A. Gruger

Infrared and Raman study of polyaniline Part II: Influence of ortho substituents on hydrogen bonding and UV/Vis—near-IR electron charge transfer

Abstract IR and Raman spectra of chemically (electrochemically) prepared protonated forms of polymers of 2-methyl (PANIME), 2-methoxy (PANIMEO), 2-chloro (PANICL) and 2-fluoroanilines (PANIF) and their corresponding bases have been investigated in the 4000−100(200)cm−1 region. Exciting lines at 457.94, 514.53, 632.81 and 1064 nm have been used for the investigation of Raman resonance spectra in order to characterize the benzenoid and (semi)quinoid (SQ) parts of the repeat unit. Highly conducting poly-2-methyl and poly-2- methoxyaniline on the one hand show spectral features which can be interpreted in terms of a strong NH⋯N hydrogen bond (dNN ≈ 0.25 nm) with a NH stretching broad absorption centred near 1300 cm−1 cut by numerous Evans transmission bands. On the other hand, poly-2-chloro and poly-2-fluoroaniline which have much lower conductivity have much weaker NH⋯X hydrogen bonds characterized by NH stretching bands near 3000 cm−1 (dNN ≈ 0.28/0.285 nm). Conductivity mechanisms are discussed. UV/Vis—IR spectra show that conductivity is essentially associated with interchain coupling. IR spectra suggest that in PANIME and PANIMEO as in PANI the interchain charge carriers constitute bipolaronic species, i.e. −NH+SQNH+−, cross-linked by NH⋯N hydrogen bonds. In weakly conducting PANIF and PANICL, however, disproportionation of the bipolaronic species, no longer stabilized by H-bonding, leads to localized polaronic moieties. Possible mechanisms for proton conduction are discussed.

Infrared and Raman study of polyaniline Part I. Hydrogen bonding and electronic mobility in emeraldine salts

Abstract Infrared and Raman spectra of chemically prepared (doped) protonated forms of polyaniline and its deuterated ND and —C 6 D 4 —derivatives have been examined. Broad infrared absorption centered near 1100 cm −1 and cut by numerous Evans holes has been observed and interpreted as an NH stretching band of a strong asymmetric interchain NH + ⋯ N hydrogen bond (d N-N ≈ 2.5 A). Evans holes, which have corresponding Raman bands, have been assigned to various benzenic and semiquinoidal vibrations. A model of highly conducting polyaniline taking into account the proton transfer is proposed and discussed. The inversion of the NH + ⋯ N hydrogen bond leads to interchain conversions which can generate charge carriers.

Optical and vibrational spectra of sols/solutions of polyaniline: water as secondary dopant

Abstract The nature of polyaniline (2A or 2S form) dissolved in concentrated sulfuric and methanesulfonic acid has been investigated by (UV-Vis-NIR) optical absorption and Raman scattering using various exciting laser lines. Two different palaronic species have been evidenced and selectively analysed, using 15 N labelled and perdeuterated ring derivatives. Addition of water induces formation of 2S-like materials, probably forming a sol and not a true solution. The achievement of the metallic state seems to be related to the ordering of the chains interacted with water, oxonium ions and anions.

Spectres de vibration du trans-azoxybenzène et de quelques dérivés isotopiques

Abstract The I.R. (50–3200 cm −1 ) and Raman (100–3200 cm −1 ) spectra of the trans -azoxybenzenes C 6 H 5 NONC 6 H 5 , C 6 D 5 NONC 6 D 5 and C 6 H 5 N 15 ON 15 C 6 H 5 have been analyzed. The infrared dichroism of an oriented crystal of C 6 H 5 NONC 6 H 5 and C 6 D 5 NONC 6 D 5 has been also observed between 300 and 1600 cm −1 . An assignment of the fundamental vibrations is proposed. The electronical delocalization between the phenyl and the azoxy groups is discussed.

Chain length effect on intrachain electronic excitation and interchain coupling in poly- and oligo-anilines

Abstract The electronic absorption profiles of a polyaniline base (using NMP as a solvent) and of a polyaniline salt (dissolved in sulfuric and methanesulfonic acid) have been studied from 0.5 to 5.5 eV and compared with those of some constitutive bricks of the polyaniline repeated units: diphenylamine (BB), N , N ′-diphenyl-1,4-phenylene-diamine (BBB) and -diimine (BQB) and the oxidized tetramer (BQBBa, a stands for a terminal amine group). Standard (∼100 nm) and short (10–20 nm) polyaniline chain lengths are considered. Resonant Raman profiles were determined for exciting energies ranging from 1 to 3 eV. In the cationic chains, the electronic absorbance per cycle is increased with the chain length but the formation of NH 2 + groups in strong acids destroys the conjugation, leading to isolated phenyl rings. On the other hand, for polymer base chains, constant behavior is observed with length ≥16 nm and pernigraniline chain portions are evidenced from the analysis of the Resonant Raman profile. Although a semi-conduction amorphous-like behavior is deduced from Urbachs tail analysis for polyaniline base nanofilaments, a crystalline behavior is recognized for HCl intercalated films.

