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SzcmmaryThe electrochemical polymerization of pyrrole on platinum produces a strongly adhered, durable film with enhanced conductivity and good electrode properties.MANY workers have been interested in the modification of electrode surfaces by covalently attaching organic monolayers or depositing polymer films.1 While the electrochemical preparation of films on a surface could conceivably produce superior material, this approach has not been pursued rigorously. Our interest in the preparation of durable, conducting organic electrode surfaces led us to this approach. We now present the preliminary results on the electrochemical polymerization of pyrrole2 under controlled conditions. The polypyrrole films were synthesized galvanostatically on a platinum surface in a two-electrode cell containing 0-1 M Et4NBF4 and 0.06 M pyrrole in 99% aqueous MeCN. The films strongly adhere to the surface. These films can be prepared in a variety of aprotic solvents. The yield of deposited film is 0-5 mol F-1 but can be less if the nucleophilic character of the solvent or electrolyte is enhanced. For example, no film is produced in the presence of 0-1 M Et4NBr in MeCN.A polypyrrole film (0.8,um thick) on platinum was used as an electrode in cyclic voltammetry measurements. In MeCN and Me,SO containing Et4NBF,, only a broad background current is observed in the range + 1.0 to -2.0 V.The voltammogram of M ferrocene in MeCN resembles that obtained on platinum. The E" value equals $0.450 us. NaCE (sodium chloride calomel electrode) (0.450 V on Pt3), ip,/ipa equals unity, and I Ep, --E.pcI equals 90 mV a t 2OmVs-l scan rate, with ca. 70% zR compensationFor the reduction of tetracyanoquinodimethane (TCNQ) the signals appear a t +0.260 and -0-290 V vs. NaCE (+0-265 and -0.300 V on Pt). I values equal 70mV a t 2OmVs-l scan rate and ipa/zpc values equal unity for both reaction steps.The I E,, --E PC

Highly stable, flexible films of polypyrrole with p-type conductivities of 100 Q-1 cni-l have been prepared by electrolytic oxidation of the appropriate pyrrole monomers; similarly prepared films of mixtures of pyrrole and N -m e t h y l p~~r o l e have conductivities between 5 xand 100 Q-l cm-1 depending upon the composition. ~~ THE discovery of metallic properties in polymers such as polysulphur nitride, (SN) .,l and doped polyacetylene, (CH)z,2 has encouraged the search for other polymeric systems with greater chemical stability and plasticity.In 1968 Dall'Olio et al.3 reported a value of 8 W 1 cm-l for the conductivity of an oxy-pyrrole polymer. IVe have prepared shiny blue-black flexible films of polypyrrole* which, like (SN), and doped (CH)., show metallic properties. The room temperature conductivity of the present films, measured by four-probe technique is as high as 100 R-' cm-l. The films are stable in air and can be heated to 250 "C with little effect on their conducting properties. Thermal analysis confirms that the material is thermally inert below 250 "C after which the rate of weight loss increases and becomes very rapid above 600 "C. The films are prepared by the electrolytic oxidation of pyrrole in acetonitrile solution using a tetraethylammonium tetrafluoroborate electrolyte.4 Though the chemical composition of the films depends on the conditions of preparation, a typical composition is C4.0N,,.87H3.5(BF4) o.25 indicating that the pyrrole rings remain intact. This is consistent with polypyrrole being a polymer formed by linking the pyrrole units zria the cc carbon atoms as suggested previously.The pvrrole units carry a partial positive charge balanced by the BF4-ions somewhat akin to the AgBF, and AgClO, treated polyacetylene films reported by Clarke ef nZ.6 Raman and reflection i.r. spectra show bands characteristic of pyrroles7 confirming the presence of the pyrrole rings in the polymer. The films are less crystalline than pristine polyacetylene. Electron diffraction data show onlyo diffuse rings which correspond to a lattice spacing of 3.4 A , close to the value expected if the separation between the pyrrole chains were determined by the thickness of the pyrrole rings. Normally the chains of pyrrole rings are randomly oriented in the plane of the film. However, under certain circumstances the diffraction rings are arced as a result of the preferred orientation of the pyrrole chains. Scanning electron microscopy shows that the polypyrrole, in contrast to both (SN) . and (CH) does not consist of fibres but is a continuous film with a flotation density of 1.48 g ~m -~.The temperature dependence of the four-probe conductivity is weakly activated, decreasing from a value of 100 R-lcm-l a t room temperature to 30 t2-lcm-l on cooling to -193 "C. Thermop3wer measurements show t h a t the polymer is p-type which is consistent with the fact that it is partially oxidized. The room temperature value of the thermopower is 7pVdeg-l. This small value of the thermopower together with its linear temperature d...

