Dielectric constant and ac conductivity in polyaniline derivatives

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Solid State Communications, Vol. 97, No. 12, pp. 1029-1031, 1996 Copyright © 1996 Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-1098/96 $12.00+.00

Pergamon

0038-1098(95)00853-5

Dielectricconstantand ac conductivityin polyanilinederivatives N.J. Pinto§t, P. D. Shah§, P. K. Kahol# and B. J. McCormickCr §Department of Physics, Wichita State University, Wichita, KS 67260, U.S.A. ?Department of Physics and Electronics, University of Puerto Rico, CUH Station, Humacao, PR 00791, U.S.A. #Department of Physics and National Institute for Aviation Research, Wichita State University, Wichita, KS 67260, U.S.A. ¢rDepartment of Chemistry, Wichita State University, Wichita, KS 67260, U.S.A.

(Received 25 September 1995; accepted 12 December 1995 by J. Kuhl) We report our results on ac conductivity and dielectric constant measurements at 10 kHz on polyaniline and derivatives of polyaniline having alkyl ring-substituents. A comparison with de conductivity is made to gain further insight into the conduction mechanism. The room temperature de and ac conductivities are similar; however, deviations at lower temperatures characteristic of conduction mechanisms typical of variable range hopping are observed. This deviation is larger for samples having greater disorder. Disorder also suppresses the temperature dependence of the dielectric constant. Keywords: A. Polymers, D. Dielectric Response, D. Electronic Transport

I. INTRODUCTION The fully protonated alkyl ring-substituted derivatives of polyaniline, namely poly(o-toluidine) (POT), poly(oethylaniline) (POEA), poly(m-ethylaniline) (PMEA) and poly(o-propylaniline) (POPA), have been recently studied 1"3to obtain an understanding of the conduction mechanism in polyaniline. In particular, electron localization along the polymer chain and the origin of hopping behavior have been investigated via de conductivity, magnetic susceptibility, and electron-spin-resonance measurements. 13 These studies confirm that the presence of longer side chains on the polyaniline backbone lead to charge localization along the chain. While the ac conductivity ofemeraldine base polyaniline (PAN-EB) as well as that ofemeraldine salt polyaniline (PANCI) has been studied in the past, ~ to our knowledge no study has been done on the ac conductivity ofpolyaniline derivatives at radio frequencies. Epstein and coworkersI have, however, measured the microwave conductivity of POT in order to characterize the effect of electron localization on the transport properties. In this communication, we report measurements and analysis of our ac conductivity and dielectric constant data on polyaniline derivatives having much larger pendant groups attached to the polyaniline backbone (at 10 kHz) with the underlying objective of studying electron localization effects. The ac conductivity is also compared with our de results published earlier on the same samples3.

toluidine), poly(o-ethylaniline), poly(m-ethylaniline) and poly(o-propylaniline). All the samples were synthesized using the procedure of Chiang and MacDiarmid 7, and they were protonated in HC1. The results presented in this paper are on PAN-EB (x = 0), PAN-CI (x = 0.10), POT (x = 0.20), POEA (x = 0.5), PMEA (x = 0.5), and POPA (x = 0.5), where x = [H+]/[N]. Samples used in this study were in the form of pellets pressed under a pressure of 10 kbar. Typical sample geometrical capacitances were of the order of 0.1 - 0.2 pF. Acheson Electrodag 502 was used as electrodes and to make electrical contacts with the sample. A general radio capacitance bridge model 1615-A was used to measure the capacitance and dissipation factor as functions of temperature. From the data, the real and imaginary parts of the dielectric constant were extracted. The imaginary part of the dielectric constant yielded the ac conductivity. All measurements were performed in vacuum (to remove superficial moisture) at a frequency of 10 kHz. For heavily doped PAN and POT the conductivity was too high at the measured frequency of I0 kHz to give reliable measurements using the GR capacitance bridge and hence measurements were not performed. The emeraldine base form of polyaniline (PAN-EB) was also studied as a reference for the other samples and compared to results obtained from other investigators 4'6. III. RESULTS AND DISCUSSION Figure 1 shows the temperature dependence of the dielectric constant e for fully protonated POEA, POPA and PMEA. The temperature dependence for PAN-EB is also shown in this figure. Similar results were obtained by others4'6 for PAN-EB to within experimental uncertainty. At an applied

