Anodic shrinking and compaction of polypyrrole blend: electrochemical reduction under conformational relaxation kinetic control

We describe, for the first time, the anodic compaction of the polypyrrole/p-toluenesulfonate blend. The subsequent electrochemical reduction until increasing negative potentials give chronoamperograms showing a shoulder at decreasing times for increasing potentials. In the context of the electrochemically stimulated conformational relaxation (ESCR) model, developed for cathodic compaction of basic polymers like polypyrrole, polyaniline or polythiophene, those shoulders can be attributed to a structural swelling with penetration of cations from the solution, occurring under conformational relaxation–nucleation kinetic control of the compacted polymeric chains. We checked the application of the ESCR model obtaining the charge consumed to relax one mol of polymeric segments from experiments performed by anodic compaction followed by a cathodic polarization.     

Electrochemical oxidation of poly[(hexamethyltrisilanylene)oligo(2,5-thienylene)] films

Electrochemistry of σ–π-conjugated polymers, poly[(hexamethyltrisilanylene)oligo(2,5-thienylene)] has been investigated in acetonitrile by using cyclic voltammetric and spectroelectrochemical techniques. Cyclic voltammograms of these polymers films display generally two pairs of main redox peaks, indicating the doping and dedoping processes of the polymers. The electrochemical oxidation of the polymer films results in their partial decomposition, besides the doping, which is shown by electrochemical quartz crystal microbalance and spectroelectrochemical measurements. The decomposition products are characterized by UV-visible absorption spectroscopy, fluorescence spectroscopy, FT-IR, and GPC measurements. It is concluded that the decomposition originates mainly from electrochemical cleavage of silicon–silicon bonds in the polymer chains, and the electrochemical stability of the σ–π-conjugated polymers may be improved by decreasing the oligosilanylene block size in the polymer chains.     

Analysis of the impedance spectra of Pt|poly(o-phenylenediamine) electrodes – hydrogen adsorption and the brush model of the polymer films

Electrochemical impedance spectra obtained for poly(o-phenylenediamine) films on platinum in contact with 0.5 M H₂SO₄ solution were analyzed on the basis of the brush model elaborated for conducting polymer films on electrodes. It is assumed that the polymer film consists of short and long polymer chains forming bundles in such a way that only a small part of the metal substrate is covered by the polymeric material. The analysis of the impedance spectra taken at several potentials in the region of hydrogen adsorption on platinum supports the validity of the model. The model has been tested also by varying the film thickness and the roughness of the platinum substrate.     

Electrochemical and photoelectrochemical behaviour of poly-3-methylthiophene and polybithiophene in non-aqueous solutions: 1. Dark and light processes in systems with and without cathodic doping

The electrochemical as well as the photoelectrochemical behaviour of poly-3-methylthiophene and polybithiophene in systems with and without the possibility of cathodic doping was studied. Using electrochemical impedance measurements it was demonstrated that in both types of systems the photoelectrochemical processes on such polymers involved photoelectrochemical undoping of the polymer itself as well as the photoelectrochemical reduction of a component of an external redox couple in the solution. In systems with cathodic doping, in addition to these two processes, photoassisted cathodic doping of the polymer is observed. The model of photoelectrochemical processes on polymer electrodes is discussed.     

The electrochemistry of fluorine-substituted polyphenylthiophenes for charge storage applications

A series of thiophene-based conducting polymers has been synthesized. The properties of these polymers were characterized using electrochemical techniques. The polymerization potentials of the monomers were found to be related to the corresponding Hammett σ values. The maximum n-doping levels of the polymers were also found to be correlated with the Hammett σ values. No correlation was found with the maximum p-doping levels of the polymers. The doping densities were found to decrease on repeated cycling and were correlated with irreversible morphological changes in the films. Test cells were fabricated using conducting polymer films for both anode and cathode. These cells exhibited discharge voltages of about 2.5 V and capacities of 9.5 to 11.5 mA h g^?1.     

