Synthetic Metals (v.150, #2)

Polyaniline (PANI) has been prepared in water-dispersed medium in the presence of nonionic surfactants. Large range of surfactants was selected based on their chemical composition and their hydrophile–lipophile balance (HLB). Three types of nonionic surfactants were chosen: three nonylphenolethoxylates, a surfactant bearing an amide group, and surfactants containing an amine function. The experimental conditions were optimized and a dispersion process was developed for the direct synthesis of a doped PANI. The polymerizations were monitored by pH measurements and UV spectroscopy and the kinetic behavior was determined. The conductive PANI was systematically characterized and the effect of the surfactant established. The PANI/surfactant affinity was estimated. The doping level and the surface composition of the PANI particles have been studied by X-ray photoelectron spectroscopy (XPS). Absorption spectra of PANI in a large range of pH showed the transition from emeraldine salt to emeraldine base form. All the results were compared to the properties of a conventional HCl doped PANI and a dodecylbenzenesulfonic acid (DBSA) doped PANI.
Keywords: Polyaniline; Nonionic surfactant; Kinetics; Dispersion;

The uniform polyaniline nanoparticles with size of about 80 nm were electropolymerized using the method of constant potential of 0.7 V. The effects of erbium chloride and magnetic field on properties of polyaniline deposited on two different platinum electrodes were investigated. The resulting films were characterized by FT-IR, UV–vis spectra and scanning electron microscope (SEM). The experimental results show that the magnetic field has an orientation effect on polyaniline chain and there exists the interaction between Er3+ and polyaniline chain due to the electrostatic interaction. The longer polymerization time and the higher concentration of erbium chloride can lead to the more distinct morphology of polyaniline films.
Keywords: Polyaniline; Nanoparticles; Cyclic voltammetry; FT-IR spectra; UV–vis spectra; SEM;

Thin films of polyaniline (PAn)-oligomers were gradually converted into thick conductive PAn films using a two-pot, alternate oxidation and reduction strategy. The well known redox mechanism of aniline polymerization was used to design independent oxidation and reduction processes. An important aspect of the strategy is the recyclable nature of raw materials, aniline and oxidant in the pot. Using appropriate dopants, well-defined PAn nanostructures were generated.
Keywords: Polyaniline; Two-pot process; Nanostructures;

Observation of photoluminescence in polypyrrole micelles by Kwan Sik Jang; Heung Cho Ko; Bongjin Moon; Hoosung Lee (127-131).
Photoluminescence (PL) was observed from micelles of polypyrrole without substituent groups. CHCl3 and CCl4 as well as 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) showed a typical fluorescence quenching behavior that can be observed when quencher molecules are added to a micellar solution of fluorescent material. At higher concentrations, the PL intensity decreased due to quenching in the aggregate phase and self-absorption. The PL intensity varied depending upon the solvent in the order of dimethyl sulfoxide (DMSO) >  N,N-dimethylformamide (DMF) >  N-methyl-2-pyrrolidinone (NMP) > ethanol > toluene > tetrahydrofuran (THF).
Keywords: Polypyrrole micelles; Photoluminescence of polypyrrole; Photoluminescence quenching in micelles; Highly soluble polypyrrole; Anionic surfactant-doped polypyrrole;

Volume changes of polypyrrole doped with poly(styrene sulfonate) (PPy/PSS) during redox switching were investigated in an aqueous solution of CsClO4, utilizing electrochemical scanning tunneling microscopy (EC-STM). PPy/PSS was synthesized on a micropatterned Au substrate for the quantitative measurement of volume changes with doping. It was found that the polymer volume is greatly increased due to solvent swelling and the magnitude of anisotropic volume changes during redox switching ranges from −0.95 to 15.5%, depending on the film thickness. The potential cycling between −0.7 and +0.1 V (versus Ag wire) also revealed that the PPy/PSS volume is not linearly related to the potential. The volume decreases with doping up to −0.05 V and begins to rise above −0.05 V.
Keywords: Polypyrrole; Scanning tunneling microscopy; Volume change; Film thickness;

