In this paper we report the functionalization of conductive polypyrrole (PPY) films via electrochemical reduction of the aryl diazonium salts in a manner that is similar to the one employed for other conductive surfaces. To understand the general trends of the grafting behavior of diazonium salts and to establish the optimal conditions for the covalent functionalization of the PPY films, we have compared the grafting behavior of four p-substituted phenyldiazonium salts: p-nitrophenyl diazonium tetrafluoroborate (PNBDBF 4 − ), p-tolyl diazonium tetrafluoroborate (TDBF 4 − ), p-fluorophenyl diazonium tetrafluoroborate (FPDBF 4 − ) and 4-diazo-N,N-dimethylaniline tetrafluoroborate (DDMABF 4 − ). The selection of the molecules to be grafted was done both for their electroactivity after grafting and the contrasted electronegativityof the substituents at the benzene ring. For all investigated diazonium salts, a linear relationship between their reduction potential at the PPY electrodes and Hammett substituent constants was obtained, suggesting a similar electrochemical reaction mechanism. The functionalization of the polypyrrole films has been evaluated using electrochemical methods like EQCM, CV and EIS. The presence at the polymeric films surface of the functional groups introduced by the electrochemical reduction of diazonium salts was evidenced also by XPS. This approach enables new functionalities on PPY that could otherwise not withstand the polymerization conditions.

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We investigate the dielectric properties of hydrogel by means of the impedance spectroscopy technique. Our experimental data relevant to the frequency dependence of the real and imaginary parts of the electrical impedance of the cell indicate that, in the low frequency region ͑f Ͻ 20 Hz͒, the electric response of the cell is dominated by surface effects, weakly dependent on the thickness of the sample. On the contrary, in the high frequency region ͑f Ͼ 100 Hz͒, the electric response of the sample is mainly due to the bulk properties. The observed frequency dependence of the electrical impedance of the cell can be well interpreted by means of the Poisson-Nernst-Planck model taking into account the Ohmic character of the interface electrodes-hydrogel.

The electric response of a hydrosolution is investigated. We show that our experimental data can be interpreted by means of the standard drift-diffusion model only if the frequency dependence of the viscosity of the hydrosolution is taken into account. From this result, it follows that measurements of impedance spectroscopy can give information on the non-newtonian character of complex fluids.

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