Evaluation of the potential dependence of 2D–3D growth rates and structures of polypyrrole films in aqueous solutions of hexafluorates

Polypyrrole hexafluorosilicate, PPYSiF₆, and polypyrrole hexafluoroaluminate, PPYAlF₆, were found to follow the mixed 2DI–3DI kinetic model of electrodeposition in the potentials range (+0.80 V, +0.60 V) and (+0.95 V, +0.55 V) vs. SCE, respectively. The potentiostatic depositions of polypyrrole were performed on polycrystalline gold and on the single layer or bilayer polypyrrole films. Only in the case of the third layer deposition of PPYSiF₆ (on PPYSiF₆) at +0.6 V, the experimental current was found to fit better with the 2DP–3DI model. For electrodeposition of PPYAlF₆ on gold, a slightly better quality fitting with 2DP–3DP or 2DP–3DI kinetic models was observed only for some samples synthesized at 0.55 V. In general, contribution of the 2D structure in the polypyrrole deposits was observed to decrease with an increase of the polarization potential. Conclusions of the mathematical analysis of the current–time functions are in accordance with a texture of the outer surface of the polymer layers visible in SEM micrographs. The apparent rate constants of the lateral and outward growths of polypyrrole increase with potentials in (+0.55, +0.80) V range. Under assumption of the classical electrochemical kinetics law, the anodic transfer coefficients were found lower than 0.5. Values of the outward growth rate constant were of the order of 10⁻⁸–10⁻¹⁰ mol/(s cm²); being approximately one order of magnitude higher for polypyrrole hexafluorosilicate than for polypyrrole hexafluoroaluminate. The lateral growth rates of PPYAlF₆ on gold were found to depend on the electrodeposition potential more significantly than the outward growth rate. An increase in the growth rates with the potential was observed to diminish/vanish at E > +0.80 V, probably due to concurrent degradation processes of polypyrrole chains. The analysis of the deposition currents was done under conditions of the masking double layer currents that were characterized by the relaxation times of the order of seconds or longer.     

A polyaniline + polyethylene oxide mixture as a composite polymer positive electrode in solid-state secondary batteries

We present here a physico-chemical and electrochemical study of a polyaniline (PAni) + polyethylene oxide (PEO) mixture prepared from a codissolution of both polymers in 1-methyl-2-pyrrolidinone. SEM observations showed that the best homogeneity of the polymer material was obtained when PAni was protonated directly in solution. FT-IR spectroscopic experiments showed that the entanglement of the PEO and PAni chains hindered the return of the PEO chains to their more stable helical form. XPS analyses of the films surface gave a 40% doping of the aniline units by the anions of the salt Li[N(CF₃SO₂)₂]. High gravimetric capacities of the order of 200 A h kg^?1 (related to the PAni mass in the mixture) were extracted during the charge/discharge of the polymer material in a PEO based solid electrolyte. The best performances were obtained for PEO proportions comprised between 30 and 60 wt.%. Moreover, good cycling stability was observed. The PAni + PEO material could be suitable for use as an electrode material in high cycle life electrochemical devices.     

Electrophoresis with electrochemical detection in a polymer microdevice

This work presents the integration of an electrochemical detector in a photoablated polymer microdevice with electroosmotic driven flow. The structure of the device features a separation capillary with two side arms forming the injection capillaries, an electrical decoupler and an electrochemical detector. The decoupler is composed of a microhole or microhole array perpendicular to the separation microchannel. The system allows the electrical high voltage connection between the inlet of the capillary and the decoupler; the remaining end of the separation capillary being almost free of electric field, facilitating an electrochemical detection at the end of the microchannel. Separation parameters have been varied comprising the electrophoretic potential, the time of the injection and the buffer composition. Separation of model phenolic compounds is shown here as a demonstration of the system.     

