DC ELECTRICAL CONDUCTIVITY AND MAGNETIC SUSCEPTIBILITY STUDIES ON POLYANILINE-PVC BLENDS AT LOW TEMPERATURES

D.Srinivasan^a, Anasuya Raghunathan^b, T.S.Natarajan^a C.K.Subramaniam^d

B.Wessling^c and G.Rangarajan^a

^aDepartment of Physics, Indian Institute of Technology, Madras-600036, India

^bDepartment of Physics, Wichita State University, Wichita, KS 67260-0032

^cOrmecon Chemie GmbH & Co., KG, Kornkamp 50, 22926 Ahrensburg, GERMANY.

^d Center for Electrochemical and Energy research, SPIC Science Foundation, 110,Mount road, Madras 600 032, India

Abstract

The reduced activation energy of polyaniline (PANI) - PVC blends (33% and 40% PANI weight) decreases upon decreasing temperature below 1K and the blends are found to be in the metallic regime of Metal-Insulator (MI) transition. Magnetic susceptibility measurements on PANI and PANI-PVC blends show a temperature independent Pauli susceptibility down to 50 K and a crossover from a temperature independent susceptibility to temperature dependent Curie-like behaviour is observed below 50K. A finite density of states at the Fermi surface is observed and the PANI-PVC blends are found to be a Fermi glass in the metallic regime near the MI transition.

I. INTRODUCTION

Thermally stable and electrically conducting polymer, polyaniline(PANI) dispersed in the insulating polymer polymethylmethacrylate (PMMA) and polyvinyl chloride(PVC) is widely used in various applications such as electromagnetic shielding etc.[1]. Recent reports show that blending PANI with PMMA and PVC increases the conductivity especially at low temperatures[2]. It is found that PANI(40%)-PMMA(60%) is more conducting than unblended PANI at room temperature[2]. In the presence of strong disorder, the Anderson localization and Coulombic interactions play an important role in understanding the properties of the disordered metals and doped semiconductors near the metal insulator(MI) transition. We report on the d.c. electrical conductivity and magnetic susceptibility of PANI-PVC blends.

II. EXPERIMENT

The Commercially available polyaniline, ORMECON^® [Ormecon Chemie GmbH & Co., KG, FRG] was used for preparing the PANI-PVC blends. The blends containing 20% and 47% PANI weight in PVC were prepared by dispersion techniques used for the production of commercially available INCOBLEND [Ormecon Chemie GmbH & Co., KG, FRG]. The magnetic susceptibility of unblended PANI and PANI-PVC blends was measured by using a commercial SQUID magnetometer in the temperature range 2.0K to 300K in an applied magnetic field of 50 milli Tesla. A non-magnetic quartz tube was used as a sample holder. The magnetic susceptibility of the sample holder was measured separately in the same temperature range and magnetic field range. The d.c electrical conductivity (s ) measurements were carried out in the temperature range 5K-300 mK using ³He evaporation cryostat [HELIOX-3] Oxford Instruments Temperature Controller ITC4 with a calibrated Lakeshore Ge resistor(GRT 200A). Above liquid helium temperatures up to room temperature, s measurements were carried out by using a manual insertion probe with the MPMS system.. A linear four probe technique was adopted for the electrical conductivity measurements. Electrical contacts were made with a conductive silver paint and the measurements were made in the ohmic region. Sample heating was avoided by adjusting the constant current source so that the power dissipated into the sample was less than 1 m W.

III. RESULT AND DISCUSSION

a) D.C ELECTRICAL CONDUCTIVITY OF PANI-PVC BLENDS

In the presence of strong disorder the metallic features such as is not observed at low temperatures. The temperature dependence electrical conductivity is better explained by plotting the log-log plot between reduced activation energy, and temperature[3,4] The temperature dependence of W in various regimes are described as follows,

(i) W has a negative temperature coefficient in the insulating regime

(ii) W is temperature independent for a wide range of temperatures in the critical regime.

