Influence of Structural Parameters Of Conducting Polymers on Their Microwave Properties
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THEIR MICROWAVE PROPERTIES P. HOURQUEBIE CEA / Centre d'6tude du Ripault / BP 16
37260 Monts, France ABSTRACT The complex dielectric constant &*(e* = '-is") of conducting polymers (polyaniline and poly(3-alkyl thiophene)) is studied over a frequency range spanning from 130 MHz to 20 GHz. The effects of structural parameters (counter-anion size, molar mass, length of the alkyl chain on the substituted monomer) on electrical properties (Cdc and &* values) have been investigated. INTRODUCTION The unusual dielectric properties of conducting polymers make them a new class of microwave absorbing materials [1]. In order to optimise the use of such materials we have tried to make correlations between structural (counter-anion size, molar mass, length of the alkyl chain on the substituted monomer) and electrical properties (Odc and s* values). In this paper we present dielectric measurements on emeraldine salt samples at various doping levels for three different counter-anions, over a wide range of frequency ( 130 MHz to 20 GHz ). By changing the counter-anion it is possible to vary the distance between neighbouring chains and we will show how the variation of this parameter will affect microwave properties.
Three groups of poly(3-alkyl thiophene) will also be studied. The comparison of results between poly(3-octyl thiophene) (POT(FeC13)) and poly(3-butyl thiophene) (PBT(FeC13)) synthesised by FeC13 oxidation will reveal the consequence of an increasing length of the alkyl chain on the substituted thiophene monomer. As a comparison of results between poly(3-butyl thiophene) synthesised by FeC13 oxidation (PBT(FeC13 )) and polycondensation of the dibrominated monomer (PBT(MgX)) is not able to reveal the effect of molar mass, a more accurate study on fractionated samples of poly(3-butyl thiophene) has to be carried out. EXPERIMENTAL TECHNIQUES
Emeraldine salt was synthesised by oxidation of aniline monomer using ammonium peroxidisulfate and converted to Emeraldine base. Emeraldine base was protonated by equilibrating the powder samples with acid solutions for one day. Solutions of three different
acids ( hydrochloride acid (HCl), p-toluene sulfonic acid (pTSA) or naphthalene 1,5-disulfonic acid (NDSA) ) with various pH were used. For each sample doping level was given by
elemental analyses ([Cl]/[N] for HCI doping, [S]/[N] for pTSA and NDSA doping). Synthesis of 3-alkyl thiophene monomers was carried out by condensation of 3-bromo thiophene and of the corresponding bromoalcane. Synthesis of polymers were carried out by 239 Mat. Res. Soc. Symp. Proc. Vol. 328. ©1994 Materials Research Society
oxidation of the monomer using FeCI 3 [2] leading to Poly(3-octyl thiophene) (POT(FeC13 )) and Poly(3-butyl thiophene) (PBT(FeCI 3 )) or by polycondensation of the dibrominated monomer [3] leading to Poly(3-butyl thiophene) (PBT(MgX)). Products were then precipitated, washed and dried under vacuum. Elemental analyses of materials obtained were in good agreement with theoretical results although a residual amount of Fe was found in
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