calculations were performed basing

on voltage/current (V/I) characteristics measured with an

electrometer by Keitley (USA) model 6517, driven by

computer. The samples were obtained by depositing 1,

10 and 30 monolayers in both the undoped and doped

forms. Contacting was set up by means of silver wires and

silver paint.

2.5. AFM imaging

AFM measurements were obtained in intermittent con-

tact mode by using a PSI Autoprobe CP microscope

(Thermomicro-scopes, Sunnivale, CA, USA), equipped with

100 and 5-Am scanners and polysilicon cantilevers (Ultra-

levers, Thermomicro-scopes, Sunnivale, CA, USA). Previ-

ous to use, both the scanners where calibrated in x, y and z

coordinates by using reference standard gratings (VLSI

Standards, CA, USA).

Films 468 (2004) 17–22

3. Results and discussion

3.1. Specific resistance LS films measurements

The study of V/I characteristics revealed linear behav-

iour for all samples, as shown in Figs. 1 and 2, differently

from the curves already seen by other authors [15]. How-

ever in this work, Schottky contacts were not formed in the

experimental set-up [15]. The linear behaviour allowed us

to calculate the specific conductivity of the materials by

Fig. 2. V/I characteristics for POAS-MWNTs nanocomposite in both the

undoped and doped forms [(a) = 1 monolayer; (b) = 10 monolayers; (c) = 30

monolayers].

V. Bavastrello et al. / Thin Solid Films 468 (2004) 17–22 19

applying the Ohm’s first and second laws. In Table 1 are

illustrated the results obtained from the specific conductiv-

ity measurements of POAS-MWNTs nanocomposite and

POAS pure polymer in both the undoped and doped forms.

The experimental data always showed that, in the case of

the undoped films, the increment of the number of mono-

layers deposited upon the substrate decreased the specific

conductivity. These results can be attributed to the insulator

properties of the undoped form of the conducting polymer.

In fact, for very thin films (few nanometers in thickness),

the close vicinity to the substrate can determine the rather

high level of the conductivity due to the presence of some

defects resulted from the interaction of the monolayers with

Table 1

Specific conductivity parameters of POAS and POAS-MWNTs LS films as a fun

forms

Undoped form specific conductivity (S/cm)

1 layers 10 layers 30 lay

POAS 9.3� 10� 5 4.1�10� 6 6.6� 1

POAS-MWNTs 1.6� 10� 3 7.6� 10� 5 2.8� 1

the substrates. On the contrary, when the thickness of the

film is increased, the electrical properties are determined by

the POAS itself, which is insulating in the undoped form.

The presence of MWNTs in the polymeric matrix showed

to decrease the mere insulator capability of the undoped

form, but it was not able to change the film conductivity

upon increasing the number of layers. In the case of the

doped forms, the doping process did not seem to provide

better results for the pure polymer, where the increment of

the number of layers still resulted in a scarce decrement of

the specific conductivity. These data can be explained by

an increased disorder generated in thicker films that con-

trasts the enhanced conductivity we should obtain by

means of the doping process. An interesting phenomenon

was observed in the case of the nanocomposite material,

where thicker films provided better conducting properties.

In fact, the value of the specific conductivity calculated for

the nanocomposite in the doped form was three orders of

magnitude higher than that obtained from the pure polymer

for films of 30 monolayers. These results thus highlighted

the effective presence of MWNTs inside the polymeric

matrix, as well as the capability to enhance the conducting

properties of the polymer in the doped form. It is important

to point out that during the process of polymerisation

carried out for the synthesis of the nanocomposite material,

the oxidising agent is able to oxidise only the monomer,

since the temperature and the strength of the oxidising

agent cannot operate an oxidation of the nanotube surface.

In other words, the oxidising agent is only employed for

the polymerisation and it is consumed by the monomers

and the growing polymeric chains. The MWNTs dispersed

inside the medium of reaction are then constantly wrapped

up by the forming conducting polymer macromolecules

and the two materials interact by means of non-covalent

bonds. It is also important to point out that the best

enhancement of the conducting properties is obtained when

the nanocomposite is in its doped form. This fact suggests

that the conducting polymer is the major responsible for the

increased conductivity of the nanocomposite in the doped

form, while the MWNTs provide a support in the mecha-

nism of conduction as well as a better alignment of the

polymeric chains.

3.2. AFM imaging

Several images of different regions, carried out on

different samples, were acquired by means of AFM

ction of different numbers of layers, in the case of the undoped and doped

Doped form specific conductivity (S/cm)

ers 1 layers 10 layers 30 layers

0� 7 1.7� 10� 3 1.4� 10� 3 1.1�10� 3

0� 5 7.0� 10� 3 2.2� 10� 1 2.0

Fig. 3. AFM image with section of POAS-MWNTs [(a) = 1 monolayer;

(b) = 10 monolayers] in the undoped form. The image was acquired in

intermittent contact mode. The image size was 5� 5 Am.

