Electronic Conduction in Polymers. I. The Chemical Structure of Polypyrrole (original) (raw)
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On the structure and transport properties of polypyrroles
1992
Conducting organic polymers have attracted much attention as electronic materials, since Shirakawa et al. reported [1] an increase of several orders of magnitude in the electrical conductivity of polyacetylene after reaction with oxidants. This oxidative process giving positively charged structures is known as ,,doping,, owing to its analogy with the doping process for inorganic semiconductors. However, from a chemical point of view, the two types of doping correspond to different types of chemical transformation [2]: the oxidative process upon the non-conducting neutral polymer results in an positively charged oxidized molecule which in its solid structure requires the incorporation of a counter anion. Obviously, the nature of this anion determines the physical properties of the material. In spite of the high electrical conductivities reported for polyacetylene [3] (see Scheme 1) it rapidly degenerates in air. Other it conjugated organic conductors are thus attracting more attentio...
The in situ conductivity vs p-doping charge of low-defect polypyrroles, N-substituted polypyrroles, and polythiophenes has been investigated in acetonitrile in the presence of the weakly coordinating perchlorate ion as supporting electrolyte. In-situ ESR and EQCM measurements have given supporting information on polymer structure and conduction carriers. The structures of the polymers cover a wide range of conjugative, geometrical, and solvation conditions, but the conductive pattern follows simply the polymer ring type (pyrrole, N-substituted pyrrole, or thiophene). In polythiophenes an initial region of low conductivity, due to strongly spin-dimerized polarons, is followed by an increase of conduction to a plateau of high conductivity. N-substituted polypyrroles display a linear increase of conductivity with charge followed by a plateau of conductivity. Polypyrroles without N-substitution show an increase of conductivity to a maximum followed by a symmetrical decrease to zero at a charge corresponding to one bipolaron per tetrapyrrole unit. A redox-type bipolaron model of conduction, based on stabilization of the bipolaron positive charge by H-bonding with the counteranion, is suggested. The parent polypyrrole shows the uncomplicated conductivity pattern (increase of conductivity to a plateau) due to a uniquely strong stabilization of the π-stacked polymer chains. 10.
Electrical Properties of Polypyrrole Conducting Polymer at Various Dopant Concentrations
Polypyrrole conducting polymer was prepared by chemical reaction method with various concentrations of iron (III) chloride (FeCl3) as dopant. The dc conductivity was obtained from current-voltage characteristic by using parallel-plate techniques in the temperature range of 100-300K. With the involvement of chloride, Cl -in the polymeric chain, the conductivity increased as temperature and the dopant concentration increased. To describe the electrical transport process, Mott's 1-D, 2-D and 3-D variable range hopping (VRH) models have been considered. The result gives evidence of transport mechanism based on Mott's 3-D VRH mechanism for all various dopant concentrations studied.
Electrical Conductivity of Polypyrrole Films at a Temperature Range of 70 K to 350 K
Materials Research Bulletin, 1998
The dc conductivity of electrochemically synthesized polypyrrole films doped from light to intermediate levels with p-toluene sulfonic acid was measured in the temperature range of 77 to 300 K, using a modified four-probe rig. Plots of dc conductivity vs. temperature were parameterized by fitting Mott's Variable Range Hopping conduction model. The localization length of localized electrons was assumed to be 3 Å, which is approximately equal to the length of a pyrrole monomer. Mott parameters of polypyrrole films doped with p-TS were evaluated at 300 and 10 K. Results were found to be consistent with Mott's requirement that ␣R Ͼ Ͼ 1. Microwave (10 GHz) conductivity measurements were carried out on the same set of polypyrrole samples at a temperature range of 90 to 473 K. Both microwave and dc conductivities were found to increase with temperature. The large values of microwave conductivity compared to dc conductivity over the temperature range tested suggests the existence of more charge hopping that does not contribute to the dc conductivity.
Evaluation of electrical conduction in iodine-doped polypyrrole
Journal of Materials Science, 1992
Electrical conductivity of polypyrrole has been measured after doping with different iodine concentrations. A thermally activated electrical conductivity was found which was pseudoohmic and increased with doping level. The results can also be fitted by log cr versus T -1/2 and tog o-versus T-1/4 dependences, instead of the Arrhenius log cr versus T -1 dependence. From these results it was concluded that within the experimental scatter no significant distinction can be made between these different temperature dependence laws. Hence these data can only enable one to speculate about the true underlying transport model, rather than to draw decisive conclusions. Electrical conductivity results predicting the role of iodine dopant concentration on the conduction process of semiconducting polypyrrole are discussed.