(iii) W has a positive temperature coefficient in the metallic regime

The reduced activation energy ) is plotted as a function of temperature in log-log plot for PANI(20%)-PVC(80%) (Fig.1) and PANI(47%)-PVC(53%) (Fig.2). For PANI-PVC blends W decreases upon decreasing the temperature below 1K and the systems are found to be in the metallic side of MI transition.

b) TEMPERATURE DEPENDENCE OF PARAMAGNETIC SUSCEPTIBILITY

The total magnetic susceptibility, is expressed as a sum of core diamagnetic susceptibility, and paramagnetic susceptibility, .

(1)

The dopant is PTSA and the core value of PANI-PTSA(y=0.5) is calculated as . The core susceptibility of PANI(20%)-PVC(80%) and PANI(47%)-PVC(53%) are calculated by using the core value of PVC. After subtracting the core value from the experimental values, the total paramagnetic susceptibility of PANI and its blends are plotted as a function of temperature. This is shown in Fig.3 and 4.

In the case of unblended PANI a nearly temperature independent Pauli susceptibility is observed down to 50 K . Below 50 K, a Curie like susceptibility is observed.

The total paramagnetic susceptibility

(2)

where c is a constant and

(3)

where N(E_F) is the density of the states at the Fermi energy. Figure 5 and Figure 6 show the vs 1/T plot for unblended PANI and PANI-PVC blends respectively. The temperature independent Pauli susceptibility is calculated from the above plot and the density of the states at the Fermi energy is calculated using Eq.(3). The result is shown in the Table.1. In the case of PANI(20%)-PVC(80%) and PANI(47%)-PVC(53%), a temperature dependent Curie susceptibility is observed at lower temperatures (below 40 K) and a temperature independent Pauli susceptibility is observed (above 40 K). Pauli susceptibilities of unblended PANI and PANI-PVC blends are shown in Table. 1. It is seen that the PANI(Ormecon) sample has about 20 times more number of spins per 2 rings than PANI-CSA[5]. The same is true also of the PANI-PVC blends.

The general formula for the static spin susceptibility in the Anderson localized regime was derived by Kamimura[6] and is given by:

(4)

where , is the chemical potential and are the energies of the localized states labelled by and , the average intrastate electron-electron coulomb interaction energy.

The numerical solution of equation 4 shows that at low temperatures when is less than , the states near the Fermi energy become singly occupied and the spin susceptibility obeys the Curie law

(5)

where N_S is the number of singly occupied states. Since a finite density of states is observed at the Fermi energy, the Curie type behaviour of the spin susceptibility arises from single occupancy of localized states near E_F. N_S values are calculated from the slope of Vs 1/T for PANI and PANI-PVC blends for T< 50 K. The values are shown in Table.1. For unblended PANI N_S is equal to / mole-2rings and for blends it is one order less than that of unblended PANI.

IV. CONCLUSION

At low temperatures, activation energy, W decreases with decreasing temperature shows PANI-PVC blends are in the metallic regime of the MI transition. PANI and PANI-PVC blends show a temperature independent Pauli susceptibility down to 50 K and below 50 K a Curie-like behaviour is observed. A finite density of states present at the Fermi energy implies that PANI and its blends form a "Fermi glass". The Curie like behaviour arises from the single occupancy of localized states at the Fermi energy.

REFERENCES

(1) Bernhard Wessling, Synthetic Metals 85,1313(1997)

(2) C.K.Subramaniam et al, Journal of Polymer Science B,31,1425(1993)

D.Srinivasan et al., Czechoslovak Journal of Physics, 46 (suppS4), 2035 (1996)

(3) Reghu Menon, C.O.Yoon,D.Moses, A.J.Heeger and Y.cao,Phys.Rev. B

48,17685(1993).

(4) R.S Kohlman, A. Zibold, D.B. Tanner, G.G. Ihas, T. Ishiguro, Y.G. Min,

A.G. MacDiarmid, and A.J. Epstein, Phys. Rev. Lett. 78, 3915(1997)

(5)N.S. Saricittchi, A.J. Heeger and Y. Cao, Phys.Rev. B 49 ,5988 (1994).

(6) H. Kammimura , Philos. Mag. B 42, 763 (1980); in Electron-Electron

Interactions in Disordered Systems, edited by A.L. Efros and M. Pollak

(North- Holland, Amsterdam, 1985).

TABLE - 1

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