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Materials Chemistry and Physics 130 (2011) 760– 768

Contents lists available at ScienceDirect

Materials Chemistry and Physics

jo u rn al hom epage : www.elsev ier .com/ locate /matchemphys

tructure–property relationship in aliphatic polyamide/polyaniline surfaceayered composites

. Fatyeyevaa,b,c,∗, A.A. Puda, J.-F. Bardeaub, M. Tabelloutb

Institute of Bioorganic Chemistry and Petrochemistry of National Academy of Science of Ukraine, 50 Kharkivske shose, Kyiv 02160, UkraineLaboratoire de Physique de l’Etat Condensé, UMR CNRS 6087, Université du Maine, Av. Olivier Messiaen, 72085 Le Mans cedex 9, FranceLaboratoire Polymères, Biopolymères et Surfaces, UMR 6270 & FR 3038 CNRS, Université de Rouen, Bd. Maurice de Broglie, 76821 Mont Saint Aignan cedex, France

r t i c l e i n f o

rticle history:eceived 24 February 2010eceived in revised form 18 July 2011

a b s t r a c t

Conducting polymer composite films based on different aliphatic polyamides (PA) (PA-6, PA-11 andPA-12) have been synthesized by in situ aniline polymerization inside a surface layer of the PA hostmatrix. Dielectric permittivity and dielectric loss of these films are explained in terms of the interfacial

ccepted 23 July 2011

eywords:. Interfaces. Composite materials. Electrical properties

polarization. The real part of permittivity is found to be higher in the PA/polyaniline (PANI) compositefilms than in the virgin PA polymer matrix. Such behaviour is attributed to the interaction between PA andPANI molecular chains and to the conductivity increase after the aniline polymerization. The performedconfocal Raman spectrometry and X-ray diffraction studies also confirmed the presence of interactionsbetween PA and PANI molecular chains.

. Dielectric properties

. Introduction

Conducting polymers have been a focus of considerable researchince the first observation of polyacetylene by MacDiarmid ando-workers [1]. Their remarkable properties, namely optical, mag-etic characteristics and conductivity behaviour, offer tremendouspportunities for the development of fundamentally new materialystems to be applied in fields of energy storage and conver-ion, photovoltaics, visualisation of information (electrochromicisplays), corrosion protection, sensors, membrane filtrations, etc.2–4]. PANI (Fig. 1) has been revealed as one of the most promis-ng materials due to its excellent properties including easinessf the synthesis, environmental stability, relatively high conduc-ivity (10−4–102 S cm−1 depending on synthesis conditions) andossibility to be chemically modified [3,5]. However, in spite ofumerous advantages of PANI, its fragility, infusibility and poorolubility in common organic solvents often hinder its practicalpplication. Some ways to overcome these problems are PANI dop-ng by functionalized acids (e.g. dodecyl benzene sulphonic acid,amphor sulphonic acid, etc.), using substituted monomers and

orming composites with conventional polymers [2,6]. The prepa-ation of conducting polymer composites seems to be the mostromising option, as in this case a material, which combines the

∗ Corresponding author at: Laboratoire Polymères, Biopolymères et Surfaces, UMR270 & FR 3038 CNRS, Université de Rouen, Bd. Maurice de Broglie, 76821 Mont Saintignan cedex, France. Tel.: +33 2 35 14 66 98; fax: +33 2 35 14 67 04.

E-mail address: kateryna.fatyeyeva@univ-rouen.fr (K. Fatyeyeva).

254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2011.07.057

© 2011 Elsevier B.V. All rights reserved.

intrinsic conductivity of conducting polymer and good mechanicalproperties of insulating polymers (e.g. poly(vinyl alcohol) (PVA),poly(vinylidene fluoride) (PVDF), poly(methyl methacrylate), etc.)will be obtained.

In spite of the numerous articles concerning differentmethods of synthesis of conducting composites [2,6–11], onlysome of them deal with their dielectric properties although thedielectric function ε*(ω) can provide information about transportmechanism in the system. It was found that dielectric proper-ties of PANI are in good agreement with the physicochemicalparameters correlated to statistical distribution of insulating andconductive segments along the chain [12–14]. Taking all this intoaccount, it was important to understand the fundamental mecha-nism of the dielectric effect in conducting polymers and to providestructure–function relationship, which may help in creation of newmaterials.

It should be mentioned that the PANI composite proper-ties (especially, electrical and dielectric ones) differ from thoseobserved for pure PANI because they are strongly dependent onthe nature of the polymer matrix [15–22]. The conductivity anddielectric properties of these heterogeneous systems depend on anumber of factors including the concentration of conducting fillers,their shape and distribution into the matrix, the orientation andinterfacial interaction between filler molecules and host matrix[16,18–22], as well as on the method of preparation [14,17].

Nowadays, good conductivity results are received for the poly-mer matrix/PANI systems [2,16–19]. However, the compositematerials with polymers of basic nature (specifically, polyamides)have not been sufficiently studied. Although a few studies on this

K. Fatyeyeva et al. / Materials Chemistry

NN

x

N

H

N

H

x

N N N N

H H H H

-

Emeraldine base (E B) (blue)

2xHA

+. +.