Electrochemical behavior of Ni particles modified polypyrrole films studied by EQCN technique

Polypyrrole films modified with Ni Particles were eletrodeposited using a constant potential on a Pt/quartz crystal electrode with the aim to investigate the changes in the voltammetric and mass profiles compared with pure polypyrrole. The electrochemical behavior of the polypyrrole with Ni showed a new cathodic peak at ?0.73 V associated to Ni²⁺ reduction. Besides, it was characterized that there is a redox-pair of the polymer and Ni and a model was proposed to explain the phenomenon considering the eletroneutrality principle. The transport of the ${\mathrm{ClO}}_4^{-}$ ion into (or outside) the polymer is opposite to the expected direction. This occurs due to the redox-pair between the PPy chain and Ni species which can be explained by a loss of anions during Ni oxidation and the gain of anions during the Ni reduction.     

Molar enthalpy of the polypyrrole electrochemistry

The kinetics of the angular movement of a triple-layer muscle working potentiostatically under different temperatures allowed the obtention of both the activation energy and the molar enthalpy of the process. By compaction of a polypyrrole film under constant temperature and at a constant cathodic potential, followed by a subsequent oxidation at a constant anodic potential under different temperatures, the molar enthalpy was obtained by application of the ESCR model. Similar results, 27.1 kJ/mol, from muscles, versus 27.7 kJ/mol, from relaxation experiments, were obtained.     

Electrochemical anion doping of poly[(tetraethyldisilanylene) oligot 2,5-thienylene)] derivatives and their p-type semiconducting properties

Chemically synthesized poly[(tetraethyldisilanylene)oligo(2,5-thienylene)] derivatives (DS/mT; m = 3 to 5) have been successfully anion-doped by electrochemical oxidation. Band-gap energies of 2.52, 2.65, 2.82 and 3.27 eV were evaluated for DS5T, DS4T, DS3T and DS2T respectively. The DS5T, DS4T and DS3T films exhibited electrical conductivities of the order of 10^?3 to 10^?4 S cm^?1 when doped with BF₄^?. The work functions of the films changed from 5.1 to ca. 5.5 eV with electrochemical anion doping. In cyclic voltammograms of the polymer films for anion doping and dedoping, an anodic peak potential and a cathodic one (E_pc.) shifted to the positive direction as the number m of thienylene units decreased. E_pcs at a sweep rate of 100 mV s^?1 were about 0.8, 0.9 and 1.0V for DS5T, DS4T and DS3T respectively. Reversible electrochemical doping and dedoping of the DS5T film were feasible when the potential was cycled between 0 and 1.2 V. At potentials more positive than 1.2 V, however, both overoxidation of the oligo(thienylene) unit and Si-Si bond cleavage took place, leading to decreases in conductivity and work function of the film.     

Resonant Raman spectroscopy as a tool for determining the formation of a ladder structure in electropolymerized poly(5-amino-1-naphthol)

Electropolymerization of poly(5-amino-1-naphthol) onto different conducting substrates was studied in HCl, HClO₄, PTSA (p-toluene sulfonic acid) and HCSA (camphorsulfonic acid) media. In situ UV–vis spectroelectrochemical studies have shown spectral changes that demonstrate that the oxidation process is accompanied by the formation of polaronic and bipolaronic structures due to the ejection of electrons from the polymeric matrix. Resonance Raman spectroscopy was used to follow structural changes that occur during the redox processes of the polymer and it allows us to infer the formation of imine groups and the possible ‘ortho’ coupling during electropolymerization.     

Equilibrium potential of potentiometric ion sensors under steady-state current by using current-reversal chronopotentiometry

Potentiometric ion sensors based on a solvent polymeric ion-sensing membrane and a conducting polymer as the solid contact were investigated by current-reversal chronopotentiometry. The low-frequency electrochemical behavior of the electrodes resembles a series RC equivalent circuit. The resistance R was attributed primarily to the bulk resistance of the ion-sensing membrane and the capacitance C was attributed to the bulk redox capacitance of the conducting polymer layer acting as an ion-to-electron transducer. It is shown that the equilibrium potential of the ion sensors can be obtained in the presence of a steady-state current by using current-reversal chronopotentiometry (double current step chronopotentiometry).     

