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The influence of the polymer matrix on the dielectric and electrical properties of conductive polymer composites based on polyaniline
Journal of Non-Crystalline Solids 351 (2005) 2835–2841 www.elsevier.com/locate/jnoncrysol
The influence of the polymer matrix on the dielectric and electrical properties of conductive polymer composites based on polyaniline M. Tabellout a
a,*
, K. Fatyeyeva
a,b
, P.-Y. Baillif a, J.-F Bardeau a, A.A. Pud
b
Laboratoire de Physique de lÕEtat Condense´, UMR CNRS 6087, Universite´ du Maine, Avenue Olivier Messiaen, 72085 Le Mans cedex 9, France b Institute of Bioorganic Chemistry and Petrochemistry, National Academy of Sciences of Ukraine, 50 Kharkovskoye Shosse, 02160 Kiev, Ukraine Available online 2 August 2005
Abstract The dielectric properties of conducting polymer composite films based on polyaniline located in a thin layer in the vicinity of the surface of different insulating polymer matrices (polyamide and poly(ethylene terephthalate)) have been studied using dielectric relaxation spectroscopy in a wide temperature and frequency range. Several relaxation processes related to the film surface conductivity and influenced by the nature of the polymer matrix have been found in these composites. The nature of the polymer matrix is found to influence the rate of aniline polymerization, the distribution of polyaniline clusters and the depth of aniline penetration as shown, respectively, by Conducting Probe AFM and Raman spectroscopy. These features are correlated to the dielectric behavior of the composite films. Ó 2005 Elsevier B.V. All rights reserved. PACS: 73.61.Ph
1. Introduction The development of intrinsically conducting polymers (ICP) has attracted great attention from the scientific community due to the increasing potential for technological applications in electrochromic devices, sensors, electrolytic capacitors, rechargeable batteries, etc. Among these polymers, polyaniline (PANI) generates a special interest owing to its relative low cost, stability, and a good combination of optical, catalytic, conductive and sensor properties. However, a major problem still exists in a practical use of PANI and other
*
Corresponding author. Tel.: +33 243833552; fax: +33 243833518. E-mail address: [email protected] (M. Tabellout). 0022-3093/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.04.085
ICP because of their infusibility, insolubility and poor mechanical characteristics. An alternative approach is the use of PANI in a form of conducting composites or blends with conventional polymers [1]. The obtained composites combine good conductive properties with mechanical and even optical properties of the polymer matrix. This allows a development of polymeric composites with specific properties and shapes. The composite films used in this study were obtained by in situ polymerization of absorbed aniline in thin surface layers of a polymer matrix leading to a double layer film [2,3]. The layer containing PANI becomes reversibly conductive/non-conductive by acid doping/ dedoping. Obviously, conductive properties in these composites will depend on the molecular organization of the conductive clusters with respect to the polymer matrix. In this work, in order to investigate the influence of
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M. Tabellout et al. / Journal of Non-Crystalline Solids 351 (2005) 2835–2841
the matrix nature on the dielectric properties of the composites, two different composite films based on polyamide 6 (PA6) and poly(ethylene terephthalate) (PET) matrices have been synthesized and studied using dielectric relaxation spectroscopy (DRS), conducting probe AFM (CPAFM), X-ray scattering and Raman spectroscopy.
Electrode Insulating layer Layer containing PANI Layer containing PANI Insulating layer Electrode
2. Experimental 2.1. Materials The studied composites were prepared with both PA6 and PET films as the polymer matrix. These films purchased from Goodfellow Cambridge Ltd are 25 lm thick and unaxially oriented for PA6 and 20 lm thick and biaxially oriented for PET. The films were swelled up to 10 wt% content of aniline and were further immersed into an oxidant solution to polymerize in accordance with [2–4]. The final composite film consists of two layers. The first one, with a thickness varying from 2 to 4 lm depending on the matrix, consists of PANI clusters distributed in the host matrix. The second one is the pure matrix. Composite film doping is performed by exposition of the film surface containing PANI to a 37% fuming HCl acidic atmosphere for PET/PANI composite or by immersion into a 5% solution of HCl for PA6/PANI film. The doping process leads to a change of the film color from blue to green related to an increase of the film surface conductivity by several orders of magnitude. Pure polyaniline powder samples were obtained by aniline oxidative polymerization procedure using ammonium persulfate in an HCl solution [5]. Dedoping process was performed in 5% NH4OH solution. 2.2. Experimental techniques 2.2.1. Dielectric relaxation spectroscopy measurements Dielectric permittivity measurements were performed using a Novocontrol broadband dielectric spectrometer (Novocontrol GmbH, Germany) in a wide frequency (0.1 Hz–10 MHz) and temperature (153–293 K with an accuracy of ±0.1 K) ranges using a plan capacitor geometry. Two pieces of film were placed in between two round plate electrodes according to the scheme in Fig. 1. The surfaces containing PANI were facing each other in order to respect the sample symmetry. For pure PANI samples, the electrical and dielectric measurements were performed on cylindrically shaped and compressed pellets using an HP 4291A impedance analyzer in the frequency range 1 MHz–1 GHz. The sample placed in between two round plate electrodes is used as a termination of a golden coaxial line and the electric impedance was calculated from the complex reflection factor at the end of the line [6].
Fig. 1. Sample cell configuration for dielectric measurements of the composite films.
2.2.2. AFM electrical imaging The electrical images of the conductive surface of the film were obtained with a home-built extension of a Digital Instruments Nanoscope III in contact mode (called ÔResiscopeÕ) [7]. 2.2.3. Raman spectroscopy A Jobin-Yvon Horiba T64000 spectrometer (excitation line k = 514.5 nm) was used to determine the thickness of the layer containing PANI in the two composite films. 2.2.4. X-ray scattering X-ray diffraction measurements using a Philips XÕpert diffractometer were performed on polymer matrix samples to get structural informations, mainly, a qualitative comparison of the crystallinity extent of the polymer matrices.
