Thin Solid Films 520 (2012) 5877–5883

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Influence of substituents in electrochemical and conducting properties of polyanilinederivatives and multi walled carbon nanotubes nanocomposites

Valter Bavastrello a,⁎, Tercio Bezerra Correia Terencio a, Luca Belmonte a,Pierluigi Cossari c, Claudio Nicolini a,b

a Laboratories of Biophysics and Nanobiotechnology, Department of Medical Science, University of Genova, Via Pastore 3, 16132 Genova, Italyb Nanoworld Institute, Fondazione EL.B.A. Nicolini, Largo Redaelli 7, Pradalunga, Bergamo, Italyc Istituto di Geologia Ambientale e Geoingegneria - Area della Ricerca del CNR di Roma1, Via Salaria Km 29.300 - 00015 Rome, Italy

⁎ Corresponding author. Tel.: +39 010 3538206; fax:E-mail address: bavastrello_valter@yahoo.it (V. Bava

0040-6090/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.tsf.2012.05.006

a b s t r a c t

a r t i c l e i n f o

Article history:Received 28 October 2011Received in revised form 3 May 2012Accepted 4 May 2012Available online 11 May 2012

Keywords:PolyanilineMulti-walled carbon nanotubesNanocompositesCyclic voltammetryConductivityLangmuir–Schaefer films

Poly(o-methoxyaniline) and poly(o-methylaniline) were synthesized by oxidative polymerization in the pres-ence of multi-walled carbon nanotubes (MWNT) for the fabrication of chloroform processable nanocompositesobtained by embedding MWNT in the polymer matrix without the formation of covalent bonds. The study ofpressure–area isotherms highlighted different substituents along the aromatic rings affected the packing gradeof macromolecules when spreading on different subphases in relation to the associated sterical hindrance. Thepresence of MWNT inside the polymer matrix showed to favor a more stretched conformation of macromole-cules with a subsequent increment of area/molecule values with respect to the corresponding pure conductingpolymers. Furthermore, the sterical hindrance affected the nanocomposite electrochemical properties andconducting polymers containing less hindering substituents along the aromatic rings turned out to be faster elec-trochemical systems. Less hindering substituents were also able to enhance the conducting properties ofnanocomposite materials in association with MWNT.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

In recent years, two classes of organic materials like conductingpolymers and carbon nanotubes (CNT) have gained great interestfor their unique physical chemistry properties [1–4]. In the light of asynergistic application of these organic materials in order to take ad-vantages of their peculiar properties, it is possible to embed littlequantity of CNT, either single-walled carbon nanotubes or multi-walled carbon nanotubes (MWNT), inside the polymer matrix of con-ducting polymers for the fabrication of nanocomposites with interest-ing applications [5,6]. One method of synthesis can be the oxidativepolymerization of monomers in the presence of dispersions of CNT.This method is very simple and requires easily reproducible set upconditions.

Among conducting polymers, polyaniline and its derivatives havebeen deeply studied in the last decades for their good electrical prop-erties, easy methods of synthesis and high environmental stability[7–10]. The chemistry of polyanilines is generally more complexwith respect to other conducting polymers because of their depen-dence on both the pH value and the oxidation states, described bythree different forms known as leucoemeraldine base (fully reducedform), emeraldine base (EB) (50% oxidized form), and pernigraniline

+39 010 3538215.strello).

rights reserved.

base (fully oxidized form). The most important is the EB form sincethe doping process obtained from the protonation of imine groupsbasic sites located along the polymer backbone by means of H+ ions[11,12] issues the emeraldine salt form, responsible of the strong in-crement in conducting properties [13]. The polyanilines doping pro-cess is always associated to conformational modifications of thepolymer chains, due to the local distortions created by addition ofH+ ions to the basic sites [14]. These distortions are even able to af-fect the morphology of the deposited films by varying their organiza-tion and play an important role in the electrical properties of theconducting polymer [15].

The number and kind of substituents along the aromatic rings andthe presence of CNT in the medium of reaction proved to affect boththe chemical structure and the final physical and chemical propertiesof synthesized materials [16]. The polymer backbone conformationseems also to affect the molecular rearrangement occurring duringthe doping process and the sterical hindrance generated by substitu-ents “too close” to the aromatic ring is even responsible of the spon-taneous undoping process [17].