It is supposed that there should be a thermal electric effect if a dc current is applied across two dissimilar conducting polymers, similar to so called “Peltier effect” in metals or semiconductors. However, this hypothesis has not been tested on conducting polymers and using these materials to make cooling fabrics has never been attempted before. Polypyrrole coated fabrics were used to test the hypothesis in this preliminary study. Seebeck and the Peltier effects were proven to exist. However, thermoelectricity effect between two conducting polymer coated fabric samples was only about 10 μV/°C. Cooling effect by conductive polymer powder was achieved but performance was unsteady due to electrical degradation of the conducting polymer. Nevertheless, the concept was demonstrated and the development of a cooling fabric is possible.
Keywords: Cooling effect; Peltier effect; Conducting polymers; Textiles;

Thermal degradation of the dielectric relaxation of 10–90% (w/w) zeolite-conducting polypyrrole composites by A.N. Papathanassiou; J. Grammatikakis; I. Sakellis; S. Sakkopoulos; E. Vitoratos; E. Dalas (145-151).
The effect of thermal aging of 10–90 wt% zeolite-conducting polypyrrole composite on its dielectric properties is studied in the frequency range 10−2 to 2 × 106  Hz from room temperature to liquid nitrogen temperature. A dielectric relaxation mechanism, which appears in the fresh samples, is influenced by the thermal annealing. The frequency f max where a maximum of a dielectric loss peak is located decays exponentially with the aging time and the intensity of the loss peaks shows a maximum at intermediate aging time. A modified Williams–Landel–Ferry law describes the temperature variation of f max in all specimens. Increasing activation energy values on increasing the aging duration are obtained. The temperature dependence of f max and the activation energy (regarded as the height of a potential barrier) are different from those characterizing the macroscopic conductivity, which is described by the charging energy limited tunneling model. The intensity of the dielectric mechanism in thermally treated samples deviates from the linear decrease with inverse temperature occurring in fresh polypyrrole. Although the thermal degradation of the logarithm of the dc conductivity decays proportional with the root of the aging time, the equivalent conductivity obtained from the dielectric data decays exponentially with aging duration. Time constants are obtained in both cases. The model of Barton–Nakajima–Namikawa (BNN) can hardly interconnect the dc conductivity with the relaxation process in fresh sample. The divergence augments with the aging time. The thermal aging law and the inadequacy of the BNN model probably indicates that the dc process is probably irrelevant to the relaxation process.
Keywords: Conducting polymers; Zeolite; Polypyrrole; Electrical conductivity; Permittivity; Thermal aging;

Nickel oxide/carbon nanotubes nanocomposite for electrochemical capacitance by Ji Yeong Lee; Kui Liang; Kay Hyeok An; Young Hee Lee (153-157).
We have fabricated supercapacitor electrodes with nickel oxide (NiO)/carbon nanotubes (CNTs) nanocomposite formed by a simple chemical precipitation method. The presence of CNT network in the NiO significantly improved (i) the electrical conductivity of the host NiO by the formation of conducting network of CNTs and (ii) the active sites for the redox reaction of the metal oxide by increasing its specific surface area. This increased the specific capacitance by 34% at a percolation limit of 10 wt.% of CNTs. In addition, we also found that the power density and cycle life were improved. We will further show that the specific surface area of the composite is closely related to the specific capacitance.
Keywords: Carbon nanotubes; NiO composite; Supercapacitor; Electrochemical capacitance;

