Frequency Dependent Dielectric Permittivity Studies in Emeraldine Base and Weakly Doped Polyaniline and its Deriviatives
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olyaniline. Steric effects of the larger substitutent group will lead to greater interchain separation thus leading to smaller overall dc conductivity. Few studies are reported on the radio frequency dependence of the conductivity and dielectric response in weakly doped samples of the polyaniline family[2-5]. The presence of polarons and bipolarons in these weakly doped systems play an important role in contributing to the conductivity and towards dielectric relaxation at radio frequencies. At low doping levels charge hopping among fixed polaron and bipolarons (their creation/annihilation) including spinless charge defect states is responsible for the observed conductivity[2]. We report our dielectric permittivity results on weakly doped polyaniline and its derivatives (POT and OPEA) in an effort to study the effect of introducing greater disorder by increasing interchain distance on the charge transport properties. Samples for which y = 0.00, 0.03, 0.07 (PAN, POT, OPEA) and 0.50 (OPEA) are studied and compared to reveal differences attributed to larger interchain separation. Our results are consistent in that the transport of charge is significantly affected as the substitutent group on the main chain polymer backbone gets larger. EXPERIMENTAL DETAILS Samples of polyaniline and its derivatives were synthesized by the oxidative polymerization of the corresponding monomer (aniline) with ammonium persulfate in acidic media[1]. Doping was achieved by soaking the polymer in HCl of pH=2.68 (y=0.03), pH=2.10 (y=0.07) and pH=0.0 (y=0.50) for 2 - 4 days and vacuum dried. Pressed pellets of about 100 mg of the sample were prepared in a high pressure cell and were used in these experiments. Measurements of the real (ε') and imaginary (ε") parts of the complex dielectric permittivity (ε*) in the frequency range 10-3 Hz - 106 Hz, were carried out using the Schlumberger Technologies 1260 Impedance/Gain-Phase Analyzer in combination with Novocontrol Broad Band Dielectric Converter and an active sample cell (BDC-S). The BDC-S containing the sample holder, the sample capacitor, high precision reference capacitors and active electronics optimize the overall performance and reduces the typical noise in the measurements, particularly at low frequencies. The sample was mounted between two gold plated parallel plates and placed in the sealed cell at atmospheric pressure. The applied electric field was held at the same value for all samples. RESULTS Figure 1 shows the frequency dependence of the absolute conductivity [σ = (σ ′2+σ″2)1/2] of all samples studied here at 300 K. This conductivity was extracted from the measured real and imaginary parts of the complex dielectric permittivity ε* (= ε'- iε") using the following relation:
σ = σ ′ − iσ ′′ = i 2πfε o (ε * −1)
(1)
where f is the frequency, i=%-1 and εo is the permittivity of vacuum. In each case, higher doping is seen to lead to an increase in the conductivity. The larger disorder in POT and OPEA as compared to PAN suppresses the transport of charge and hence results in smaller co
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