Conductivity and thermopower of blends of polyaniline with insulating polymers (PETG and PMMA)

Solid State Communications, Vol. 97, No. 3, pp. 235-238, 1996 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-1098196 $9.50+.00 0038-1098(95)00653-2

Pergamon

CONDUCTIVITY

AND THERMOPGWER INSULATING

C.K. Subramaniam,

OF BLENDS

POLYMERS

OF PGLYANILINE

WITH

(PETG AND PMMA)

A.B. Kaiser, P.W. Gilberd and C.-J. Liu

Physics Department, Victoria University of Wellington, P 0 Box 600, Wellington, New Zealand and B. Wessling Zipperling Kessler & Co., Postfach 1464, D-22904 Ahrensburg, (Received

18 August

1995; accepted

21 September

Germany

1995 by H. Eschrig)

We have measured the temperature dependence of the conductivity and thermoelectric power of conducting polyaniline (PAni) dispersed in PETG copolyester and in poly(methylmethacrylate) (PMMA). Near room temperature, the PAni/PMMA blends (like unblended PAni) show a change to metallic sign for the conductivity temperature dependence, whereas this sign change does not occur for the PAni/PETG blends. As temperature decreases, the PAni/PMMA blends show a much smaller decrease of conductivity than the PAni/PETG blends and even unblended PAni. A simple model involving metallic conduction in addition to tunnelling between metallic particles can give a good account of all the conductivity data. The thermopower of all the blends is small and increases as temperature increases. Keywords:

A. polymers, D. electronic transport

1. INTRODUCTION

blends, but both are consistent, like the conductivity of unblended PAni, with a simple picture of tunnelling between metallic particles separated by nonmetallic barriers (i.e. an amorphous particle shell, adsorbed “dirt” or gases).

The electronic transport properties of conducting polymers show both metallic and nonmetallic features (e.g. [l-3]). The temperature dependence of conductivity is predominantly of nonmetallic sign (i.e. conductivity increases with temperature), but often shows a change to metallic sign at higher temperatures. The thermopower, however, is typically small and increases as temperature increases, as expected for metallic diffusion thermopower. To account for the observed transport properties, the conducting emeraldine salt form of polyaniline has been characterized [2] as a collection of metallic regions separated by amorphous polyaniline barriers that dominate the temperature dependence of the macroscopic conductivity; the scale of the metallic crystalline regions appears to be mesoscopic (close to 10 nm) rather than molecular, and identical to the primary particles [4,5].

2. CONDUCTIVITY The conductivity of the PAni/PMMA blends, which contained 60% and 67% by weight of PMMA, was measured down to helium temperatures (experimental details are similar to those for our earlier study of PAni/PVC blends [lo]). The data are shown in Fig. 1. compared to our earlier data [lo] for a pellet of compressed PAni powder similar to that used to make the blends. The most striking result is that the conductivity of the PAnilPMMA blend with 60% PMMA exceeds that of the pure PAni at all temperatures, despite the fact that the major component of the blend is nonconducting. It appears that the barriers to conductivity in the PAni sample are reduced in the process of blending. We found earlier [lo] that in a PAnilPVC blend with 60% PVC, although the room temperature conductivity was depressed relative to that of pure PAni, the conductivity of the blend below about 100 K was greater. Our new results indicate that the blending with PMMA was even more effective in lessening barriers to conductivity than blending with PVC.

In this paper, we investigate and compare the case where polyaniline particles are dispersed [6,7] in insulating polymers to deliberately create conducting “3D islands” in a Polyaniline (PAni-ES, the nonconducting matrix. emeraldine salt form, protonated with an organic acid H+X-) [8] was dispersion blended with two insulating polymers: PETG copolyester, based on polyethylene tetephthalate [9], and poly(methylmethacrylate) (PMMA). We find a striking contrast between the behaviour of conductivity in the PAni/PETG blends and the PAni/PMMA

Fig. 1 also shows that even in the 67% PMMA blend, 235

CONDUCTIVITY

236

AND THERMOPOWER

35 60%

PAni I PMMA

The situation for the PAni/PETG blends, however, is quite different. Fig. 2 shows our data for blends with 60%, 70%, 80% and 85% of PETG. In this case, the conductivity of the blend with 60% nonconducting polymer is several times less than for unblended PAni. The conductivity then drops dramatically as the fraction of nonconducting polymer increases: the room-temperature conductivity of the blend with 15% PETG is more than three orders of magnitude less than that of unblended PAni.

