Degradation of microporous polyaniline film by UV-ozone treatment

Thin microporous polyaniline films in the emeraldine-base form (PANI-EB) have been prepared electrochemically at a potential of 0.8 V. The effect of ozone treatment on the structural changes of polyaniline thin films has been investigated using X-ray photoelectron spectroscopy, SEM and EPR. Surface analysis showed that oxidation occurs mainly at the carbon atoms, resulting in the formation of CO and COOH species. The degradation reaction seems to be preferentially directed towards the quinoneimine units. At high extent of carbon oxidation, UV-ozone treatment may involve the direct oxidation of the nitrogen atoms to form -NO_x species.     

The effects of thermal treatment on the antioxidant activity of polyaniline

The thermal stability of chemically synthesized polyaniline (PANI) was examined, including granular (G) polyaniline powders formed conventionally in an HCl medium, and nanorod (NR) samples prepared using a falling-pH synthesis. The samples were examined before and after dedoping (dd) using thermogravimetric analysis (TGA), which showed small mass losses in the 200-300 °C temperature range, and greater mass losses due to oxidative degradation at higher temperatures. Furthermore, samples were treated thermally at 100, 125, 150, 175, 200, 250 and 300 °C for 30 min in air. SEM images did not show any pronounced effect on the morphologies of the samples from thermal treatment up to 300 °C. The ratios of the intensities (Q/B) of the predominantly quinonoid (Q) and benzenoid peaks (B) from FTIR spectroscopic analysis revealed that NR-PANI and NR-PANIdd underwent cross-linking upon thermal treatment up to 175 °C and were oxidized after treatment above 175 °C. G-PANI and G-PANIdd also underwent the same chemical changes with oxidation occurring above 200 °C. The free radical scavenging capacity of the samples was evaluated using the 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay, and was found to be independent of the spin concentrations of the samples. All samples exhibited a rapid decline in free radical scavenging capacity when exposed to temperatures above 200 °C, indicating that any polymer processing should be undertaken at temperatures less than this value to achieve high antioxidant activity.     

Electrochemical stability of polyaniline

The kinetics of the electrochemical degradation of polyaniline (PANI) films has been investigated in 0.5 M sulfuric acid solution at different electrode potentials ranging from 0.3 to 1.0 V vs. Ag/AgCl. Two kinds of PANI films were used, one doped with poly(styrene sulfonate) (PSS), and the other—with indigotetrasulfonate (ITS). Within a range of relatively low electrode potential (0.3-0.6 V), the degradation was found to proceed at a first-order rate constant of 4×10⁻⁵ to 5×10⁻⁵ s⁻¹, corresponding to degradation half-period of 4-5 h. A sharp increase in the degradation rate proceed by extending the electrode potential to higher values, with a maximum rate constant of ≈2.5×10⁻³ s⁻¹ for PANI-PSS, and ≈1.2×10⁻³ s⁻¹ for PANI-ITS films, obtained at a higher potential of 0.9-1.0 V. The data obtained are interpreted by different degradation rate for two distinct redox forms of PANI—emeraldine (slow degradation), and pernigraniline (fast degradation).     

Thermal degradation mechanism of dodecylbenzene sulfonic acid- hydrochloric acid co-doped polyaniline

Co-doped polyaniline (PANI) was synthesized in microemulsion by hydrochloric acid (HCl) and dodecylbenzene sulfonate (SDBS) then thermal treated in air at 160 and 200 °C for 0.5 h, respectively. The changes of structure, thermal stability, micromorphology and electrical conductivity after thermal treatment were studied by Fourier transformed infrared (FT-IR), Thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscope (SEM) and four-probe technique. It was found that the conductivity of PANI decreased about 50% after thermal treated at 160 °C, and droped by 2 orders of magnitude at 200 °C. This may be explained by that only a fraction of total mass of HCl losses during thermal treatment at 160 °C, but after heating at 200 °C, the dedoping of dodecylbenzene sulfonic acid (DBSA) along with cross-linking, chain scission and oxygen incorporation in a form of carbonyl groups take place, resulting in destruction of crystal structure, decrease of the emeraldine sequence, lower thermal stability and heterogeneous micromorphology.     