A- A-

Emeraldine salt (ES) (gree n)

+2xHA

F(

sr

csiwptfphfoasm

tPatf1oeptb

P(tipoa

2

2

i(t((

2

tl

ig. 1. Transition between PANI base form and the protonated emeraldine formone of the possible chemical representations). A− represents an arbitrary anion.

ubject which can be found in the literature show quite promisingesults [23–30].

As it was previously mentioned, composites based ononducting polymers can be used for antistatic applications. Forome of these applications, e.g. in the photographic industry, coat-ngs require a high level of transparency. Therefore, the promising

ay to combine the electrical properties of PANI and the trans-arency of common polymers is the aniline polymerization insidehe swelled polymer matrix with an oxidant solution in order toorm surface layered composites [20,22,29–32]. During the matrixolymerization, a conducting path of PANI through the transparentost matrix is formed. The kinetic of PA-12/PANI composite films

ormation [20] as well as the dielectric properties of films basedn poly(ethylene terephtalate) (PET) and PA-6 [21,22] have beenlready reported. Several relaxation processes related to the filmurface conductivity and influenced by the nature of the polymeratrix are found in these composites.Raman spectroscopy is quite sensitive to the electronic struc-

ural changes of PANI [33]. The oxidation and protonation states ofANI affect the Raman signals collected from the polymer. Therere a number of reports which mention Resonance Raman spec-roscopy as a valuable tool for analyzing the behaviour of differentorms of PANI with excitation wavelengths ranging from 457 to064 nm [33–38]. It has been shown that often either the benzenoidr quinoid units are resonance enhanced depending on the laserxcitation wavelength. So, when studying oxidation and reductionrocesses of PANI it is of great importance to choose an excita-ion wavelength that equally enhances vibrations originating fromenzenoid and quinoid forms.

Thus, the aim of the present work is to synthesize the surfaceANI composite materials based on different aliphatic polyamidesPA-6, PA-11 and PA-12) and to investigate their structural, elec-rical and dielectric properties in order to better understand thenfluence of the polymer matrix structure. The studies of dielectricroperties of these systems are of interest for possible applicationsf the surface layered composite films in actuators, sensors and asntistatic coatings.

. Experimental

.1. Materials

Aniline (Merck) was distilled under vacuum and stored under argon atmospheren a refrigerator at 3 ◦C. Ammonium persulphate (APS) (Ukraine), hydrochloric acid37 wt.%, Ukraine), HClO4 (70 wt.%, Ukraine), n-hexane (Ukraine) and ammonia solu-ion (25 wt.%, Ukraine) were of reagent grade and used as received. Films of PA-625 �m) were purchased from Goodfellow Cambridge Ltd. Films of PA-11 BESHVO50 �m) and PA-12 (50 �m) were kindly donated by Arkema (France).

.2. Preparation of the PA/PANI surface layered composite films

The surface layered conducting polymer composite films were prepared usinghe procedure described above [20]. The pristine PA films were immersed into ani-ine at 20 ± 1 ◦C in argon atmosphere to be swelled during definite time period as

and Physics 130 (2011) 760– 768 761

a function of the PA matrix (for example, for PA-12 during 2 h in order to reach12 wt.% of aniline that corresponds to 7.5 wt.% of PANI [20]). Swelled in that wayPA films were washed by n-hexane to remove the surplus of aniline from their sur-face. Then, one side of these swelled films was treated with the oxidizing solution of0.1 M (NH4)2S2O8 in 1 M water solution of HCl in the home-made cell to run chemi-cal aniline polymerization inside the surface layer of the PA matrix in order to formthe PA/PANI composite film. After completion of the polymerization process, thegreen transparent composite film was washed by distilled water and placed in Soxletapparatus with n-hexane for 6 h to extract the non-polymerized aniline residua andby-products from the film matrix. The procedure was followed by drying the film at60 ◦C under dynamic vacuum up to a stable weight.

Surface composite films obtained in that way were then treated with the 5%ammonia aqueous solution for 24 h in order to convert (dedope) PANI inside thePA matrix to the non-conducting emeraldine base (EB) state (Fig. 1). The dedopedcomposite films were then redoped in 1 M water solution of appropriate acid (HClor HClO4) to prepare the PA/PANI doped composite films.

2.3. Dielectric relaxation spectroscopy

Dielectric permittivity measurements were performed using Novocontrolbroadband dielectric spectrometer (Novocontrol GmbH, Germany) in wide fre-quency (0.1 Hz to 10 MHz) and temperature (173–423 K) ranges. The sample cellwas composed of two round golden parallel electrodes filled with a material to forma capacitor. The polymer composite film was placed between two electrodes. Thesurfaces containing PANI faced each other in order to respect the sample symmetry[22].

The values of imaginary and real parts of dielectric permittivity were analyzedby using WinFit 2.4 (1996) software of Novocontrol GmbH (Germany) according tothe empirical Havriliak–Negami (HN) function [39]:

ε∗(ω) = εu +N∑

i=1

�εi

(1 + (iω�i)˛i )ˇi

+ �dc

iε0ω(1)

where ω = 2�f is the angular frequency with f being the frequency; ε0 denotes thevacuum permittivity; �dc is the dc-conductivity; �εi , � i , ˛i and ˇi are the relax-ation strength, the relaxation time, the symmetrical and asymmetrical distributionparameters for the ith relaxation process, respectively. εu is the unrelaxed dielectricpermittivity.

