Conductivity electropolymerization

Electropolymerization can be used to coat other substrates, not just planar electrodes, resulting in the production of conducting polymer "composites." For example, electrochemical methods have been used to coat spherical graphite particles in a pulsed bed reactor. In fact the substrate to be coated need not be conductive. Electropolymerization can be used to coat other structures by producing polymer in solution that is subsequently deposited onto other surfaces. This is possible because conducting polymers are formed... 

Entrapment of biochemically reactive molecules into conductive polymer substrates is being used to develop electrochemical biosensors (212). This has proven especially useful for the incorporation of enzymes that retain their specific chemical reactivity. Electropolymerization of pyrrole in an aqueous solution containing glucose oxidase (GO) leads to a polypyrrole in which the GO enzyme is co-deposited with the polymer. These polymer-entrapped GO electrodes have been used as glucose sensors. A direct relationship is seen between the electrode response and the glucose concentration in the solution which was analyzed with a typical measurement taking between 20 to 40 s. 

Other usefiil gas sensors include the potentiometric ammonia (64) or hydrogen cyanide probes (65), and amperometric carbon monoxide (66) and nitrogen dioxide (67) devices. The hydrogen cyanide probe is an example of a modem device that relies on changes in the conductivity of electropolymerized film (polyanihne) in the presence of a given gas. 

This means that we can follow the empirical kinetics of the electropolymerization process, at a constant overpotential (Fig. 6), by tracking the weight of the rinsed and dried polymer film,37 41 as we do in homogeneous polymerization processes of conducting or nonconducting poly-... 

These facts are different demonstrations of the same event degradation reactions occur simultaneously with electropolymerization.49-59 These reactions had also been called overoxidation in the literature. The concept is well established in polymer science and consists of those reactions between the pristine polymer and the ambient that promote a deterioration of the original polymeric properties. The electrochemical consequence of a strong degradation is a passivation of the film through a decrease in the electrical conductivity that allows a lower current flow at the same potential than the pristine and nondegraded polymer film did. Passivation is also a well-established concept in the electrochemistry of oxide films or electropainting. 

The final conclusion from the different kinetic studies that simultaneously followed productivity, consumed current, storage capacity of the obtained films, and the current efficiency in generating electroactive polymer in the final film is that any electropolymerization of conducting polymers occurs together a partial degradation of the electroactive polymer. The final film is a mixed material. From the kinetic studies we know the variables that increase or deplete the degradation reaction in relation to the polymerization reaction. 

In 1979, Diaz et al. produced the first flexible, stable polypyrrole (PPy) film with high conductivity (1(X) Scm ). The substance was polymerized on a Pt-electrode by anodic oxidation in acetonitrile. The then known chemical methods of synthesis " usually produced low conductivity powders from the monomers. By contrast, electropolymerization in organic solvents formed smooth and manageable films of good conductivity. Thus, this technique soon gained general currency, stimulating further electropolymerization experiments with other monomers. In 1982, Tourillon... 

Galvanostatic, potentiostatic as well as potentiodynamic techniques can be used to electropolymerize suitable monomeric species and form the corresponding film on the electrode. Provided that the maximum formation potentials for all three techniques are the same, the resulting porperties of the films will be broadly similar. The potentiodynamic experiment in particular provides useful information on the growth rate of conducting polymers. The increase in current with each cycle of a multisweep CV is a direct measure of the increase in the surface of the redoxactive polymer and, hence, a suitable measure of relative growth rates (Fig. 5). 

Besides synthesis, current basic research on conducting polymers is concentrated on structural analysis. Structural parameters — e.g. regularity and homogeneity of chain structures, but also chain length — play an important role in our understanding of the properties of such materials. Research on electropolymerized polymers has concentrated on polypyrrole and polythiophene in particular and, more recently, on polyaniline as well, while of the chemically produced materials polyacetylene stih attracts greatest interest. Spectroscopic methods have proved particularly suitable for characterizing structural properties These comprise surface techniques such as XPS, AES or ATR, on the one hand, and the usual methods of structural analysis, such as NMR, ESR and X-ray diffraction techniques, on the other hand. 