Synthesis and polymerization of 2- and 3-substituted thiophene derivatives linked by polyether bridges

New compounds consisting of 2- and 3-thienyl units linked by polyether bridges have been synthesized and their electrochemical polymerization was performed via constant potential electrolysis (CPE) in an electrolytic solution containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF₆) dissolved in CH₃CN. 2-Thienyl monomers (I and II), but not 3-thienyl monomers (III and IV), were also polymerized via chemical oxidation, which yielded broken π-conjugated polymer products. The polymers were characterized using ¹H NMR and FT-IR spectroscopic techniques. It was found that both chemical and electrochemical oxidation of 2-thienyl monomers gave mainly poly(2,2′-bithiophenemethylene) due to elimination of polyether chains during the polymerization reaction. On the other hand, electrochemical oxidation of 3-thienyl monomers resulted in corresponding polymers without any cleavage of polyether bridges. Spectroelectrochemical (SPEL) properties of the products were investigated using UV–Vis spectroscopic techniques.     

Investigation of the cathodic electropolymerization of acrylonitrile,ethylacrylate and methylmethacrylate by coupled quartz crystal microbalance analysis and cyclic voltammetry

The cathodic electropolymerization of acrylonitrile (AN), ethylacrylate (EA) and methylmethacrylate (MMA), has been monitored for the first time by coupled electrochemical quartz crystal microbalance (QCM) and cyclic voltammetry analyses. These data have been compared to the previously published analyses for methacrylonitrile (MAN). At the potential E_p1 of the less cathodic of the two voltammetric peaks observed (peak I), the polymer formed is anchored firmly to the cathode even in a good solvent for it, and it remains adsorbed when the voltammetric scan is repeated up to E_p1. In parallel, a linear relationship is observed between the frequency change recorded in situ by the QCM up to E_p1 and the PAN film thickness measured ex-situ by ellipsometry. However, when potentials more negative than peak I are scanned, the polymer desorption (degrafting) occurs as assessed by solubilization in a good solvent. Polymerization is also resumed but in solution and no longer as grafted chains. The major difference between the acrylic monomers (AN and EA) and the methacrylic ones (MMA and MAN) is that part of the methacrylic chains are not grafted at E_p1. This coexistence of adsorbed and desorbed chains is not observed for the polyacrylic chains in that potential range. The mass of PAN deposited onto the cathode has been approximated from the film thickness and the Sauerbrey equation, so allowing the M_n of the grafted chains to be estimated.     

Solid-state electrochemical investigation of poly[2-methoxy, 5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene]

The solid-state electrochemical behavior of the luminescent conjugated polymer, poly[2-methoxy, 5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and its blend with polyethylene oxide (PEO) was investigated by cyclic voltammetry and potential step experiments. It was found that the nature of the ions affects the ionic conductivity and the electrochemical redox behavior of the polymer(particularly on electrochemical n-doping). An electrochemical loop, which is similar to that observed in the electrochemical crystallization, was observed in the cyclic voltammetry of MEH-PPV/PEO(Li⁺) blend. The diffusion of the electrolyte ions is slow, but it was improved by the addition of PEO. The diffusion coefficient of the counterion in the blend is up to 10^?8 cm² s^?1. These solid-state electrochemical results help to shed light on the mechanism of light-emitting electrochemical cells.     

Electrochemical doping of poly(vinyl chloride) obtained by photodehydrochlorination from laminated poly(vinyl chloride) + polypyrrole films

The poly(vinyl chloride) (PVC) in laminated PVC + polypyrrol (PPy) films has been converted to an electrically conducting structure by photodehydrochlorination and subsequent electrochemical doping. The maximum conductivity (6 × 10^?1 S cm^?1) of the PVC film was obtained by potentiostatic oxidation at 0.3 V vs. SCE in HCl solution. The reflection spectra of the PVC film showed a new band around 600 to 800 nm after the electrochemical doping, indicating the formation of a charge-transfer complex. The maximum conductivity of the PVC film was reached with the highest concentration of the charge transfer complex, polyene⁺ … X^?, formed between the photogenerated polyenes and dopant anions.     

