Ion exchange behaviour and charge compensation mechanism of polypyrrole in electrolytes containing mono-, di- and trivalent metal ions

The electrochemical quartz crystal microbalance technique was used to examine the ion exchange behaviour and charge compensation mechanism of polypyrrole (PPy) doped with polystyrene sulfonate (PSS) immersed in electrolytes containing singly, doubly and triply charged metal ions. New insights were obtained concerning the mechanism of charge compensation and ion exchange behaviour of PPy/PSS films in electrolytes with triply and doubly charged transition metal ions. Understanding the ion exchange behaviour of conducting polymers immersed in these types of aqueous media could play a pivotal part in the development of methods for removing toxic metals from water supplies. Charge compensation occurred predominantly by means of cation movement for PPy/PSS films when the electrolyte was KNO₃, Ca(NO₃)₂, Mn(NO₃)₂ or Co(NO₃)₂. After prolonged redox cycling the electroactivity of the polymer decreases slightly, and the charge compensation mechanism becomes complex owing to movement of ions and neutral species in opposite directions. The charge compensation mechanism for PPy/PSS films immersed in aqueous Cr(NO₃)₃ and Al(NO₃)₃ solutions was also complex, with the ion exchange behaviour shifting towards anion movement to and from the polymer.     

Synthesis and redox behavior of PEDOT/PSS and PPy/DBS structures

Electrochemical synthesis of dodecylbenzenesulfonate (DBS⁻) doped polypyrrole (PPy/DBS) onto polystyrenesulphonate (PSS⁻) doped poly(3,4-ethylenedioxythiophene) (PEDOT/PSS) modified gold EQCM electrode was studied. Monitoring of mass and potential response during PPy growth onto PEDOT underlayer revealed at least three different stages in this process. AFM study confirmed that PEDOT film morphology constantly changed during the synthesis of PPy film onto its surface. Studying of redox properties of PEDOT/PSS + PPy/DBS structures showed that PEDOT influenced the redox processes of the structure after its complete coverage with PPy suggesting the formation of three-dimensional electrode.     

Charge carrier mobility in substituted polythiophene-based diodes

We have investigated the transport properties of the semiconducting polymer poly(3-(2′-methoxy-5′-octylphenyl)thiophene) (POMeOPT). We have measured the current–voltage (C–V) characteristics of single polymer layer devices in two regimes contact limited current and bulk-limited current. The passage from one regime to the other was done upon insertion of a conducting polymer poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT–PSS) between the metallic electrode and the semiconducting polymer. With PEDOT–PSS as electrode, the polymer gave space–charge limited current (SCLC) with the mobility dependent on electric field. Fitting the data, we were able to obtain important parameters, such as the zero-field mobility and the characteristic field. We have compared our results with the well-studied polymer poly(2-methoxy-5-(2′-ethyl-hexyloxy)–1,4-phenylene vinylene) (MEH–PPV) in similar experiments earlier reported.     

Electrical properties and stability of polypyrrole containing conducting polymer composites

Conducting polymer composites of polyethylene and polypyrrole (PE/PPy), polypropylene and polypyrrole (PP/PPy), and poly (methyl methacrylate) and polypyrrole (PMMA/PPy) were prepared by means of a chemical modification method, resulting in a network-like structure of polypyrrole embedded in the insulating polymer matrix. The content of polypyrrole determined by elemental analysis varied from 0.25 to 17 wt.%. Electrical conductivity of compression-moulded samples depends on the concentration of polypyrrole and reached values from 1 × 10⁻¹¹ to 1 S/cm. The morphology of the composites was investigated by low-voltage scanning electron microscopy (LVSEM). Potential contrast measurements as a function of the acceleration voltage were used to prove the perfection of the PPy network structure. The electrical transport mechanism in PP/PPy composite was studied. The data of the temperature dependence of conductivity were fitted following the function for a charge-energy-limited tunnelling (CELT) model. There is only a small drop in conductivity caused by annealing of PP/ PPy composites in air at temperatures up to 80 °C. A stabilizing effect of PPy on thermal stability of polypropylene is shown by thermogravimetric analysis. The antistatic properties of PE/PPy and PMMA/PPy composites were demonstrated.     