PMMA

25

20

15

10

5

-I

0

0

200

100

Vol. 97, No. 3

OF BLENDS OF POLYANILINE

300

The overall pattern is illustrated in Fig. 3, which shows using a logarithmic scale the enhanced conductivity in the PAnilPMMA blends and the depressed conductivity in the PAni/PETG blends. The approximate linearity of the plots in the figure also shows that the conductivity over a wide temperature range (but not at high temperatures) is generally consistent with the usual form [ 1 l] for granular metals: CJ = cro exp[- (TO/ T)t/2]

(1)

T W)

Fig. 1. Conductivity of two PAnilPMMA blends (with 60% and 67% PMMA) and unblended PAni. The lines are fits of the data to Equ. (2). with parameter values as listed in Table 1. the conductivity of the blend is greater than that of unblended PAni below about 150 K. However, the reduced proportion of conducting PAni leads to an overall reduction in conductivity compared to the 60% PMMA blend, as might be expected.

4 3.5

The conductivity above 250 K deviates strongly from Equ. ( 1) for the more highly-conducting blends, and even changes to a metallic sign for the temperature dependence in PAni and the PAni/PMMA blends as we reported earlier [ 121. This change in sign has also been observed recently for PAni protonated with camphor sulfonic acid (PAniCSA) and blends with PMMA [ 131. It is common in very highly conducting polyacetylene, and can be well described [3] by including the resistivity term [14,15] representing highly-anisotropic metallic conduction in series with the tunnelling term. For a case such as the present where conductivity reduces to zero in the zero-temperature limit. the composite expression for resistivity is [ 121: o -I = r exp(-T,

3 f

where cru and To are constants. (In fact, the data for the PAni/PMMA blends give a slightly better fit for an exponent lower than l/2).

/T) + t exp[(To /T)1/2]

(2)

2.5

%

2

z D

1.5 1 0.5 0 (r 0

100

200

300

T WI

10-s ,

, 0.1

,

85%PETG’ , ,. 02 T.“’

Fig. 2. Conductivity of four PAni/PETG blends (note the change in conductivity scale for the lower-conductivity blends). The lines are fits of the data to Equ. (2), with parameter values as listed in Table 1.

/ 0.3

,

. 0.4

(&l/2)

Fig. 3. Conductivity of PAni and PAni blends (with labels showing the percentage of non-conducting polymer component) as a function of lmt/2.

Vol. 97, No. 3

CONDUCTIVITY

AND THERMOPOWER

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237

where the coefficients r and t determine the magnitude of the metallic and tunnelling resistivity terms respectively, but depend on morphology in a complex fashion. The fits in Figs 1 and 2 show that Equ. (2) gives a surprisingly good account of all our conductivity data. Instead of the coefficients r and t, the fits were made in terms of the conductivity ~(300) at 300 K and the fraction m of the resistance at 300 K arising from the first (metallic) term in Equ. (2), given by: m = r o(300)

exp(-T,/

300).

(3)

The value of ~(300) determines the overall conductivity magnitude, while m indicates the extent of the reduction in slope near room temperature (whether or not a change to metallic sign occurs also depends on the value of To). The resulting fit parameters are listed in Table 1 for each of the blends and for pure PAni. The value of T, represents the energy of phonons with wavevector spanning the Fermi surface of the highly-anisotropic metal; it is about 1400 K in polyacetylene [ 1.51,but higher values give a slightly better fit for the PAni case. Since Tm is not determined accurately by the data, we have taken a value of 2000 K for all samples.

Table 1. Parameter values for the fits of Equ. (2) to the conductivity data for polyaniline blends with PMMA and PETG. The value ofTm was taken as 2000 K. Blend

o(300)

60% PMMA 67% PMMA 60% 70% 80% 85%

PETG PETG PETG PETG

100% PAni

(S cm-l)

30 13 3.6 0.91 0.10 0.013 18

To (K)

m

130 60

0.09 0.11

770 650 950 1350

0.11 0.06 0.03 0.02

1040

0.28

The fitted values of m in Table 1 show the largest metallic contribution in PAni and in PAni-rich blends, and small values in the PETG blends of ‘low conductivity, as might be expected. The values of the parameter To in the tunnelling term are much smaller in the PMMA blends, reflecting the much smaller decrease of conductivity in these samples as temperature decreases. The values of To in the PETG blends, however, show only a relatively small change from that in unblended PAni, representing the similarity of slope of the data in Fig. 3. This lack of change in To for the PETG blends suggests that the PAni particles retain their original properties to a greater extent than in the PMMA blends, which could be consistent with the more granular appearance of the PETG blends compared to the PMMA blends in SEM images of a fracture surface (Fig. 4).

Fig. 4. Scanning electron microscope images of a fracture surface for the PAni/PETG blend with 70% PETG (top), and the PAni/PMMA blend with 67% PMMA (bottom).