The characteristic relaxation time � was taken at the position of the maximumof dielectric loss for each relaxation process. The temperature dependences wereanalyzed using Arrhenius equation:

� = �0 exp(

EakBT

)(2)

where �0 is the relaxation time at very high temperature; Ea is the activation energy;kB is Boltzmann’s constant (8.616 × 10−5 eV K−1).

2.4. Electrical conductivity measurements

The four-electrode technique was used in order to measure the surface resis-tance R of the conducting composite films as a function of the temperature. Fourcopper wires were attached in parallel on the sample surface with conducting silverpaint for better electrical contact. The resistance values were measured using AgilentE3634A electrometer. Measurements were performed by changing the temperatureat the rate of 0.2 ◦C min−1 in the cryostat equipped with a temperature-controlledheater. The temperature control was realized with the help of Novocontrol broad-band dielectric spectrometer. The resistance measurements were averaged out ofthe results of (i) three measurements for each film and (ii) two films for each sample.

2.5. Resonance Raman spectrometry measurements

The Raman experiments were carried out by multichannel Horiba Jobin-YvonT64000 spectrometer connected to a liquid N2 cooled CCD detector and equippedwith a confocal microscope. The 514.5 nm line of an Argon-Krypton ion laser wasused as an excitation source for the Raman spectra. This excitation line has beenchosen since it allows studying PANI in emeraldine salt (ES) as well as in EB oxida-tion states (Fig. 1) [20,40]. It was previously shown that both phenyl and quinoneunits of PANI could simultaneously be enhanced by 514.5 nm excitation wavelengthwhereas vibrations of these units are resonance enhanced in the blue and red exci-tation wavelengths, respectively [35,37,40].

To avoid any local degradation and perturbation of the polymer samples, thelaser beam power was limited to 0.04 mW on the sample surface and the spectrawere collected during the integration time of 300 s. The laser beam was focused on

the sample in a ∼1 �m spot size with an ×100 objective. The scattered Raman signalwas collected by the same objective lens and was focused onto a pinhole (100 �mopening aperture) to reject out-of-focus signals. The diverging light coming out ofthe pinhole was then refocused onto the entrance slit of the spectrograph. Underthese conditions, the depth resolution was determined to be 1.5 �m.

762 K. Fatyeyeva et al. / Materials Chemistry

Wavenumber (cm-1)

35003250300027502500150012501000750

Ram

an in

tens

ity (a

.u.)

F2

2

tc42dB

n

w

3

3

ipoaIto

Fi

ig. 2. Raman spectra of the PA films (� = 514.5 nm, t = 240 s, P = 1.67 mW): 1 – PA-6; – PA-11; and 3 – PA-12.

.6. X-ray diffraction measurements

X-ray diffraction measurements were performed using a Panalytical diffrac-ometer Xpert Pro at room temperature to characterize the structure of theomposite polymer films. The Cu K� radiation source (� = 0.154 nm) operated at5 kV and 35 mA. X-ray patterns were recorded with the step size of 0.03◦ from = 3.5–70◦ . The 2 values were reproduced within ±0.1◦ variation. The inter-planar-spacing was calculated by substituting the scattering angels (2) of the peak inragg equation:

� = 2d sin (3)

here 2 is the X-ray diffraction angle and � is the X-ray wavelength (� = 1.54 A).

. Results and discussion

.1. Resonance Raman spectrometry study

The Raman behaviour of the virgin PA films was preliminarynvestigated in order to compare vibrational bands of PA in com-osite films with those observed in virgin PA (Fig. 2). For all typesf PA (PA-6, PA-11 and PA-12) the Raman spectra are rather similar

nd all vibrational bands corresponding to polyamide are present.n particular, we observed the C–N–C stretching mode at 930 cm−1,he C–C stretching modes at 1064–1127 cm−1, the stretching bandf the carbonyl group vibration at 1642 cm−1, the stretching vibra-

Wavenumber (cm-1 )

100 0 1200 140 0 160 0 18 00

Ram

an in

tens

ity (a

.u.)

ig. 3. Raman spectra of the composite films based on (a) PA-12 and (b) PA-6 films (� = 5n the form of EB; 4 – doped composite PA/PANI–HCl film; 5 – dedoped composite PA/PA

and Physics 130 (2011) 760– 768

tions of the CH2-groups at 2852, 2889 and 2932 cm−1 as well asthe symmetric deformation at 1372 cm−1 and the in-phase twist ofthe CH2-groups at 1298 cm−1 [41,42]. The spectra also contain thevibrations of the N–CH2-groups at 2722 cm−1 and the N–H stretch-ing at 3300 cm−1 [42]. It should be noted that in the case of the PA-6film the vibrational bands, which correspond to the carbonyl andN–H groups are more intense than those observed for the PA-11and PA-12 films (compare, for example, the peaks’ relative inten-sity at 930, 1642 and 3300 cm−1, Fig. 2) because of the differencein the molecular structure of the studied polymer matrix.