Novel zinc‐rich epoxy paint coatings with hydrated alumina and carbon nanotubes supported polypyrrole for corrosion protection of low carbon steel: Part I: Inhibitor particles and their dispersions

In part 1 of this work, preparation, structure, spectroscopic, and electrochemical characteristics of the polypyrrole (PPy) deposited alumina/multi‐walled carbon nanotubes (MWCNTs) inhibitor particles (PDAMIPs) are presented. TEM observations evidenced uniform deposition of thin PPy film on the functionalized nanotubes, whereas co‐deposition of PPy and polystyrene sulfonic acid (PSS) lead to thick polymer coverage on hydrated alumina. Modification with polymer complexes resulted in moderately dispersed PDAMIPs, which is due to the various degrees of aggregation and coalescence. FTIR revealed compact and dense PPy structure on the functionalized MWCNTs while it was not the case on alumina and the PSS modified nanotubes. Closer interaction of PPy with the MWCNTs resulted in enhanced charge mobility, whereas greater electroactivity and reversibility of PPy were noted to samples containing functionalized nanotubes and low amount of PSS. Rheological study verified moderate micron‐scale dispersity and the modification caused various degrees of aggregations of the PDAMIPs. These were recognized to be valid in the suspensions at a solid phase concentration with component contents similar to the corrosion tested hybrid coatings. Rheological percolation of the nanotubes (with anisotropic factor of ～100) was confirmed at volume fractions of 3.30 × 10^?3 and 6.0 × 10^?4 which were under the dilute/semi‐dilute boundary type theoretical and experimental thresholds. This is related to the extensive interconnection of the nanotube‐supported filaments. Thus, overlapping of the nanotubes should contribute to the electrical percolation thereby galvanic corrosion prevention function of the zinc‐rich hybrid coatings, which is discussed in the 2nd part.     

Electrospinning PVDF/PPy/MWCNTs conducting composites

Conducting PVDF/PPy composites (PPy composites) were prepared by using the highly porous electrospun (e-spun) nonwoven web as a host polymer. E-spun nonwoven web was made by electrospinning a solution of PVDF and CuCl₂·2H₂O in solvent of N,N-dimethylacetamide (DMAc). The PPy composites were fabricated by exposing a nonwoven web containing oxidant to pyrrole vapors. Field-emission scanning electron microscopy (FE-SEM) analysis was conducted to show the microstructure of the nonwoven webs and the uniform coating of PPy on the e-spun fiber surfaces of the PPy composite. The information of PPy on the e-spun fibers surface was confirmed by attenuated Fourier-transform infrared spectrometer (ATR FT-IR) and X-ray photoelectron spectroscope (XPS). The thermal property of PPy composites was also investigated by differential scanning calorimeter (DSC) and thermogravimetric analyzer (TGA). The electrical conductivity of the PPy composites was affected by the fabrication method and oxidant content in the nonwoven web. The electrical conductivity and mechanical strength of the PPy composites were improved when surface-modified multi-walled carbon nanotubes (MWCNTs) were added to the e-spun fibers. Energy-filtered transmission electron microscopy (EF-TEM) results confirmed that the MWCNTs were well arranged and embedded in the e-spun fibers. The observed conductivity of the conducting PPy-MWCNTs composite was 10^?1 S/cm.     

Self-strengthened conducting polymer hydrogels

Conducting polymer hydrogels of poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT–PSS) with self-strengthening functions have been synthesized via a facile one-step procedure. The enhanced mechanical toughness, directly established in the materials in their formation process, results from the effective transfer of the local stress between the two components of PEDOT and PSS, which is closely related to the homogeneity of the microstructure and the phase structure. The experimental conditions for the fabrication of the hydrogels have been optimized, and samples prepared with suitable reagent concentrations can sustain a stress in the order of megapascals and a strain higher than 90%. The electrical conductivity of the hydrogels, in the order of 10^?2 S/cm, is demonstrated to be determined by the surface composition and the compactness of the samples. The contribution of this work lies in not only the invention of a new type of conducting polymer hydrogels with outstanding mechanical properties, but also the creation of a novel method to achieve facile preparation of multifunctional mechanically strong hydrogels through the combination of suitable components into proper structures.     

Preparation of polypyrrole/sulfonated-poly(2,6-dimethyl-1,4-phenylene oxide) conducting composites and their electrical properties

The conducting composites were prepared by chemical oxidative polymerization using pyrrole and poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) or sulfonated-poly(2,6-dimethyl-1,4-phenylene oxide) (SPPO) in chloroform. The pyrrole was protonated and polymerized using iron(III) chloride (FeCl₃). The electrical conductivities of PPy/SPPO composites were increased up to 1 order with the amount of PPy compared to PPy/PPO composites. The introduction of sulfuric group to PPO results in the Coulombic interaction between each phase of composites. As a result, the electrical conductivities might be increased due to the effect of miscibility between each phase. The electrical conductivity of PPy/SPPO composite was increased up to 2.14 S/cm with amount of 25 wt.% PPy. The performance of charge–discharge of PPy/SPPO electrode was much higher than that of PPy/PVdF electrode because SPPO act as a dopant as well as a binder.     

