K.R. Cromack

Polyaniline: Oriented Films and Fibers

Abstract The dependency of the conductivity of polyaniline (emeraldine oxidation state) on its molecular weight has been determined. Uni- and biaxially oriented films and uniaxially oriented fibers of emeraldine base have been studied and the dependency of their degree of crystallinity, tensile strength, and conductivity (after doping) on draw ratio has been determined.

N-substituted derivatives of polyaniline

Fully N-methylated polyaniline was obtained analytically pure by the oxidative polymerization of C6H5N (CH3) (H) with (NH4)2S2O8 in 1.0M aqueous HCl. The resulting black-green polymer (σ ∼10−4 s/cm) was shown to have a similar backbone structure to parent polyaniline, and upon heating, lost HCl and H2O, yielding a black-brown solid, (σ < 10−8 s/cm) containing ∼14% oxidized groups. “Pseudo-protonic” acid doping of “emeraldine” base with neat (CH3)2SO4 yielded polymers whose extent of doping could be monitored continuously. The ∼50% methylated “emeraldine” base (σ = ∼10−2 s/cm) had a χPauli approximately half that of 50% proton doped “emeraldine” base, consistent with the symmetrically effect of the methyl groups.

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.

High resolution ESR of pernigraniline base solutions : the shape of neutral solitons

Abstract High resolution ESR measurements of solutions of pernigraniline base (PNB) form of polyaniline reveal direct evidence of neutral solitons as the origin of unpaired spins. Our ESR spectra show for the first time rich hyperfine structure from which the spin distribution has been determined for a defect in a conducting polymer. The spin density is found to reside largely on a nitrogen site and is confined primarily to the two adjacent rings. Bredas et al. have proposed two types of charged solitons, QNQ and BNB (Q = quinoid, B = benzeniod) with the charge distribution having a node and antinode at the nitrogen site, respectively. Our ESR spin distribution is in accord with that calculated by Su and Epstein for neutral BNB solitons, which they found to be more stable than neutral QNQ solitons. No experimental evidence was found for the presence of QNQ solitons.

Photoexcitations in emeraldine base

Abstract The emeraldine base form of polyaniline has been studied by photoinduced absorption (PA) spectroscopy and by light-induced electron spin resonance (LESR). The PA spectrum shows the presence of an induced bleaching at 2 eV, three photoinduced absorptions at 0.9, 1.4, and 3.0 eV, and an associated bleaching of the bandedge transition starting at 3.5 eV. The two induced absorptions at 1.4 and 3.0 eV are ascribed to the photogeneration of positive polarons. The origin of the “low energy” induced absorption at 0.9 eV is less certain, though it is possibly due to the formation of bipolarons or some other interchain species. Time-resolved studies of the exciton bleaching indicate that the excitons form within 10 ps or less after light strikes the polymer, and that their fast decay dynamics are independent of temperature. The measured LESR signal for pump light of energy ∼ 2.5 eV is consistent with the dissociation of some excitons into polarons.

Photoexcited defects in poly(3-methylthiophene): Light induced electron spin resonance and photoinduced absorption

Abstract Light induced electron spin resonance (LESR) and photoinduced absorption (PA) are reported for poly(3-methylthiophene) (P3MT). The LESR is composed of a single line at g≅2 whose position, width and intensity are dependent on the exciting photon energy, ηωL. The LESR line intensity shows a sharp peak at 2.05 eV, where the line is the narrowest and it has the smallest g. The PA spectrum is composed of infrared active vibrations (IRAV) absorption bands and two midgap electronic bands, at 0.39 and 1.28 eV. The intensity of these three features as a function of ηωL is similar to each other and shows a sharp peak at 2.10 eV. Based on these observations, the possible spin-charge relationships for the photoinduced defects are discussed.

ESR of pernigraniline base solutions revisited

Abstract A new electron spin resonance solution spectrum for intrinsic paramagnetic defects in pernigraniline base (PNB) is reported. Our previously reported ( Synth. Met. , 55–57 (1993) 648) solution spectra for PNB were found to correspond to a nitroxyl radical contaminant. The correct isotropic splitting constant for the nitrogen hyperfine interaction in PNB is measured to be 9.8 ± 0.4 G. The π-electron spin density on the central nitrogen atom for intrinsic defects in PNB is estimated to be 0.36. This experimentally determined density is in good agreement with calculated densities for the neutral soliton in PNB.

Light induced electron spin resonance in polyaniline

Abstract Light induced electron spin resonance (LESR) is reported for members of the polyaniline family of polymers. The LESR is composed of a single line at g≈2 whose width and intensity are dependent on the time of exposure and temperature. The LESR line intensity shows a very long growth and decay time that can be fit to a Kohlrausch stretched exponential type law with coefficients that follow a Vogal-Fulcher law. There is a direct dependence of the linewidth of the photo-induced spins on the concentration of induced spins suggesting a strong interaction between induced defects and leading to the postulation of a phase segregation of defect.