3. Results and discussion It has been shown by standard AFM [8] that the doping of films based on PANI and a polymer matrix such as PET drastically modifies the film surface topography showing a re-organization of PET and PANI flexible chains under the influence of the acid. This morphological change is combined with a modification of the electrical properties of the film surface. The electrical images (Fig. 2) performed on the surfaces of the layers containing PANI revealed a distribution of conducting clusters, with a resistance value around 105 X dispersed in a matrix having a high resistance value (over 1011 X). In the case of PET matrix based film (Fig. 2(a)), the conducting clusters are rather dispersed and no clear evidence of percolation is observed in the lateral dimensions of the layer containing PANI. In contrary, for PA6 based samples, the conductive clusters occupy a large part of the film surface (Fig. 2(b)). It has been shown [9] that the percolation threshold is, in the latter case, reached for only 4 wt% of PANI. As a result, the electrical macroscopic properties are different: the film surface resistance, measured with a standard 4 probes technique, is found to be 20 kX and 4 MX [10], respectively, for
M. Tabellout et al. / Journal of Non-Crystalline Solids 351 (2005) 2835–2841
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Fig. 2. Electrical images of doped PET/PANI (a) and PA6/PANI (b) film surfaces obtained by CP-AFM.
e ¼ eu þ
N X
Dei ai bi
i¼1
ð1 þ ðixsi Þ Þ
þ
rdc ; ie0 x
ð1Þ
where x = 2pf is the angular frequency with f being the frequency. Dei, si, ai and bi are, respectively, the relaxation strength, the relaxation time, the symmetrical and asymmetrical distribution parameters for the ith relaxation process. rdc is the dc conductivity and eu the unrelaxed dielectric permittivity. A Debye type relaxation corresponds to a = b = 1.
0.07
τ1
153K 173K 193K 213K 233K 253K 273K 293K
0.06 0.05 0.04
ε"
τ2
0.03 0.02 0.01
τ3
(a)
0.00 10-1
100
101
102
103
104
105
106
Frequency (Hz)
(a) 0.16 0.14 0.12 0.10
ε"
PA6 and PET based films at room temperature. This difference cannot be attributed to the initial concentration of aniline in the swelled film and, correspondingly, to the PANI content. The samples being prepared with the same route, in both cases the virgin films took up roughly the same amount of aniline (around 10 wt%). It was also shown by Raman spectroscopy [10,11] that the thickness of the layer containing PANI was larger in PA6 (4 lm) than in PET (2 lm). The nature of the polymer matrix may most probably explain the encountered differences, as it will be discussed later. The observed drastic modifications occurring in the surface layer containing PANI, consequently to acid doping, may result in significant changes in the dielectric behavior of the doped films. The layered structure of the film (one layer being conductive and the other one insulating) and the cluster organization of PANI in the conductive layer will be traduced by the appearance of polarization phenomena. Therefore, in order to check the above assumptions, dielectric relaxation measurements were performed on both films in a wide temperature (173–293 K) and frequency (0.1 Hz–10 MHz) ranges (Fig. 3). To account for the relaxation times and their temperature dependence, dielectric relaxation spectra were analyzed according to the empirical Havriliak–Negami (HN) function [12]:
τ1
173K 183K 193K 203K 213K 223K 233K 243K
τ3
0.08 0.06 0.04 0.02 0.00 10-1
(b)
100
101
102
103
104
105
106
Frequency (Hz)
Fig. 3. Dielectric relaxation spectra (dielectric loss e00 ) of doped PET/ PANI (a) and PA6/PANI (b) films at different temperatures. The solid lines represent the best fit to Eq. (1).
The spectra obtained for the doped PET/PANI and PA6/PANI films at different temperatures are depicted in Fig. 3(a) and (b), respectively. The solid lines are fits
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M. Tabellout et al. / Journal of Non-Crystalline Solids 351 (2005) 2835–2841
Table 1 HN fit parameters for the relaxations processes labelled s1, s2 and s3 in doped PET/PANI film T (K)
s1 s (s)
De
a
b
s (s)
De
a
b
s (s)
De
a
b
153 173 193 213 233 253 273 293
3.68E02 2.11E02 1.43E02 8.50E03 5.79E03 4.15E03 3.30E03 2.83E03
0.10 0.09 0.10 0.10 0.09 0.08 0.08 0.08
1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1
1.16E03 6.59E04 4.59E04 2.76E04 1.78E04 1.14E04 7.93E05 7.11E05
0.05 0.04 0.06 0.03 0.05 0.04 0.03 0.02
0.96 0.94 0.71 0.93 0.83 1 1 1
0.84 1 1 1 0.78 0.53 0.42 0.76
5.99E06 2.45E06 7.85E07 1.29E06 3.86E07 2.28E07 1.06E07 1.25E07
0.09 0.12 0.11 0.19 0.20 0.30 0.32 0.29
0.50 0.38 0.41 0.26 0.28 0.21 0.22 0.29
0.51 0.58 0.75 1 0.89 1 1 0.89
s2
s3
Table 2 HN fit parameters for the relaxations processes labelled s1 and s3 in doped PA6/PANI film T (K)
s1 s (s)
De
a
b
s (s)
De
a
b
173 183 193 203 213 223 233 243
1.88E05 1.54E05 1.35E05 1.25E05 1.11E05 1.01E05 9.10E06 8.60E06
0.17 0.23 0.24 0.22 0.19 0.18 0.16 0.14
0.95 0.86 0.86 0.90 0.96 0.98 1 1
1 1 1 1 1 1 1 1
3.72E07 2.31E07 2.06E07 1.93E07 1.91E07 1.67E07 1.38E07 1.06E07
0.075 0.17 0.20 0.21 0.24 0.26 0.25 0.22
0.96 0.96 0.89 0.69 0.66 0.64 0.65 0.68
0.52 0.29 0.33 0.66 0.66 0.66 0.66 0.69
s3
with k being the Boltzman constant, s0 the relaxation time at very high temperature and E the activation energy. The temperature dependencies of the relaxation times are shown in the graphs of Fig. 4(a) and (b). The corresponding fit parameters are collected in Table 3. In order to understand the origin of the observed relaxation processes, the spectra obtained in the virgin, non-doped and doped films at T = 233K for PET/PANI are depicted in Fig. 5. The virgin film of PET exhibits a large relaxation attributed to a b relaxation process commonly associated to local movements such as side groups or chain segments. This peak is quite broad and symmetric and the relaxation times are charac-
-5
Relaxation time [s]
τ1 τ2 τ3
10-2
-6
10
10-3
Relaxation time [s]
10-1
10
-7
10
10-4 4
3
5
6
7
1000/T [K-1]
(a) 1e-4
τ1 τ3 Relaxation time [s]
to Eq. (1) and the derived fit parameters are collected in Tables 1 and 2, respectively, for PET and PA6 based composites. No ohmic losses are observed in the imaginary part of the dielectric permittivity due to the layered structure of the films, one layer being insulating and the other conductive. With this sample configuration, only ionic conductivity could be observed at temperatures above the glass transition temperatures which values are 348 and 323 K for PET and PA6, respectively. The characteristic relaxation times s were taken at the position of the maximum of dielectric loss for each relaxation process. The temperature dependences of these relaxation times were analyzed using an Arrhenius law: E s ¼ s0 exp ð2Þ kT
1e-5
1e-6
1e-7
1e-8 4
(b)
5
6
1000/T [K-1]
Fig. 4. Arrhenius plot of the characteristic relaxation times for the different relaxation processes in doped PET/PANI (a) and PA6/PANI (b) films. The solid lines represent the best fit to Eq. (2). The arrows indicate the corresponding Y-axis.