3. THERMOELECTRIC

POWER

As shown in Fig. 5, the thermopower of PAni and all the blends is small, and (apart from PAni at very low temperatures) increases with temperature. This behaviour resembles metallic diffusion thermopower, although we note that there is in fact a small curvature of the thermopower. The behaviour of the thermopower of all the blends is remarkably similar, in contrast to the huge difference in conductivities. There is a slight tendency for the thermopower of the PMMA blends to lie above that for the PETG blends, but this is hardly significant. Additional measurements were made on two PETG blends using different portions of the original sample, with virtually identical results for the 70% PETG blend, and slightly larger values for the 60% PETG blend, as shown in Fig. 5. This similarity of the blend thermopowers could arise partly from the fact that thermopower is an intrinsic property much less dependent on the electrical barrier regions than conductivity [16]. We would expect thermopower to depend to a larger extent on the metallic fraction of the samples without being influenced as much as conductivity by the greater reduction in barriers around the PAni particles in the PMMA blends Our earlier thermopower data for PAni/PVC blends [lo] are similar to that for the blends in Fig. 5, except for negative values at very low temperatures. In the only other data on PAni blends of which we are aware, the thermopower of

CONDUCTIVITY

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vol. 97, No. 3

as well as temperature dependence (except for small negative values seen at very low temperatures). These data further emphasize the similarity of PAni blend thermopowers. The thermopower of our pure PAni does appear to be significantly smaller than that of the blends; small thermopowers (of either sign) have been seen for some PAni samples by other authors [ 1,2].

..

7

Y >

4. CONCLUSION

J

0

100

200

300

T 6)

Fig. 5. Thermopower of PAnifPMMA and PAniiPETG blends, and unblended PAni. Data for all four PAni/PETG blends are included in the band labelled “PETG blends” (including two data sets for the 70% PETG blend), and a second data set for the 60% PETG blend is labelled separately.

PAni-CSAiPMMA blends measured by Yoon et al. [13] is very close to that of our PAnilPMMA blends in magnitude

The pattern of conductivities shown in Fig. 3 indicates that the conductivity in our PAni/PETG blends has a relative decrease as temperature decreases similar to that in unblended PAni, but with a reduced metallic term and a smaller fraction of material involved in the conduction paths in the blends (consistent with the conclusions of Pelster et al. [4]). In our PAni/PMMA blends, on the other hand, the conduction barriers around PAni particles appear to be lessened, especially at lower temperatures. Possible reasons include better dispersion and optimal network structure for PAni in PMMA, and hydrolytic effects in PETG (PAni will catalyze the hydrolytic cleavage of ester bonds, which might lead to increased shell “dirt”). The consistency of our conductivity data with a combination of tunnelling and metallic conduction, and the character and similarity of thermopower for the blends, support a key role for metallic PAni particles in electronic transport in the blends. Acknowledgements - This research was supported (CKS, ABK and CJL) by the Foundation for Research, Science and Technology, New Zealand. We thank Joe Trodahl for advice and assistance.

REFERENCES

1.

2. 3. 4. 5. 6. 7. 8.

Y.W. Park, Y.S. Lee, C. Park, L.W. Shacklette & R.H. Baughman, Solid State Commun. 63, 1063 (1987). Z.H. Wang, E.M. Scherr, A.G. MacDiarmid & A.J. Epstein, Phys. Rev. B 45,419O (1992). A.B. Kaiser, Synth. Met. 45, 183 (1991). R. Pelster, G. Nimtz & B. Wessling, Phys. Rev. B 49, 12718 (1994). B. Wessling, R. Hiesgen & D. Meissner, Acta Polymer 44, 132 (1993). B. Wessling & H. Volk, Synth. Met. 18,671 (1987). B. Wessling, Advanced Materials 5,300 (1993). VERSICON, Allied Signal Inc.

9. KODAR PETG Copolyester, Eastman Chemical. 10. C.K. Subramaniam, A.B. Kaiser, P.W. Gilberd & B. Wessling, J. Polymer Sci.: Part B: Polymer Physics 31, 1425 (1993). 11. P. Sheng, Phil. Mag., 65, 357 (1992). 12. A.B. Kaiser, C.K. Subramaniam, P.W. Gilberd & B. Wessling, Synth. Met. 69 197 (1995). 13. C.O. Yoon, M. Reghu, D. Moses, A.J. Heeger & Y. Cao, Synth. Met. 63.47 (1994). 14. L. Pietronero, Synth. Met. 8 225 (1983). 15. S. Kivelson & A.J. Heeger, Synth. Met. 22, 37 1 (1990). 16. A.B. Kaiser, Phys. Rev. B 40, 2806 ( 1989).