Synthesis and electrochemical properties of composite films based on poly-3,4-ethylenedioxythiophene with inclusions of silver particles

The electrochemical behavior of PEDOT/Ag composite films obtained by chemical deposition of ultrafine Ag particles into the poly-3,4-ethylenedioxythiophene (PEDOT) matrix was studied. The film morphology was characterized by transmission electron microscopy (TEM). The changes in the mass of the films during the chemical deposition of silver into the polymer structure were evaluated microgravimetrically. The mass of the included metallic silver particles depends on the synthesis time and the initial concentration of silver ions in solution. The cyclic voltammograms (CVs) of PEDOT/Ag films in sodium nitrate solutions and sodium nitrate solutions with additions of chloride ions were studied. The cyclic voltammograms of PEDOT/Ag films in chloride-containing solutions showed the peaks of the oxidation of silver and reduction of the oxidation product, which were absent on the CVs of the starting PEDOT film.     

Electrochemical synthesis, characterization and some properties of a polymer derived from thioflavin S

Polymeric films derived from thioflavin S were electrosynthesized on mild steel and silver electrodes in sulfuric acid and lithium perchlorate-containing aqueous solutions. The introduction of thioflavin S in an acidic solution protected the surface of steel from corrosion. The electrochemical behavior of the steel coated with a layer of poly(thioflavin) was examined by electrochemical impedance spectroscopy. The films exhibited a capacitive behavior and were semi-conductive in nature. Infra red reflectance measurements of the polymer films at the steel surface showed that the polymer structure retained the aromatic structure of the benzene and thiazole rings with the distinction of a nitrogen quinone vibrational band. Surface morphology of the polymer film was examined with scanning electron microscopy. The films are yellow, compact and dense when electrochemically formed onto steel surfaces when compared to a blue rather porous when formed onto silver electrode. The mechanism of electropolymerization of thioflavin is given and found similar to that of aniline with the possibility of metal chelation with the sulfur and/or nitrogen in the thiazole ring.     

Influence of the electropolymerisation mode on PEDOTh films morphology and redox behaviour—an AFM investigation

The morphologies of PEDOTh films deposited on platinum electrodes, using different electrochemical modes but conditions selected to obtain layers with similar electroactivities, have been analysed by atomic force microscopy (AFM). Ex situ AFM showed a fibber-like topography for the potentiodynamically synthesised films, contrasting to the granular character of those formed at constant potential. In situ contact mode AFM, used to examine the initial stages of the polymer growth and the surface morphology evolution, disclosed the development of long segments with the number of potential cycles, and the formation of identical globular nodules, which diameter increases with the growth charge. The morphological changes imparted by the switching process revealed to be more marked in films grown at constant potential. Thickness measurements performed on oxidised and reduced films unveiled the swelling/shrinking of the polymers during the ingress/egress of the perchlorate anions, with film volume changes of ca. 20% upon oxidation.     

Ultrathin conjugated polymer network films of carbazole functionalized poly(p-phenylenes) via electropolymerization

Ultrathin films of a cross-linked and chemically distinct conjugated poly(p-phenylene) network via electropolymerization are described. The amphiphilic network precursor was synthesized by incorporating the alkoxy carbazole group (-O(CH2)5Cb) to a poly(p-phenylene) (C6PPPOH) backbone. In order to investigate the combined thin film electrochemical and photophysical properties of poly(p-phenylene)s and polycarbazole conjugated polymers, C6PPPC5Cb was deposited on substrates using the Langmuir Blodgett Kuhn (LBK) method. The monolayer isotherm of the polymer, C6PPPC5Cb, showed a liquid expanded region slightly different from the parent polymer C6PPPOH. Multilayers (up to 30 layers) were transferred to different substrates such as quartz, gold coated LaSFN9 and ITO substrates for analysis. For conversion to a conjugated polymer network (CPN) film, the electroactive carbazole group was electropolymerized using cyclic voltammetry (CV) resulting in polycarbazole linking units. The differences in the film properties and corresponding changes in the electrochemical behavior indicate the importance of film thickness and electron/ion transport process in cross linked network films. From the electrochemical studies, the scan rate was found to have a considerable effect on electropolymerization with higher oxidation and reduction peak values found for the rigid network polymer compared to the uncrosslinked polymers.     

Measuring the ionic flux of an electrochemically actuated conducting polymer using modified scanning ion conductance microscopy

We propose a modification of a scanning ion conductance microscope suitable for probing an electrode in an operating electrochemical cell. We demonstrate its use by measuring salt concentration variations near a conducting polymer electrode as the polymer is electrochemically oxidized and reduced. The electrochemical control circuit is opened to isolate the working electrode, at a frequency sufficiently high that the electrode capacitance maintains the electrode potential. The local solution conductivity variations are detected through the probe current during the open-circuit time. We demonstrate two-stage ion exchange during oxidation and reduction of poly(3,4-ethylenedioxythiophene) films that develops strongly with repeated cycling and is correlated with actuation changes. Spatial composition variations of the film, caused by redox current distribution over the surface, and electromigration to the probe tip, causing local solution composition changes, have clear and characteristic effects on the measured transients.     