M. Tabellout et al. / Journal of Non-Crystalline Solids 351 (2005) 2835–2841
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Table 3 The fit parameters to Eq. (2) for doped PET/PANI and doped PA6/PANI films T (K)
PET/PANI
PA6/PANI
s1
s2
s3 (T < Tc)
s3 (T > Tc)
s1
s3
s0 (s) E (meV)
1.5E4 74
1.5E10 87
1.9E10 118
1.86E10 147
1.28E6 40
1E8 52
0.06
τ1
virgin PET non-doped PET-PANI doped PET-PANI
0.05 0.04
τ2
0.03
ε"
τ3
0.02 0.01 0.00 10-2
10-1
100
101
102
103
104
105
106
107
Frequency (Hz) Fig. 5. Frequency dependence of dielectric loss for virgin PET, dedoped PET/PANI and doped PET/PANI at T = 193 K. The solid lines represent the best fit to Eq. (1).
terized by a relatively high activation energy (E = 500 meV) typical of sub-glass relaxations in polymers. While the spectrum of non-doped PET/PANI is quite similar to that of virgin PET, the spectrum of doped PET/PANI film exhibits a totally different behavior showing 3 additional relaxation processes with low activation energy values (Table 3) characteristic of electronic transport. The first 2 peaks, starting from the low frequency side and labelled s1 and s2, are nearly Debye type relaxation processes (a 1, b 1) with activation energies of 74 meV for s1 and 87 meV for s2 (Fig. 4(a)). These values are much lower than those characterising dipolar re-orientation processes in polymers. These peaks are most likely related to interfacial polarization relaxations effects [13]. Specifically, one of these relaxations may be understood if we consider the doped layer constituted by conductive PANI clusters surrounded by less or non-conducting medium as it is observed on electrical images (Fig. 2). Such a dispersion is known to lead to a Maxwell–Wagner–Sillars (MWS) effect [13,14]. The other relaxation process may arise from the significant difference in conductive properties between the two layers, the first one being the conducting doped PET/PANI layer and the second one being the pure polymer matrix. This interface polarization leads also to a Debye type relaxation process. The third peak is rather broad (Table 1). Two Arrhenius laws in the investigated temperature range are used to describe the temperature dependence of the associated relaxation
time. The change of slope occurs at a critical temperature Tc = 240 K (Fig. 4) showing 2 processes: one with an activation energy of 147 meV at high temperatures and the other one, smaller, of 118 meV. This behavior has also been observed by EPR measurement [8] and was attributed to the thermally activated conversion between localized and delocalized paramagnetic centres. This relaxation might be connected to the hopping conduction process inside the PANI clusters. This hypothesis is supported by the electrical and dielectric measurements performed on pure PANI in its doped and dedoped forms in the frequency range 1 MHz– 1 GHz at different temperatures ranging from 143 to 243 K. The real part of the dielectric permittivity exhibits a relaxation process of a high dielectric strength in the hundred MHz region consequently to the doping process (Fig. 6(a)) whereas, in the dedoped form of PANI only a flat dielectric response is observed. Correspondingly, the real part of the conductivity spectrum (Fig. 6(b)) of doped PANI is dominated at low frequency by a dc-conductivity and exhibits at higher frequency an ac-conductivity increasing with frequency. The temperature dependences of the derived relaxation time and dc-conductivity are fitted to Eqs. (2) and (3), respectively: E rdc ¼ r0 exp kT
ð3Þ
with k being the Boltzman constant, r0 the conductivity at very high temperature and E the activation energy. A nearly identical activation energy (E = 28 meV) is found for both dc-conductivity and relaxation time indicating that the two processes originate from the same electronic transport mechanism. These values are much lower than that found for s3 relaxation process in PET/PANI. This difference may be attributed to the higher doping level reached in pure PANI in comparison to PET based composite due to the difficulties for the ions to diffuse through the matrix. The PA6/PANI film exhibits a rather different behavior. Two relaxation processes of low dielectric strength are visible in the virgin and non-doped PA6/PANI films (Fig. 7). The high frequency one can be attributed to a c relaxation process associated with local chain motions. The second process is most likely corresponding to a b relaxation process characterized by an activation energy of 600 meV and a symmetric distribution of
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M. Tabellout et al. / Journal of Non-Crystalline Solids 351 (2005) 2835–2841 140 143K 163K 193K 233K dedoped PANI
120 100
ε'
80 60 40 20 0 106
107
(a)
108
109
Frequency (Hz) 10-1
σ' [S/cm]
10-2
10-3 143K 163K 193K 233K
10-4 106
107
108
109
Frequency (Hz)
(b)
Fig. 6. Frequency dependence of the real parts of dielectric permittivity (a) and conductivity (b) for PANI doped by HCl at different temperatures and for dedoped PANI at 193 K. Solid lines are the best fit to Eq. (1).