The simultaneous detection of heat flow and chemiluminescence intensity during oxidation of unstabilised polypropylene particles

A new experimental technique combining highly sensitive heat flow measurements (microcalorimetry) with the simultaneous detection of chemiluminescence intensity was developed. A perfusion ampoule was connected by fibre optics to an external photo-multiplier and photon counter device. The perfusion ampoule was placed in a 201 calorimetric unit used with a 2277 TAM thermostat. The simultaneous approach offers the advantage of using the same sample and experimental conditions, which reduces the risk for experimental artefacts. Results from the combined equipment confirmed the existence of a time shift between the oxidation profiles of unstabilised polypropylene (PP) previously reported using non-simultaneous MC and CL techniques. The results demonstrate that microcalorimetry is a highly sensitive and versatile tool in studying oxidation of polymers. The success of implementing fibre optics into a perfusion ampoule implies further the possibilities of introducing other probes for the simultaneous detection of oxygen uptake, formation of oxidation products and for studying the effects of UV irradiation among others.     

Fabrication of polyaniline with hierarchical structures in alkaline solution

Nanosheet- or nanorod-based microspheres and nanorod-based microrods of polyaniline (PANI) with hierarchical structures were successfully prepared by oxidation polymerization of aniline in alkaline solution. Temperature was found to have important influence on the morphology of PANI hierarchical structures and their building blocks. The concentration of alkali (NaOH) could be used to guide the morphological evolution of PANI, from leaf-like structures to nanosheet-based particles, and to nanorod-based microspheres and nanorod-based microrods with increasing concentration of NaOH in synthesis. The chemical structures of product were characterized by FTIR, UV-vis spectra and XRD, and its solubility was also studied in this report.     

Efficient oxidation of hydroquinone and alcohols by tailor-made solid polyaniline catalyst

We described the efficient and convenient catalytic oxidation of hydroquinone and primary/secondary alcohol compounds into benzoquinone and aldehyde/ketone using various oxidation states of polyaniline. We analyzed the oxidation catalyzing capabilities of the pernigraniline base polymer by designing a tailor-made catalyst to be used as a powerful oxidant, and it was easily recovered and regenerated for recycling.     

Intrinsic redox states of polyaniline studied by high-resolution X-ray photoelectron spectroscopy

High-resolution X-ray photoelectron spectroscopic (XPS) measurements of the various intrinsic redox states of polyaniline (PANI), using a monochromatized Al—Kα source, were carried out. The presence of the imine, amine and positively charged nitrogen species corresponding to a particular intrinsic redox state and protonation level of the polymer was resolved quantitatively and unambiguously. The result confirmed the peak assignments of former XPS core-level studies using the lower resolution non-monochromatized Mg—Kα X-ray source. Thus, the high-resolution XPS using a monochromatized Al—Kα X-ray source is a truly unique tool for the convenient and quantitative analysis of the various intrinsic redox states of PANI. Received: 16 May 2000/Accepted: 29 August 2000     

Electrical and structural analysis of conductive polyaniline/polyacrylonitrile composites

Conducting composites of polyacrylonitrile (PAN) copolymer containing 10% mass ratio methylacrylate and dodecylbenzene sulfonic acid doped polyaniline (PANI-DBSA) were prepared by solution blending. Electrical properties of the blends were characterized by means of electrical conductivity measurements and the phase structures were investigated via scanning electron microscopy (SEM), X-ray diffraction (XRD), FT-IR spectroscopy, differential scanning calorimetry (DSC) and dynamical mechanical analysis (DMA). It was found that the electrical conductivity of the composites increased with the increase of PANI-DBSA content and the percolation threshold lay around 3.2 wt%. DSC and DMA measurements showed that there was only one T_g for each blend and the values of T_g varied with the PANI-DBSA content, implying that the PANI-DBSA/PAN blend was at least partially compatible. The formation of the hydrogen bonding between the carbonyl groups in PAN copolymer and the imine groups in PANI-DBSA was identified by the FT-IR spectra. XRD demonstrated that the intrinsic layered arrangement of PANI-DBSA was disaggregated in the blends. Nanosize network structure of PANI-DBSA dispersing in PAN matrix and the so-called phase reverse occurring in the skin layer of the film samples at low PANI-DBSA loading were observed by SEM.     