Comparative behavior of electrodes coated with thin films of structurally related electroactive polymers

General methods are described for the synthesis of electroactive polymers and the preparation of uniform, stable, polymer-coated electrodes. The electrochemical behaviour of thin films of eight different functionalized polystyrenes, differing in the identity and amount of attached electroactive species, and polyvinylferrocene is presented. A wide range of electrochemical properties can be observed by varying parameters such as film thickness, nature of the bound redox couple, extent of polymer functionalization and oxidation state. The deviations from ideal surface behavior are investigated in detail. These are shown to be consequences of cooperative electronic interactions, structural reorganization or uncompensated resistance within the film, depending upon the material. The film resistance varies greatly among the polymer films studied and is shown, in some cases, to be a sensitive function of the extent of oxidation and prior treatment of the film. This variable resistance is shown to be a consequence of slow ion transport through the film. The mechanism of electron transport in such materials is considered and a model of the metal/polymer/electrolyte interface is proposed.     

Electrocatalytic oxidation of some biologically important compounds at conducting polymer electrodes modified by metal complexes

Conducting poly(3-methylthiophene) electrodes were electrochemically prepared. The resulting polymer films were modified with an inorganic complex, ferrocene. The incorporation of the ferrocene/ferrocenium moiety into the polymer film resulted in enhanced charge transfer towards the oxidation of some organic molecules of biological interest. The electrochemical response of the complex-containing polymer electrode was compared to that of the unmodified polymer electrode and that of the substrate. Apparent diffusion coefficients of the redox species were estimated from the cyclic voltammetric data for different biological molecules at the ferrocene-containing polymer electrode. Infra-red spectroscopic measurements for the “as-grown” films revealed the presence of the inorganic complex within the polymer. The modified polymer electrode showed noticeable enhancement for the charge transfer across the film interface and can be used as an electrochemical sensor for biological compounds. Received: 3 June 1997 / Accepted: 7 July 1997     

Quasi-equilibrium volume changes of polyaniline films upon redox switching. Formal potential distribution and configurational modeling

The quasi-equilibrium electrochemomechanical behavior of relatively thick polyaniline films in sulfuric acid is investigated through experimental measurements and theoretical modeling. The leucoemeraldine (LE)-emeraldine (EM) conversion, or redox switching, is studied. The dependence of film volume and electrochemical charge is determined as a function of applied potential. It is observed that the film volume follows the charge, showing an expansion during the second half of the LE-EM oxidation. The model postulates the existence of a stable intermediate, protoemeraldine (PE), with a formal potential distribution for the PE-EM reaction. The volume change is modeled statistically considering contributions from mixing, polymer deformation, and electrostatic charge. The model shows very good agreement with the experiments, indicating that, in the conditions studied, the deformation contribution dominates the volume changes as a result of the conformational modifications undergone by the polymer in the PE-EM oxidation.     

Mechanical properties of films from polymer latices

The mechanical behavior of latex films is governed by their macromolecular nature as well as by their origin from particles dispersed in an aqueous medium. When monomers of different polarity are copolymerized in emulsion copolymerization, a heterogeneous distribution of the polar groups in the latex and the film can occur, owing to the different water solubilities of the comonomers. Films from these latices in many cases show a two-phase morphology, first, consisting of the main polymer within the particles and, second, a phase which is concentrated in the interphase between the original particles and which has a strong influence on the mechanical properties of the films. Films from latices with crosslinked particles behave like homogeneous networks in the linear viscoelastic range, i.e. at small strains. Structured networks are found when latex films are interparticularly crosslinked during or after film formation, e.g. by polar bifunctional monomers or metal salts. Tensile tests of films show that the mechanical strength of latex films develops in the last stage of film formation by interdiffusion and entaglement formation across particle boundaries.     

Effects of preparation temperature on the conductivity of polypyrrole conducting polymer

An attempt has heen made to investigate the effect of temperature on the conductivity of polypyrrole conducting polymer films prepared by an electrochemical method in an aqueous medium using camphor sulfonate as the dopant. The polymer was grown from aqueous solutions employing a range of temperatures (l–60°C). It was found that with increase in temperature the conductivity decreased and the optimum temperature was found to be between 10 and 30°C. The results showthatthe polymer formed at low temperature has higher conductivity and is stronger than that formed at higher temperatures. Characterization by X-ray scattering shows that interlayer distance, d_Bragg (?), increases with increasing temperature. The morphology of the films formed was studied by using a scanning electron microscope (SEM). The changes in conductivity and physical appearance were interpreted as being due to compactness in the molecular packing and formation ofαβ linkages in the film.     