X-ray absorption spectroscopy of iron-doped conducting polymers

X-ray absorption spectroscopy (XAS) measurements were performed at the Fe K edge to determine the iron local structure in chemically prepared polyaniline (PANI) and polypyrrole (PPy) samples prepared with FeCl₃ as an oxidant. The samples were conditioned at different pHs by an acid–base treatment. In both as synthesized-doped polymers, the observed dispersed Fe atoms were predominantly coordinated to chlorine as Fe(III)Cl_x species, where x=6 for PANI and x=4 for PPy samples suggesting that the polymer–counteranion bonding is weaker than the iron–chelate one. For the PANI samples submitted to basic treatment, three different sets of Fe distances were found: five Fe–O at 2.00 Å, three Fe–Fe at approximately 2.80 Å and five Fe–Fe at 3.00 Å and the near-edge spectra showed the presence of octahedrally coordinated Fe⁺³. These results strongly suggest the presence of small oxide/hydroxide aggregates. Similar data were obtained for PPy treated with NH₄OH.     

Electrochromic properties of conducting polymer metal nanoparticles composites

Electrochromic devices were made using a nanocomposite blend of conducting polymer poly(3,4 ethylenedioxythiophene):poly styrene sulfonate (PEDOT:PSS) and metal nanoparticles of silver or gold. Microscopic analysis showed a random distribution of metal nanoparticles with little aggregation in the matrix. The two terminal devices exhibited an increase in absorption that is dependant on the loading of nanoparticles in the polymer. Further, the active electrochromic spectral window showed a bias dependant tuning and a broadened spectral response. All devices exhibited slow relaxivities which we interpret as resulting from the high capacitance of the metal nanoparticles embedded in the polymer matrix.     

Doped polymeric cathodes for PPV/Al based LEDs

The effect of Li-doping in poly(para-phenylenevinylene) (PPV) based light emitting devices has been studied. In a standard structure with an indium tin oxide (ITO) anode, poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT–PSS)-layer and an active PPV-layer, the effects of a thin (around 1 Å) Li-layer and a thin layer, (50 Å), of a large bandgap polymer, poly(2,5-diheptyl-1,4-phenylene-alt-1,4-naphthylene) (P14NHP) between the PPV and the aluminum cathode have been studied in terms of IV-characteristics and efficiency. The Li-atoms dope the interfacial layer of the PPV as seen by photoelectron spectroscopy. A thin layer of Li improves the charge balance by decreasing the energy barrier for injection of electrons for the Al/Li/PPV/PEDOT–PSS/ITO device. The efficient electron injection originates from a Fermi level alignment between the doped polymer and the aluminum cathode, which reduces the energy barrier. A thin layer of the large bandgap polymer P14NHP, between the PPV and Al contact, increases the light output and efficiency by blocking the holes. In addition, it may also reduce the light quenching by moving the region of recombination away from the Al-contact. The addition of a Li-layer on top of P14NHP leads to an increase of the quantum efficiency, because of better electron injection.     

The effect of variable chain length on the electronic properties of electroluminescent polymers

We have used self-consistent molecular dynamics calculations with semi-empirical quantum chemistry at the complete neglect of differential overlap (CNDO) level to discuss some of the issues relating to the electronic processes involved in electroluminescence of a particular conjugated polymer: poly(p-phenylene-vinylene) (PPV). Specifically addressed are the effects of chain length and molecular charge on the electronic properties of individual PPV strands, such as chemical potential, charge induced defects and charge rearrangement among the polymer atoms. The effect of high electric fields on charge-induced defects already formed is also discussed.     