X-ray photoelectron spectroscopic studies of charge transfer interactions in electroactive polyaniline

Four modes of charge transfer interactions in polyaniline (PAN), viz. acid protonation, self-doping, charge transfer interactions with organic acceptors, and charge transfer interactions with surface grafted functional polymers have been studied by X-ray photoelectron spectroscopy (XPS). In the case of acid protonation, the protonation behavior of volatile and non-volatile acid differs. The structures of sulfonated leucomeraldine (LM) and nigraniline (NA) are similar to those of sulfonated and self-protonated emeraldine (EM). A substantially higher degree of charge transfer interaction with the organic acceptors is observed for EM film that has been subjected to one cycle of acid/base treatment. The charge transfer interactions with the organic acceptors have proceeded further than the pure formation of molecular complexes. Both pristine and Ar plasma or O₃ pretreated EM films are susceptible to surface modifications by graft copolymerization. The protonic acid functional groups of the graft readily give rise to a self-protonated EM surface.     

X-ray study of plasticized polyaniline

Polyaniline (PANI) doped with diisooctyl phosphate (DiOHP) has been investigated by X-ray diffraction measurements. It was found that a lamellar-like structure is stabilized when x, the molar ratio of DiOHP to PANI is equal to 0.2. For x=0.3 the samples are singularly disordered. For 0.3<x<0.5, some molecular ordering is recovered and results in an ill defined layered structure. Finally, for molar ratios greater than 0.5 this ordering is progressively lost. The aged samples show exactly the same sequence as a function of the doping rate but globally with a higher degree of crystallinity for the ordered phases. Results are qualitatively discussed in terms of interplaying of protonation and plasticization effects induced by the doping process.     

Raman spectroelectrochemical study on the kinetics of electrochemical degradation of polyaniline

The kinetics of electrochemical degradation of polyaniline and a copolymer of aniline and metanilic acid have been studied by in situ Raman spectroscopy at a gold electrode. It has been concluded that probably no drastic changes in polymer structure occur on prolonged electrochemical treatment of polymer films at a high electrode potential (0.8 V vs. Ag/AgCl). Instead, most prominent changes relate to a gradual decrease of an overall intensity of spectra, viz. to gradual degradation of a polymer layer. The degradation proceeds faster at pH 1.0, compared to pH 7.0. The kinetic results obtained have been analyzed following simple 2- or 3-parameter exponential decay equations, and compared with the known degradation rate constants.     

Evolution of wood surface free energy after heat treatment

Surface free energies of pine and beech wood were investigated before and after heat treatment using the Lifshitz-van der Waals/acid-base approach from contact angles measured by the Wilhelmy method. The results obtained showed that the decrease of the electron-donating component of the acid-base component was the major parameter affecting the wetting of the modified wood's surface. The Lifshitz-van der Waals component was slightly modified after heat treatment indicating that the atomic and molecular interactions due to permanent or induced dipoles between wood macromolecules were weakly modified. Modification of the surface chemical composition was studied by X-ray photoelectron spectroscopy (XPS) and titration of acidity. XPS indicated an important decrease of the O/C ratio after heat treatment explaining the decrease of the electron-donating component (γ⁻) of the surface free energy. The decarboxylation and degradation of glucuronic acids present in hemicelluloses, demonstrated by titration of carboxylic acid functions of wood, had only limited effect on the electron-accepting component (γ⁺).     

Electrochemical study of polyaniline deposited on a titanium surface

The electrochemical synthesis of polyaniline on a titanium surface in aqueous sulfuric acid solutions with various concentrations of added aniline has been investigated by cyclic voltammetry. By utilizing a more cathodic potential range (up to −0.6 V) for the cyclization than is usual (up to −0.2 V) on Pt and Au electrodes, the new voltammetric waves have been deconvoluted from the already well-known ones for polyaniline. By simultaneous electrochemical and in situ Raman spectroscopic measurements, the Raman bands of polyaniline electrodeposited on a Ti electrode, were assigned for potentials of −0.15 V and −0.6 V. It was found that the new monitored waves were closely related to the so-called “middle” peaks and appear only when the polyaniline reaches an overoxidized state. Received: 7 August 1997 / Accepted: 4 November 1997     

Spectroscopic characterization of the structural changes of polyaniline nanofibers after heating

The thermal behavior of PANI nanofibers doped with β-naphthalenesulfonic acid (β-NSA) was investigated and their morphological and structural changes after heating were monitored by SEM, XRD and Raman techniques, respectively. By using electron-scanning microscopy it is possible to verify that the nanofiber morphology is stable and no polymer degradation is observed in thermogravimetric (TG) data up to 200 °C. Nevertheless, the heating promotes the formation of cross-linking structures (phenazine and/or oxazine-like rings), that is clearly demonstrated by the presence of bands at ca. 578, 1398, and 1644 cm⁻¹ in resonance Raman spectra of heated PANI-NSA samples. The most important consequence of the formation of cross-linking structures in PANI-NSA samples is that these samples retain their nanofiber morphology upon HCl doping in contrast to PANI-NSA nanofibers without heating.     