For many applications, it is desirable to have these materials inthin films structure, preferably with known thicknesses and molecu-lar packing [18]. Langmuir–Blodgett and Langmuir–Schaefer (LS)techniques offer a unique control over architecture, thickness andmolecular orientation and have been proven to be powerful toolsfor the fabrication of ultra-thin polymeric films with controlledstructures [19–21]. Generally, when a monolayer is fabricated at the

Table 1Amounts of reagents used for the synthesis of the materials carried out in 200 ml of1 M HCl aqueous solution.

Nanocomposite MWNT(mg)

Monomer(g)

Oxidant(g)

POAN–MWNT 100.2 10.02 4.64POTO–MWNT 99.8 9.98 5.31

R R R R

R R

NH2NH2

H H

+n n

R R R R

R R

NH2NH2

H H

+n n

5878 V. Bavastrello et al. / Thin Solid Films 520 (2012) 5877–5883

gas–liquid or liquid–liquid interface, the film is named Langmuir film.A Langmuir film can be deposited on a solid surface and is thereaftercalled either Langmuir–Blodgett film, in the case of vertical deposi-tion, or LS film, in the case of horizontal deposition. LS is often seenjust as a variant of Langmuir–Blodgett depositions. LS assembliesare particularly attractive as they allow a very high control of thelayer thickness, and require a very small amount of polymer materialin contrast to the solution casting or spin coating techniques [22],respectively.

Taking into account the considerations so far discussed, the aimof thepresent workwas to investigate the possible variation in electrochemicaland conducting properties of polyaniline derivatives and MWNTnanocomposites, deposited in thin films by LS technique, due to the spa-tial rearrangement of conducting polymers. For this purpose, we studiedthe conformational changes occurring in the doping process in relation todifferent substituents along the aromatic rings constituting the polymerbackbone and the presence of MWNT inside the polymer matrix.

2. Experimental details

2.1. Materials

We obtained monomers of ortho-methylaniline and ortho-methoxyaniline, ammoniumpersulfate [(NH4)2S2O8] as oxidizing agentsand other reagents from Sigma. We purchased high purity MWNT fromNANOCS Inc, New York, USA. MWNT had a diameter ranging between30 and 40 nm and length between 40 and 50 μm. Fig. 1 shows the chem-ical structure of monomers used in the syntheses.

2.2. Syntheses of nanocomposites

We carried out the standardized synthesis of nanocomposites byoxidative polymerization of monomers under controlled conditions,maintaining the temperature between 0 °C and 4 °C by means of anice bath for 24 h. For the synthesis, we dispersed 100 mg of CNT bysonication in 200 ml of 1 M HCl aqueous solution of monomers byusing a SONIC 300 VT equipment, set at 10% power of sonication for1 min in order to avoid breaking process of CNT [16]. We used amonomer/MWNT weight ratio of 100/1 and a monomer/oxidantmolar ratio of 4/1, following methods of polymerization previouslyperformed [16]. Table 1 summarizes the amounts of reagents usedfor the syntheses. Particular care was taken on adding the oxidizingagent in order to have the most reproducible conditions at the verybegin of the polymerization.

We then filtered and subsequently treated the crude materials in theinsoluble doped form (emeraldine salt form)with ammoniumhydroxidefor 1 h to obtain the solvent processable undoped form (EB form).

The undoped materials underwent further filtrations and twotreatments with methanol and diethyl ether in order to eliminatethe oligomers, followed by the evaporation of the residue solventsunder vacuum. The final purified nanocomposites EB forms werecompletely soluble in chloroform.

Fig. 2 illustrates the schemeof the synthesis leading to the formationof an arbitrary unit (arb. unit) starting from monomers. Basing on thefraction of imine nitrogen groups per arb. unit it is possible to obtainthe three different reduced/oxidized leucoemeraldine base, EB, andpernigraniline base forms previously described.

NH2

CH3

NH2

OCH

NH2

CH3

NH2

OCHa b

3

Fig. 1. Chemical structure of monomers used in the synthesesa) ortho-methylaniline;b) ortho-methoxyaniline.

