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Physical characterization of polyaniline-Na+-montmorillonite nanocomposite intercalated by emulsion polymerization
Synthetic Metals 117 (2001) 115±118
Physical characterization of polyaniline-Na-montmorillonite nanocomposite intercalated by emulsion polymerization B.H. Kima, J.H. Junga, J.W. Kimb, H.J. Choib, J. Jooa,* a
Department of Physics and Center for Electro and Photo Responsive Molecules, Korea University, Seoul 136-701, South Korea b Department of Polymer Science and Engineering, Inha University, Inchon 402-751, South Korea
Abstract We report the results of temperature dependence of dc conductivity (sdc(T)), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and X-ray diffraction (XRD) experiments for dodecylbenzenesulfonic acid (DBSA) doped polyaniline (PAN)-Na-montmorillonite (MMT) nanocomposite. The PAN-Na-MMT nanocomposite was synthesized by intercalating the emulsion of aniline monomer with zylene and DBSA in the layer of Na-MMT with distilled water and then initiating the polymerization of aniline inside the layer of Na-MMT. From the results of X-ray diffraction experiments, we observe the insertion of PAN-DBSA between the interlayer of the Na-MMT, whose separation consequently becomes larger (1.52 nm) than in a polymer-free clay (0.96 nm). The sdc of the systems is 10ÿ1 S/cm at room temperature, and its temperature dependence follows a quasi one-dimensional variable range hopping (VRH) model. The slope of sdc is 7.0103 K. From the result of XPS experiment, we obtain the doping level of the system quantitatively. We measure temperature dependence of EPR linewidths. Base on those results, we discuss the effect of the layer of montmorillonite on PAN-DBSA samples. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Polyaniline and polyaniline-Na-montmorillonite; Conductivity; XPS; EPR; X-ray diffraction
1. Introduction After the report of physical and chemical properties of polyaniline (PAN) doped with hydrochloric acid in 1987 [1], intensive efforts have been made to increase the conductivity of doped PAN systems. Various dopants and solvents to control the charge transport properties of PAN system have been introduced for the past decade [2±4]. For commercial applications, PAN is the best promising material in conducting polymers, because of environmental stability, easy processing, and economical ef®ciency [5,6]. PAN has been used for electrode of light emitting diode and Li ion rechargeable battery, corrosion protection, and RF and microwave absorber [7±9]. For the use of commercial products, it is necessary for PAN to be mix with other polymers or inorganic materials. Suspensions of PAN in silicone oil have been studied as potential candidates for dry-base electrorheological (ER) ¯uid system [10,11]. An ER ¯uid is a colloidal dispersion whose rheological properties are changed by an imposed electrical ®eld. It has potential applications such as torque transducers, valves without moving parts, and vibration *
Corresponding author. Tel.: 82-2-3290-3103; fax: 82-2-927-3292. E-mail address: [email protected] (J. Joo).