Effect of moisture (in solid polymer electrolyte) on the photosensitivity of conducting polypyrrole sensitized by prussian blue in solid-state photocells

The effect of small concentration of dispersed prussian blue (PB) [in solid polymer electrolyte viz polyvinyl alcohol (PVA) with phosphoric acid] on the photocurrents were studied (in wet state and in dry state of solid polymer electrolyte) in solid state photocells fabricated using conducting polypyrrole (PPy). These exhibit good photoresponse to visible light: photocurrents being 20 to 25 times greater than the dark currents depending upon the concentration of the sensitizer. The current–voltage (I–V) characteristics in such cells reveals that the charge transport is mainly governed by the space charge effects and the sensitization effects are due to the lowering of the potential barrier formed at the polypyrrole (PPy)/polyvinyl alcohol (PVA) interface under photoexcitation. Also, sensitization effects were studied in the wet state and in dry state of solid polymer electrolyte (PVA). It has been observed that the moisture content in the solid polymer electrolyte reduces the photosensitivity factor (S=I_l/I_d, where I_l is the light current and I_d is the dark current), since it increases the ionic conductivity in the solid polymer electrolyte thereby increasing the dark current values which results in overall decrease in the photosensitivity factor (S). The importance of the other factors such as dye concentration, dye aggregation etc. have been explained on the basis of the experimental observations. The sensitization effects are explained on the basis of the energy band diagram of the materials forming the photocells.     

Influence of dopant ions on properties of conducting polypyrrole and its electrocatalytic activity towards methanol oxidation

The polypyrrole (PPy) films were deposited on vacuum metallized substrates by electro-oxidation of pyrrole monomer. These electrodes were then modified with a range of metal halides having different electronegativities. The modified polypyrrole electrodes were employed to investigate electrocatalytic activity towards methanol electrochemical oxidation by means of cyclic voltammetry in 0.1 M HClO₄ as supporting electrolyte. It was found that the electronegativity of the dopant ion incorporated in the PPy film governs the electrocatalytic activity towards methanol oxidation. Among different dopant anions used in the present work, the PPy doped with zirconium chloride gave the highest anodic current of 10 mA cm⁻² at the oxidation potential of methanol. Electrical property and the charge created due to doping in the polymers were measured using X-ray photoelectron spectroscopy (XPS) and Electron spin resonance spectroscopy (ESR). Electrocatalytic activity of the modified electrodes was correlated with various factors obtained from different polymer characterization experiments. The results were explained on the basis of the charge-transfer efficiency at the electrodeelectrolyte interface, which was associated with the acceptor state created by the dopant in the semi-conducting polymer.     

An in situ Raman spectroelectrochemical study of the controlled doping of semiconducting single walled carbon nanotubes in a conducting polymer matrix

The interaction of semiconducting single wall carbon nanotubes (SWCNT) and the conducting polymer poly(3,4-ethylendioxythiophene/polystyrenesulfonate) (PEDOT/PSS) was studied by Raman spectroscopy and in-situ Raman spectroelectrochemistry. The mixing of SWCNT with PEDOT/PSS caused a partial doping of SWCNT as indicated by the change of the relative intensity of the Raman features of SWCNT. The in situ Raman spectroelectrochemical measurements showed that semiconducting tubes could be doped even if they are embedded in PEDOT/PSS. However, in contrast to the neat SWCNT, the doping of SWCNT in composite is less efficient. For SWCNT in composite it is necessary to apply larger potential (by about 0.4–0.6 V) to achieve the same level of bleaching of Raman bands as in the case of neat SWCNT.     

Effect of polystyrenesulphonate and electrolyte concentration for electrical properties of polypyrrole film on aluminium alloy using conductive AFM

The electrochemical synthesis of polystyrenesulphonate (PSS) doped polypyrrole (PPy) film onto aluminium alloy (AA 2024-T3) under galvanostatic conditions at current densities of 1 mA cm^?2 was studied. In this study, conductive atomic force microscopy (C-AFM) was performed to investigate the electrical properties of PPy film on AA 2024-T3 depending on the concentration of PSS as dopant and nitric acid as electrolyte during electrochemical synthesis. The addition of HNO₃ to aqueous electrolyte solution is found to allow the electrochemical synthesis of well adhering homogeneous PPy film in the presence of PSS on AA 2023-T3. The PPy film in the presence of nitric acid alone can be synthesised, although this film showed the poor quality of the electrical properties of PPy film. According to C-AFM, the study confirmed that the conductivity of PPy film is significantly increased with increasing the PSS, nitric acid concentration and electrochemical deposition time.     