0.12 0.10
virgin PA6 non-doped PA6-PANI doped PA6-PANI
ε"
0.08
relaxation strength. The HCl doping of PA6/PANI composite leads to the appearance of only two additional relaxation processes (Fig. 7) in the experimental temperature and frequency ranges. Above the glass transition temperature these relaxations are no longer visible due to the appearance of the ionic conductivity and the a relaxation process which dominate the spectra. The two additional peaks observed in doped PA6/ PANI are labelled s1 and s3. As compared to PET based film, the second relaxation process s2, if any, may have shifted to higher frequency or disappeared. As no relaxation process was found at higher frequency, the relaxation process s2 may have, most probably, disappeared due to the quasi-homogeneity of the layer containing PANI as it can be observed in Fig. 2(b). The relaxation time s1, which is a Debye type relaxation process (a 1, b 1) decreases by 3 orders of magnitude in comparison to PET/PANI implying a higher conductivity of the PA6/PANI layer. Moreover, its activation energy (E = 40 meV) (Fig. 4(b)) is also two times lower than that of PET/PANI composite. The second peak s3 has an activation energy (E = 52 meV) (Fig. 4(b)) closer to that found in pure PANI and hence is attributed to conductivity relaxation in PANI as discussed before. The general feature resulting from this comparison is the decrease of the relaxation time s1 by 3 orders of magnitude accompanied by a decrease of the activation energy in PA6 based composite if compared to the PET based one. This result is in conformity with the conductivity values obtained by the standard 4 probes technique and correlated with the electrical images of the surface of the two films. In doped PA6/PANI film, the electrical images clearly show a percolation of the conductive clusters in the layer containing PANI. The explanation of this behavior can be found in the matrix structure. X-ray diffraction measurements were performed for this aim on both virgin films in order to qualitatively determine the relative extent of crystallinity. It is clear from the difference in the peak widths in
0.06 1
0.02 0.00 10-2
10
-1
10
0
10
1
10
2
10
3
10
4
10
5
10
6
10
7
Frequency (Hz) Fig. 7. Frequency dependence of dielectric loss for virgin PA6, dedoped PA6/PANI and doped PA6/PANI at T = 233 K. The solid lines represent the best fit to Eq. (1).
Normalized intensity
0.04
PA6 PET
0.8 0.6 0.4 0.2 0 -10
-5
0
5
10
Shifted diffraction angle ( 2θ)
relaxation time (a 0.4, b = 1). The only effect of the presence of non-doped PANI is the increase of the b
Fig. 8. Normalized X-ray scattering spectra of PA6 and PET virgin films.
M. Tabellout et al. / Journal of Non-Crystalline Solids 351 (2005) 2835–2841
Fig. 8 that PA6 is of less crystallinity extent. Therefore, although the preparation route and the aniline content were the same (10 wt%), the resulting amount of PANI, its organization and even its doping level are strongly affected by the crystallinity of the matrix. The more flexible molecular chains (PA6) allow a freer organization of PANI leading to a percolation of the conductive clusters and enable a higher doping level. 4. Conclusion Surface conductive films, based on doped and dedoped PANI located in two different polymer matrices (PA6 and PET) have been studied in a wide temperature and frequency ranges using dielectric relaxation spectroscopy. At low temperature, in addition to sub-glass relaxation processes occurring in the polymer matrix and in the non-doped composite as well, 3 relaxation processes are observed in the doped one related to its conductive properties. In the low frequency region, interfacial polarization relaxations are attributed to the layered and clustered structure. The clustered structure is confirmed by the electrical images obtained by CP-AFM on the film conductive surface. At higher frequency, conductivity relaxation appears to be connected to the conductivity in the PANI clusters. The nature of the polymer matrix is found to influence these relaxations by frequency shift, change in relaxation strength and activation energy. In order to investigate these effects, X-ray scattering and Raman spectroscopy studies have been performed and have shown that the matrix crystallinity may contribute to these effects. In the most amorphous PA6 matrix, the PANI containing layer is thicker and percolation between the conductive clusters is reached. Thus, these results give evidence that the polymer matrix structure
2841
plays an important role in the conductive properties of the film.
Acknowledgements The authors are grateful to O. Schneegans and F. Houze´ of the Laboratoire de Ge´nie Electrique de Paris, UMR CNRS 8507, Supe´lec, Universite´ Paris VI et Paris XI, rue Juliot Curie, 91192 Gif-sur-Yvette Cedex, France for providing the CP-AFM measurements.