reducer-like shock absorber [10]. Recently, semiconducting polymers have been adopted as dry-base, nearly anhydrous ER ¯uids. Among ER ¯uids, PAN has advantages over other materials, for example, pyrolyzed polyacrylonitrile, with respect to density, control of conductivity, and thermal stability. In this paper, we report the results of temperature dependence of dc conductivity, XPS, EPR, XRD experiments for dodecylbenzenesulfonic acid (DBSA) doped PAN-NaMMT (PAN-DBSA/clay). A quasi one-dimensional variable range hopping (VRH) model provides the best ®tting for temperature dependence of dc conductivity (sdc(T)) of PANDBSA/clay samples. We observe that PAN-DBSA/clay materials are less conducting than PAN-DBSA ones. The results of XRD experiments show the insertion of PANDBSA between the interlayers of the Na-MMT. The doping rate of the PAN-DBSA/clay system is obtained through XPS experiment. The results of temperature dependence of EPR linewidths are discussed. 2. Experimental Particles of PAN-Na-MMT clay nanocomposite were synthesized via emulsion polymerization [12,13]. The Na-
0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 0 5 4 9 - X
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B.H. Kim et al. / Synthetic Metals 117 (2001) 115±118
montmorillonite (Southern Clay Product, USA) in aqueous medium was prepared and then sonicated using an ultrasonic generator. Dodecylbenzenesulfonic acid (DBSA) dissolved in distilled water was mixed with aniline monomer solution and emulsion solution were mixed in the 4-neck reactor by stirring, and the temperature was kept at 258C. The oxidant initiator solution was then dropped into the reactor. After terminated reaction, we obtained the particles by washing, ®ltering, drying, milling and sieving, sequentially. A four-probe method and a Janis closed-cycle refrigerator system were used for measuring sdc(T) from 300 to 10 K. Four thin gold wires (0.05 mm thick and 99% pure gold) were attached in parallel on the sample surface by graphite glue (Acheson Electrodag 502) for better electrical contact. The samples were prepared with pellet form under the pressure of 2.1107 Pa. The EPR linewidths were obtained using a Bruker ESP300 spectrometer (X-band). The temperature range was from 300 to 100 K. For the EPR experiments, the powder sample was put in an EPR tube (Wilmad 707) and pumped with a diffusion pump (<10ÿ4 Torr). For X-ray diffraction patterns, we used X'pert-MPD (model pw Ê ) diffractometer. The materials for XRD 3020, l 1:542 A experiments are in powder form. The diffuse background due to the air and the glassy tube was periodically evaluated and subtracted from the diffractograms in order to keep the signal scattered from the sample. The XPS data were measured by using VG ESCALAB MKII spectrometer (Al Ka 1486.6 eV photons). All binding energies were referenced by C 1s neutral carbon peak at 284.6 eV to compensate for surface charging effect. The area ratio corrected by the sensitivity factor was used for qualitative analysis of XPS data. 3. Experimental results and discussion Fig. 1 compares temperature dependence of sdc(T) of PAN-DBSA and PAN-DBSA/clay samples. Room tempera-
ture sdc of PAN-DBSA and PAN-DBSA/clay is measured to be 3 and 0.3 S/cm, respectively. Temperature dependence of sdc of PAN-DBSA/clay follows a quasi one-dimensional (1D) variable range hopping (VRH) model [14,15]: " # T0 1=2 : sdc T s0 exp ÿ T Here T0 8a=zN EF kB , aÿ1 is the localization length, N(EF) is the density of states at the Fermi level, kB is the Boltzmann constant, and z is the number of the nearestneighbor chains. From the slope of sdc(T), we obtain that T0 for PAN-DBSA is 1.7103 and T0 7:6 103 K for PANDBSA/clay which indicates that PAN-DBSA is more highly conducting state. From the result of sdc(T), we analyze that the insertion of the clay layer induces the weak interchain interaction resulting in low sdc of PAN-DBSA/clay systems. One can determine the metallic, critical, or insulating state of the materials from the slope of temperature dependence of the reduced activation energy de®ned as [16] W T
d ln sdc : d ln T
Fig. 2 compares temperature dependence of W of PANDBSA and PAN-DBSA/clay samples. The negative slope of PAN-DBSA/clay indicates that the systems are in insulator regime, while PAN-DBSA samples are in critical or metallic regime. Fig. 3 shows the temperature dependence of EPR linewidths (DHP±P). EPR line shapes (i.e. linewidths) are determined by spin±spin (hyper®ne, dipolar, etc.) interactions, narrowing mechanisms (spin diffusion, rotation, and exchange) and spin-lattice relaxation. In systems that do not have complicated line shapes, i.e. homogeneously broadened (Lorentzian) lines. The spin±spin interactions and effects of narrowing mechanisms can be described in terms of an effective spin±spin relaxation time T2. For an EPR spectrum with a Lorentzian line shape, DHP±P can be
Fig. 1. Comparison of temperature dependence of dc conductivity for Pan-DBSA and PAN-DBSA/clay.