Fig. 3 shows the Raman spectra of the composite PA-12/PANIand PA-6/PANI films in ES and EB oxidation states. It is clearly seenthat the Raman spectra of the composite films are mainly characte-rized by the PANI bands. However, the PANI bands in the case of thecomposite films are shifted in comparison with those of pure PANI.In the spectrum of the doped sample based on PA-12 (Fig. 3a, curve4) the semiquinone radical >C–N+• band at 1320–1340 cm−1 [34](Fig. 3a, curve 2) is significantly shifted towards 1347–1378 cm−1,while the dedoped composite sample (Fig. 3a, curve 5) shows a neg-ligible shift of the band at 1165 cm−1 (C–H bending of the quinoidring) [29] (Fig. 3a, curve 3) to 1169 cm−1 and the band at 1409 cm−1

(C–C stretching of the quinoid ring) to 1413 cm−1. Moreover, the>C–N+· band at 1351 cm−1 can be observed on the Raman spectrumof the composite PA-12/PANI in the EB state (Fig. 3a, curve 5). Thismay be explained by the spatial difficulties of the doping-dedopingprocess realization, because in the case of the composite materialthe polymer matrix hinders the diffusion of ions of the dopant anddedoping agent out of and in it. This feature can explain the uncom-pleted dedoping process of PANI in the composite film, that resultsin the appearance of the >C–N+· stretching vibrations on the Ramanspectrum.

In the case of the PA-6 film as a polymer matrix (Fig. 3b) onecan observe similar behaviour as in the case of the composite filmbased on PA-12. Namely, on the Raman spectra of the compositefilm only characteristic bands of PANI are present (Fig. 3b, curves 4and 5). For both the PA-6/PANI composite film and the PA-12/PANIone, the shift of the vibrational bands is observed. However, forthe PA-12 film the bands are broader (Fig. 3a) than for the PA-6film (Fig. 3b). The other interesting observation is the presence ofvery strong bands at 1193 and 1625 cm−1 for the doped compositePA-6/PANI–HCl film (Fig. 3b, curve 4). These bands correspond tothe C–H in-plane bending vibrations of the benzenoid rings andC–C ring stretching vibration of the benzenoid ring, respectively.

For the dedoped samples (Fig. 3b, curve 5) the band at 1169 cm−1,corresponding to the C–H bending of the quinoid ring is also presentin the case of the composite PA-6/PANI film. Such behaviour maybe explained by, at least, two reasons: by a higher quantity of the

Wavenumber (cm-1)1000 1200 1400 1600 1800

14.5 nm, t = 300 s, P = 0.04 mW): 1 – virgin PA; 2 – PANI in the form of ES; 3 – PANINI film. The intensity of the PA spectrum (1) was multiplied by a factor 10.

K. Fatyeyeva et al. / Materials Chemistry

2 θ (deg.)403020100

Inte

nsity

(a.u

.)

12

3

Fc

fi

ogocfimeott

stepcfi

3

opao1c

sp

TV1

ig. 4. X-ray diffraction patterns of the virgin PA-11 (1), PANI–HCl (2) and theomposite PA-11/PANI–HCl film (3).

ormed PANI in the PA-6 film and by a higher doping level of PANIn the case of the polymer matrix based on PA-6.

It should be noticed that the conducting PANI containing layerbtained for such surface conducting composites is rather homo-eneous [20]. The thickness of this layer is found to be dependentn the polymer matrix – 2.2 ± 0.2 �m in the case of the PA-12/PANIomposite film [20] and 4.7 ± 0.2 �m in the case of the PA-6/PANIlm. These results also indicate the influence of the polymer matrixolecular structure on the properties of the obtained surface lay-

red composite materials. It should be noted that the Raman spectraf the PA-11/PANI composite films (not shown) are very similar tohe Raman spectra of the composite films based on PA-12 due tohe similarity of the PA molecular structure.

One can suppose that the presence of the PANI conductingurface layer confirmed by the confocal Resonance Raman spec-rometry may result in the appearance of interfacial polarizationffects. Besides, the observed modifications occurring in the com-osite films due to the acid doping also may result in significanthanges in the dielectric and electric behaviour of the compositelms.

.2. X-ray diffraction study

The X-ray diffraction analysis was used to identify the structuref conducting PA/PANI composite films. The measurements wereerformed using the composite film based on the PA-11 matrix asn example. Fig. 4 shows the wide-angle X-ray diffraction patternsf the PA-11 virgin film and pure PANI as well as of the doped PA-1/PANI composite film. The 2 and d-spacing values of the samplesalculated according to Eq. (3) are summarized in Table 1.

The X-ray diffraction pattern of the PA-11 film (Fig. 4, curve 1)hows the characteristic of both � and � phases, i.e. the principalolymorphic structures found in polyamides [43]. The � phase con-

able 1alues of the inter planner distance for the PA-11, PANI–HCl and composite PA-1/PANI films.