Effects of polyaniline chain structures on proton conduction in a PEM host matrix

This study examined the effects of the conjugated chain structure of polyaniline (PAn) on proton transport in a proton exchange membrane (PEM) containing a small amount of PAn colloidal particles. The PEM host matrix consisted of a hydrophobic three-component polymer blend (TCPB) of poly(4-vinylphenol-co-methylmethacrylate) P(4-VP-MMA), poly(butyl methacrylate) (PBMA), and Paraloid^® B-82 acrylic copolymer resins; in which a hydrophilic network of 2-acrylamido-2-methyl propanesulfonic acid (AMPS), 2-hydroxyethyl methacrylate (HEMA) and poly(ethylene glycol)dimethylacrylate (PEGDMA) was formed upon embedded polymerization. Colloidal PAn particles were added to the PEM matrix during the embedded polymerization of PEM. Two types of PAn colloidal particles with different chain structures and morphologies were synthesized by inverse miniemulsion polymerization and interfacial polymerization. The PAn(1) particles from inverse miniemulsion polymerization were bar-shaped, contained a higher fraction of quinoid diimine units than the scaffold-like PAn(2) particles from interfacial polymerization, and displayed a strong promotional effect on proton conduction. The oxidation state of the PAn particles was also varied by post-synthesis treatments to evaluate the effect of oxidation state on proton conduction. It was found that a mixed oxidation state such as the emeraldine form of PAn had the best enhancement effect. The PAn loading optimal for proton conductivity enhancement of the composite PEM was determined to be about 2 wt% of PAn(1).     

Enhanced magnetoconductivity and electrical property of MWCNT-CdS nanocomposite embedded in polyaniline

PANI/MWCNT-CdS nanocomposites with different content of CdS wt.% has been synthesized by the chemical oxidative in-situ polymerization reaction of aniline in the presence of multi-walled carbon nanotubes (MWCNT). TEM, XRD, FTIR, and TGA studies were done for the structural and thermal characterization of the samples respectively. The particle size of CdS nanoparticles distributes in between 2.7 and 4.8 nm. XRD spectrum reveals that the co-existence of MWCNT, CdS in PANI matrix, where CdS forms a hexagonal structure. TGA result shows that nanocomposite becomes more thermally stable with the increase in CdS content. The dc electrical transport property of PANI/MWCNT-CdS nanocomposites has been investigated within a temperature range 77 ≤ T ≤ 300 K. The dc conductivity follows a 3D variable range hopping (VRH) model. A large magnetoconductivity change (19%) is observed for 2 wt% CdS content in PANI/MWCNT-CdS, which is explained by the wave function shrinkage model.     

Thermal and structural stability of composite systems based on polyaniline deposited on porous polyethylene films

Conducting composite systems containing polyaniline layers produced on the surface and inside the pores of polyethylene support have been prepared. Microporous polyethylene films were obtained by melt extrusion with subsequent annealing, uniaxial extension, and thermal fixation. Polyaniline layers were formed by in-situ polymerization of aniline onto polyethylene porous support placed into the aqueous reaction mixture. Structural and chemical transformations upon heating of these systems in air in free state and in vacuum under load have been investigated by thermo-mechanical tests, IR spectrometry, and electron microscopy. Changes in mechanical properties of composites after heating have been analyzed. Composite systems have been found to demonstrate a considerably lower shrinkage upon heating than microporous polyethylene substrates. It has been discovered that the composites preserve mechanical integrity on heating up to temperatures much higher than the polyethylene melting point. It is concluded that thermo-mechanical behaviour of the composites is determined by the space-continuous phase of polyaniline on the surface and in the bulk of polyethylene support.     

Electrochemical and Raman spectroscopic study of polyaniline; influence of the potential on the degradation of polyaniline

The polymerization of aniline has been studied employing in-situ electrochemical and Raman spectroscopical techniques. Aniline was polymerized by cyclic voltammetry on a Pt surface in sulfuric acid solutions of aniline. The Raman bands were assigned for degradation products of the overoxidized form of polyaniline. A discussion of the degradation mechanism is given. Received: 12 November 1997 / Accepted: 20 January 1998