B.H. Kim et al. / Synthetic Metals 117 (2001) 115±118
117
Fig. 2. Temperature dependence of the reduced activation energy W of PAN-DBSA and PAN-DBSA/clay samples.
written [17,18] DHPÿP
Fig. 3. Temperature dependence of EPR linewidths (DHP±P) of (a) PANDBSA and (b) PAN-DBSA/clay samples.
1 1 / ; T2 2T1
where T1 is the spin-lattice relaxation time. In conducting polymers, the hyperfine and dipolar broadening of the resonance will be limited by narrowing due to spin diffusion. Thus increases in conductivity will increase the effectiveness of spin diffusion to narrow the EPR linewidth. This is reflected as a decrease in T2ÿ1. Above 160 K, DHP±P linearly increases with increasing temperature for both samples, which is consistent with a (1/2T1)-dominated linewidth. However, below 160 K, DHP±P of PAN-DBSA/clay samples decreases as temperature increases as shown in Fig. 3(b), which is a 1/T2-dominated linewidth. Fig. 4 shows the N 1s XPS core level spectra of PANDBSA/clay samples. The N 1s main peak is decomposed into three lines. The line of nitrogens in imine and amine sites of PAN-DBSA/clay are centered at 398.2 and 399.2 eV
Fig. 4. N 1s XPS core level spectra of PAN-DBSA/clay samples.
Fig. 5. X-ray diffraction patterns of PAN-DBSA/clay and clay samples.
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B.H. Kim et al. / Synthetic Metals 117 (2001) 115±118
[19], respectively. Based on the analysis of the area ratio of the N 1s peak, PAN-DBSA/clay is fully doped (i.e. amine nitrogen:imine nitrogen 20:1). Fig. 5 shows X-ray diffraction patterns of clay and PANDBSA/clay samples studied here. We estimate the variation of d-spacing, which is induced from the angular position 2y of the observed peaks according to the Bragg formula l 2d sin y [20]. The d-spacing in the direction of Ê , and that of PAN-DBSA/ (0 0 1) of clay sample is 9.6 A Ê clay sample is 15.2 A. This result demonstrates the insertion of PAN-DBSA between the clay layers of Na-MMT. 4. Conclusion We synthesized the DBSA doped polyaniline-Na-montmorillonite nanocomposite by emulsion polymerization. The sdc(T) of PAN-DBSA/clay samples follows the quasi 1D VRH model. From the slope of W(T), PAN-DBSA samples are in critical or metallic regime, while PAN-DBSA/clay samples are in insulating regime. The low conducting state of PAN-DBSA/clay samples originates from the insertion of the clay layer, which weakens the interchain interaction. The results of temperature dependence of EPR linewidths qualitatively agree with those sdc(T) and W(T). From XRD experiments, we con®rm the insertion of PAN-DBSA between the interlayers of Na-MMT. Acknowledgements This work was supported in part by the CRM-KOSEF (1998) and Basic Science Research Grant (BSRI-97-2444, BSRI-98-2444) of the Ministry of Education.
