ESR and magnetic susceptibility studies of polyaniline and its blend with poly (methyl methacrylate)

 

B. Wessling¹, P.K. Kahol², A. Raghunathan², B.J. McCormick²

1Ormecon Chemie GmbH & Co., Ferdinand-Harten-Straße 7, 22949 Ammersbek, Germany
²Physics Department, Wichita State University, Wichita, Kansas 67260, U. S. A.

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Abstract

We present an analysis of electron spin resonance, magnetic susceptibility, and dc conductivity measurements on polyaniline and polyaniline-PMMA blend to understand the reason for enhanced N(E_F) in the case of the blend. A number of experiments performed under different conditions can be understood using a model in which metallic particles are surrounded by disordered regions or shells. Our results are in agreement with an earlier investigation that found different structural arrangement in the blend compared with polyaniline.

Keywords: polyaniline, poly (methyl methacrylate), electron spin resonance, magnetic susceptibility, conductivity

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1. Introduction

Polyaniline (PANI) is a disordered polymer which can be brought to conductive state by doping with aqueous and organic acids [1]. After doping with para-toluene sulfonic acid (PTSA), PANI and its dispersion in poly (methyl methacrylate) in the weight ratio 40:60 (to be abbreviated as PANI-PMMA) have provided some interesting results [2-6]. For example, PANI-PMMA yields 17 states/(eV 2-rings) for the density of states at the Fermi level, N(E_F), which is an order of magnitude larger compared with the value obtained on usually doped polyanilines [3,4]. The PANI-PMMA blend exhibits higher dc conductivity (s) compared with unblended PANI [5]. A number of other measurements have also been interpreted as providing evidence for the existence of a metallic state in both PANI and PANI-PMMA [2-6]. Note that the blend PANI-PMMA is a dispersion of PANI in PMMA and that PANI in this dispersion has crossed the insulator-to-metal transition into the metallic side [7].

We present here an electron spin resonance (ESR), magnetic susceptibility (c) and dc conductivity study of PANI and PANI-PMMA with the objective of finding electronic differences between PANI and PANI-PMMA.

 

2. Experimental

A computer controlled X-band Bruker EMX 6/1 spectrometer, with necessary attachments for low temperature measurements, was used for ESR measurements. Magnetic susceptibility measurements were made using a "force" magnetometer in a dc field of 5 kG. dc conductivity measurements were made using a four-probe technique.

 

3. Results

Room temperature dc conductivity was measured to be around 18 S/cm and 12 S/cm for PANI-PMMA and PANI, respectively.

Pumping PANI at room temperature under a vacuum of 10 mTorr decreases peak-to-peak ESR linewidth (D_pp), but has no significant effect on either the asymmetry ratio (A/B) or dc conductivity. On the other hand, PANI-PMMA shows an increase in D_pp up to a factor of 1.5, decrease in A/B, and a small decrease in s.

The temperature dependence of D_pp for PANI and PANI-PMMA is shown in Figure 1 (top panel). The two linewidths differ by a factor of 8 at room temperature, but this difference increases as temperature is lowered. The lineshapes are Dysonian for both PANI and PANI-PMMA. On thermal annealing these samples at 170 ^oC for 15 hours, PANI-PMMA shows a drastic reduction in linewidth from approximately 5.5 G to about 0.6 G. The temperature dependence of D_pp for both annealed PANI-PMMA and annealed PANI is also shown in Fig. 1 (bottom panel).

Magnetic susceptibility measurements, from cT versus T plots, give 1.9 states/(eV 2-rings) and 12.9 states/(eV 2-rings), respectively, for N(E_F) in the case of unannealed samples of PANI and PANI-PMMA. No significant change in the values of N(E_F) is found on the annealed samples compared to respective values for unannealed PANI and PANI-PMMA.