Sample 2 angle (deg.) (inter-planner d-spacing distance (Å))

PA-11 7.8 (11.3); 11.5 (7.7); 21.8 (4.1)PANI–HCl 10.8 (8.2); 15.9 (5.6); 20.8 (4.3); 24.4 (3.6)PA-11/PANI–HCl 7.6 (11.6); 11.4 (7.7); 21.9 (4.1); 22.4 (3.7)

and Physics 130 (2011) 760– 768 763

sists of planar sheets of hydrogen-bonded chains, with these sheetsbeing stacked upon one another and displaced along the chaindirection by a fixed amount. Pleated sheets of methylene units withhydrogen bonding between sheets rather than within sheets char-acterize the � phase. The strong diffraction peak at 2 = 21.8◦ is thedistinctive feature of the � phase of PA-11. This peak arises from thedistance between the hydrogen-bonded chains [44]. The character-istic feature of the � phase is proven by the presence of diffractionpeaks at 7.8◦ and 11.5◦. Bragg diffraction peaks at 2 angles around10.8◦, 15.9◦, 20.8◦ and 24.4◦ can be found in the X-ray diffractionpattern of PANI–HCl (Fig. 4, curve 2). The diffraction peaks are rel-atively sharp and strong. The peak centred at 2 = 20.8◦ may beascribed to the periodicity parallel to the polymer chain, while thepeak at 2 = 24.4◦ may be caused by the periodicity perpendicular tothe polymer chain [45]. The peak at 2 = 20.8◦ represents either thecharacteristic distance between the ring planes of benzene ringsin adjacent chains or the close-contact interchain distance [46]. Ithas been found that the peak at 2 = 24.4◦ is stronger than that at2 = 20.8◦, which is similar to the case of highly doped ES [47].

The X-ray diffraction patterns of the virgin and PA-11/PANI–HClcomposite films indicate a slight increase of the inter-planner dis-tance in the case of the composite film (compare curve 1 and 3for the virgin and composite films, respectively, in Fig. 4) (Table 1).This expansion is likely due to the aniline polymerization inside thesurface layer of the PA matrix. So, this result suggests the hydrogen-bonding interactions between the oxygen atoms of polyamide andhydrogen atoms on nitrogen atoms of doped PANI. This kind ofinteractions is supposed to be the main one. At the same time, theelectrostatic attraction may play a secondary part. Deka et al. havefound that in the case of composite films based on poly(vinylidenefluoride-hexafluoropropylene) and PANI, the addition of dedopedPANI in the polymer matrix increases the broadening and decreasesthe intensity of the X-ray diffraction peaks [48]. This is explainedby the fact that the addition of PANI prevents the polymer chainreorganization and causes significant disorder in polymer chains[48]. In our case, i.e. in the case of the doped composite film, thehydrogen-bonding interaction between the polyamide matrix andhydrogen atom on nitrogen atoms of PANI is weakened owing to theexistence of doped ions (Cl−) in the PANI chains. But in any case, theobtained X-rays diffraction patterns testify to the existence of theinteraction between the polymer matrix and the PANI molecularchains.

3.3. Electrical and dielectric properties

It is obvious that the electrical properties of the PA films havechanged after the aniline polymerization and doping processes.Indeed, the surface dc-conductivity of the PA-12/PANI–HCl film isnine orders higher (namely ∼10−5 S cm−1 at 303 K) than that ofvirgin PA-12 (∼10−14 S cm−1). The temperature dependence of thesurface resistance of the composite film doped by HCl (Fig. 5, curve1) displays the semiconductor behaviour of such a composite in thewhole range of temperatures. The activation energy for the conduc-tivity as derived from the slope of Arrhenius plot (Fig. 5, curve 2)is estimated to be 0.075 ± 0.004 eV which is similar to the valuesobtained for the pure PANI powders. In particular, for PANI–HClthe activation energy has been found to be 0.076 ± 0.004 eV.

The dopant nature is very important for the electricalproperties of the composite films. The replacement of HCl dopantby HClO4 results in the decrease of the surface conductivity from5 × 10−5 S cm−1 to 6 × 10−6 S cm−1, respectively. This fact can be

explained by the size of the anion-dopants and, consequently, bytheir facility to penetrate inside the polymer matrix. As one can seefrom Fig. 6, the matrix nature also affects the conductivity value ofthe composite films depending on the quantity of methylene units

764 K. Fatyeyeva et al. / Materials Chemistry and Physics 130 (2011) 760– 768

Fig. 5. Temperature dependence of resistance (1) and corresponding Arrheniusplot (2) for the composite PA-12/PANI–HCl (7.5 wt.%) film. The value variation ispresented for each 10th value for clearness.

1000/T (K-1)3,0 3,5 4,0 4,5 5,0 5,5 6,0 6,5

1/R

(Ohm

-1)

10-6

10-5

10-4

1

2

3

Fig. 6. Arrhenius plot of conduction of composite films, based on different types ofPT

i(

RshuPiermwfibp

TA

10-2 10-1 100 101 102 103 104 105 106 107

ε"

0.00

0.02

0.04

0.06

0.08

0.10 PA-6PA-11 PA-12 HN fit

observed for the composite films with PANI in both dedoped anddoped states. This may be due to the additional polarization which

A: 1 – PA-6/PANI–HCl film; 2 – PA-11/PANI–HCl film; 3 – PA-12/PANI–HCl film.he value variation is presented for each 20th value for clearness.

n the chain. This agrees with the variation of the activation energysee Table 2).

As it was previously shown by the confocal Resonance micro-aman spectrometry [20] and by the X-ray diffraction study, theurface conducting PA/PANI films have a layered film structure andydrogen-bonding interaction between the PA and PANI molec-lar chains. Therefore, the layered film structure of the compositeA/PANI films and the PANI cluster organization within the insulat-ng host matrix [20] suggest an existence of interfacial polarizationffects causing the relaxation processes. Moreover, the additionalelaxation associated with the dipolar reorientation of chains seg-ents or side groups in the polymer matrix may influence thehole spectrum of the composite PA/PANI film. In order to con-

rm these assumptions the measurements of dielectric and electricehaviours of the synthesized conducting layered PA/PANI com-osite films were performed.

able 2ctivation energy for the doped PA/PANI films calculated from Fig. 6.