V. Bavastrello et al. / Thin Solid Films 468 (2004) 17–2220

technique. The performed investigations showed that the

morphology of POAS-MWNTs nanocomposite always

changed with respect to the conducting properties of the

sample. Thus, AFM images of films containing 1 and 10

monolayers of material in both the undoped and doped

forms were obtained. The images related to the undoped

form are presented in Fig. 3a and b, respectively. It can

be observed that the morphology of a single monolayer is

characterised by the presence of linear strips, evidenced

by the lighter colour, that seem to define some kind of

order of the deposited film. The morphology is slightly

different in the case of 10 monolayers where the linear

strips previously discussed are substituted by agglomer-

ations (grains) always evidenced by the regions with

lighter colour. This result is the natural consequence of

further depositions, where the molecules tend to deposit

randomly on the substrate. It is interesting to point out

that a previous work carried out on polyanilines deriva-

tives thin films evidenced morphologies based on fine

grain structures [16]. For the POAS, an average size of

68F 17 nm was determined [16]. By the analysis of the

AFM images of the nanocomposite, it is possible to

observe that the ‘‘particles’’ characterising the morphology

of the deposited films have a smaller size. This fact can

be explained by the presence of MWNTs inside the

polymeric matrix, which play an important role in stretch-

ing the polymeric chains, thus avoiding the formation of

globular assembles. Figures also report line section pro-

files. The analysis of the profiles allowed us to attribute

to the deepness of the fractures a value of about 3.5 nm

in both cases. This result was important to verify that the

fractures corresponded to shallow breaking along the film

even in the case of 10 monolayers. AFM images of the

films containing 1 and 10 monolayers of the nanocom-

posite in the doped form are shown in Fig. 4a and b,

respectively. In the case of a single monolayer, we obtain

the morphology of the film generated by highly separated

areas, individuated by the lighter spots similar to wide

strips. The same feature was obtained from the deposition

of 10 monolayers, where the major thickness of the film

was able to produce a better coverage of the substrate.

The analysis of the morphologies thus evidenced that the

films in the doped form were composed of conducting

islands, contacting each other, and having wider areas

with respect to the features present in the images of the

films in the undoped form. Grain-like features disappeared

giving proof of an unwrapping effect due to the confor-

mational changes occurred during the in situ doping

process carried out during the deposition itself, and

possibly supported by the presence of MWNTs inside

the polymeric matrix. The unwrapping process of the

polymeric chains, which occurs when they are spread

onto acid subphase instead of water, was previously

confirmed by the determination of the corresponding areas

per molecule [16]. It is very interesting to observe that

the section profile related to Fig. 4b shows fractures

Fig. 4. AFM image with section of POAS-MWNTs [(a) = 1 monolayer;

(b) = 10 monolayers] in the doped form. The image was acquired in

intermittent contact mode. The image size was 5� 5 Am.

V. Bavastrello et al. / Thin Solid Films 468 (2004) 17–22 21

having a deepness of roughly 35 nm, and thus the

grooves were as deep as the whole thickness of the film.

This fact then confirmed an increased uniformity of the

morphology in the case of the doped films associated to a

major rigidity of the material attributed to the presence of

MWNTs inside the polymeric matrix. This phenomenon

was already seen by using electrochemical impedance

spectroscopy in films of POAS-MWNTs nanocomposite

obtained by solution-casting technique [17]. Furthermore,

the AFM images highlighted the stress undergone by the

polymeric chains during the doping process capable to

deeply affect the morphology of the deposited films.

Taking into account all the results so far obtained, it is

possible to assess that the presence of MWNTs inside the

polymeric matrix can provide a support for the mecha-

nism of conduction of the polymer, as well as a better

alignment of the polymeric chains. The result is the

improvement of the conductivity of the material [18,19].

Anyway the doping process, indispensable to impart

conducting properties to the polymer, affects the morphol-

ogy of the deposited film with the formation of non-

conducting gaps individuated by deep fractures. On the

ground of these considerations, it will be our future task

to carry out further investigations to study the possibility

of reducing the number of non-conducting gaps by

‘‘tuning’’ the concentration of MWNTs inside the poly-

meric matrix.

4. Conclusions

POAS pure polymer and POAS-MWNTs nanocomposite

were synthesised by oxidative polymerisation technique.

Mono and multilayers LS films were fabricated by depos-

iting onto water and 1 M HCl aqueous solution as sub-

phases. Experimental data of the resulting specific

conductivity revealed that the presence of MWNTs inside

the polymeric matrix improves the electric properties of the

conducting polymer. The morphology of the deposited films

was investigated by AFM microscopy, and the related

images showed an unwrapping process of the polymeric

chains in the nanocomposite material. Furthermore, the

AFM images also demonstrated that the doping process,

indispensable to increase the specific conductivity of the

polymer, deeply affects the morphology of the deposited

film with the formation of non-conducting gaps. Further

study to reduce the formation of these gaps will take into

account the possibility of ‘‘tuning’’ the concentration of

MWNTs inside the polymeric matrix and the related rigidity

of the deposited films.

Acknowledgements

Authors are thankful to FIRB-MIUR on Organic Nano-

science and Nanotechnologies for the financial support.

V. Bavastrello et al. / Thin Solid Films 468 (2004) 17–2222

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