References [1] A. Bhattacharya, A. De, Prog. Solid State Chem. 24 (1996) 141. [2] A.A. Pud, S.P. Rogalsky, G.S. Shapoval, A.A. Korzhenko, Synth. Met. 99 (1999) 175. [3] A.A. Pud, G.S. Shapoval, V.P. Kukhar, Ukrainian Claim for Invention Rights N94010153 of 11.05.93. [4] A.A. Pud, N. Ogurtsov, A.A. Korzhenko, G.S. Shapoval, Prog. Polym. Sci. 28 (2003) 1701. [5] A.G. MacDiarmid, Y.N. Xia, J.M. Wiesinger, US Patent 1995.-IC6 C08F 6/00.-No. 5403913. [6] F. Kremer, M. Arndt, in: J.P. Runt, J.J. Fitzgerald (Eds.), Dielectric Spectroscopy of Polymeric Materials, American Ceramic Society, 1997 (Chapter 2). [7] F. Houze´, R. Meyer, O. Schneegans, L. Boyer, Appl. Phys. Lett. 69 (1996) 1975. [8] A.A. Pud, M. Tabellout, A. Kassiba, A.A. Korzhenko, S.P. Rogalsky, G.S. Shapoval, F. Houze´, O. Schneegans, J.R. Emery, J. Mater. Sci. 36 (2001) 3355. [9] S.W. Byun, S.S. Im, Polymer 39 (1998) 485. [10] M. Tabellout, K. Fatyeyeva, P.-Y. Baillif, J.-F Bardeau, P. Ruello, A.A. Pud, in press. [11] A.A. Korzhenko, M. Tabellout, J.R. Emery, A.A. Pud, S.P. Rogalsky, G.S. Shapoval, Synth. Met. 98 (1998) 157. [12] A.K. Jonscher, Dielectric Relaxation in Solids, Chelsea Dielectric, 1996. [13] B.K.P. Scaife, Principles of Dielectrics, Clarendon, Oxford, 1989. [14] E. Tuncer, J. Phys. D: Appl. Phys. 37 (2004) 334.
The influence of the polymer matrix on the dielectric and electrical properties of conductive polymer composites based on polyaniline M. Tabellout a
a,*
, K. Fatyeyeva
a,b
, P.-Y. Baillif a, J.-F Bardeau a, A.A. Pud
b
Laboratoire de Physique de lÕEtat Condense´, UMR CNRS 6087, Universite´ du Maine, Avenue Olivier Messiaen, 72085 Le Mans cedex 9, France b Institute of Bioorganic Chemistry and Petrochemistry, National Academy of Sciences of Ukraine, 50 Kharkovskoye Shosse, 02160 Kiev, Ukraine Available online 2 August 2005
Abstract The dielectric properties of conducting polymer composite films based on polyaniline located in a thin layer in the vicinity of the surface of different insulating polymer matrices (polyamide and poly(ethylene terephthalate)) have been studied using dielectric relaxation spectroscopy in a wide temperature and frequency range. Several relaxation processes related to the film surface conductivity and influenced by the nature of the polymer matrix have been found in these composites. The nature of the polymer matrix is found to influence the rate of aniline polymerization, the distribution of polyaniline clusters and the depth of aniline penetration as shown, respectively, by Conducting Probe AFM and Raman spectroscopy. These features are correlated to the dielectric behavior of the composite films. Ó 2005 Elsevier B.V. All rights reserved. PACS: 73.61.Ph
1. Introduction The development of intrinsically conducting polymers (ICP) has attracted great attention from the scientific community due to the increasing potential for technological applications in electrochromic devices, sensors, electrolytic capacitors, rechargeable batteries, etc. Among these polymers, polyaniline (PANI) generates a special interest owing to its relative low cost, stability, and a good combination of optical, catalytic, conductive and sensor properties. However, a major problem still exists in a practical use of PANI and other
*
Corresponding author. Tel.: +33 243833552; fax: +33 243833518. E-mail address: [email protected] (M. Tabellout). 0022-3093/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.04.085
ICP because of their infusibility, insolubility and poor mechanical characteristics. An alternative approach is the use of PANI in a form of conducting composites or blends with conventional polymers [1]. The obtained composites combine good conductive properties with mechanical and even optical properties of the polymer matrix. This allows a development of polymeric composites with specific properties and shapes. The composite films used in this study were obtained by in situ polymerization of absorbed aniline in thin surface layers of a polymer matrix leading to a double layer film [2,3]. The layer containing PANI becomes reversibly conductive/non-conductive by acid doping/ dedoping. Obviously, conductive properties in these composites will depend on the molecular organization of the conductive clusters with respect to the polymer matrix. In this work, in order to investigate the influence of
2836
M. Tabellout et al. / Journal of Non-Crystalline Solids 351 (2005) 2835–2841
the matrix nature on the dielectric properties of the composites, two different composite films based on polyamide 6 (PA6) and poly(ethylene terephthalate) (PET) matrices have been synthesized and studied using dielectric relaxation spectroscopy (DRS), conducting probe AFM (CPAFM), X-ray scattering and Raman spectroscopy.
Electrode Insulating layer Layer containing PANI Layer containing PANI Insulating layer Electrode
2. Experimental 2.1. Materials The studied composites were prepared with both PA6 and PET films as the polymer matrix. These films purchased from Goodfellow Cambridge Ltd are 25 lm thick and unaxially oriented for PA6 and 20 lm thick and biaxially oriented for PET. The films were swelled up to 10 wt% content of aniline and were further immersed into an oxidant solution to polymerize in accordance with [2–4]. The final composite film consists of two layers. The first one, with a thickness varying from 2 to 4 lm depending on the matrix, consists of PANI clusters distributed in the host matrix. The second one is the pure matrix. Composite film doping is performed by exposition of the film surface containing PANI to a 37% fuming HCl acidic atmosphere for PET/PANI composite or by immersion into a 5% solution of HCl for PA6/PANI film. The doping process leads to a change of the film color from blue to green related to an increase of the film surface conductivity by several orders of magnitude. Pure polyaniline powder samples were obtained by aniline oxidative polymerization procedure using ammonium persulfate in an HCl solution [5]. Dedoping process was performed in 5% NH4OH solution. 2.2. Experimental techniques 2.2.1. Dielectric relaxation spectroscopy measurements Dielectric permittivity measurements were performed using a Novocontrol broadband dielectric spectrometer (Novocontrol GmbH, Germany) in a wide frequency (0.1 Hz–10 MHz) and temperature (153–293 K with an accuracy of ±0.1 K) ranges using a plan capacitor geometry. Two pieces of film were placed in between two round plate electrodes according to the scheme in Fig. 1. The surfaces containing PANI were facing each other in order to respect the sample symmetry. For pure PANI samples, the electrical and dielectric measurements were performed on cylindrically shaped and compressed pellets using an HP 4291A impedance analyzer in the frequency range 1 MHz–1 GHz. The sample placed in between two round plate electrodes is used as a termination of a golden coaxial line and the electric impedance was calculated from the complex reflection factor at the end of the line [6].
