ac conduction properties of conducting polymer blends based on polyaniline

ELSEVIER

Synthetic

ac Conduction

Properties JBrBme

Laboratoire

CEA-Grenoble, de Physique

Planes,

Dgpartement des Me’taux

Metals

84 (1997)

of Conducting Ewa

Barika,

de Recherche Synthitiques,

797-798

Polymer Blends based on Polyaniline

Raphael

Senis

and

Fondamentale sur 17, rue des Martyrs,

Adam

Pron

la Mat&e F-38054

* Condense’e/SB’M Grenoble Cedes

9, France

Abstract The composition and temperature dependence of the ac-conductivity of films of conducting polymer blends The frequency dependence of the complex permittivity dramathas been studied in the frequency range : 10 Hz - 18 GHz. ically changes from a dielectric to a conducting behavior for a weight fraction of conducting polymer corresponding to the The high frequency dispersion of the ac-conductivity, percolation threshold pc determined by dc-conductivity measurements. characteristic of disordered conductors is recovered. In addition, a “relaxation” process is observed at intermediate frequencies, which can be of rather high amplitude for weight fractions slightly above pc. The “fractal” nature of the conducting polymer net seems to be responsible for this behavior. Keywords:

1.

Polyaniline,

Solution

selj

assembly,

Phase-segregated

INTRODUCTION

Conducting polymers (CP) have been attracting great interest thanks to the promising association of electrical and mechanical properties. This has been reinforced by the POSsibility of processing some of them, in their doped i.e. conducting - form, from solution. The pionneering works demonstrate it for polyaniline (PANI) doped with camphor sulphonic acid (CSA) in the solvent m-cresol [I]. By codissolution, the CP can be blended with an insulating polymer. By varying either the doping agent or the insulating matrix, many conducting composite films have been realized [1,2], that combine exciting properties : very low percolation threshold - always lower than 1 wt % PAN1 -, high conductivity 10d3 to 10-l S.cm-l at p = 1 wt % PAN1 -, great flexibility and optical transparency. Other preparations of low percolation CP blends have been reported in the literature [3]. In each case it seems that the mesostructure the conducting phase is seen as a fibrillar or globular self-assembled network - plays a key role in this behavior. It is therefore natural to explore the conduction mechanisms at intermediate lengthscales, between that of a single chain for which the blending effect is very small and that of the sample itself. This is achieved through ac-conduction measurements (a,,). To our knowledge, very few work has been done in this field, maybe due to the technical difficulty especially on thin self-standing films. In Ref.[l] only low frequency (V < 1 MHz) data are available; the spectra show no structure except for low PAN1 concentrations below the percolation threshold, where a stepwise ‘also

at Academy

of Mining

0379-67791971%17.00 0 1997 Else&r PII

SO379-5779(96)04151-3

and

Metallurgy,

Science S.k

Cracow,

Poland

All rights resewed

composite

interfaces,

ac

Conductivity

increase appear at a concentration-dependant frequency. A more complete study has been done [4] on pressed powder pellets of blended PANI, with a much higher percolation threshold around 8 wt %. Above p,, a,,(~) shows a strong increase attributed to a relaxation process. cac(v, with 2.

In this T,p) the

paper we present the first results of a study of for two series of PAN1 based composites doped “classical”

CSA

and

the di-i-octyl

phosphate

(DOP).

EXPERIMENTAL

Composites were prepared by mixing a solution of 0.5 wt % polyaniline doped with CSA or DOP with a solution of 5 wt % cellulose acetate (CA) and plasticizers, both in mcresol [2]. Then films were cast by slow evaporation of the solvent at 50-6O*C. Their thickness vary from 25 to 80 ,um. Broadband ac-conductivity measurements were performed by the reflexion method with three vectorial network analysers covering the frequency range : 10 Hz-18 GHz. A special cell compatible with the coaxial APC7 standard was designed to fit all the apparatus so that a given sample can be measured on the whole frequency range without unmounting. Discs of 7 mm diameter were cut from the films and gold evaporated in order to improve the electrical contacts with the inner conductor and the short-circuited part of the line. At high frequency, wave propagation in the sample has to be considered for a correct determination of 6. The low temperature measurements were done in a liq~uid~helium to insure