The characterization of the optical functions of BCP and CBP thin films by spectroscopic ellipsometry by Z.T. Liu; C.Y. Kwong; C.H. Cheung; A.B. Djurišić; Y. Chan; P.C. Chui (159-163).
The optical properties of 2,9-dimethyl-4,7-diphenyl-l,10-phenanthroline (bathocuproine or BCP) and 4,4′-bis(carbazol-9-yl)biphenyl (CBP) thin films were characterized by spectroscopic ellipsometry (SE). Thin films of BCP and CBP were evaporated on silicon substrates. Spectroscopic ellipsometry measurements were performed in the wavelength range of 300–1000 nm. The refractive indices and extinction coefficients of BCP and CBP were determined by fitting the obtained data using Lorentz model and Relaxed Lorentz (RL) model with variable thicknesses. The extinction coefficients obtained from the fitting of ellipsometry data were in excellent agreement with absorbance measurement results.
Keywords: Organic light emitting diodes (OLEDs); Microcavity;

An attempt was made to electrochemically immobilize tetrasulfonated phthalocyanine (CoTsPc) into polypyrrole in presence of three types of surfactants (cationic, anionic, and neutral) during the polymerization of pyrrole in an aqueous solution with different potential regions. The surface characteristics of anodically/cathodically prepared electrodes with cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and Triton X-100 were investigated using the scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) measurements. The SEM and XPS measurements revealed the position and orientation of CoTsPc and surfactant, respectively, in the prepared films. The film conductivity was deduced from AC impedance and its conductivity values were found to be very low when compared to the literature results. The decrease of film conductivity in presence of the surfactant and CoTsPc could be associated with the fewer amounts of counter ions in the film and also the absence of conjugation for electron hopping within the film.
Keywords: Electropolymerization; ppy/CoTsPc/surfactant; Films; Cathodic; Anodic; Surfactants;

Polyaniline thin films in situ polymerized under very high pressure by D.W. Gu; J.S. Li; J.L. Liu; Y.M. Cai; L.J. Shen (175-179).
The conducting PANI films with network morphology were synthesized on glass substrate by in situ polymerization under various very high pressures (>108  Pa). The effects of applied pressure on films were investigated by using UV–vis absorption spectroscopy, SEM, film thickness measurement and conductivity measurement. The high pressure used in polymerization played an important role in the formation of PANI films. Inferred from the analysis of our experimental results, there was a competition between the adsorption and transference of the cation radicals during the initial growth of films under high pressure. The applied high pressure was favorable to improve the conjugation length in PANI and/or electrical contacts between the fibrils in network of PANI films.
Keywords: Polyaniline film; High pressure; Polymerization; Electrical conductivity;

Charge transport mechanism in bis(double-decker lutetium(III) phthalocyanine) (Lu2Pc4) thin film by Ahmet Altındal; Şaziye Abdurrahmanoğlu; Mustafa Bulut; Özer Bekaroğlu (181-187).
The dc and ac dark conductivity measurements have been performed on spin-coated Lu2Pc4 thin film in the frequency range 40–105  Hz and in the temperature range 50–290 K. Measurements revealed that dc behaviour of the film can be described by Mott's three-dimensional variable range hopping (VRH) model, T −1/4 law, in the whole temperature range. The ac conductivity σ(ω) varied as ω s , where the exponent s is found temperature dependent variable and always less than unity. The measured ac conductivity data are discussed in terms of classical models based on pair approximation and a unified theory. Although the ac behaviour can be approximately explained by the correlated barrier hopping model below the 190 K, the temperature dependencies are stronger than any classical model predicts. The log–log plot of the normalized conductivity σ(ω)/σ(0) with reduced frequency ω ˜ / σ ˜ ( 0 ) showed that, the ac conductivity data agrees fairly well with the predictions of the extended pair approximation (EPA) model due to Summerfield and Butcher.
Keywords: ac conductivity; Variable range hopping; Lutetium phthalocyanine; EPA;

White light emission from two blue emitters of an equimolar (1:1) blend in single-layer device by O.-K. Kim; W.H. Kim; Z. Huang; J.T. Je; C.S.P. Sung (189-193).
A white light emission was observed from a single-layer emitting system consisting of an equimolar blend of two blue emitters of a bipolar oligomer, 9,9-spirobifluorene-centered bioxadiazole (303) and polyvinylcarbazole (PVK), featuring a single broad emission (ranging in 400–800 nm), which is distinctly different from those of large PVK-fraction blends (λ em  ≈ 490 nm) and that of pristine 303 (λ em  ≈ 555 nm) as well.
Keywords: White light emission; Electroluminescence; Photoluminescence;