References [1] A.J. Epstein, J.M. Ginder, F. Zuo, R.W. Bigelow, H.S. Woo, D.B. Tanner, A.F. Rechter, W.S. Huang, A.G. MacDiarmid, Synth. Met. 18 (1987) 303. [2] J. Joo, Z. Oblakowski, G. Du, J.P. Pouget, E.J. Oh, J.M. Weisinger, Y. Min, A.G. MacDiarmid, A.J. Epstein, Phys. Rev. B 49 (1994) 2977. [3] Y. Cao, A.J. Heeger, Synth. Met. 48 (1992) 91. [4] Y. Cao, A.J. Heeger, Synth. Met. 52 (1992) 193. [5] A.G. MacDiarmid, J.C. Chiang, M. Halpern, W.L. Mu, L.D. Somasiri, W. Wu, S.I. Yaniger, Mol. Cryst. Liq. Cryst. 121 (1985) 173. [6] J. Joo, V.N. Prigodin, Y.G. Min, A.G. MacDiarmid, A.J. Epstein, Phys. Rev. B 50 (1994) 12226. [7] N.F. Colaneri, L.W. Shacklette, IEEE Trans. Instru. Meas. IM41 (1992) 291. [8] L. Yang, W. Qiu, Q. Liu, Solid State Ionics 86 (1997) 1235. [9] R. Racicot, R. Brown, S.C. Yang, Synth. Met. 85 (1997) 1236. [10] M.S. Cho, H.J. Choi, K. To, Macromol. Rapid Commnun. 19 (1998) 271. [11] K. Koyama, K. Minagawa, T. Yoshida, N. Kuramoto, in: Proceedings of the Fourth International Conference on ER Fluids, 1993, p. 129. [12] V. Mehrotra, E.P. Giannelis, Solid State Ionics 51 (1992) 115. [13] J. Sterholm, Y. Cao, F. Klavetter, P. Smith, Polymer 35 (1994) 2903. [14] N.F. Mott, E.A. Davis, Electronic Processes in Non-Crystalline Materials, Clarendon Press, Oxford, 1979. [15] Z.H. Wang, A.R. Ray, A.G. MacDiarmid, A.J. Epstein, Phys. Rev. B 43 (1991) 4373. [16] J. Joo, G. Du, J. Tsukamoto, A.J. Epstein, Synth. Met. 88 (1997) 1. [17] J. Joo, S.M. Long, J.P. Pouget, E.J. Oh, A.G. MacDiarmid, A.J. Epstein, Phys. Rev. B 57 (1998) 9567. [18] J. Joo, H.G. Song, C.K. Jeong, J.S. Baeck, J.K. Lee, K.S. Ryu, Synth. Met. 98 (1999) 215. [19] E.T. Kang, K.G. Neoh, K.L. Tan, Prog. Polym. Sci. 23 (1998) 227. [20] J.P. Pouget, M.E. Jozefowicz, A.J. Epstein, X. Tang, A.G. MacDiarmid, Macromolecules 24 (1991) 779.
Physical characterization of polyaniline-Na-montmorillonite nanocomposite intercalated by emulsion polymerization B.H. Kima, J.H. Junga, J.W. Kimb, H.J. Choib, J. Jooa,* a
Department of Physics and Center for Electro and Photo Responsive Molecules, Korea University, Seoul 136-701, South Korea b Department of Polymer Science and Engineering, Inha University, Inchon 402-751, South Korea
Abstract We report the results of temperature dependence of dc conductivity (sdc(T)), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and X-ray diffraction (XRD) experiments for dodecylbenzenesulfonic acid (DBSA) doped polyaniline (PAN)-Na-montmorillonite (MMT) nanocomposite. The PAN-Na-MMT nanocomposite was synthesized by intercalating the emulsion of aniline monomer with zylene and DBSA in the layer of Na-MMT with distilled water and then initiating the polymerization of aniline inside the layer of Na-MMT. From the results of X-ray diffraction experiments, we observe the insertion of PAN-DBSA between the interlayer of the Na-MMT, whose separation consequently becomes larger (1.52 nm) than in a polymer-free clay (0.96 nm). The sdc of the systems is 10ÿ1 S/cm at room temperature, and its temperature dependence follows a quasi one-dimensional variable range hopping (VRH) model. The slope of sdc is 7.0103 K. From the result of XPS experiment, we obtain the doping level of the system quantitatively. We measure temperature dependence of EPR linewidths. Base on those results, we discuss the effect of the layer of montmorillonite on PAN-DBSA samples. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Polyaniline and polyaniline-Na-montmorillonite; Conductivity; XPS; EPR; X-ray diffraction
1. Introduction After the report of physical and chemical properties of polyaniline (PAN) doped with hydrochloric acid in 1987 [1], intensive efforts have been made to increase the conductivity of doped PAN systems. Various dopants and solvents to control the charge transport properties of PAN system have been introduced for the past decade [2±4]. For commercial applications, PAN is the best promising material in conducting polymers, because of environmental stability, easy processing, and economical ef®ciency [5,6]. PAN has been used for electrode of light emitting diode and Li ion rechargeable battery, corrosion protection, and RF and microwave absorber [7±9]. For the use of commercial products, it is necessary for PAN to be mix with other polymers or inorganic materials. Suspensions of PAN in silicone oil have been studied as potential candidates for dry-base electrorheological (ER) ¯uid system [10,11]. An ER ¯uid is a colloidal dispersion whose rheological properties are changed by an imposed electrical ®eld. It has potential applications such as torque transducers, valves without moving parts, and vibration *
Corresponding author. Tel.: 82-2-3290-3103; fax: 82-2-927-3292. E-mail address: [email protected] (J. Joo).