II. EXPERIMENTAL The samples studied were polyaniline, polv(o1029

1030

DIELECTRIC CONSTANT AND AC CONDUCTIVITY

electric field of 10 kHz, ~ for PAN-EB is seen to be ~ 5 at room temperature and it approaches 4 at low temperatures. In the absence of conducting electrons (which are present in the doped sample) this is the dielectric response from the core electrons which are bound tightly to the polymer backbone. Epstein and coworkers ~ have found a value of 5 for the low temperature dielectric constant of the emeraldine base form of polyaniline. We also note that ~ for the emeraldine base is very weakly temperature dependent when compared to the results on a PAN sample (doped corresponding to x = 0.10) at 10 kHz measured in our laboratory; this weak temperature dependence is the expected response from the core electrons present as background charge in the polymer. The room temperature values ofe for PAN-EB and PAN-CI (x = O . 10) at 10 kHz differ by a factor of 80, whereas the low temperature (T = 0 K) value of our PAN-CI sample is ~ 8 compared to a value of ~ 4 for the PAN-EB sample. This increase in the temperature independent dielectric constant with increasing protonation level at low temperatures supports the fact that it is a sum of the dielectric response of the backbone emeraldine base polymer plus a term related to the contribution of isolated polarons and bipolarons formed at these doping levels, oscillating about their pinning center. 4 Assuming a pinning model~'~ and averaging the dielectric constant over three directions, =

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Vol. 97, No. 12

per ring, m" is the polaron effective mass, and coo is the polaron pinning frequency. For x=0.10, using V=125 A 3 , ~= ~e = ( 8 - 4) = 4, and using9 m*-50n% we obtain 6aff2g= 3.6x10 ~3Hz or a pinning energy of 0.15 eV, which is close to the one obtained earlier from e at microwave frequency? The low temperature values o f e at 10 kHz for fully protonated POEA, POPA and PMEA are 11, 7 and 5.5, respectively. The dielectric constant for POT (x > 0.20) as well as for PAN-C1 (x > 0.10) could not be measured up to 2 MHz on our equipment due to high conductivity of these samples. The measured dielectric constant for POT (x = 0.20) is, however, shown in the inset of Fig. 1 alongwith the results for POEA (x = 0.50) in the entire temperature range. Since = 7 for POT (x = 0.50) at microwave frequencies~ and because e increases with a decrease in frequency, we conclude that is larger than 7 for fully protonated POT. Furthermore, since the dielectric constant increases with the doping level5 and because our measured low temperature value o f e for POT (x = 0.20) is 20, we expect c to be larger than 20 for fully protonated POT, i.e., at least 3 times larger than the microwave value. Similarly, since the microwave dielectric constant at low temperature (T = 0 K) for PAN-CI (x = 0.50) is 15,~the same factor of 3 as above would yield a value for greater than 45. That is, the low temperature values of e for fully protonated PAN-CI, POT, POEA and POPA are >45, >20, 11 and 7, respectively. The value ofe for PMEA is 5.5. It should be noted that the dielectric constant ofemeraldinesalt polyanilineis a function of crystallinity as well as the local structural behavior of the synthesized samples.I° An earlier systematic study~3 of the electrical conductivity, magnetic susceptibility, and ESR of PAN, POT, POEA and POPA showed increased electron localization along the chains with increased size of the alkyl group on benzene rings. This behavior was argued to arise from increased interchain structural disorder which progressively increases with increasing size of the substituents. We believe that decreasing dielectric constant with increasing size of the substituent group is also due to increased electron localization as mentioned above. In particular, the dielectric constant for a one-dimensional disordered solid in the limit T = 0 K is TM

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Figure 1. Temperature dependence oftbe dielectric constant e for poly(o-ethylaniline) (hexagons), poly(o-propylaniline) (circles), poly(m-ethylaniline) (squares) and emeraldine base polyaniline (triangles) at 10 kHz. The inset shows the temperature dependence of e for poly(o-toluidine) corresponding to x = 0.20 (circles) and poly(o-ethylaniline) (inverted triangles) at 10 kHz.

where C = 1.2~f/A, f is a geometrical factor, A is average cross sectional area of each chain, N(EF) is the density of states at the Fermi level for a one-dimensional chain, a-i is the electron localization length, and ¢(core) is the contribution from core polarization. Assuming that e(core) = 4.0, the values ofe - e(core) for POEA, POPA and PMEA are 7.0, 3.0 and 1.5, respectively. The corresponding values of ct"2 for these systems are 25, 1.7 and 0.4, respectively. It'it is argued that N(F~) is the same for POEA, POPA and PMEA, we do not find a linear dependence between e - ¢(core) and ct"2as predicted by equation (2). Nonetheless, e - e(core) is seen to decrease with increased size of the alkyi groups on the benzene rings. Fig.2 shows the temperature dependence of ac conductivity (o,~) for fully protonated POEA, POPA and PMEA at 10 kHz. The inset of Fig. 2 includes our measurements at 10 kHz for PAN-CI (x = 0.I0), POT (x =