Efficient light emitting diodes from polyfluorene copolymer blends by Ali Cirpan; Liming Ding; Frank E. Karasz (195-198).
Highly efficient light emitting diodes were obtained by blending blue-emitting poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(N,N′-diphenyl)-N,N-di(p-butyl-oxyphenyl)-1,4-diaminobenzene] (I) with green-emitting poly[(9,9-dioctyl-2,7-divinylene-fluorenylene)-alt-co-{2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene}] (II). Only green emission was obtained from the films of polymer blends and from corresponding double-layer LEDs, indicating an almost complete Förster energy transfer from I to II. These devices show enhanced external quantum efficiency, higher brightness and lower turn on voltages. Luminance and efficiency of 2.6 × 104  cd/m2 and 2.55%, respectively, were obtained.
Keywords: Light emitting diode; Electroluminescence; Polyfluorene; Blending;

The morphology and thermal behaviour of model charge transport composites based on the hole transport molecule tri-p-tolylamine (TTA) in bisphenol-A polycarbonate (BPAPC), cyclohexyl polycarbonate (PCZ) and polystyrene (PS) were investigated. A very large decrease in the glass transition temperatures (T g) of the host polymers is seen with the dispersion of TTA in the polymer matrix. The T g reduces almost close to ambient temperature. This is much more pronounced than in the case of TPD (a tetraphenyl diamine) based composites [F. Khan, A.-M. Hor, P.R. Sundararajan, J. Phys. Chem. B 108 (2004) 117–126]. It is found that the extent of T g depression is higher when the polymer is conformationally less flexible. FTIR study indicates a molecular level of interaction between TTA and the polymers used here. Upon annealing, phase separation and crystallization of TTA occurs at temperatures as low as 80 °C, and leads to a recovery of the T g of the polymer. The T g recovery is again much more significant than in the case of TPD. Sublimation of TTA occurs when the polycarbonate based composites are annealed at ≥120 °C. Re-plasticization upon annealing as suggested in the case of TPD/BPAPC is not observed with TTA/BPAPC. Due to such sublimation and the accompanying depletion of TTA, a photoreceptor, annealed at 140 °C failed to discharge. With polystyrene as the binder, sub-micron crystals of TTA are seen even with a low concentration of 30%. Chaining of such crystals is seen with a concentration of 50%, which could be a contributing factor for the enhanced charge mobility noted for polystyrene based charge transport composites.
Keywords: Tritolylamine; Charge transport; Phase separation; Sublimation; Photoreceptor;

Unusual behaviours of photoluminescence by long-term illumination in alkoxy-substituted phenylene vinylene polymer by Bin Nal Yoon; In-Hee Chang; Suk-Ho Choi; Dong Hoon Choi; Min Ju Cho; Jae-Hong Kim (213-216).
We have studied photo-induced effects of photoluminescence (PL) for dialkoxy phenylene vinylene polymer (P(MHPV)) films with different thickness under irradiation of 488 nm laser line. The samples show photo-induced PL enhancement or quenching depending on excitation level, temperature, and sample thickness. Increasing behaviour of PL intensity with exposure time is observed in relatively thicker samples and the rising rate of the PL intensity is larger under higher intensity of the excitation light. The increasing behaviour of PL intensity with exposure time is changed into the decreasing one at temperatures ≤200 K. It is proposed that these unusual behaviours are attributed to the chemical structural variation through degradation of the bridge double bonds linking the phenyl rings, resulting in partial disconnection of conjugation in the polymers during illumination and to the spatial diffusion of excited carriers to non-degraded P(MHPV) regions by energy transfer.
Keywords: Phenylene vinylene polymer; Photoluminescence; Enhancement; Quenching;