Polypyrrole films morphology

Glatzhofer et al.1281 describe the reduction of conductivity of polypyrrole films, doped with poly(styrene-p-sulfonate), as increasing concentrations of dioxane are added to the aqueous electrolyte. They claim that the solvent induces a conformational change in the counterion, which modifies the film morphology. 

Comparable to thiophene, pyrrole is a five-membered heterocycle, yet the ring nitrogen results in a molecule with distinctly different behavior and a far greater tendency to polymerize oxidatively. The first report of the synthesis of polypyrrole (PPy) 62 that alluded to its electrically conductive nature was published in 1968 [263]. This early material was obtained via electrochemical polymerization and was carried out in 0.1 N sulfuric acid to produce a black film. Since then, a number of improvements, which have resulted from in-depth solvent and electrolyte studies, have made the electrochemical synthesis of PPy the most widely employed method [264-266]. The properties of electrosynthesized PPy are quite sensitive to the electrochemical environment in which it is obtained. The use of various electrolytes yield materials with pronounced differences in conductivity, film morphology, and overall performance [267-270]. Furthermore, the water solubility of pyrrole allows aqueous electrochemistry [271], which is of prime importance for biological applications [272]. 

As mentioned in the introduction, the electrical conductivity upon doping is one of the most important physical properties of conjugated polymers. The conductivity ranges from lOOOOOS/cm for iodine-doped polyacetylene [41], 1000 S/cm for doped and stretched polypyrrole [42], to 500 S/cm for doped PPP [43], 150 S/cm for hydrochloric acid doped and stretched polyaniline [44], and 100 S/cm for sulfuric acid doped PPV [45] to 50 S/cm for iodine-doped poly thiophene [46]. The above listed conductivities refer to the unsubstituted polymers other substitution patterns can lead to different film morphologies and thus to a different electrical conductivity for the same class of conjugated polymer in the doped state. 

T. Silk, Q. Hong, J. Tamm, and R.G. Compton, AFM studies of polypyrrole film surface morphology -I. The influence of film thickness and dopant nature. Synth. Met., 92,59 (1998). 

In order to immobilize enzymes in conducting polymers to fabricate biosensors, the electrochemical synthesis of polypyrrole films was studied under different conditions. It was found that the size and morphology of polypyrrole films synthesized using cyclic voltammetry were affected by the concentration of the supporting electrolyte at a scan potential range between 0.0 and 1.0 V (vs. SCE), and at a scan rate of 48 mV s [47]. The diameters of particles prepared in a solution containing 0.10 M pyrrole and 0.10 M NaCl... 

In Sect. 2.7.3.7.1, appropriate control of polymer composition of a redox polymer (PVF) was shown to lead to the introduction of viscoelastic phenomena and to thermal sensitivity. For polypyrrole, deposition from micellar surfactant media (dodecylsulfate and dodecylbenzenesul-fonate) also leads to changes in film morphology and viscoelastic behavior [139]. 

Garcia-Belmonte, G., and J. Bisquert. 2002. Impedance analysis of galvanostatically synthesized polypyrrole films. Correlation of ionic diffusion and capacitance parameters with the electrode morphology. Electrochim Acta 47 (26) 4263. 

A. Witkowski, M. S. Freund, A. Brajter-Toth, Effect of electrode substrate on the morphology and selectivity of overoxidized polypyrrole films, Analytical Chemistry 1991, 63, 622. 

Otero, T. F., and Larreta-Azelain, E., Electrochemical control of the morphology, adherence, appearance and growth of polypyrrole films, Synth. Metals, 26, 79-88 (1988). 

Truong, V. T, Ennis, B. C., and Forsyth, M., Enhanced thermal properties and morphology of ion-exchanged polypyrrole films. Polymer, 36, 1933-1940 (1995). Kivelson, S., Electron hopping in a soliton band conduction in lightly doped (CH)., Phys. Rev. B, 25, 3798-3821 (1982). 

Cyclic voltammetry can be used to estimate the charge transfer rate and also evaluate how this rate depends on parameters such as morphology and the chemical structure. The cyclic voltammetric examination of electroactive polymers is usually done in monomer-free solutions containing only the solvent and supporting electrolyte. In order to avoid the complication of mixed electrolytic equilibria, the supporting electrolyte and the solvent are usually the same as employed for the polymerization. Figure 3 shows the cyclic voltammogram (CV) of a polypyrrole film prepared in acetonitrile/tetra-w-butyl ammonium fluoborate medium. The anodic peak corresponds to polypyrrole oxidation, while the cathodic one corresponds to the reduction of this species. 

In addition, we have recently described a procedure for controlling the morphologies of electronically conductive polymers (32). This procedure involves the electrochemical growth of the conductive polymer at an electrode surface which has been masked with a microporous polymer membrane. The pores in this membrane act as templates for the nascent electronically conductive polymer. Because the template membrane contains linear cylindrical pores, cylindrical conductive polymer fibrils are obtained (32). We will show in this manuscript that microfibrillar polypyrrole films prepared via this approach can support higher rates of charge-transport than conventional polypyrrole. 

Oxidation at the anode results in the formation of blue to black oxidized materials (thickness 1 mm — 0.61 pm) of high conductivity. The films show a yellow colour in the uncharged low conductivity state upon undoping. The mechanical properties of 1 mm thick polypyrrole films (obtained on platinized electrodes in propylene carbonate partly in the presence of water) were reported Polypyrrole (PP) was obtained in two different morphologies Electrochemical polymerization in an aqueous solution of results in a compact structure of aggregated spheres... 

This polymerization method, based on the application of square waves of potential, has been also optimized to control the morphology, adherence and homogeneity of polypyrrole films on platinum [67]. 

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