2.3. Fabrication of Langmuir–Schaefer films and study of pressure–areaisotherms

We fabricated LS films in a Langmuir–Blodgett trough (MDT Corp.,Russia) 240 mm×100 mm in size and 300 ml in volume at a compres-sion speed of 1.67 mm/s (100 cm2/min),maintaining a surface pressureof 25 mN/mat the air–liquid interfaces [15–17,23]. The nanocompositesspreading solutions were obtained by dissolving 5 mg of materials in20 ml of chloroform. For the deposition, we used either distilled wateror 0.1 M HCl aqueous solution (pH=1) as subphases to obtain filmsof nanocomposites in both undoped and doped forms, respectively.

We also utilized the Langmuir–Blodgett trough to study the surfacepressure–area isotherms of nanocomposites in both forms by spreadingon the liquid subphases previously described. Surface pressure–areaisotherms, π–A isotherms or simply isotherms can be defined as a mea-surement, at constant temperature, of the surface pressure as a functionof the available area per eachmolecule in afloatingmonolayer (Langmuirfilm).We thus employed the surface pressure–area isotherms to calculatethe related area/arb. unit, where for arb. unit we intended the structureshown in Fig. 2.

2.4. UV–vis spectroscopy

We used an UV–vis spectrophotometer Jasco V530 with softwareto record UV–vis spectra of the nanocomposite LS films on quartzsubstrates in both undoped and doped forms. For the preparation ofthe samples, we applied the methods of deposition described in theprevious section. Spectra were collected by ranging between 250and 1000 nm, setting 100 nm/min scanning speed and 1.0 nm datapitch. We carried out UV–vis acquisitions also to study the methodsof deposition by monitoring the layer-by-layer growing process ofthe deposited thin films.

2.5. Cyclic voltammetry

The electrochemical measurements were carried out by usinga potentiostat/galvanostat (EG &G PARCmodel 163) throughM270 sup-plied software. We employed a standard three-electrode configuration,

N N N N

y 1-yn

N N N N

y 1-yn

Fig. 2. Scheme of the synthesis leading to the formation of an arbitrary unit startingfrom monomers. Basing on the fraction of imine nitrogen groups per arbitrary unitit is possible to obtain different reduced/oxidized polymer chains as following: Fullyreduced form (leucoemeraldine base) for y=1; Fully oxidized form (pernigranilinebase) for y=0; Half oxidized form (emeraldine base) for y=0.5.

0

0.2

0.4

0.6

0.8

Wavelength (nm)

Ab

sorb

ance

UndopedDoped

- *n- *

polaron- * -polaron

POAN-MWNT

0.0

0.1

0.2

0.3

0.4

0.5

250 500 750 1000

250 500 750 1000

Wavelength (nm)

Ab

sorb

ance

UndopedDoped

-polaron

n- *

polaron- *

- * POTO-MWNTπ

ππ

π

π

ππ

π

π π

Fig. 4. UV–vis spectra of POAN–MWNT and POTO–MWNT nanocomposites in undopedand doped forms.

5879V. Bavastrello et al. / Thin Solid Films 520 (2012) 5877–5883

where nanocomposite LS films onto Indium Tin Oxide (ITO) glass platesacted as working electrodes, platinum wire as counter electrode, andAg/AgCl as reference electrode. We collected cyclic voltammograms ofnanocomposites LS films in 0.1 M HCl aqueous solution by applyingsweep rates of 5 mV/s and 10 mV/s, respectively.

2.6. Specific conductivity measurements

We obtained specific conductivity calculations by basing on voltage/current (V/I) characteristics measured with an electrometer Keitleymodel 6517, driven by computer. For thesemeasurements, we deposit-ed 1, 10, and 30monolayers of nanocomposites onto glass substrates inboth undoped and doped formsbyusing themethods of deposition pre-viously described and then contacting the fabricated device to the elec-trometer by means of silver wires and silver paint. We carried outmeasurements of current by applying a potential ranging between−10 V and 10 V. Fig. 3 illustrates the experimental set up used for thedetermination of specific conductivities.