reducer-like shock absorber [10]. Recently, semiconducting polymers have been adopted as dry-base, nearly anhydrous ER ¯uids. Among ER ¯uids, PAN has advantages over other materials, for example, pyrolyzed polyacrylonitrile, with respect to density, control of conductivity, and thermal stability. In this paper, we report the results of temperature dependence of dc conductivity, XPS, EPR, XRD experiments for dodecylbenzenesulfonic acid (DBSA) doped PAN-NaMMT (PAN-DBSA/clay). A quasi one-dimensional variable range hopping (VRH) model provides the best ®tting for temperature dependence of dc conductivity (sdc(T)) of PANDBSA/clay samples. We observe that PAN-DBSA/clay materials are less conducting than PAN-DBSA ones. The results of XRD experiments show the insertion of PANDBSA between the interlayers of the Na-MMT. The doping rate of the PAN-DBSA/clay system is obtained through XPS experiment. The results of temperature dependence of EPR linewidths are discussed. 2. Experimental Particles of PAN-Na-MMT clay nanocomposite were synthesized via emulsion polymerization [12,13]. The Na-
0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 0 5 4 9 - X
116
B.H. Kim et al. / Synthetic Metals 117 (2001) 115±118
montmorillonite (Southern Clay Product, USA) in aqueous medium was prepared and then sonicated using an ultrasonic generator. Dodecylbenzenesulfonic acid (DBSA) dissolved in distilled water was mixed with aniline monomer solution and emulsion solution were mixed in the 4-neck reactor by stirring, and the temperature was kept at 258C. The oxidant initiator solution was then dropped into the reactor. After terminated reaction, we obtained the particles by washing, ®ltering, drying, milling and sieving, sequentially. A four-probe method and a Janis closed-cycle refrigerator system were used for measuring sdc(T) from 300 to 10 K. Four thin gold wires (0.05 mm thick and 99% pure gold) were attached in parallel on the sample surface by graphite glue (Acheson Electrodag 502) for better electrical contact. The samples were prepared with pellet form under the pressure of 2.1107 Pa. The EPR linewidths were obtained using a Bruker ESP300 spectrometer (X-band). The temperature range was from 300 to 100 K. For the EPR experiments, the powder sample was put in an EPR tube (Wilmad 707) and pumped with a diffusion pump (<10ÿ4 Torr). For X-ray diffraction patterns, we used X'pert-MPD (model pw Ê ) diffractometer. The materials for XRD 3020, l 1:542 A experiments are in powder form. The diffuse background due to the air and the glassy tube was periodically evaluated and subtracted from the diffractograms in order to keep the signal scattered from the sample. The XPS data were measured by using VG ESCALAB MKII spectrometer (Al Ka 1486.6 eV photons). All binding energies were referenced by C 1s neutral carbon peak at 284.6 eV to compensate for surface charging effect. The area ratio corrected by the sensitivity factor was used for qualitative analysis of XPS data. 3. Experimental results and discussion Fig. 1 compares temperature dependence of sdc(T) of PAN-DBSA and PAN-DBSA/clay samples. Room tempera-
