A. Régis

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.

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.

Chromogenic WPA/TiO2 hybrid gels and films

A novel low-temperature electrochromic and photochromic material in the form of gels and thin solid films composed of H3PW12O40·nH2O (WPA) incorporated in titanium oxide gel issued of Ti(OPri)4 with addition of acetic acid were made via the sol-gel route by the dip-coating technique. Gels in bulk and thin film forms have been studied with the help of FT-IR transmission, reflection absorption and Raman spectroscopy. Similarity of the characteristic band frequencies with pure WPA confirmed that entrapped Keggin salt is well preserved inside the gel. Photochromism of WPA/TiO2 gels which appears after short exposure to UV (λ=366 nm) radiation is related to the excitation effect of branched alcohol leading to formation of carbonyl modes due to the alcohol transformation to aldehyde. Electrochromic properties of WPA/TiO2 gel films were tested in aqueous 0.001M HClO4 and 0.1M LiClO4 electrolytes as well as in the sol-gel electrochromic device (ECD) composed of LiClO4 doped ormolyte and Sb:Mo:SnO2 counter electrode film with optically passive response. Slight improvement of the WPA/TiO2 intercalation/deintercalation kinetics was obtained with the films exhibiting one order of magnitude higher electronic conductivity which were made with addition of in situ polymerized pyrrole with Fe(NO3)3.

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.

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.

Polymorphism and disorder in oligo- and polyanilines

Abstract The short-range structure and disorder of polyanilines is discussed by considering backbone geometry, conformation and sp 2 /sp 3 hybridization of nitrogen atom. The comparison between polymer and oligomer spectra shows that ring deformation modes are very sensitive to static/dynamic disorder. Low energy Raman and inelastic neutron allow to distinguish two class of emeraldine salts (ES-I/ES-II) already distinguished from X-Ray Powder patterns.

Inelastic neutron scattering study of proton dynamics in polyanilines

Abstract The inelastic neutron scattering (INS) spectra between 16 and 4000 cm −1 of chemically prepared non-doped (PANI 2A) and 50% protonated (PANI 2S) polyaniline at 20 K are reported. Band frequencies are compared to those estimated from the IR reflectivity. Different types of protons are distinguished in the spectra: (i) For the ‘water-containing’ 2S sample, INS features are consistent with the presence of a crystalline arrangement of the H 2 O network within the PANI lattice. (ii) The spectra of the 2S and 2A samples are dominated by bands assigned to vibrations of the benzoic and quinoidal protons. (iii) A continuum of intensity, underneath the bands of the aromatic protons, is tentatively attributed to the recoil of ‘quasi-free’ protons. These protons could be of great relevance to the explanation of the correlation between the protonation degree and the electronic and protonic conductivity.