DIELECTRIC CONSTANT AND AC CONDUCTIVITY

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charge. The larger deviation in the case of PAN-EB reflects a greater disorder of chain conformation in comparison to PANCI. As far as the derivatives of polyani!ine are concerned, this deviation is greater in the ease of POEA, POPA and PMEA as compared to POT since the disorder introduced by the larger pendant groups is greater. Such evidence of greater disorder is clearly seen in our earlier studies, z3 We also see that PMEA has the greatest disorder and fewer isolated polarons and bipolarons if we assume that the contribution to the dielectric constant from the emeraldine base backbone is the same. Further evidence &this behavior is also provided by greatly different de conductivities 3 and dielectric constants of POEA and PMEA. Our earlier EPR results 3 also showed that the line shape of PMEA was closer to being a Gaussian than Lorenztian as compared to OPPA and OPEA, suggesting significant disorder and electron localization. We, however, would like to caution the reader against over-interpretation of the temperature independence of the ac conductivity at low temperatures for poly(o-propylaniline) (circles) and poly(methylaniline) (squares). This behavior could partly be due to leakage current. IV. CONCLUSION

Figure 2. Temperature dependence of o plotted as In(o) vs. T ~afor poly(o-ethylaniline) (hexagons), poly(o-propylaniline) (circles) and poly(m-ethylaniline) (squares) at I0 kHz. The inset shows a plot of In(aT 1/2) vs T"~/4PAN-CI (x = 0.10) (inverted triangles), PAN-EB (triangles) and POT (x = 0.20) (diamonds) at 10 kHz. 6120) and PAN-EB. The results for dc conductivity are also shown here for comparison. In view of the quasi-onedimensional variable-range-hopping nature of charge, ~'~z~3 represented by oH = ao exp[-(2To/T)ta] with To = 8/[~'lN(Ev) zkB], we have plotted both o,¢ and oH as In(o) vs T "~a. The room temperature conductivity for PAN-EB at 10 kHz is of the order of 10-9 - 10"l° S/cm and drops by two orders of magnitude in the temperature range 77 K< T< 300 K. We see that although the de and ac conductivity values agree to within experimental uncertainty at room temperature there is a deviation in the ac conductivity at lower temperatures. The higher value of a= especially at lower temperatures as compared to oH is an indication of the presence of some more probable hops that do not contribute to de conductivitys. A behavior of this sort is typical for disordered conductors where the conductivity is governed by hopping transport of

We have presented our results of the ac and dc conductivity in polyaniline and polyaniline derivatives together with the dielectric constant measurements at 10 kHz. On the basis of the above, we can summarize our conclusions as follows. First, the addition of larger pendant groups to the polymer backbone lead to greater localization of charge thus leading to reduced conductivity and dielectric constant. Second, at lower temperatures, the ac conductivity is seen to be greater than the dc counterpart suggesting the presence of electron hops that do not contribute to de conductivity. This deviation between the ac and dc conductivities increases with the size oftbe substituent groups on the benzene rings. Third, the increasing disorder due to increased size of the alkyl groups on benzene rings suppresses the temperature dependence of the dielectric constant. ACKNOWLEDGEMENTS We would like to thank Prof. Taher for loan of the capacitance bridge. Thanks are also due to the National Science Foundation (Grant No. 9255223) for their financial support of this work.

REFERENCES ~Z.H. Wang, A. Ray, A.G. MacDiarmid and A.J. Epstein, Phys. Rev. B 43, 4373 (1991). 2N.J. Pinto, PK. Kahol, B.J. McCormick, N.S. Dalai and H. Wan, Phys. Rev. B 49, 18983 (1994). 3P.K. Kahoi, N.J. Pinto and B.J. McCormick, Solid State Commun. 9_.~L21 (1994). +F. Zuo, M. Angelopoulos, A.G. MacDiarmid and A.J. Epstein, Phys. Rev. B 39, 3570 (1989). SH.H.S. Javadi, KR. Cromack, A.G. MaeDiarmid and A.J. Epstein, Phys. Rev. B 39, 3579 (1989). qVI.K. Ram, R. Mehrotra, S.S. Pandey and B.D Malhotra, J. Phys.: Condens. Matter 6, 8913 (1994). ~J.C. Chiang and A.G. MacDiarmid, Synth. Metals 13, 193

(1986). 8P.A. Lee, T.M Rice and P.W. Anderson, Solid State Commun. 14, 703 (1974). 9R.p. McCall, M.G. Roe, J.M. Ginder, T. Kusumoto, A.J. Epstein, G.E. Asturias, EM. Scherr and A.G. MacDiarmid, Synth. Met. 29, 433 (1989). ~oj. Joo, Z. Oblakowski, G. Du, J.P. Pouget, E.J. Oh, J.M. Wiesinger, Y. 1Vfin,A_G. MaeDiarmid, and A.J. Epstein, Phys. Rev. B 49, 2977 (1994), ~A.A. Gogolin, Phys. Rep.1, 1 (1982); 5, 269 (1988). aZP.K. Kahol, N.J. Pinto, E.J. Berndtsson and B.J. McCormick, J. Phys: Condensed Matter 6, 5631 (1994). ~3Z.H. Wang, E.M. Scherr, A.G. MacDiarmid and A.J. Epstein, Phys. Rev. B45, 4190 (1992).