Reversible electrochemical switching of polymer brushes grafted onto conducting polymer films

We demonstrate the electrochemical switching of conformation of surface-bound polymer brushes, by grafting environmentally sensitive polymer brushes from an electrochemically active conducting polymer (ECP). Using atom transfer radical polymerization (ATRP), we grafted zwitterionic betaine homopolymer and block copolymer brushes of poly(3-(methacryloylamido)propyl)-N,N'-dimethyl(3-sulfopropyl)ammonium hydroxide) (PMPDSAH) and poly(methyl methacrylate)-b-PMPDSAH, from an initiator, surface-coupled to a poly(pyrrole-co-pyrrolyl butyric acid) film. The changes in ionic solution composition in the surface layer, resulting from oxidation and reduction of the ECP, trigger a switch in conformation of the surface-bound polymer brushes, demonstrated here by electrochemical impedance spectroscopy (EIS) and in a change of wettability. The switch is dependent upon temperature in a way that is analogous to the temperature-dependent solubility and aggregation of similar betaine polymers in aqueous solution but has a quite different dependence on salt concentration in solution. The switch is fully reversible and reproducible. We interpret the switching behavior in terms of a transition to a "supercollapsed" state on the surface that is controlled by ions that balance the charge state of the ECP and are adsorbed to the opposite charges of the zwitterionic graft, close to the graft-ECP interface. The behavior is significantly modified by hydrophobic interactions of the block copolymer graft. We speculate that the synergistic combination of properties embodied in these "smart" materials may find applications in electrochemical control of surface wetting and in the interaction with biomolecules and living cells.     

Model experiments for a molecular understanding of polymer crystallization

Time‐resolved real‐space observations of morphology and pattern formation resulting from crystallization of ultrathin films of low‐molecular‐weight poly(ethylene oxide) (PEO) or diblock copolymers containing PEO shed light on the mechanisms of how polymer crystals are formed. We used simple but restricted geometries like thin films of controlled thickness or confinement resulting from block copolymer mesotructures. Under such conditions, we were able to relate the observed morphology and its temporal evolution directly to molecular processes and the kinetics of crystal growth. We demonstrate that changes in the morphology with time are due to different thermal histories and are the consequence of the mestable nature of polymer crystals. Information about the nucleation process was obtained by examining crystal formation in 12‐nm small spherical cells of a block copolymer mesostructure. We discuss the advantages of thin‐film studies for a better understanding of polymer crystallization. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1869–1877, 2003     

Highly ordered polymer films of amphiphilic, regioregular polythiophene derivatives

The fabrication and characterization of highly ordered thin films made from amphiphilic, regioregular polythiophene derivatives are described. Films of poly(3-(11-(2-tetrahydropyranyloxy)undecyl)thiophene (PTHPUDT) were prepared by the Langmuir-Blodgett technique. The amphiphilic nature of the polymer affords layer-by-layer deposition and the formation of multilayer films of head-to-head and tail-to-tail Y-type structure. X-ray diffraction studies indicate bilayer separations of approximately 30 A. Anisotropic optical absorption in the plane of the film indicates that the thiophene backbones are preferentially oriented along the dipping direction. Further, polarized light microscopy studies indicate that these films are highly birefringent and that the optical retardation is uniform over the entire film. Ellipsometry studies confirm the sizable magnitude of the birefringence. Optical second-harmonic generation studies of multilayer films provide information regarding both the thiophene orientation within the film and the anisotropic distribution of chromophores in the surface plane. Taken together, these data offer strong evidence of highly ordered films in which the hydrophobic polythiophene chains lie parallel to the substrate surface with their alkyl chains oriented normal to the surface, as dictated by the hydrophilic nature of the alkyl chain's terminal tetrahydropyran functional group. As such, these films offer the potential for elucidating the connection between polymer morphology and physical property in materials that are otherwise subject to a sufficiently complex distribution of morphologies that such a correspondence is precluded.     

Photochromism and electrochemistry of a dithienylcyclopentene electroactive polymer

A bifunctional substituted dithienylcyclopentene photochromic switch bearing electropolymerisable methoxystyryl units, which enable immobilization of the photochromic unit on conducting substrates, is reported. The spectroscopic, electrochemical, and photochemical properties of a monomer in solution are compared with those of the polymer formed through oxidative electropolymerization. The electroactive polymer films prepared on gold, platinum, glassy carbon, and indium titanium oxide (ITO) electrodes were characterized by cyclic voltammetry, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). The thickness of the films formed is found to be limited to several monolayer equivalents. The photochromic properties and stability of the polymer films have been investigated by UV/vis spectroscopy, electrochemistry, and XPS. Although the films are electrochemically and photochemically stable, their mechanical stability with respect to adhesion to the electrode was found to be sensitive to both the solvent and the electrode material employed, with more apolar solvents, glassy carbon, and ITO electrodes providing good adhesion of the polymer film. The polymer film is formed consistently as a thin film and can be switched both optically and electrochemically between the open and closed state of the photochromic dithienylethene moiety.     