~cryostat. low thermal

A stainless conductivity.

steel

coaxial

line

was

built

798

J. Plan&s et al. /Synthetic

3. RESULTS

AND

DISCUSSION

A typical set of a,,(v) curves for various PAN1 weight fractions is shown in Fig.1. For very low concentrations (10m3 and 5.10m4, not shown) there is no measurable dcconductivity and the dielectric losses (E”) are in the range 0.2-0.3. The percolation scaling law fits the low frequency data with pC = 1.7 10m3 in agreement with dc measurements. At p = 1.5 10e3 there is no well defined low frequency plateau, contrary to all the higher PAN1 contents. The salient feature of this curve is the appearance of a relaxation process of huge amplitude, prior to the high frequency dispersion traditionally observed in amorphous semi-conductors. As PAN1 content increases, the relaxation amplitude tends to decrease - but not in an obvious monotonic way - whereas the dispersion characteristic frequency is shifted out of the measured range.

Metals

84 (I 997) 797- 798

relaxation process is hardly seen around pC and has a greater amplitude for higher PAN1 contents. In some cases where both mechanisms are clearly evidenced, the relaxation part can be extracted and proves to be of the Debye form. This is not always true and anomalous relaxation should sometimes be invoked, which is not unexpected for those geometries. Nevertheless, the “simpler” Debye form could result from the effective medium approximation applied to a cluster of conducting inclusions in a insulating matrix. This procedure, coupled with percolation theory, has been succesfully applied to micro-emulsion systems [5] and could be considered here. The ramified geometry also induces numerous interfaces between conducting and insulating regions, leading to polarization effects whose relaxation frequency can be widely distributed [4]. loo

lo2

I I

1o-6 “,“,,’ lo1

“““’ “““’ lo3

IX

/

““3 “““’ “*““’ ““‘” lo5 lo7

Frequency

c lo9 Frequency

I Hz

I Hz

1 - ac conductivity of PANI-CSA/CA blends for PAN I weight fractions ranging from low3 to 4.10-‘.

Fig.2 - Temperature dependence of the ac conductivity PANI-CSA/CA blend at 0.8 wt % PANI. The fitting are explained in the text.

Considering the temperature dependence, Fig.2 shows that both characteristic frequencies increase with increasing T. A fitting procedure has been applied to each curve following (w = 2xv):

REFERENCES

(UT-)” o(u) = CJO 1+ a 1+ (UT)” (

w u + c-1wo )

where the second term is for the relaxation of amplitude a and frequency 7-r and the third term for the dispersion. r -r and we are found to be proportional to ae. a is kept to 1 except for T < 25K and u N 0.7 as predicted by percolation theory. At higher concentration, the low temperature measurements unambiguously show that no inflexion point appears. This claims for a geometrical origin for the relaxation. We obtain similar results in the case of DOP doped PANI, with quantitative differences: o) pC = 6.10A3; b) the

of a lines

[l] M. Reghu, C.O. Yoon, C.Y. Yang, D. Moses, P. Smith, A.J. Heeger, Phys. Rev. B 50 (1994) 13931; C.O. Yoon, M. Reghu, D. Moses, Y. Cao and A.J. Heeger, Synth. Met. 69 (1995) 255. [2] A. Pron, Y.F. Nicolau, F. Genoud and M. Nechtschein, submitted to J. Appl. Polym. Sci. (1996). [3] A. Fizazi, J. Moulton, K. Pakbaz, S.D.D.V. Rughooputh, P. Smith and A.J. Heeger, Phys. Rev. Lett. 64 (1990) 2180; P. Banerjee and B.M. Mandal, Synth. Met. 74 (1995) 257. [4] R. Pelster, G. Nimtz and B. Wessling, Phys. Rev. B 49 (1994) 12718. [5] F. Bordi, C. Cametti, J. Rouch, F. Sciortino and P. Tartaglia, J. Phys.:Condens. Matter 8 (1996) A19.