ture sdc of PAN-DBSA and PAN-DBSA/clay is measured to be 3 and 0.3 S/cm, respectively. Temperature dependence of sdc of PAN-DBSA/clay follows a quasi one-dimensional (1D) variable range hopping (VRH) model [14,15]: " # T0 1=2 : sdc T s0 exp ÿ T Here T0 8a=zN EF kB , aÿ1 is the localization length, N(EF) is the density of states at the Fermi level, kB is the Boltzmann constant, and z is the number of the nearestneighbor chains. From the slope of sdc(T), we obtain that T0 for PAN-DBSA is 1.7103 and T0 7:6 103 K for PANDBSA/clay which indicates that PAN-DBSA is more highly conducting state. From the result of sdc(T), we analyze that the insertion of the clay layer induces the weak interchain interaction resulting in low sdc of PAN-DBSA/clay systems. One can determine the metallic, critical, or insulating state of the materials from the slope of temperature dependence of the reduced activation energy de®ned as [16] W T
d ln sdc : d ln T
Fig. 2 compares temperature dependence of W of PANDBSA and PAN-DBSA/clay samples. The negative slope of PAN-DBSA/clay indicates that the systems are in insulator regime, while PAN-DBSA samples are in critical or metallic regime. Fig. 3 shows the temperature dependence of EPR linewidths (DHP±P). EPR line shapes (i.e. linewidths) are determined by spin±spin (hyper®ne, dipolar, etc.) interactions, narrowing mechanisms (spin diffusion, rotation, and exchange) and spin-lattice relaxation. In systems that do not have complicated line shapes, i.e. homogeneously broadened (Lorentzian) lines. The spin±spin interactions and effects of narrowing mechanisms can be described in terms of an effective spin±spin relaxation time T2. For an EPR spectrum with a Lorentzian line shape, DHP±P can be
Fig. 1. Comparison of temperature dependence of dc conductivity for Pan-DBSA and PAN-DBSA/clay.
B.H. Kim et al. / Synthetic Metals 117 (2001) 115±118
117
Fig. 2. Temperature dependence of the reduced activation energy W of PAN-DBSA and PAN-DBSA/clay samples.
written [17,18] DHPÿP
Fig. 3. Temperature dependence of EPR linewidths (DHP±P) of (a) PANDBSA and (b) PAN-DBSA/clay samples.
1 1 / ; T2 2T1
where T1 is the spin-lattice relaxation time. In conducting polymers, the hyperfine and dipolar broadening of the resonance will be limited by narrowing due to spin diffusion. Thus increases in conductivity will increase the effectiveness of spin diffusion to narrow the EPR linewidth. This is reflected as a decrease in T2ÿ1. Above 160 K, DHP±P linearly increases with increasing temperature for both samples, which is consistent with a (1/2T1)-dominated linewidth. However, below 160 K, DHP±P of PAN-DBSA/clay samples decreases as temperature increases as shown in Fig. 3(b), which is a 1/T2-dominated linewidth. Fig. 4 shows the N 1s XPS core level spectra of PANDBSA/clay samples. The N 1s main peak is decomposed into three lines. The line of nitrogens in imine and amine sites of PAN-DBSA/clay are centered at 398.2 and 399.2 eV
Fig. 4. N 1s XPS core level spectra of PAN-DBSA/clay samples.
Fig. 5. X-ray diffraction patterns of PAN-DBSA/clay and clay samples.