In this work, the electrical admittance of this plane-parallel resonator loaded on one face with a viscoelastic medium is described directly in terms of the physical properties of the system. This description is based on the detailed matrix equations for the piezoelectric quartz and includes the piezoelectric effect of the quartz transducer and the shear modulus and viscosity of the overlayer. This physically based analysis has proven invaluable in relating the admittance behavior directly to the materials’ properties. Initial comparisons of the theory with experimental data are given, including the appearance of even harmonics as described by Benes [E. Benes, J. Appl. Phys. 56, 608 (1984)] for the asymmetrically loaded resonator. Additionally, measurements on a series of perfluoropolyether fluids of differing molecular weights indicate that they cannot be described as simple viscous liquids in the MHz range. The assignment of a frequency-dependent viscosity with a relaxation time proportional to the molecular weight resolves some of the discrepancies. Because this study relates the complete admittance spectrum of the loaded resonator to the overlayer material properties, it opens the possibility of studying more than just the changes in resonant frequency. A detailed analysis of the resonance spectrum using an impedance analyzer would provide data which can be fit to the material’s properties and would be particularly helpful with viscoelastic media.

By analyzing the viscoelastic properties of two distinct layers, a layer of "soft" vesicles and a "rigid" bilayer, we have created a model system to permit the study of film behavior in the region of nonlinear mass and frequency change (non-Sauerbrey). The structural transformation of lipid vesicles to a bilayer is shown to be accompanied by significant changes in their physical properties. After the adsorption and saturation of intact vesicles on gold surfaces, the adsorbed vesicle layer exhibits a soft, water-rich, viscoelastic state. The AH peptide, a vesicle-destabilizing agent, is then added to trigger the formation of a much thinner (approximately 5 nm), compact, and rigid bilayer. In this study, we used the quartz crystal microbalance with dissipation technique. Large non-Sauerbrey frequency and energy dissipation changes characterize the viscoelastic nature of adsorbed intact vesicle films thicker than approximately 10 nm. Once the transformation is complete, the frequency changes along with zero energy dissipation for sufficiently thin films (t approximately 5 nm) were effectively modeled with the Sauerbrey equation. Furthermore, we checked the validity of the Voigt-Voinova model in which the quartz substrate is treated as a Voigt element, which is beyond the Sauerbrey description. The calculations treating the film as having a constant viscosity agreed well with the Voigt-Voinova model. These results were compared to calculations done using the electromechanical (EM) model, which does not require a series expansion. The Voigt-Voinova results were in excellent agreement with the EM model, providing evidence that the expansion used in their study is quite accurate.

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We present a study which reveals that the switching speed for the redox reaction of polyaniline is very fast. With a 1200Å film in 2M sulfuric acid, switching speeds of up to 100 μs with current densities of 100 A/cm2 are observed with the appropriate compensation for IR loss. The true switching speed is higher still and has not been measured. The oxidation and reduction processes have different electrical behaviors. For the reduction process, the reaction is controlled by the RC constant of the cell given by the resistance of the solution and the capacitance of the double layer. The oxidation process is more complicated and a full description is lacking.