Bulk–surface nanostructure and defects in polyaniline films and fibres

Polyaniline films and fibres were prepared by different ways using meta-cresol and camphor sulfuric acid. Their microstructures were studied by scanning electron microscopy and compared to those of films prepared using the NMP route and a usual emeraldine chloride salt powder. Raman scattering under various exciting conditions was used to determine the structure of the different polymorphs (Types I and II), which was correlated with the process. Structure #I is observed at the surface preferentially. Partial surface de-protonation and formation of quinoic segments with special geometry are evidenced.

Inelastic neutron scattering studies of polyanilines and partially deuterated analogues

Abstract Inelastic neutron scattering spectra have been measured from 16 to 4000 cm−1 for various polyaniline samples at 30 K: the emeraldine-base and the emeraldine-salt, the ring-deuterated analogues and their hydrated forms. The spectra of the totally hydrogenated samples are dominated by bands due to protons bound to the aromatic rings. The spectrum of the ring-deuterated base reveals that most of the remaining protons are not bound to nitrogen atoms. There is a continuum of intensity due to the recoil of free particles with mass 1 amu. In the emeraldine-salt additional protons (H+) are trapped in very shallow potential-wells with dissociation threshold ≈ 300 cm−1. At energy transfer greater than this threshold, these protons are free to recoil. The spectra of the hydrated samples reveal that free entities are not trapped by water molecules. These new dynamics are tentatively related to the electronic structure and conductivity of these polymers. These are supposed to be determined by the position of the electronic state of “H0” entities relatively to the half-filled π-band of the metal-like form of the pemigraniline-base. In the emeraldine-base electrons transferred from “H0” entities to the conduction band give an insulator with a totally-filled band and a gas of H+ entities. In the emeraldine-salt the electronic state of “H0” entities is lowered. The structure is that of a metal-like half-filled band with a gas of “H0” entities and weakly bound protons.

Polymorphisme Cristallin des Sept Premiers Termes de la Serie des 4,4′-Di-n-alcoxyazoxybenzenes—I—Enthalpimetrie Differentielle—Spectroscopie Raman (Modes de Reseau)

Abstract A crystalline polymorphism has been detected in the seven first members of the homologous series 4,4′-di-n-alkoxyazoxybenzenes (n = 1 to 7) by D.S.C and Raman spectroscopy. In the case of the compounds n = 1,3,4,5, the crystalline polymorphism is characterized in the high temperature phase by a statistical disorder of the azoxy group, the molecules being pseudo-centrosymmetrical. Besides, for n = 3 and 5, con-trarily to n = 1 and 4, an important increase of the molecular volume due to conformational changes appears at the transition. As for the members n = 2, 6 and 7, the crystalline systems of the two solids are practically similar.

Pressure—temperature-induced conductivity in polyaniline base and salts

Abstract The conductivities of polyaniline (and poly(2-methoxyaniline), poly(2-methylaniline)) and of the sulfonated derivative have been studied as a function of pressure (0 to 30 kbar) and of temperature (20 to 100–400 °C). Conductivity measurements were made with the aid of impedance spectroscopy in a piston—cylinder high-pressure cell. The conductivity of emeraldine chloride is quite stable up to 10 kbar and then increases continuously up to a factor 50 (1 to 60 S cm −1 at 100 °C, 25 kbar). Similar behaviour is observed for the ortho -substituted derivatives (poly(2-methylaniline) and poly(2-methoxyaniline)) as well as for the self-doped sulfonated polyaniline. The conductivity increases regularly with temperature and then drops irreversibly: highest values are observed at 90, 70 and 60 °C for the self-doped material and the chlorides, poly(2-methoxyaniline chloride) and poly(2-methylaniline chloride), respectively. A strong increasing in conductivity is observed versus temperature and pressure, up to 10 −4 S cm −1 , at 300 °C under 20 kbar for the emeraldine base. The nature of the charge carriers (electrons or protons) is discussed.

The O2/H2O redox couple as the origin of the structural/electronic defects in polyanilines

Abstract The protonation of N , N ′-diphenyl-1,4-phenylenediimine (BQB oligomer, representative of the polyaniline (PANI) unit) by H 2 SO 4 and non-oxidative acids like H 3 PO 4 , HCl and CH 3 COOH is studied by UV-Vis–NIR and resonance Raman spectroscopies. Emphasis is put on the water content effect. The acid-solubilised BQB is reduced to the amine oxidation state: in diluted acids, protonated (BBB) 2+ species are formed while a mixture of radical cations and protonated (BQB) 2+ species is observed for higher concentrations. The results show that redox reactions involving the O 2 /H 2 O couple are present as side-reactions. In concentrated HCl acid, the presence of protonated (BBB) species chlorinated on the ring indicates that the Cl 2 /Cl − redox couple participates to the redox phenomena. Side-reactions randomly affect the quinoid or the benzenoid segments of the polyaniline, changing the backbone chain linkage during EB and ES synthesis. This prevents ideal linking for the final ES material and the resulting materials failed to achieve the intrinsic conductivity expected for ideal ES products.