3. Results and discussion

3.1. Syntheses of nanocomposite materials

The analysis of UV–vis spectra, aswill be described in the next section,showed the syntheses issued nanocomposite containing conductingpolymers in the EB form. It is important to point out that during thepolymerization process (NH4)2S2O8 was able to oxidize only the mono-mer, since the temperature and the strength of the oxidizing agentwere not capable to operate an oxidation of the CNT surface, as widelydemonstrated in our previous work [16]. The growing macromoleculesthus constantly wrapped up around MWNT dispersed in the medium ofreaction, interacting by means of non-covalent bonds [16]. All synthe-sized nanocomposites were easily soluble in chloroform.

3.2. UV–vis spectroscopy

UV–vis spectroscopy of nanocomposites was carried out on LSfilms in both undoped and doped forms deposited on quartzsubstrates. The study of recorded UV–vis spectra, illustrated in Fig. 4, rev-ealed the formation of the typical bands associated to polyaniline deriva-tives. In the case of undoped nanocompositeswe had the presence of twobands assigned to π–π* interband transition associated to the benzoid/quinoid ring structure and to n–π* transition from the nonbonding nitro-gen lone pair to the conduction band π* [24]. In the case of dopednanocomposites we assessed the formation of the polaronic state sincethe presence of the relative bands due to the polaron–π* and π–polarontransitions, respectively. For a complete vision of all experimental data,we summarized the wavelength of the bands related to both undopedand doped forms in Table 2.

Glass substrate

Silver paint

Nanocomposite filmSilver wire

Glass substrate

Silver paint

Nanocomposite filmSilver wire

Fig. 3. Schematic of the experimental set up used for the determination of the specificconductivity.

UV–vis spectroscopy was also useful to check the thin films fabrica-tion by LS technique. For this purpose, UV–vis spectra were collectedevery 5 deposited monolayers up to a total of 60. Fig. 5 shows UV–visspectra related to POAN–MWNT nanocomposite in both undoped anddoped forms and evidenced a proportional increment in the UV–visabsorbance in relation to the growing film thickness.

3.3. Fabrication of Langmuir–Schaefer films and study of pressure–areaisotherms

We studied the pressure–area isotherms by spreading 200 μl ofPOAN–MWNT and POTO–MWNT nanocomposites chloroform stocksolutions at the air/liquid interface. Fig. 6 shows the nanocompositesbehavior obtained on spreading at air/water (undoped monolayers)and air/0.1 M HCl aqueous solution (doped monolayers) interfaces,respectively, while for the calculation of the area per molecule webased on the arb. unit of conducting polymers shown in Fig. 2.

The extrapolation of the area per molecule from isotherms, issuedvalues of 57 Å2/arb. unit and 53 Å2/arb. unit for POAN–MWNT andPOTO–MWNT nanocomposites by spreading at air/water interface andvalues of 39 Å2/arb. unit and 42 Å2/arb. unit by spreading at air/0.1 MHCl aqueous solution, respectively. The experimental data thus highlight-ed a minor macromolecules packing, corresponding to major area permolecule values, for POAN–MWNT and POTO–MWNT nanocompositeswhen they spread on water (undoped form) with respect to the acid so-lution (doped form). The area per molecule value was therefore affectedby the structural change occurring during the doping process taking placeon the imine group basic sites, characterized by a sp2-hybridized planarconfiguration, with consequent protonation of imine nitrogen atomsand generation of sp3-hybridized tetrahedral configuration. In fact, thedouble bonds associated to sp2-hybridized configuration not only

Table 2Wavelength of bands related to π–π* and n–π* transitions for nanocomposites in theundoped form and polaron–π* π*–polaron transition for nanocomposites in thedoped form, respectively.

Nanocomposite Undoped form Doped form

π–π* n–π* Polaron–π* π–polaron

POAN–MWNT 326 nm 656 nm 396 nm 850 nmPOTO–MWNT 316 nm 595 nm 406 nm 852 nm

0

0.1

0.2

0.3

0.4

0.5

250 500 750 1000

250 500 750 1000

Wavelength (nm)

Ab

sorb

ance

Undoped

00.10.20.30.40.50.60.7

Wavelength (nm)

Ab

sorb

ance

Doped

Fig. 5. UV–vis spectra of undoped and doped forms of POAN–MWNT nanocompositescollected every 5 monolayers up to a total of 60.