PPy was the first conducting polymer to be structurally analyzed. The discovery that a,a -disubstituted pyrroles did not electropolymerize led to the conclusion that the pyrrole units in PPy are a-linked Magic angle spinning C-NMR... 

Chain length is another factor closely related to the structural characterization of conducting polymers. The importance of this parameter lies in its considerable influence on the electric as well as the electrochemical properties of conducting polymers. However, the molecular weight techniques normally used in polymer chemistry cannot be employed on account of the extreme insolubility of the materials. A comparison between spectroscopic findings (XPS, UPS, EES) for PPy and model calculations has led some researchers to conclude that 10 is the minimum number of monomeric units in a PPy chain, with the maximum within one order of magnitude n9- 27,i28) mechanical qualities of the electropolymerized films,... 

In the field of soluble conducting polymers new data have been published on poly(3-alkylthiophenes " l They show that the solubility of undoped polymers increases with increasing chain length of the substituent in the order n-butyl > ethyl methyl. But, on the other hand, it has turned out that in the doped state the electro-chemically synthesized polymers cannot be dissolved in reasonable concentrations In a very recent paper Feldhues et al. have reported that some poly(3-alkoxythio-phenes) electropolymerized under special experimental conditions are completely soluble in dipolar aprotic solvents in both the undoped and doped states. The molecular weights were determined in the undoped state by a combination of gel-permeation chromatography (GPC), mass spectroscopy and UV/VIS spectroscopy. It was established that the usual chain length of soluble poly(3-methoxthythiophene) consists of six monomer units. 

Meanwhile, the R-R coupling (see Sect. 2.2) has evidently found general acceptance as the main reaction path for the electropolymerization of conducting polymers The ionic character of the coupling species explains why polar additives such as anions or solvents with high permittivity accelerate the rate of polymerization and function as catalysts. Thus, electropolymerization of pyrrole is catalyzed in CHjCN by bromide ions or in aqueous solution by 4,5-dihydro-1,3-benzenedisulfonic acid The electrocatalytic influence of water has been known since the work... 

Electropolymerization is also an attractive method for the preparation of modified electrodes. In this case it is necessary that the forming film is conductive or permeable for supporting electrolyte and substrates. Film formation of nonelectroactive polymers can proceed until diffusion of electroactive species to the electrode surface becomes negligible. Thus, a variety of nonconducting thin films have been obtained by electrochemical oxidation of aromatic phenols and amines Some of these polymers have ligand properties and can be made electroactive by subsequent inincorporation of transition metal ions... 

Even the simplest A -substituted pyrrole, A -methylpyrrole, electropolymerizes much slower than pyrrole itself Furthermore, the conductivity of polymers... 

Some progress has also been achieved in the use of chiral polymer films at electrodes. Conductive polythiophenes containing optically active substituents in the 3-positions were prepared by electropolymerization of suitable monomers without apparent lc s of optical activity The polymer of exhibits distinct... 

Conduction in the raw doped polymer can be improved by devices such as compression and stretching and judicious after-synthesis dopant ion exchange. Matrix-guided electropolymerization also yields superior products. 

The possibility for electropolymerization on the top surface of Prussian blue films was probably first shown in [126] describing the high oxidizing ability of Berlin green, the fully oxidized form of Prussian blue. Afterwards non-conducting polymers were synthesized on the top surface of transition metal hexacyanoferrate-modified electrodes for immobilization of the enzyme [127],... 

W. Sung and Y. Bae, A glucose oxidase electrode based on electropolymerized conducting polymer with polyanion-enzyme conjugated dopant. Anal. Chem. 72, 2177-2181 (2000). 

X. Zhang, J. Zhang, and Z. Liu, Conducting polymer/carbon nanotube composite films made by in situ electropolymerization using an ionic surfactant as the supporting electrolyte. Carbon 43, 2186—2191 (2005). 

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