dc Conduction in electrochemically synthesized polypyrrole films
1998
DC conductivity measurements were performed by modified four-probe rig on electrochemically synthesized polypyrrole films at a temperature range of -30 • C to 120 • C. Conductivity increased with temperature. The temperature dependence of conductivity was very high for lightly doped polymers, decreasing as the doping level increased. The model used to describe the conduction process was the conduction model originally developed for amorphous silicon by Mott and Davis. When applied to conducting polymers, it assumes that electron transport originates from localized or fixed states within the polymer chain. The charge transfer between the chains takes place by hopping, referred to as phonon-assisted hopping, between two localized states. Plots of DC conductivity versus temperature can be parametrized by Mott's Variable Range Hopping conduction model. The DC conductivity of polypyrrole films doped from light to intermediate levels with p-toluene sulphonic acid were measured in the temperature range of 77K to 300K. The localization length of localized electrons was assumed to be 3Å, which is approximately equal to the length of the pyrrole monomer. Mott parameters of polypyrrole films doped with p-TS were evaluated at 300K and 10K. Results were found to be consistent with the Mott's requirement that αR >> 1 . Theoretical values of α and N (EF ) have been determined at approximately 10 8 cm −1 and 10 19 -10 20 cm −3 eV −1 , respectively. Hence according to Mott parameters determined by the experimental data for the p-TS doped polypyrrole samples, Mott parameters are seen to have a better agreement with those expected from disordered systems, particularly for lightly doped samples, indicating the suitability of Mott's model to these samples. The average hopping distance R decreased from 16Åto 4.4Åwith the increase in the doping level from 0.006 M to 0.03 M at 300K, whereas at 10K, R decreased from 37Å to 10Å over the same dopant range.
Polymer, 2000
The electrical conductivity of chemically prepared polypyrrole in aqueous solution was found to be strongly dependent on the preparation technique and polymer additive. Owing to the hygroscopic nature of polypyrrole, it is essential to remove residual water. Accordingly, the conductivity can be enhanced by about two orders of magnitude when using a preparation technique that includes a washing treatment with organic solvents and drying under vacuum at elevated temperatures to attain maximum removal of water. Thus, the electrical conductivity of polypyrrole is affected not only by reported factors such as the ratio of oxidant to pyrrole, reaction temperature, and reaction time, but also by the preparation technique. Additionally, a significant enhancement of the conductivity up to 90 S cm Ϫ1 by using of poly(ethylene glycol) as an additive during the polymerization could be achieved. ᭧
Structure–conductivity relationships in chemical polypyrroles of low, medium and high conductivity
Synthetic Metals, 2006
Chemically synthesized polypyrroles of low (σ < 75 S/cm), medium (75 < σ < 200 S/cm) and high (σ > 200 S/cm) electrical conductivity (σ) with the same dopant and degree of doping have been investigated by means of Wide Angle X-ray Scattering (WAXS), 13 C Cross Polarized Magic Angle Spinning Nuclear Magnetic Resonance ( 13 C CP/MAS NMR) spectroscopy and Fourier Transform Infrared (FTIR) Spectroscopy to establish structure-conductivity relationships useful for industrial applications. A similar amorphous structure was found by WAXS even for the higher conducting PPy (σ = 288 S/cm). WAXS spectra for polypyrroles of medium and high conductivity showed a weak peak at 2θ = 10-11 • due to improved order of the counterions in these materials. The effect of the counterion size in the asymmetry of the PPy main WAXS peak was elucidated by performing ion exchange of the Cl − dopant with counterions of larger size such as BF 4 − and ClO 4 − . From 13 C CP/MAS NMR measurements predominantly ␣-␣ bonding was found in these materials. The main 13 C CP/MAS NMR resonance peak of PPy located at 126-128 ppm was broadened upon increasing conductivity. Interestingly, a linear relationship was observed between the half-width at half-height (HWHH) of the 13 C CP/MAS NMR peak and conductivity where a doubling of the polypyrrole conductivity leads to an increase of HWHH by 6-7 ppm. FTIR data of these materials were analysed in the framework of the Baughman-Shacklette theory describing the dependence of conductivity on conjugation length. By comparison of model predictions and experimental results, the PPy samples were found to be in the regime of long conjugation lengths, L K 2 /k B T, where K 2 is a parameter related to the energy change on going from j − 1 to j charges on a conjugated segment of conjugation length L, k B the Boltzman constant and T is the absolute temperature.
Synthetic Metals, 1992
Polypyrrole (PPy) is one of the most extensively investigated of the electronically conductive polymers. Nevertheless, there is still much we do not understand about this interesting and useful material. One of the most persistent mysteries involves simple chemical composition. Doped PPy should have the empirical formula C4H3NXz, where X-is the dopant counterion and z is the fractional doping level; z is typically on the order of 0.3. However, elemental analyses of PPy typically show empirical formulae like C4H3NXz O r, where y ranges from 0.3 to 0.7 unaccounted for oxygen atoms per pyrrole ring. This paper presents IR spectral data which suggest that the oxygen in PPy is present as covalently bound hydroxide. Analogous IR investigations of poly(N-methylpyrrole) films show that this polymer is also hydroxylated. The hydroxy substitution present in these polymers undoubtedly results from nucleophilic attack by water on the nascent polycationic chains.