 

 

 

 

 

 

 

 

 

 

 

Fig. 1. Temperature dependence of peak-to-peak ESR linewidth for PANI (open circles) and PANI-PMMA (solid circles) on unannealed (top) and annealed (bottom) samples

 

4. Discussion and Conclusions

Here we first summarize some of the findings of our results. First, dc conductivities of PANI-PMMA and PANI are different to within a factor of two, although earlier results showed a difference of up to five. Second, pumping under vacuum increases PANI-PMMA’s linewidth but has very little effect on that of PANI. Third, PANI-PMMA’s linewidth is larger compared to PANI by an order of magnitude, although N(E_F) for PANI is an order of magnitude smaller. Fourth, annealing causes ESR linewidth to decrease in the case of PANI-PMMA, but N(E_F) does not change significantly on annealing.

PANI-PMMA is a colloidal dispersion of polyaniline. The following general points help us find a consistent interpretation for the observed ESR, magnetic susceptibility and dc conductivity behavior. First, charge exists as delocalized polarons in "ordered" regions [8,9]. Second, disordered/ amorphous regions contain localized bipolarons [8,9]. Third, under suitable conditions, bipolarons can be destabilized into localized polarons or/and partly delocalized polarons [10]. Fourth, ordered regions are surrounded by a disordered region or shell [8,9,11].

Particles in PANI and PANI-PMMA are comprised of ordered regions surrounded by disordered or partly disordered regions. The disordered region in the case of PANI is very disordered and contains predominantly localized spinless bipolarons. On the other hand, PANI-PMMA’s disordered region is relatively less disordered and contains partly delocalized polarons. Relatively narrow ESR line in PANI thus arises from delocalized polarons in the ordered regions. What could have been a narrow ESR line in the case of PANI-PMMA is broadened by partly delocalized polarons in the partly disordered shell. Annealing PANI-PMMA introduces further disorder in the partly disordered shell that transforms partly delocalized polarons into localized bipolarons, thus narrowing the ESR line. Because PANI’s shell is already disordered, its ESR line stays the same.

As annealing does not affect the ordered regions, Pauli susceptibility and N(E_F) remain unchanged at their respective unannealed values for both PANI and PANI-PMMA. Due to a large difference in the values of N(E_F) between PANI and PANI-PMMA even for the annealed samples, and based upon the above ESR behavior especially on annealed samples, we are led to conclude that our results provide support to an earlier investigation [7 ], which showed a different crystal structure for PANI-PMMA compared with PANI.

 

References

[1] Handbook of Conducting Polymers, edited by T. A. Skotheim, R. L. Elsenbaumer, and J. R. Reynolds (Marcel-Dekker, New York, 1998)

[2] B. Wessling, Handbook of Organic Conductive Molecules and Polymers, edited by H. S. Nalwa, Vol. 3, (1997) 499.

[3] D. Srinivasan, A. Raghunathan, T. S. Natarajan, G. Rangarajan, C. K. Subramaniam, and B. Wessling, Czech. J. Phys. 46 (1996) 2035.

[4] A. Raghunathan, P. K. Kahol, J. C. Ho, Y. Y. Chen, Y. D. Yao, Y. S. Lin, and B. Wessling, Phys. Rev. B 58 (1998) R15955.

[5] A. B. Kaiser, C. K. Subramaniam, P. W. Gillberd, and B. Wessling, Synth. Met. 69 (1995) 197.

[6] D. Srinivasan, T. S. Natarajan, G. Rangarajan, S. V. Bhat, and B. Wessling, Solid State Commun. 110 (1999) 503.

[7] B. Wessling, D. Srinivasan, G. Rangarajan, T. Mietzner and W. Lennartz, Europ. Phys. J. E., in press.

[8] J. M. Ginder, A. F. Richter, A. G. MacDiarmid, and A. J. Epstein, Solid State Commun. 63 (1987) 97.

[9] P. K. Kahol, A. J. Dyakonov, and B. J. McCormick, Synth. Met. 89 (1997) 17.

[10] P. K. Kahol, A. Raghunathan, B. J. McCormick, and A. J. Epstein, Synth. Met. 101 (1999) 815.

[11] B. Wessling, Synth. Metals 102 (1999) 1396.

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