Type of PA Activation energy Ea (eV)

PA-6 0.051 ± 0.004PA-11 0.064 ± 0.003PA-12 0.075 ± 0.004

Frequency (Hz)

Fig. 7. The dielectric loss ε′′ of the PA films as a function of frequency at 233 K.

Firstly, the dielectric behaviour of the virgin PA host matrix wasstudied. The dielectric spectra obtained for the virgin PA-6, PA-11and PA-12 films at 233 K are shown in Fig. 7. The solid lines arethe best fit to HN equation (Eq. (1)). At least two polarization pro-cesses related to molecular motions are observed for all types ofPA. The polarization process of weak intensity at high frequen-cies is known to be a �-relaxation. It is obviously caused by thelocal motion of chain segments, mainly (CH2) segments, locatedbetween the interchain hydrogen bonds [49,50]. A much more pro-nounced polarization process at lower frequencies is characterizedby a symmetric distribution of relaxation time ( = 0.5, = 1) andit may be related to �-relaxation due to a motion (rotation) of theamide bonds with water molecules that are bonded to them [49].As one can see, the film of PA-6 displays more distinct �-relaxation,which may be connected with a higher content of the amide groupsin comparison with PA-11 and PA-12 (Fig. 7). A freedom of motionof these amide bonds is assumed to be the cause of this relaxationprocess [49]. The temperature dependences of these relaxations(� and �) follow Arrhenius law (Eq. (2)) and the correspondingactivation energies are listed in Table 3. The calculated activationenergy for the � process is in good agreement with the previouslyreported activation energy for PA-12 [49] and for the aryl-aliphaticcopolyamides [50].

Fig. 8 presents frequency dependences of real and imaginaryparts of permittivity (ε′ and ε′′, respectively) for the PA-12/PANIfilm in the doped and dedoped states. The spectrum of the virginPA-12 film is given for comparison. The solid lines in both ε′ and ε′′

dependences fit to the measured data using HN function (Eq. (1)).It should be noticed that spectra in Fig. 8 have been depicted at193 K since at higher temperatures the �-relaxation process andionic conductivity hid all relaxations. The results indicate that thepresence of PANI in the composite film causes a great increaseof ε′ and ε′′ in the whole frequency region as compared to theinitial PA-12 film. It is important to note that these features are

emerges because of the presence of charge carriers in PANI evenin the dedoped state. However, the observed effects depend on

Table 3Activation energies of polyamides.

Type of PA Activation energy Ea (eV)

ˇ

PA-6 0.326 0.525PA-11 0.436 0.737PA-12 0.379 0.688

K. Fatyeyeva et al. / Materials Chemistry and Physics 130 (2011) 760– 768 765

) as a function of frequency for the composite PA-12/PANI film (7.5 wt.%) at 193 K.

bao(ntr(oa

ictAsvfitdi[

xaiPiacfsiiPtft(irsbtct

Table 4Fit parameters according to Arrhenius equation (Eq.(2)) for the doped PA-12/PANIfilms.

Acid-dopant HCl HClO4

parameter �1 �� �1 ��

composite film doped by HCl as a function of the temperaturehave shown that at high temperatures (more than 293 K) all therelaxations are hidden or overlapped by the conductivity and �-relaxation process associated with the chain motion in the polymer

ε"

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

173 K193 K213 K 233 K 253 K HN fit

Fig. 8. The variation of dielectric constant ε′ (a) and dielectric loss factor ε′′ (b

oth a doping degree and a dopant nature. In the virgin PA filmnd dedoped composite film only two relaxation processes arebserved (Fig. 8) – the �-relaxation in the high frequency regionclose to ∼105 Hz) and the �-relaxation process related to a mecha-ism of motion (rotation) of the amide bonds with water moleculeshat are bonded to them [49]. Thus, a higher strength of the �-elaxation in the case of the dedoped PA/PANI composite filmFig. 8) can be explained by the hydrogen-bonding interactions ofxygen atoms of polyamide and the hydrogen atoms on nitrogentoms of dedoped PANI (Fig. 1).

In the case of the doped PA-12/PANI composite film, the greatncrease in ε′ (Fig. 8a) can be explained by an additional polarizationaused by the high concentration of charge carriers in conduc-ive PANI particles dispersed within the insulating polymer matrix.s the conductivity of the dedoped PA-12/PANI composite film isimilar to the conductivity of the virgin PA-12 film such greatariation of ε′ and ε′′ is not observed in the dedoped PA-12/PANIlm. A similar behaviour was reported by Rocha et al. in the case ofhe PVDF/poly(o-methoxyaniline) (PVDF/POMA) blends with bulkistribution of the conducting phase, which were prepared by cast-

ng the blended solution of POMA and PVDF in dimethylacetamide51].