5880 V. Bavastrello et al. / Thin Solid Films 520 (2012) 5877–5883

“flattened”wide segments of thepolymer backbone but also impeded therotation around the atom–atom bond with consequent decremented de-grees of freedom, associated to a minor tangling grade of macromole-cules. On the contrary, the doping process due to protonation leads tothe formation of single bondswith consequent change in the atom spatialdisposition, since the rotation around the atom–atom bond was not im-peded and the tetrahedral conformation allowed a major “curling effect”and consequently an increased tangling grade of macromolecules,corresponding to diminished area per molecule values.

The kind of substituents along the aromatic rings also affectedthese values. The presence of CH3O\ groups seemed to have minorinfluence than CH3\ ones in the case of undoped nanocompositeswhile we observed an opposite behavior for doped ones. This phe-nomenon occurred since for undoped materials the planar configura-tion present in the oxidized segments along the polymer backboneruled the atoms spatial disposition by limiting the macromoleculestangling process. The consequence was a stretched configuration inboth cases but the presence of an oxygen atom in the CH3O\ groupgenerated a slight increment in the values of area per molecule withrespect to the CH3\ one that was closer to the aromatic rings [25].

010203040506070

Area per molecule (Å2/arb. unit)

Area per molecule (Å2/arb. unit)

Su

rfac

e p

ress

ure

(mN

/m) 1

2 1 = POAN-MWNT2 = POTO-MWNT

a

01020304050607080

0 10 20 30 40 50 60 70 80 90

0 10 20 30 40 50 60 70 80 90

Su

rfac

e p

ress

ure

(mN

/m)

b

1 = POAN-MWNT2 = POTO-MWNT

1

2

Fig. 6. Pressure–area behavior of nanocomposites (barriers speed=1.67 mm/s): a) air/water interface (undoped form); b) air/1 M HCl aqueous solution interface (doped form).

On doping, the decrement in oxidized segments along the polymerbackbones favored the tangling process and, in this case, the stericalhindrance was more important and ruled the final conformation.Thus, the major distance due to the presence of an oxygen atomwas able to reduce the sterical hindrance in the case of the CH3O\group with respect to the closer CH3\ one and accentuated the dif-ference in the area per molecule values.

Ram and co-workers found a similar behavior for POAN and POTOpure conducting polymers in the case of undopedmaterials, while thedoped form generated some differences. Specifically, they obtainedvalues of 55 Å2/arb. unit and 45 Å2/arb. unit for POAN and POTOpure conducting polymer by spreading at air/water interface, whilethey had values of 21 Å2/arb. unit and 25 Å2/arb. unit by spreadingat air/0.1 M HCl aqueous solution, respectively [26]. The comparisonamong pure conducting polymers and related nanocomposites areaper molecule values showed that the presence of MWNT inside thepolymer matrix generally limited the tangling process of macromole-cules. For undoped materials, this phenomenon was less accentuatedfor POAN–MWNT nanocomposite (57 Å2/arb. unit) with respect toPOAN pure conducting polymer (55 Å2/arb. unit) than for POTO–MWNT nanocomposite (53 Å2/arb. unit) with respect to POTO(45 Å2/arb. unit) pure conducting polymer, because of a generalmore stretched configuration, mainly in the latter case. For dopedma-terials, where the macromolecules tangling process is more relevant,the presence of MWNT in the polymer matrix deeply affected themacromolecules atoms spatial disposition with respect to the pureconducting polymers. This phenomenon was consistent with thehigher values of area per molecule found for the nanocompositeswith respect to the corresponding pure conducting polymers. Inother words, the presence of MWNT into the polymer matrix mainlyaffected the polymer chains conformation in the doped form sincethey gave a greater contribution in stretching the conducting polymerbackbones, while for the undoped form the presence of a “natural”more stretched conformation was mainly ruled by the substituentsalong the aromatic rings. Table 3 summarizes the area per moleculesof nanocomposites and related pure conducting polymers in bothundoped and doped forms.

3.4. Cyclic voltammetry

Cyclic voltammetry was performed to investigate the redox transi-tions and the influence in this process of the substituents along thearomatic ring in association with MWNT inside the polymer matrix. InFig. 7 are shown the voltammograms obtained by depositing 60 LSmonolayers of POAN–MWNT and POTO–MWNT nanocomposites ontoITO glass substrates, respectively. The cyclic voltammetrieswere carriedout in a cell containing 0.1 MHCl aqueous solution by previously apply-ing a sweep rate of 5 mV/s and then of 10 mV/s, respectively.

