LT 21 Proceedings of the 21st International Conference on Low Temperature Physics Prague, August 8-14, 1996

Part $4 - LT Properties of Solids 1: Magnetism (experiment)

E L E C T R I C A L C O N D U C T I V I T Y AND M A G N E T O - R E S I S T A N C E S T U D I E S ON

P O L Y A N I L I N E - P M M A B L E N D S AT V E R Y L O W T E M P E R A T U R E

D.Srinivasan a, Anasuya Raghunathan d, T.S.Natarajan', G.Rangara_ian ~, C.K.Subramaniam b, B.Wessling r

a Department of Physics, Indian Institute of Technology, Madras 600036 b Center for Electrochemical and Energy research, SPIC Science Foundation, 110,Mount road, Madras 32 r Zipperling Kessler & Co., Postfach 1464, D-22904, Ahrensburg, Germany d Department of Physics, Indian Institute of Science, Bangalore 560 012

D.C. electrical conductivity of polyaniline(33%,40%) blended with PMMA was measured from 5K to 300mK. The conductivity behaviour is consistent with fluctuation induced tunneling. Magneto-resistance (MR) was measured between 300K and 2K. From 20K to 2K, a large positive MR was observed. At 2K, for low magnetic fields(<l Tesla), a deviation from the normal H" behaviour was observed.

1. I N T R O D U C T I O N 2 . E X P E R I M E N T A L M E T H O D S

Polymer blends are superior to polymer composites because of their homogeneity and processablity[1]. The critical fraction of conducting material required to reach the percolation threshold for electrical conduction in these materials is found to be low. Three different regions of conduction, viz. metallic, critical and insulating, are found to occur. They are characterised by the sign of the function W(T) = -d lnp(T)/d lnT which is positive for an insulator and negative for a metal[2]. A positive and a quasi-linear thermoelectric power[3], a temperature independent susceptibility and a Dysonian shape for the electron spin resonance absorption are additional features observed in these systems which indicates the existence of metallic islands. As the fraction of conducting material increases, a cross-over from negative to positive value of W is found to be occur. This is accompanied by a decrease in the thermoelectric power and an increase in the asymmetry parameter of the Dysonian esr line. Such a behaviour has already been reported in the case of PANI-CSA and highly conducting polyacetylene doped with potassium. We report here a similar behaviour in PANI-PMMA blends. The relatively high conductivity of these blends has enabled dc electrical conductivity measurements to be performed down to millikelvin temperatures. The results obtained are explained using some of the known models of conduction in such materials.

Commercially available PANI, VERSICON [Allied signal Inc., Morristown, New Jersey] was used for preparing the blends. PANI-PMMA blends containing 33% and 40% PANI weight were prepared by using the dispersion techniques used for the production of commercially available INCOBLEND [Zipperling Kessler & Co., Ahrensburg, FRG]. Dc electrical conductivity of the blends from 5K to 300mK was measured by using a 3He evaporation HELIOX-3 cryostat and Oxford Instruments Intelligent Temperature Controller. A linear four probe technique was used for resistivty measurements. A Keithley model constant current source was used for passing current through the sample and a Keithley model nanovoltmeter was used for voltage measurements. The electrical contacts were made using conducting silver paint and the measurements were performed in the ohmic region. To avoid sample heating at low temperatures the current source was adjusted so that the power dessipated into the sample was less than llaW. A calibrated Lakeshore Ge resistor (GRT 200A) was used for the measurement of the sample temperatures with an accuracy of+lmK in the temperature range 300mK to 1K. The magnetoresistance of the blends from 300K to 2K was measured using a manual insertion utility probe of a commercial SQUID magnetometer. Measurements were found to be reproducible to within 1%.

Czechoslovak Journal of Physics, Vol. 46 (1996), Suppl. $4 2035

Tabled Sample 3. Ts0 Tsl aso (O)nm A

(K) (K) (S/cm) (A) PMMA-PANl(33%) 0.14 0.89 8.5 3.97 ,2.30 1.9 PMMA-PANI(40%) 0.06 1.99 10.0 0.42 3.66 0.7

(CHIo,,)• 0.05113.6 150- - - . 2 . 5 1 - - - . ll[CuPc], 0.09 22.5 600 5.9

3 . R E S U L T S A N D D I S C U S S I O N S In the highly conducting disordered material

