Electrically Conducting Polymers: Science and Technology
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ectrical Conductivity of Conducting Polymers The temperature-dependent conductivity [cr(T)] of heavily iodine-doped (CH)V and hexafluorphosphate (PF6)doped PPy down to the mK range varies as a function of aging (disorder).45 The highest crdc at room temperature reported in this study is ~5 X 104 S/cm for I.-, doped T-(CH), and ~103 S/cm for the highest conducting PPy(PFf,). For both of these materials, the conductivity decreases with decreasing temperature to a minimum at Tm ~ 10 K. Below Tm, tr increases by —20% and then is constant to 1 mK. Some highly conducting preparations of PAN-CSA show similar behavior. Samples of the same chemical composition prepared from different solvents may have different local order thus very different conductivities. Less highly conducting samples of doped polyacetylene, doped polyaniline, and doped polypyrrole become insulating at low temperatures. Hydrochloric acid as well as camphor sulfonic acid-doped polyaniline prepared in chloroform often have43 log a proportional to T 1/2 as expected for quasi-onedimensional VRH, a = (70exp[-(T,,/r)1/2], where To = 16/[fcBN(EF)Lz]. Here L is the one-dimensional localization length, and z is the number of nearest neighbor
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Electrically Conducting Polymers: Science and Technology
chains. Generally the higher conductivity samples have a weaker temperature dependence at low temperatures (To ~ 700-1000 K for T < 80 K), and lower conductivity samples have a stronger temperature dependence (To ~ 4000 K). The smaller To for the more highly conducting samples has been associated with weaker localization due to improved intrachain and interchain order. Microwave-Frequency Dielectric Constant The microwave-frequency dielectric constant provides a measure of the charge delocalization in individual samples. The low-temperature dielectric constant emw for a series of emeraldine hydrochloride samples is proportional to the square of the crystalline coherence length f2, independent of the direction of orientation of the sample with regard to the microwave-frequency electric field.43 This demonstrates that the charge is delocalized three-dimensionally within the crystalline regions of these samples. Using a simple metallic-box model, e = s* + (29/7ir>2N(EF)L2, and taking for the low-temperature localization length the x-ray crystalline-correlation length determined by x-ray diffraction, we find N(EP) is 1.23 states/(eV 2-rings) for PAN-HC1. This compares very favorably with the value obtained from magnetic susceptibility experiments. The sign, magnitude, and temperature dependence of the 6.5 X lO9 Hz dielectric constant for very highly conducting T[CHfl,),,],, PPy-PF6, and m-cresol prepared PAN-CSA are quite striking.2'4*43'44 For example, PAN-CSA (m-cresol) has a metallic negative dielectric constant (-4 X 104) and features a maximum in microwave-frequency conductivity at — 180 K. A similar large and negative value of emw and temperature dependence of emw were determined for heavily iodine-doped, stretched Tsukamoto polyacetylene (-1.5 X 106) and PF6-do
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