Interpenetrating polymer network composites of polypyrrole and poly (methyl acrylate) or poly (styrene-co-butyl acrylate) with low percolation thresholds

Interpenetrating polymer network composites of polypyrrole (PPy) with poly (methyl acrylate) (PMA) and poly(styrene-co-butyl acrylate) (SBA) were prepared using FeCl₃-impregnated matrix polymer films and dipping them into a solution of pyrrole in water using a pyrrole concentration of 67 mmol/dm³ and above. No polymerization occurs in solution outside the films. Polymerization occurs rapidly, about 90% of pyrrole being polymerized in the first hour inside the films. Slow polymerization continues after that, giving approximately 8 wt.% PPy incorporation in the films in 24 h. Not all of the matrix polymer can be extracted out of the composites by solvent extraction. This indicates that grafting of PPy is taking place on some of the molecules of the matrix polymers. FT-IR studies do not reveal any H bonding between the

group in PPy and the COO group in the matrix polymers. Differential scanning calorimetry reveals that PPy and SBA are immiscible while a 13 °C shift in T_g of PMA is noticed in the PPy /PMA composites which might be attributed to grafting of PPy into PMA. The percolation threshold (f_c) occurs at 0.023 and 0.045 volume fractions of PPy for PPy/PMA and PPy/SBA composites, respectively, which are much lower than the theoretically predicted value of f_c = 0.16 for conducting blends. The control experiment shows that FeCl₃-impregnated films swell to a great extent on immersion in water due to the osmotic entry of the latter into the films. The water channels inside the films are believed to grow fractally to a dendritic structure. Pyrrole enters into these channels forming a dendritically grown PPy phase. This anisotropic growth is believed to be the cause behind the low f_c. The conductivity of the films decreases when they are exposed to O₂ atmosphere at room temperature. A 25% decrease occurs over 4–5 days, following which the rate of decrease slows down. In nitrogen the conductivity remains stable. In saturated water vapour atmosphere and in the absence of air, conductivity initially increases over 4–5 days by 60 and 20%, respectively, for PPy/PMA and PPy/SBA, and then remains stable. As regards the stability with temperature, the conductivity in air increases with temperature until about 80 °C and then some drop occurs up to the investigated temperature of 100 °C.     

Studies of photoexcitations in conducting polymers mixed with C60

We have studied photoexcitations in mixed compounds of C₆₀ with derivatives of poly (p-phenylenevinylene) and poly(phenyleneacetylene) using a variety of CW and picosecond (ps) transient spectroscopies. The CW techniques used include photoinduced absorption (PA), photoluminescence (PL) and optically detected PA (ADMR), whereas the ps transient techniques used were time-resolved PA, PL and stimulated emission (SE). Compared with the pristine polymers, all mixed C₆₀ compounds have shown enhancement of PA bands associated with charged photoexcitations. These PA bands have shown a single spin-half ADMR resonance, identified as due to spins on the polymer chains; no spin-half ADMR signal from photogenerated C₆₀⁻ has been observed. In the ps time domain we have observed a pronounced reduction of the transient PL and SE in the mixed compounds. Due to the similarity found between the ps transient and CW PA bands, we have identified the ps photoexcitations in the mixed compounds as polarons (P⁺−P⁻ ) on the polymer chains, which are indirectly photogenerated as a result of exciton dissociation in C₆₀⁻ related defect centers. The ultrafast charge transfer, claimed to exist between the polymer chains and C₆₀ molecules, has not been observed in the ps time domain.     

Optical investigation of intra and interchain charge dynamics in conducting polymers

Reflectance measurements on tensile-drawn films of polypyrrole (PPy) doped with PF₆ are analyzed in terms of intra and interchain contributions to the optical conductivity σ(ω). The intrachain σ(ω) is typical of a disordered Drude metal with σ(ω→0)≈2×10³ S/cm, while the interchain σ(ω) is dominated by spectral features of strongly localized conductors, implying the interchain charge dynamics is incoherent.     

Studies on the chemical compensation of conducting polypyrrole by NaOH solution

The ‘chemical compensation’ of conducting polypyrrole (PPy) film by NaOH solution has been studied by cyclic voltammetry and vis-n.i.r. spectrometry. Strong evidence has been obtained to consider the process as a counter anion exchange between the intercalated anion and OH⁻ in solution. The NaOH-treated PPy film, PPy (OH⁻), was found to be capable of being further electrochemically reduced to PPy⁰. PPy(OH⁻) in NaOH solution is electrochemically reversible in the potential range −0.2 to −1.0 V versus SCE. When the potential scan went more positive than −0.2 V, irreversible oxidation of the PPy chain took place. PPy(OH⁻) has mid-gap electronic states lying at 1.6 and 1.0 eV above the valence band, the band gap being 3.0 eV.