Gold electroless deposition into poly-3,4-ethylenedioxythiophene films

By electroless deposition of gold into poly-3,4-ethylenedioxythiophene (PEDOT) films, the composite films (PEDOT-Au) are synthesized. Their electrochemical properties are studied by cyclic voltammetric (CVA) method. It is shown that in contrast to the original PEDOT film, the CVA curves of composite PEDOT-Au films measured in the presence of chloride ions reveal additional redox peaks associated with the presence of gold particles. The loading of metal gold particles by its chemical deposition into the polymer film is quantitatively assessed using quartz crystal microbalance method. The film mass is shown to depend on the time of gold loading and its original concentration in solution. The gold particles are shown to be oxidized by a reaction of the first order with respect to chloride ions. Based on the results of voltammetric and microbalance methods, the formation of a poorly soluble gold oxidation product Au(I)Cl in chloride-containing solutions was inferred.     

Nanoscale degradation of polypyrrole films under oxidative stress: An atomic force microscopy study and review

Atomic force microscopy (AFM) is employed to monitor the surface morphology of polypyrrole (PPy) films grown on vitreous carbon substrates during the catalytic reduction of Cr(VI) to Cr(III). The morphology of freshly-prepared films depends on substrate characteristics. Upon reaction, uniform nodules of aggregated PPy clusters appear. No significant differences in surface morphology are found between its oxidized and reduced forms. Loss of catalytic activity after 8-9 oxidation/reduction cycles of exposure to the chromate solution (oxidation) and electrochemical recharging of the film at negative potentials (reduction) correlates well with the observed polymer film dissolution/detachment from the carbon substrate. Formation of well-defined circular features (PPy rings) at different stages leads to a model for the film degradation process that includes formation of Cl₂ gas inside the polymer matrix. In the final stages, the bulk of the film typically fractures and detaches from the electrode. A catalytically inactive, ultrathin PPy layer remains on the substrate even after prolonged exposure to the target solution. A review of techniques for the study of PPy aging/degradation is given.     

Oxidant aggregate-induced porosity in vapour-deposited polymer films and correlated impact on electrochemical properties

ABSTRACT

Persistently doped conjugated polymers are integral for energy storage, flexible electronics, and biosensors due to their unique ability to interact with both ionic and electronic currents. To maximise the performance of devices across these fields, research has focused on controlling material properties to optimise conductivities of both types of charge carriers. The challenge lies in improving ionic transport, which is typically the rate-limiting step in redox processes, without sacrificing electronic conductivity or desirable mechanical properties. Here we report on control of nanostructure in vapour deposited conducting polymer films and correlate changes in film structure with resulting electrochemical properties. Structural control is enabled by exploiting the growth of oxidant nanoaggregates during the reactive vapour deposition process. Relative to dense films, porous films exhibit faster response times in electrochemical testing. Scan rate analysis confirms a transition away from diffusion-limited charging kinetics and demonstrates the important role that porosity can play in ion transport through electroactive polymers. Advantageously, continuous polymer networks remain evident in nanostructured films, ensuring that high electronic conductivities are maintained along with high porosity. We find that such enhanced properties are retained even as polymer thickness increases ten-fold. The films reported herein may serve as robust electrodes in flexible electrochemical devices.     

Influence of the magnitude and direction of electric field on the transport and growth property of deposited polyaniline films

Polyaniline (PANI) films were deposited by electrochemical polymerization of aniline monomer on a fluorine-doped glass substrate at room temperature under different electric field directions. The as-synthesized PANI films obtained at different growth cycles were characterized by AC impedance spectroscopy and scanning electron microscopy (SEM). The results revealed that the film morphology, transport kinetics, and electrical properties are strongly dependent on the electric field direction and magnitude of the applied field during electropolymerization. The SEM morphology and AC impedance (modulus spectroscopy) indicate that a more homogeneous, high-porous, and conducting PANI film is induced by horizontal electric field direction (HEFD) electrodeposition, whereas the modulus spectroscopy of the PANI film deposited by vertical electric field direction (VEFD) reveals that VEFD deposition favours two-dimensional growth of PANI. The obtained polymer is more of dielectric in nature due to preferable dendritic growth which is supported by SEM analysis.