The study of the voltammograms obtained at a sweep rate of5 mV/s highlighted the formation of three redox couples peaks forboth POAN–MWNT and POTO–MWNT nanocomposites, and the relat-ed potentials are summarized in Table 4. According to previous cyclicvoltammetry characterizations, the redox couples corresponding tothe conducting polymer oxidation/reduction processes were thefirst (I/I') and the third (III/III') due to leucoemeraldine/emeraldine

Table 3Comparison of area per molecule values related to POAN–MWNT and POTO–MWNTnanocomposites and corresponding pure conducting polymers spread on differentsubphases.

Area per molecule (Å2/arb. unit)

Nanocomposite Undopedform

Dopedform

Purepolymer

Undopedform

Dopedform

POAN–MWNT 57 39 POAN 55 21POTO–MWNT 53 42 POTO 45 25

-600 -300 0 300 600 900 1200

E/V (mV) vs Ag/AgCl

5 mV/s

10 mV/s

20

III

III

III '

II 'I '

IV

IV '

POAN-MWNT

-600 -300 0 300 600 900 1200

E/V (mV) vs Ag/AgCl

5 mV/s

10 mV/s

20 μA

μA

III

IIII '

II '

III '

POTO-MWNT

Fig. 7. Cyclic voltammogram of nanocomposites carried out in a cell containing 0.1 M HClaqueous solution. Arrows show the direction of the scan. The scan originates at−400 mVat a sweep rate of 5 mV s−1 and 10 mV s−1. (upper) POAN–MWNT nanocomposite;(lower) POTO–MWNT nanocomposite.

-60

-40

-20

0

20

40

Potential (V)

Cu

rren

t (n

A) 1 = Undoped

2 = Doped1

2

-15

-10

-5

0

5

10

Potential (V)

Cu

rren

t (μA

)

1

21 = Undoped2 = Doped

b

a

-3

-2

-1

0

1

2

3

-15 -10 -5 0 5 10 15

-15 -10 -5 0 5 10 15

-15 -10 -5 0 5 10 15

Potential (V)C

urr

ent (

mA

) 1 = Undoped2 = Doped

c

1

2

Fig. 8. V/I characteristics for POAN–MWNT nanocomposite in the undoped and dopedforms [(a)=1 monolayer; (b)=10 monolayers; (c)=30 monolayers].

-80-60-40-20

0204060

Potential (V)

Cu

rren

t (n

A) 1 = Undoped

2 = Doped1

2

a

-1.2

-0.8

-0.4

0.0

0.4

0.8

1.2

Potential (V)

1 = Undoped2 = Doped

2

1

b

0.0

0.2

0.4

-15 -10 -5 0 5 10 15

-15 -10 -5 0 5 10 15

t (m

A) 1 = Undoped

2 = Doped1

2

Cu

rren

t (μA

)

5881V. Bavastrello et al. / Thin Solid Films 520 (2012) 5877–5883

and emeraldine/pernigraniline transitions, while the intermediate one(II/II') was related to polymer destruction and cross-linking [27–29]. Spe-cifically, we observed that the polymer chains in POAN–MWNTnanocomposite containing CH3O\ groups along the aromatic rings,were able to undergo all the oxidation transitions with limited formationof degradation products, evidenced bypeaks II, as shown in Fig. 7a. On thecontrary, the polymer chains in POTO–MWNTnanocomposite containingCH3\ groups along the aromatic rings, showed an overlapping of peaks Iand II due to an extra production of degradation products, as illustrated inFig. 7b. It is possible that the major sterical hindrance of CH3\ groupsalong the aromatic rings and the increment of planar configurationsegments along the polymer backbones during the oxidation processes,with consequentmacromolecules diminished degrees of freedom, affect-ed the conformational changes occurring in the redox transitions anddeeply strained the conducting polymer macromolecules. The conse-quence was a fusion of peaks I and II for POTO–MWNT nanocomposite,since peak II due to polymer destruction was well accentuated because

Table 4Oxidation and reduction peaks related to the synthesized nanocomposites, obtained ina standard three-electrode configuration cell containing 0.1 M HCl aqueous solution.