On the contrary, the PANI doping causes the additional rela-ation process labelled �1 (Fig. 8b), which is a consequence of thebove-mentioned additional polarization phenomena along thenterface between the conducting PANI clusters and the dielectricA matrix. In its turn, these phenomena result from the appearancen the system of positive charges at the PANI units and of negativecid anions which compensate these charges. Indeed, such ahange should lead not only to additional Coulomb interactionsollowed by new mechanical stresses in the composite, but also toome redistribution of physical and chemical intra/intermolecularnteractions inside the film resulting in some polarization of thenterface between the conducting PANI clusters and the insulatingA matrix. Hence, the additional relaxation �1 can be attributedo the interaction between PA and doped PANI. Also, it has beenound that the nature of the acid dopant significantly affects bothhe position of this relaxation peak �1 and its value to some extentFig. 8). One can see that the relaxation �1 for the composite filmn the case of PANI doped by HClO4 is shifted to a higher frequencyegion (Fig. 8b). Obviously, the dopants of different molecularize and charge distribution should affect not only the relaxation

ehaviour of the whole material, but also the charge transporta-ion ability along its conductive paths, i.e. its electrical percolationharacteristics. It has been found by the AFM measurements forhe surface composite films based on PANI and PET that HClO4

�0 (s) 2.0 × 10−9 7.1 × 10−14 1.0 × 10−11 7.1 × 10−14

Ea (eV) 0.115 0.536 0.174 0.536

induces much stronger changes in the topographical relief of thecomposite film as compared with HCl [21]. So, in the case of dopingby HClO4 the reorganization of the amorphous part of PA and PANIcan be observed, i.e. the interface between the conducting PANIclusters and the insulating PA matrix will be also changed.

The activation energy is slightly higher in the case of using HClO4acid as a dopant in comparison with the activation energy for HCl-doped PANI (Table 4). Such a feature implies that in the case of thisacid, the conductivity should be less than in the case of HCl. Andreally, as stated above, the conductivity for the film doped by HCl is5 × 10−5 S cm−1 and for the film doped by HClO4 is 6 × 10−6 S cm−1.

The performed dielectric relaxation measurements for the

Frequency (Hz)10-1 10 0 101 102 103 104 105 106

Fig. 9. Frequency dependencies of ε′′ for the composite PA-12/PANI (7.5 wt.%) filmdoped by HCl.

766 K. Fatyeyeva et al. / Materials Chemistry and Physics 130 (2011) 760– 768

1000 /T (K-1)3.5 4.0 4.5 5.0 5. 5 6.0

rela

xatio

n tim

e τ

(s)

10-8

10-7

10-6

10-5

rela

xatio

n tim

e τ

(s)

10-4

10-3

10-2

10-1

100

101

τHCl

τHClOτβ

FP

mtdtpAt

Table 5Fit parameters for the doped PA/PANI films.

Type of PA PA-6 PA-11

Parameter �1 �2 �� �1 ��

ig. 10. Arrhenius plots of relaxation processes observed in the doped compositeA-12/PANI (7.5 wt.%) film.

atrix, which dominate the spectra. As one can see in Fig. 9,he relaxation process �1 is temperature-dependent. The obtainedependencies are shown in Fig. 10 and the obtained fit parame-

ers can be found in Table 4. The activation energy of the relaxationrocess �1 has rather small values in comparison with �-relaxation.s stated above, the surface composite PA-12/PANI film consists of

wo layers – the first one being the conducting doped PA/PANI layer

Frequency (Hz)10-2 10-1 100 101 102 103 104 105 106 107

ε"

0.00

0.02

0.04

0.06

0.08PA-11PA-11/PANI dedo pedPA-11/PANI do ped HCl HN fit

β

γ

τ1

(a)

ε"

Fig. 11. The variation of dielectric loss factor ε′′ as a function of frequency

1000/T (K-1)

4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4 5.6 5.8 6.0

rela

xatio

n tim

e τ

(s)

10-4

10-3

10-2

10-1

100

101

rela

xatio

n tim

e τ

(s)

10-8

10-7

10-6

10-5

10-4

τβ

τ1

τ2

(a)

rela

xatio

n tim

e τ

(s)

Fig. 12. Arrhenius plot of relaxation processes observed in the d

�0 (s) 1.3 × 10−6 1.2 × 10−8 5.8 × 10−18 4.4 × 10−10 4.5 × 10−15

Ea (eV) 0.039 0.052 0.673 0.149 0.555

and the second one being the virgin polymer matrix. Thus, one canassume that such layered structure of the composite film leads tothe interfacial polarization that, in its turn, will lead to Debye typerelaxation process [39]. It may be supposed that the charge carriersin the PANI clusters are responsible for this phenomenon.

The polymer matrix structure can affect the properties of thefinal composite film. As shown in Figs. 8 and 11, the dielectricbehaviour of different polyamides (PA-6 and PA-11 (Fig. 11) andPA-12 (Fig. 8)) differs a little in strength of the �- and �-relaxations.Besides, the presence of dedoped PANI in the composite films leadsto some growth of the relaxation strength.

The PANI doping process changes significantly the dielectricspectra of composites. Despite the polymer matrix, the doped com-posite films preserve the relaxation process which is caused by the

�-relaxation of PA. But they also reveal the additional relaxationprocess �1 in the investigated ranges of temperature and frequency.The additional relaxation process is, obviously, connected with thepresence of charge carriers in the PANI containing layer. Moreover,

Frequency (Hz)10-2 10-1 100 101 102 103 104 105 106 107

0.00

0.02

0.04

0.06

0.08

0.10

0.12PA-6PA-6/PANI dedopedPA-6/PANI do ped HClHN fit

β γ

τ1

τ2

(b)

for the composite PA-11/PANI (a) and PA-6/PANI (b) films at 193 K.