Electrochemical preparation of polypyrrole-molybdenum trisulfide-tetrathiomolybdate electrode with various amounts of molybdenum species

Polypyrrole films doped with molybdenum trisulfide and tetrathiomolybdate anions have been prepared from an aqueous solution containing pyrrole and ammonium tetrathiomolybdate in the presence of tiron, (4, 5-dihydroxy-1, 3-benzenedisulfonic acid), in the deposition solution. In such a solution the electrodeposition of polypyrrole doped with tetrathiomolybdate anions is in competition with the electrodeposition of molybdenum trisulfide by oxidation of tetrathiomolybdate anions. Our results indicate that the relative amounts of polypyrrole and molybdenum species can be varied over larger ratios when tiron is present in the deposition solution. The variation of the amount of molybdenum species can be explained by considering the number of electrons required to generate one mole of tetrathiomolybdate as dopant for polypyrrole and one mole of molybdenum trisulfide; 14 electrons are required per mole of tetrathiomolybdate incorporated in the polymer in comparison to 2 electrons per mole of molybdenum trisulfide generated directly by electrochemical oxidation of tetrathiomolybdate. Tiron acts as a catalyst for the deposition of polypyrrole and in this case the relative amount of polypyrrole in the composite film electrode is larger. For higher tetrathiomolybdate concentration, the direct formation of molybdenum trisulfide occurs preferentially and the catalytic effect of tiron is less important.     

Monitoring acid-demineralization of human dentine by electrochemical impedance spectroscopy (EIS)

Objective

The purpose of this study was to monitor in situ acid-induced demineralization of dentine by using electrochemical impedance spectroscopy (EIS) and correlate the EIS findings with changes in the chemical composition and ultrastructure of dentine.

Method

EIS was used to monitor the process of demineralization of dentine induced by an acid model. Scanning electron microscopy (SEM) was used to examine the ultrastructure, while energy dispersive X-ray (EDX) and X-ray diffraction (XRD) analysis were employed to investigate the changes in the chemical composition of surface dentine with demineralization.

Results

Two kinds of equivalent circuits that matched the histological structure of dentine were used to fit the EIS data. The parameters R_d, which is the resistance of the bulk of dentine layer and R_ct, which is the charge transfer associated with the penetration of electrolyte into the dentinal tubules, decreased with increasing duration of demineralization. SEM images showed that the smear layer on the dentine and peritubular dentine disappeared with demineralization. The EDX results showed that the content of calcium and phosphorus decreased consistently with the decreasing content of HAP as shown by the XRD results. The findings from this study suggested that the changes in R_d and R_ct determined by the EIS corresponded well with the variation in structure and composition of dentine.

Conclusions

EIS could be employed to monitor structural and chemical compositional changes induced by acid-demineralization on dentine surface.     

Study of the DNA hybridization transduction behavior of a quinone-containing electroactive polymer by cyclic voltammetry and electrochemical impedance spectroscopy

We report the characterization of a new bifunctional electroactive polymer, poly(5-hydroxy-1,4-naphthoquinone (juglone)-co-5-hydroxy-3-thioacetic acid-1,4-naphthoquinone), which can be used for direct electrochemical detection of DNA hybridization. The polymer structure was characterized based on molecular orbital calculations and ex situ and in situ FT-IR spectroscopy. The quinone group integrated in the polymer presents very stable electroactivity in neutral aqueous medium and can act as a redox marker for hybridization. The carboxylic function allows the preparation of oligonucleotides (ODN) probe-modified films by covalent binding. The mechanism of hybridization detection was studied by cyclic voltammetry and electrochemical impedance spectroscopy. The “signal-on” upon hybridization could be attributed to a change in conformation from random ODN coils on the probe-modified film to helix double stranded ODNs upon hybridization.