Fig. 1. Sample cell configuration for dielectric measurements of the composite films.
2.2.2. AFM electrical imaging The electrical images of the conductive surface of the film were obtained with a home-built extension of a Digital Instruments Nanoscope III in contact mode (called ÔResiscopeÕ) [7]. 2.2.3. Raman spectroscopy A Jobin-Yvon Horiba T64000 spectrometer (excitation line k = 514.5 nm) was used to determine the thickness of the layer containing PANI in the two composite films. 2.2.4. X-ray scattering X-ray diffraction measurements using a Philips XÕpert diffractometer were performed on polymer matrix samples to get structural informations, mainly, a qualitative comparison of the crystallinity extent of the polymer matrices.
3. Results and discussion It has been shown by standard AFM [8] that the doping of films based on PANI and a polymer matrix such as PET drastically modifies the film surface topography showing a re-organization of PET and PANI flexible chains under the influence of the acid. This morphological change is combined with a modification of the electrical properties of the film surface. The electrical images (Fig. 2) performed on the surfaces of the layers containing PANI revealed a distribution of conducting clusters, with a resistance value around 105 X dispersed in a matrix having a high resistance value (over 1011 X). In the case of PET matrix based film (Fig. 2(a)), the conducting clusters are rather dispersed and no clear evidence of percolation is observed in the lateral dimensions of the layer containing PANI. In contrary, for PA6 based samples, the conductive clusters occupy a large part of the film surface (Fig. 2(b)). It has been shown [9] that the percolation threshold is, in the latter case, reached for only 4 wt% of PANI. As a result, the electrical macroscopic properties are different: the film surface resistance, measured with a standard 4 probes technique, is found to be 20 kX and 4 MX [10], respectively, for
M. Tabellout et al. / Journal of Non-Crystalline Solids 351 (2005) 2835–2841
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Fig. 2. Electrical images of doped PET/PANI (a) and PA6/PANI (b) film surfaces obtained by CP-AFM.
e ¼ eu þ
N X
Dei ai bi
i¼1
ð1 þ ðixsi Þ Þ
þ
rdc ; ie0 x
ð1Þ
where x = 2pf is the angular frequency with f being the frequency. Dei, si, ai and bi are, respectively, the relaxation strength, the relaxation time, the symmetrical and asymmetrical distribution parameters for the ith relaxation process. rdc is the dc conductivity and eu the unrelaxed dielectric permittivity. A Debye type relaxation corresponds to a = b = 1.
0.07
τ1
153K 173K 193K 213K 233K 253K 273K 293K
0.06 0.05 0.04
ε"
τ2
0.03 0.02 0.01
τ3
(a)
0.00 10-1
100
101
102
103
104
105
106
Frequency (Hz)
(a) 0.16 0.14 0.12 0.10
ε"
PA6 and PET based films at room temperature. This difference cannot be attributed to the initial concentration of aniline in the swelled film and, correspondingly, to the PANI content. The samples being prepared with the same route, in both cases the virgin films took up roughly the same amount of aniline (around 10 wt%). It was also shown by Raman spectroscopy [10,11] that the thickness of the layer containing PANI was larger in PA6 (4 lm) than in PET (2 lm). The nature of the polymer matrix may most probably explain the encountered differences, as it will be discussed later. The observed drastic modifications occurring in the surface layer containing PANI, consequently to acid doping, may result in significant changes in the dielectric behavior of the doped films. The layered structure of the film (one layer being conductive and the other one insulating) and the cluster organization of PANI in the conductive layer will be traduced by the appearance of polarization phenomena. Therefore, in order to check the above assumptions, dielectric relaxation measurements were performed on both films in a wide temperature (173–293 K) and frequency (0.1 Hz–10 MHz) ranges (Fig. 3). To account for the relaxation times and their temperature dependence, dielectric relaxation spectra were analyzed according to the empirical Havriliak–Negami (HN) function [12]:
τ1
173K 183K 193K 203K 213K 223K 233K 243K
τ3
0.08 0.06 0.04 0.02 0.00 10-1
(b)
100
101
102
103
104
105
106
Frequency (Hz)
Fig. 3. Dielectric relaxation spectra (dielectric loss e00 ) of doped PET/ PANI (a) and PA6/PANI (b) films at different temperatures. The solid lines represent the best fit to Eq. (1).