the electrons are delocalized over large distances. The conduction is through fluctuation induced tunneling (FIT)[4]. The electrical conductivity data of PMMA- PAN1 blends from 5K to 300mK were fitted to an expression of the form[4]:

aViT = aso exp(-Tsle'2/T - Tsl~P(~:')/Ts0) where as0 is a slowly varying function of temperature as compared with the exponential term and hence is taken as a constant, Tst = (oA(Eo)2/SxkB, (O being the width of the barrier, A the area of the barrier and Eo = Eeo/Vo, Vo is the height of the barrier and E is the electric field at which the image force corrected potential is low and dp(e) = (1-e)/(l+ete+[3 e2) and the c" is the value of c T at which the exponent is maximum. ~t and [3 being determined by the parameter k that governs the shape of the barrier, aso" 2~, Ts0 and Tsl are Sheng parameters. Tso is the temperature below which there is simple elastic temperature independent tunneling and Tsl is the temperature above which a is thermally activated. Above Ts0 the fluctuation effects become significant. The experimental data from 5K down to 300mK is shown in fig.l in the form of resistivity versus inverse temperature plot and the continuous line in the figure shows the fit. The Sheng parameters corresponding to the best fit are shown in the Table. land are compared with those of other polymeric samples. It can be seen that Sheng temperatures for PMMA-PANI(33%) are small compared with PMMA-PANI(40%). The dashed line in the figure shows the deviation from the heterogeneous model[5]. Tunneling barrier parameters are calculated from the Sheng parameters[6]. The values for o & A are given in Table-I showing that they are in agreement with that of Pelster[7] et al. for PETG blends.

The magnetoresistance(MR) of PMMA- PANI(33%) is shown in the fig.2 in the form of [p(T,H)-p(T,0)]/p(T,0) versus H 2 plot. A positive MR is observed. At low temperature, the magnitude of the MR is large. At high fields, above 3 Tesla the MR shows a H 2 dependence. It is due to the shrinkage in the overlap of the localized states in the presence of the magnetic field. Below 3T, MR decreases rapidly with magnetic field.

5 . 0 0

4 . 0 0 ,

3 . 0 0 " -

E

~ " 2 . 0 0 s

1 .00

[X21:]t]Q P M M A - P A N I ( 3 3 ~ ) - - - f i t (Ref.5)

- - f i t (F I I ) ~ zxzx~x~ P M M A - P A N I ( 4 ~ ) ~

-- - - f i t (Ref.5) ~ ~ - - f i t (FIT) ~ ~

0.00 . . . . . . . . . i . . . . . . . . . , , , ,

o.oo o.so ~.oo . . . . limb . . . . . . ~166 . . . . . . ~L~b . . . . . . .~ioo I / T (K- ' )

Fig.1

0 . 8

"• 0.4

0 . 0 . . . . . . . . , . . . . . . . . . i . . . . . . . . . , . . . . . . . . . , . . . . . . . . . , . . . . . . . . . , , , , 0 5 1 0 1 5 2 0 2 5 3 0

H ' (T =)

Fig.2 In conclusion, PMMA-PANI blends show

fluctuation induced tunneling conduction at low temperature. Elastic temperature independent tunneling is observed at millikelvin temperatures. At high temperatures, a thermally activated hopping conductivity is observed. A positive magnetoresistance which is proportional to H 2 at high fields represents the shrinkage in the overlap of the localized states in the presence of the magnetic field. At low fields MR deviates from this behaviour.

R E F E R E N C E S [1] Reghu Menon, C.O. Yoon, D. Moses and A.J. Heeger. To be published in the 2 "d edition of Hand book of Conducting Polymers. [2] M. Reghu, C.O.Yoon, C.Y.Yang, D.Moses, Paul Smith, A.J.Heeger & Y.Cao, Phys. Rev. B, 50, 13931 (1994). [3] A.B.Kaiser, C.K.Subramaniam, P.W. Gilberd & B.Wessling, Synthetic metals, 69, 197 (1995). [4] Ping Sheng, Phys. Rev. B, 21, 2180 (1980). [5] A.B. Kaiser & S.C Graham, Synthetic metals, 36, 367, (1990). [6] G. Paasch, Synthetic metals, 51, 7 (1992). [7] R. Pelster, G. Nimtz & B.Wessling, Phys. Rev. B, 49, 12718, (1994).

2036 Czech. J. Phys. 46 (1996), Suppl. $4