1000/T (K-1)4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4 5.6 5.8 6.0

10-7

10-6

10-5

10-4

rela

xatio

n tim

e τ

(s)

10-4

10-3

10-2

10-1

100

101

τβ

τ1

(b)

oped composite PA-6/PANI (a) and PA-11/PANI (b) films.

mistry

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cFtTotp[vot

rcbptitst

dp

P

pfittabofcp

4

cPpdttmlt

aPat

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K. Fatyeyeva et al. / Materials Che

t should be noted that the shape and position of this relaxationrocess depend on the PA matrix used.1 From Figs. 8b and 11a onean see that for the composite films based on the PA-12 and PA-1 matrix, respectively, one broad relaxation �1 is observed closeo ∼7 × 104 Hz. However, the composite PA-6/PANI film (Fig. 11b)xhibits somewhat different behaviour and in the case of HCloping two additional relaxation processes labelled �1 and �2 arebserved.

The temperature dependencies of these relaxation peaks for theomposites based on the PA-6 and PA-11 matrix are presented inig. 12 and the fit parameters determined by fitting the resultso Arrhenius equation (Eq. (2)) for the doped films are shown inable 5. The second peak (�2) observed in the composite film basedn PA-6 has an activation energy of 0.052 eV. This value is close tohe value found in pure PANI and, therefore, the second relaxationeak may be attributed to the interfacial polarization in PANI itself15]. On the other hand, analyzing the peak shape and the obtainedalues of the activation energy for �1 in PA-11 and PA-6 (Table 5)ne can see that the relaxation peak �1 for the former is broaderhan for the latter.

It should be noted that in the PA based composite films, theelaxation process �1 occurs at higher frequencies compared to theomposite based on the PET film [21]. This result may be explainedy higher conductivity values obtained for the PA films in com-arison with those determined for the PET films. It is known thathe relaxation frequency is proportional to the conductivity. So, thencrease of the frequency by four decades leads to the increase ofhe conductivity value by four orders. Indeed, the resistance of theurface conductive layer of the PA/PANI film is four orders lowerhan in the case of the PET/PANI film.

Taking into account that the relaxation time value variesepending on the type of PA (Tables 4 and 5), the studied com-osites can be placed in the following order:

A-6/PANI–HCl > PA-12/PANI–HCl > PA-11/PANI–HCl

It is logical to suppose that the conductivity value of these com-osite films varies in the same order. However, for the compositelm based on the PA-11 matrix, the result contradicts the conduc-ivity measurements (Fig. 6), i.e. this film is a little more conductivehan the PA-12/PANI–HCl composite film. But, as it was mentionedbove, the relaxation peak �1 in the case of the composite filmsased on PA-11 and PA-12 is rather broad and according to thisbservation one may suggest that the relaxation process �1 was inact an overlap of two relaxation processes (�1 and �2). Thus, theontribution of the relaxation peak �2 to the PA-11 matrix is moreronounced than to the PA-12 matrix.

. Conclusion

In the present study the polymer surface layered conductingomposite films based on different aliphatic PA (PA-6, PA-11 andA-12) and PANI were synthesized by in situ oxidative anilineolymerization inside the PA host matrix. Structural, electrical andielectric properties of these composite PA/PANI films were inves-igated. The performed Raman studies revealed the influence ofhe oxidation state of PANI as well as the structure of the poly-

er matrix on the vibrational spectra of the composite films. Theayered structure of the surface conductive composites results inhe appearance of an interfacial polarization relaxation process. The

1 It should be pointed out that the spectra in Fig. 11 are presented at 193 K, sincet the temperature higher than the glass transition temperature (323 K and 313 K forA-6 and PA-11, respectively) these relaxations are no longer visible because of theppearance of the ionic conductivity and the �-relaxation process, which dominatehe spectra.

[[[[[[

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[[[

and Physics 130 (2011) 760– 768 767

relaxation frequency of the process is found to be proportional tothe dc-conductivity. These two physical properties (relaxation pro-cess and conductivity) are dependent upon the dopant nature andthe matrix structure.

It has been stated that in such composite films, besides the sub-glass relaxation processes (� and �) taking place in the virgin matrixand in the dedoped PA/PANI composites, additional relaxation pro-cesses related to the composite conductive properties occur in thedoped composite. Interfacial polarization relaxations in the MHzregion are attributed to the layered and clustered structure ofthe composite. At higher frequency, the conductivity relaxationappears to be connected with the interfacial polarization in thePANI clusters.

It is known that the composite materials having electricalconductivity in the range of 10−4–10−11 S cm−1 can be used asantistatic/electrostatic discharge (ESD) materials. Therefore, as theconductivity of the obtained PA/PANI surface composite films is inthis range, these films may be also used as the ESD materials.

Acknowledgements

The authors are grateful to Anne-Marie Mercier from Labo-ratoire des Oxides et Fluorures of University of Maine (France)for providing the X-ray diffraction measurements. K. Fatyeyeva isthankful to French Government for the financial support.

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