The spectra obtained for the doped PET/PANI and PA6/PANI films at different temperatures are depicted in Fig. 3(a) and (b), respectively. The solid lines are fits
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M. Tabellout et al. / Journal of Non-Crystalline Solids 351 (2005) 2835–2841
Table 1 HN fit parameters for the relaxations processes labelled s1, s2 and s3 in doped PET/PANI film T (K)
s1 s (s)
De
a
b
s (s)
De
a
b
s (s)
De
a
b
153 173 193 213 233 253 273 293
3.68E02 2.11E02 1.43E02 8.50E03 5.79E03 4.15E03 3.30E03 2.83E03
0.10 0.09 0.10 0.10 0.09 0.08 0.08 0.08
1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1
1.16E03 6.59E04 4.59E04 2.76E04 1.78E04 1.14E04 7.93E05 7.11E05
0.05 0.04 0.06 0.03 0.05 0.04 0.03 0.02
0.96 0.94 0.71 0.93 0.83 1 1 1
0.84 1 1 1 0.78 0.53 0.42 0.76
5.99E06 2.45E06 7.85E07 1.29E06 3.86E07 2.28E07 1.06E07 1.25E07
0.09 0.12 0.11 0.19 0.20 0.30 0.32 0.29
0.50 0.38 0.41 0.26 0.28 0.21 0.22 0.29
0.51 0.58 0.75 1 0.89 1 1 0.89
s2
s3
Table 2 HN fit parameters for the relaxations processes labelled s1 and s3 in doped PA6/PANI film T (K)
s1 s (s)
De
a
b
s (s)
De
a
b
173 183 193 203 213 223 233 243
1.88E05 1.54E05 1.35E05 1.25E05 1.11E05 1.01E05 9.10E06 8.60E06
0.17 0.23 0.24 0.22 0.19 0.18 0.16 0.14
0.95 0.86 0.86 0.90 0.96 0.98 1 1
1 1 1 1 1 1 1 1
3.72E07 2.31E07 2.06E07 1.93E07 1.91E07 1.67E07 1.38E07 1.06E07
0.075 0.17 0.20 0.21 0.24 0.26 0.25 0.22
0.96 0.96 0.89 0.69 0.66 0.64 0.65 0.68
0.52 0.29 0.33 0.66 0.66 0.66 0.66 0.69
s3
with k being the Boltzman constant, s0 the relaxation time at very high temperature and E the activation energy. The temperature dependencies of the relaxation times are shown in the graphs of Fig. 4(a) and (b). The corresponding fit parameters are collected in Table 3. In order to understand the origin of the observed relaxation processes, the spectra obtained in the virgin, non-doped and doped films at T = 233K for PET/PANI are depicted in Fig. 5. The virgin film of PET exhibits a large relaxation attributed to a b relaxation process commonly associated to local movements such as side groups or chain segments. This peak is quite broad and symmetric and the relaxation times are charac-
-5
Relaxation time [s]
τ1 τ2 τ3
10-2
-6
10
10-3
Relaxation time [s]
10-1
10
-7
10
10-4 4
3
5
6
7
1000/T [K-1]
(a) 1e-4
τ1 τ3 Relaxation time [s]
to Eq. (1) and the derived fit parameters are collected in Tables 1 and 2, respectively, for PET and PA6 based composites. No ohmic losses are observed in the imaginary part of the dielectric permittivity due to the layered structure of the films, one layer being insulating and the other conductive. With this sample configuration, only ionic conductivity could be observed at temperatures above the glass transition temperatures which values are 348 and 323 K for PET and PA6, respectively. The characteristic relaxation times s were taken at the position of the maximum of dielectric loss for each relaxation process. The temperature dependences of these relaxation times were analyzed using an Arrhenius law: E s ¼ s0 exp ð2Þ kT
1e-5
1e-6
1e-7
1e-8 4
(b)
5
6
1000/T [K-1]
Fig. 4. Arrhenius plot of the characteristic relaxation times for the different relaxation processes in doped PET/PANI (a) and PA6/PANI (b) films. The solid lines represent the best fit to Eq. (2). The arrows indicate the corresponding Y-axis.
M. Tabellout et al. / Journal of Non-Crystalline Solids 351 (2005) 2835–2841
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Table 3 The fit parameters to Eq. (2) for doped PET/PANI and doped PA6/PANI films T (K)
PET/PANI
PA6/PANI
s1
s2
s3 (T < Tc)
s3 (T > Tc)
s1
s3
s0 (s) E (meV)
1.5E4 74
1.5E10 87
1.9E10 118
1.86E10 147
1.28E6 40
1E8 52
0.06
τ1
virgin PET non-doped PET-PANI doped PET-PANI
0.05 0.04
τ2
0.03
ε"
τ3
0.02 0.01 0.00 10-2
10-1
100
101
102
103
104
105
106
107
Frequency (Hz) Fig. 5. Frequency dependence of dielectric loss for virgin PET, dedoped PET/PANI and doped PET/PANI at T = 193 K. The solid lines represent the best fit to Eq. (1).
terized by a relatively high activation energy (E = 500 meV) typical of sub-glass relaxations in polymers. While the spectrum of non-doped PET/PANI is quite similar to that of virgin PET, the spectrum of doped PET/PANI film exhibits a totally different behavior showing 3 additional relaxation processes with low activation energy values (Table 3) characteristic of electronic transport. The first 2 peaks, starting from the low frequency side and labelled s1 and s2, are nearly Debye type relaxation processes (a 1, b 1) with activation energies of 74 meV for s1 and 87 meV for s2 (Fig. 4(a)). These values are much lower than those characterising dipolar re-orientation processes in polymers. These peaks are most likely related to interfacial polarization relaxations effects [13]. Specifically, one of these relaxations may be understood if we consider the doped layer constituted by conductive PANI clusters surrounded by less or non-conducting medium as it is observed on electrical images (Fig. 2). Such a dispersion is known to lead to a Maxwell–Wagner–Sillars (MWS) effect [13,14]. The other relaxation process may arise from the significant difference in conductive properties between the two layers, the first one being the conducting doped PET/PANI layer and the second one being the pure polymer matrix. This interface polarization leads also to a Debye type relaxation process. The third peak is rather broad (Table 1). Two Arrhenius laws in the investigated temperature range are used to describe the temperature dependence of the associated relaxation
time. The change of slope occurs at a critical temperature Tc = 240 K (Fig. 4) showing 2 processes: one with an activation energy of 147 meV at high temperatures and the other one, smaller, of 118 meV. This behavior has also been observed by EPR measurement [8] and was attributed to the thermally activated conversion between localized and delocalized paramagnetic centres. This relaxation might be connected to the hopping conduction process inside the PANI clusters. This hypothesis is supported by the electrical and dielectric measurements performed on pure PANI in its doped and dedoped forms in the frequency range 1 MHz– 1 GHz at different temperatures ranging from 143 to 243 K. The real part of the dielectric permittivity exhibits a relaxation process of a high dielectric strength in the hundred MHz region consequently to the doping process (Fig. 6(a)) whereas, in the dedoped form of PANI only a flat dielectric response is observed. Correspondingly, the real part of the conductivity spectrum (Fig. 6(b)) of doped PANI is dominated at low frequency by a dc-conductivity and exhibits at higher frequency an ac-conductivity increasing with frequency. The temperature dependences of the derived relaxation time and dc-conductivity are fitted to Eqs. (2) and (3), respectively: E rdc ¼ r0 exp kT
ð3Þ
with k being the Boltzman constant, r0 the conductivity at very high temperature and E the activation energy. A nearly identical activation energy (E = 28 meV) is found for both dc-conductivity and relaxation time indicating that the two processes originate from the same electronic transport mechanism. These values are much lower than that found for s3 relaxation process in PET/PANI. This difference may be attributed to the higher doping level reached in pure PANI in comparison to PET based composite due to the difficulties for the ions to diffuse through the matrix. The PA6/PANI film exhibits a rather different behavior. Two relaxation processes of low dielectric strength are visible in the virgin and non-doped PA6/PANI films (Fig. 7). The high frequency one can be attributed to a c relaxation process associated with local chain motions. The second process is most likely corresponding to a b relaxation process characterized by an activation energy of 600 meV and a symmetric distribution of
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M. Tabellout et al. / Journal of Non-Crystalline Solids 351 (2005) 2835–2841 140 143K 163K 193K 233K dedoped PANI
120 100
ε'
80 60 40 20 0 106
107
(a)
108
109
Frequency (Hz) 10-1
σ' [S/cm]
10-2
10-3 143K 163K 193K 233K
10-4 106
107
108
109
Frequency (Hz)
(b)
Fig. 6. Frequency dependence of the real parts of dielectric permittivity (a) and conductivity (b) for PANI doped by HCl at different temperatures and for dedoped PANI at 193 K. Solid lines are the best fit to Eq. (1).
