H.H.S. Javadi

ESR of protonated “emeraldine”: Insulator to metal transition

Abstract The temperature dependence of the X-band electron spin resonance for the emeraldine oxidation state of polyaniline is investigated under different protonation levels ranging from the unprotonated insulating emeraldine base to the metallic emeraldine hydrochloride. For all the samples studied, the signal is composed of two lines, one Lorentzian and the other Gaussian and broader. The different temperature dependences of the intensity of the Lorentzian and Gaussian signals for each sample allows one to distinguish between localized and mobile spins. Moreover the study of the linewidth of the Lorentzian signal as a function of the temperature and of the protonation level leads to a discussion on the nature of the conduction mechanism in polyaniline.

Conduction mechanism of polyaniline: Effect of moisture

Abstract It has recently been shown that the emeraldine base polymer form of polyaniline undergoes a transition from an insulating to a metallic state upon protonation with HCl. The conductivity of the protonated polymer is sensitive to the moisture content of the polymer, varying by as much as a factor of five upon pumping the samples. We report here the temperature-dependent microwave-frequency (1010 Hz) conductivity and dielectric constant as a function of environmental history. The results are consistent with the effect of moisture on the barriers between small metallic polymer grains. Texture of granular metal particles and localization within metallic islands are decisive in the frequency- and temperature-dependence of the conductivity and dielectric constant.

Charge transport in the “emeraldine” form of polyaniline

Abstract The conductivity and the dielectric constant of the “emeraldine” form of polyaniline is studied in a wide range of frequency (dc, 10 1 –10 5 Hz, 6.5×10 9 Hz), temperature (20K–340K) and protonation level (x ≡[C1]/[N] =0.0–0.5). Two distinct behaviors are observed: Firstly, for low protonation (x ≤ 0.13), hopping of charges between polarons and bipolarons leads to dipolar relaxation ( ϵ RF (T)). Oscillation of polarons around pinning centers is suggested to be responsible for the high frequency dielectric constant of low protonated samples. Secondly, for intermediate to highly protonated samples (x > 0.22), the presence of “metallic islands” is apparent. Oscillation of charge carriers within the coherence length contributes to ϵ μω (T=0). The intrinsic conductivity within the islands is estimated to be of the order of 250 S/cm. The T-dependence of the dielectric constant indicates an increase of the coherence length with increasing temperature. The role of localization and the nature of “textured metallic islands” are discussed.

Highly conducting polyacetylene

Abstract “New”-(CH) x and “ARA” (i.e. A dditional R educing A gent)-(CH)x have been synthesized at room temperature. “New”-(CH)x is formed by polymerizing acetylene on flat polymeric or glass substrates using an AlEt3/Ti (n-BuO)4 catalyst in silicone oil which has previously been heat-treated. Additional reducing agents may be added to the catalyst after its thermal treatment to produce “ARA”-(CH)x. The stretch-oriented “New”-(CH)x polymerized on glass and “ARA”-(CH)x polymerized on polyethylene can be p-doped with I2/CCl4 to give conductivities of ∼50,000 and ∼75,000 S/cm respectively. Optical absorption, photoinduced infrared absorption, magnetic susceptibility and conductivity studies on selected forms of the above doped and undoped types of (CH)x are discussed. The electronic structure of highly conducting “New”-and “ARA”-(CH)x is observed to be essentially identical to that of conventional Shirakawa-(CH)x.

Frequency Dependent Conductivity of Emeraldine: Absence of Protonic Conductivity

Abstract An experimental study of the frequency dependence of the conductivity of emeraldine polymer as a function of moisture content and protonation level is presented. A decrease in the conductivity and dielectric constant are observed to occur with pumping. The detailed frequency and composition behavior of these data are inconsistent with the proposed proton exchange assisted conduction of electrons (PEACE). The experimental results are shown to be consistent with the role of charging energy limited tunneling among small granular polymeric metal particles formed upon protonation of emeraldine.

Aniline tetramers: comparison with aniline octamer and polyaniline

Transport and magnetic properties of a charge transfer complex of a phenyl-end-capped aniline tetramer are studied. This oligomer is an analog of the emeraldine salt form of polyaniline. Magnetic susceptibility, four probe conductivity and thermopower of BF−4-aniline tetramer single crystals are reported together with ESR susceptibility, linewidth and g-factor of the C1O−4-aniline tetramer powder. Experimental results demonstrate that the “bipolaron” state is stabilized in the tetramer, in contrast to the polaron state being preferentially stabilized in the octamer and polymer (emeraldine salt) and in agreement with bipolaron stabilization in the protonated amorphous emeraldine and emeraldine salt in solution. Its conductivity is activated, in contrast to that of the polymer in which charging effects between metallic islands are important. Susceptibility, ESR and thermopower indicate some polarons are thermally excited amongst the bipolarons. The activation energy of polarons suggest that their origin is defect sites. The cause of the stability of the bipolaron with respect to the polaron state in the tetramer is suggested to be the reduced delocalization energy in this system.

Microwave conductivity and dielectric constant of (TMTTF)2X,X = SCN, ReO4, SbF6

Abstract Microwave conductivity (6.5 GHz) and dielectric constants for three tetramethyltetrathiafulvalene salts (TMTTF) 2 X, where X = SCN, ReO 4 , and SbF 6 are described. Good agreement is found between the high frequency data and the previously reported dc measurements. However, frequency dependence of the conductivity is observed below the anion ordering of (TMTTF) 2 SCN, and the microwave conductivity exceeds the dc value above the structureless transition of (TMTTF) 2 SbF 6 . This structureless transition of (TMTTF) 2 SbF 6 (T c = 154 K ) and of (TMTTF) 2 ReO 4 (T c = 220 K ) is reflected by a divergence of the dielectric constant contrasting with a stepwise decrease at the ordering of the ReO 4 anions (T c = 160 K ). These data support the pseudo-gap picture for the structureless transition and point out the similarity between the transition observed in (TMTTF) 2 SCN and the structureless transition.