118
B.H. Kim et al. / Synthetic Metals 117 (2001) 115±118
[19], respectively. Based on the analysis of the area ratio of the N 1s peak, PAN-DBSA/clay is fully doped (i.e. amine nitrogen:imine nitrogen 20:1). Fig. 5 shows X-ray diffraction patterns of clay and PANDBSA/clay samples studied here. We estimate the variation of d-spacing, which is induced from the angular position 2y of the observed peaks according to the Bragg formula l 2d sin y [20]. The d-spacing in the direction of Ê , and that of PAN-DBSA/ (0 0 1) of clay sample is 9.6 A Ê clay sample is 15.2 A. This result demonstrates the insertion of PAN-DBSA between the clay layers of Na-MMT. 4. Conclusion We synthesized the DBSA doped polyaniline-Na-montmorillonite nanocomposite by emulsion polymerization. The sdc(T) of PAN-DBSA/clay samples follows the quasi 1D VRH model. From the slope of W(T), PAN-DBSA samples are in critical or metallic regime, while PAN-DBSA/clay samples are in insulating regime. The low conducting state of PAN-DBSA/clay samples originates from the insertion of the clay layer, which weakens the interchain interaction. The results of temperature dependence of EPR linewidths qualitatively agree with those sdc(T) and W(T). From XRD experiments, we con®rm the insertion of PAN-DBSA between the interlayers of Na-MMT. Acknowledgements This work was supported in part by the CRM-KOSEF (1998) and Basic Science Research Grant (BSRI-97-2444, BSRI-98-2444) of the Ministry of Education.
References [1] A.J. Epstein, J.M. Ginder, F. Zuo, R.W. Bigelow, H.S. Woo, D.B. Tanner, A.F. Rechter, W.S. Huang, A.G. MacDiarmid, Synth. Met. 18 (1987) 303. [2] J. Joo, Z. Oblakowski, G. Du, J.P. Pouget, E.J. Oh, J.M. Weisinger, Y. Min, A.G. MacDiarmid, A.J. Epstein, Phys. Rev. B 49 (1994) 2977. [3] Y. Cao, A.J. Heeger, Synth. Met. 48 (1992) 91. [4] Y. Cao, A.J. Heeger, Synth. Met. 52 (1992) 193. [5] A.G. MacDiarmid, J.C. Chiang, M. Halpern, W.L. Mu, L.D. Somasiri, W. Wu, S.I. Yaniger, Mol. Cryst. Liq. Cryst. 121 (1985) 173. [6] J. Joo, V.N. Prigodin, Y.G. Min, A.G. MacDiarmid, A.J. Epstein, Phys. Rev. B 50 (1994) 12226. [7] N.F. Colaneri, L.W. Shacklette, IEEE Trans. Instru. Meas. IM41 (1992) 291. [8] L. Yang, W. Qiu, Q. Liu, Solid State Ionics 86 (1997) 1235. [9] R. Racicot, R. Brown, S.C. Yang, Synth. Met. 85 (1997) 1236. [10] M.S. Cho, H.J. Choi, K. To, Macromol. Rapid Commnun. 19 (1998) 271. [11] K. Koyama, K. Minagawa, T. Yoshida, N. Kuramoto, in: Proceedings of the Fourth International Conference on ER Fluids, 1993, p. 129. [12] V. Mehrotra, E.P. Giannelis, Solid State Ionics 51 (1992) 115. [13] J. Sterholm, Y. Cao, F. Klavetter, P. Smith, Polymer 35 (1994) 2903. [14] N.F. Mott, E.A. Davis, Electronic Processes in Non-Crystalline Materials, Clarendon Press, Oxford, 1979. [15] Z.H. Wang, A.R. Ray, A.G. MacDiarmid, A.J. Epstein, Phys. Rev. B 43 (1991) 4373. [16] J. Joo, G. Du, J. Tsukamoto, A.J. Epstein, Synth. Met. 88 (1997) 1. [17] J. Joo, S.M. Long, J.P. Pouget, E.J. Oh, A.G. MacDiarmid, A.J. Epstein, Phys. Rev. B 57 (1998) 9567. [18] J. Joo, H.G. Song, C.K. Jeong, J.S. Baeck, J.K. Lee, K.S. Ryu, Synth. Met. 98 (1999) 215. [19] E.T. Kang, K.G. Neoh, K.L. Tan, Prog. Polym. Sci. 23 (1998) 227. [20] J.P. Pouget, M.E. Jozefowicz, A.J. Epstein, X. Tang, A.G. MacDiarmid, Macromolecules 24 (1991) 779.