Nanocomposite Oxidation potential (mV) Reduction potential (mV)

POAN–MWNT 156 (I), 336 (II), 536 (III) −50 (I'), 320 (II'), 488 (III')POTO–MWNT 240 (I), 396 (II), 702 (III) 10 (I'), 192 (II'), 356 (III')

-0.6

-0.4

-0.2

-15 -10 -5 0 5 10 15

Potential (V)

Cu

rren

c

Fig. 9. V/I characteristics for POTO–MWNT nanocomposite in the undoped and dopedforms [(a)=1 monolayer; (b)=10 monolayers; (c)=30 monolayers].

0

0.001

0.002

0.003

0.004

Number of monolayers

Sp

ecif

ic c

on

du

ctiv

ity

(S/c

m)

POAN-MWNTPOTO-MWNT

Undoped

0

0.5

1

1.5

2

2.5

3

0 10 20 30 40

0 10 20 30 40

Number of monolayersS

pec

ific

co

nd

uct

ivit

y (S

/cm

) DopedPOAN-MWNTPOTO-MWNT

Fig. 10. Specific conductivity of undoped and doped forms of nanocomposites obtainedfrom 1, 10, and 30 deposited monolayers.

5882 V. Bavastrello et al. / Thin Solid Films 520 (2012) 5877–5883

of the effort performed by the polymer chains during the oxidationprocesses with consequent macromolecules degradation.

The increment of the sweep rate up to 10 mv/s showed to havescarce influence in the redox couples for POAN–MWNTnanocomposite,where all the oxidation and reduction peaks were still evident since thepresence of less hindering CH3O\ groups along the polymer backbone.On the contrary, CH3\ groups along the aromatic ringwere responsibleof the differences encountered for POTO–MWNT nanocomposite sincethe emeraldine/pernigraniline transition was characterized by majorquantities of degradation products, as proven by the overlapping ofpeaks I, II, and III, respectively. The increment in degradation productsfor POTO–MWNT nanocomposite thus highlighted that a sweep rateof 10 mV/s was “too fast” to provide the spatial rearrangement ofmacromolecules enough time for the oxidation/reduction transitions.In other words, POTO–MWNTnanocomposite turned out to be a slowerelectrochemical system than POAN–MWNT one.

Ram and co-workers obtained a general redox couple peaks thatshift to higher potentials for POAN pure conducting polymer with re-spect to POAN–MWNT nanocomposite while they found similarvalues for POTO pure conducting polymer with respect to POTO–MWNT nanocomposite [26]. This phenomenon may be related tothe presence of MWNT inside the polymer matrix in associationwith CH3O\ electron donor groups along the polymer chains,which are able to promote the electrons exchange during the redoxtransitions. Therefore, the CH3O\ substituent not only diminishedthe sterical hindrance responsible of the decrement in macromole-cules degrees of freedom but also in association with MWNT in thepolymer matrix even favored the electron exchange during theredox processes, while CH3\ non-electron donor groups gave nosupport to the electrons exchange.

3.5. Specific conductivity measurements

The study of nanocomposites LS films V/I characteristics revealedquasi-linear behaviors for all samples as illustrated in Figs. 8 and 9and therefore it was possible to calculate the related specific conductiv-ity by applying the Ohm's first and second laws. For a clear vision,Table 5 summarizes the results obtained from the specific conductivitymeasurements of POAN–MWNT and POTO–MWNT nanocompositesafter depositing 1, 10, and 30 monolayers in both undoped and dopedforms, respectively.