0.12 0.10
virgin PA6 non-doped PA6-PANI doped PA6-PANI
ε"
0.08
relaxation strength. The HCl doping of PA6/PANI composite leads to the appearance of only two additional relaxation processes (Fig. 7) in the experimental temperature and frequency ranges. Above the glass transition temperature these relaxations are no longer visible due to the appearance of the ionic conductivity and the a relaxation process which dominate the spectra. The two additional peaks observed in doped PA6/ PANI are labelled s1 and s3. As compared to PET based film, the second relaxation process s2, if any, may have shifted to higher frequency or disappeared. As no relaxation process was found at higher frequency, the relaxation process s2 may have, most probably, disappeared due to the quasi-homogeneity of the layer containing PANI as it can be observed in Fig. 2(b). The relaxation time s1, which is a Debye type relaxation process (a 1, b 1) decreases by 3 orders of magnitude in comparison to PET/PANI implying a higher conductivity of the PA6/PANI layer. Moreover, its activation energy (E = 40 meV) (Fig. 4(b)) is also two times lower than that of PET/PANI composite. The second peak s3 has an activation energy (E = 52 meV) (Fig. 4(b)) closer to that found in pure PANI and hence is attributed to conductivity relaxation in PANI as discussed before. The general feature resulting from this comparison is the decrease of the relaxation time s1 by 3 orders of magnitude accompanied by a decrease of the activation energy in PA6 based composite if compared to the PET based one. This result is in conformity with the conductivity values obtained by the standard 4 probes technique and correlated with the electrical images of the surface of the two films. In doped PA6/PANI film, the electrical images clearly show a percolation of the conductive clusters in the layer containing PANI. The explanation of this behavior can be found in the matrix structure. X-ray diffraction measurements were performed for this aim on both virgin films in order to qualitatively determine the relative extent of crystallinity. It is clear from the difference in the peak widths in
0.06 1
0.02 0.00 10-2
10
-1
10
0
10
1
10
2
10
3
10
4
10
5
10
6
10
7
Frequency (Hz) Fig. 7. Frequency dependence of dielectric loss for virgin PA6, dedoped PA6/PANI and doped PA6/PANI at T = 233 K. The solid lines represent the best fit to Eq. (1).
Normalized intensity
0.04
PA6 PET
0.8 0.6 0.4 0.2 0 -10
-5
0
5
10
Shifted diffraction angle ( 2θ)
relaxation time (a 0.4, b = 1). The only effect of the presence of non-doped PANI is the increase of the b
Fig. 8. Normalized X-ray scattering spectra of PA6 and PET virgin films.
M. Tabellout et al. / Journal of Non-Crystalline Solids 351 (2005) 2835–2841
Fig. 8 that PA6 is of less crystallinity extent. Therefore, although the preparation route and the aniline content were the same (10 wt%), the resulting amount of PANI, its organization and even its doping level are strongly affected by the crystallinity of the matrix. The more flexible molecular chains (PA6) allow a freer organization of PANI leading to a percolation of the conductive clusters and enable a higher doping level. 4. Conclusion Surface conductive films, based on doped and dedoped PANI located in two different polymer matrices (PA6 and PET) have been studied in a wide temperature and frequency ranges using dielectric relaxation spectroscopy. At low temperature, in addition to sub-glass relaxation processes occurring in the polymer matrix and in the non-doped composite as well, 3 relaxation processes are observed in the doped one related to its conductive properties. In the low frequency region, interfacial polarization relaxations are attributed to the layered and clustered structure. The clustered structure is confirmed by the electrical images obtained by CP-AFM on the film conductive surface. At higher frequency, conductivity relaxation appears to be connected to the conductivity in the PANI clusters. The nature of the polymer matrix is found to influence these relaxations by frequency shift, change in relaxation strength and activation energy. In order to investigate these effects, X-ray scattering and Raman spectroscopy studies have been performed and have shown that the matrix crystallinity may contribute to these effects. In the most amorphous PA6 matrix, the PANI containing layer is thicker and percolation between the conductive clusters is reached. Thus, these results give evidence that the polymer matrix structure
2841
plays an important role in the conductive properties of the film.
Acknowledgements The authors are grateful to O. Schneegans and F. Houze´ of the Laboratoire de Ge´nie Electrique de Paris, UMR CNRS 8507, Supe´lec, Universite´ Paris VI et Paris XI, rue Juliot Curie, 91192 Gif-sur-Yvette Cedex, France for providing the CP-AFM measurements.
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