In the case of undoped films, the increment in the number of deposit-ed monolayers issued a decrement in specific conductivity and thisphenomenon was associated to the insulator properties of conductingpolymers in the undoped form. In fact, for very thinfilms fewnanometersin thickness, the close vicinity to the substrate could determine the ratherhigh-level values in conductivity due to the presence of some defectsresulted from the interaction of the monolayers with the substrates. Onthe contrary, on increasing the film thickness the electrical propertieswere determined by the nature of the conducting polymer itself, whichwas insulating in the undoped form. It is interesting to underline the spe-cific conductivity tended to similar values for both the nanocompositesthicker films, thus highlighting thesematerials in bulk showed similar in-sulating properties, as illustrated in Fig. 10 (upper graphic). The presenceof MWNT inside the polymer matrix gave no contribute in the film con-ductivity on increasing the number of monolayers in both cases. This

Table 5Specific conductivity of nanocomposite LS films as a function of different number of layers

Specific conductivity (S/cm)

Undoped form

Nanocomposite 1 layer 10 layers 30 layPOAN–MWNT 3.65×10−3 1.14×10−4 2.16×POTO–MWNT 2.15×10−4 9.58×10−5 2.03×

result was consistent with the absence of strong interactions among con-ducting polymers and MWNT, then confirming that the polymer chainssimply wrapped up around MWNT without the formation of covalentbonds and MWNT were not able to perform any kind of doping process[16].

On the contrary, we found an opposite trend for the nanocompositesdoped form since we observed better conducting properties on increas-ing the number of monolayers. Anyway, POAN–MWNT nanocompositeshowed a steeper increment in specific conductivity on increasing thenumber of monolayers and for films of 30 monolayers this materialhad better conducting properties than POTO–MWNT nanocompositeof almost one order of magnitude, as illustrated in Table 5. In thiscase the doping process therefore issuedmaterials having different con-ducting properties in bulk, as shown in Fig. 10 (lower graphic). Thepresence of MWNT inside the polymer matrix surely played an impor-tant role for doped nanocomposites by stabilizing the formation of thepolaronic state, involving the presence of positive charges along thepolymer backbones and responsible of the increment in conductivityin polyaniline derivatives. These results were then consistent withprevious experimental data demonstrating that different substituentsalong the aromatic rings as well as the presence of MWNT insidethe polymeric matrix were capable of “tuning” the formation of thepolaronic states [16]. These results also suggested that the conductingpolymers were the major responsible for the increased conductivity of

in the case of the undoped and doped forms.

Doped form

ers 1 layer 10 layers 30 layers10−5 3.98×10−3 6.35×10−2 2.1410−5 8.95×10−3 9.08×10−3 1.4×10−1

5883V. Bavastrello et al. / Thin Solid Films 520 (2012) 5877–5883

the nanocomposite in the doped form, while the MWNT provide asupport in the mechanism of conduction as well as a better alignmentof the polymeric chains.

4. Conclusions

We carried out oxidative polymerizations to synthesize chloroformprocessable nanocomposites by embedding MWNT inside the polymermatrix. The study of the pressure–area isotherms highlighted that thesterical hindrance of substituents along the aromatic rings affected thepolymer chains conformational changes during the doping processand consequently the macromolecules packing grade. The presence ofMWNT inside the polymer matrix allowed macromolecules morestretched conformations since the nanocomposite showed higherarea/arb. unit valueswith respect to the corresponding pure conductingpolymers when spreading at the same air/liquid interface.

The sterical hindrance also affected the nanocomposites electrochem-ical properties and polymer chains containing less hindering substituentsturned out to be faster electrochemical systems, whileMWNT showed topromote the electron exchange during the redox processes in the pres-ence of electron donor substituents. Furthermore, less hindering substit-uents were able to enhance the conducting properties of nanocompositematerials in association with MWNT since their capability to favorthe formation of the polaronic states, responsible of the increment innanocomposites conductivity, while the presence of MWNT inside thepolymer matrix gave no enhancement in conducting properties fornanocomposites in the undoped form. Therefore, the insulating natureof conducting polymers ruled the conductivity of nanocomposites evenin the presence of MWNT in the polymer matrix, while MWNT wereable to support the mechanism of conduction in the doped form.

In the light of these experimental results, further investigations willtake into account the synthesis of nanocomposites by using monomerswith different substituents aswell as different concentrations ofMWNTinside the polymer matrix in order to assess the possibility of “tuning”and possibly enhancing the related physical chemistry properties.

Acknowledgments

This project was supported by grants to Fondazione EL.B.A., bythe MIUR (Italian Ministry of Education, University, and Research)

for “Funzionamento” and by a FIRB Research National Network onNanosciences e ItalNanoNet (RBPR05JH2P) from MIUR to CIRSDNNOBof the University of Genoa.

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