POLYMER-FILMS ON ELECTRODES - 28 - SCANNING ELECTROCHEMICAL MICROSCOPY STUDY OF ELECTRON-TRANSFER AT POLY(ALKYLTERTHIOPHENE) FILMS

The electrochemical doping and undoping processes of poly-(3,3 ''-dido decyl-2,2':5',2 ''-terthiophene) (poly(33''DDTT)) were studied by scan ning electrochemical microscopy (SECM) to characterize the electron tr ansfer of the polymer with a redox mediator in solution when the polym er was at different oxidation levels. SECM showed that electron exchan ge processes of oxidized poly(33 '' DDTT) were localized at the polyme r/solution interface rather than inside the film. Poly(33''DDTT) in th e neutral state did not allow the permeation of redox species to the u nderlying metal and behaved Like a completely passivating film. A modi fied expression for the effective electron-transfer rate constant, k(e ff), was obtained by grouping the terms concerned with charge transpor t across the polymeric film, and the values of k(eff) were calculated from SECM approach curves. The values of k(eff) for the electron trans fer between poly(33 '' DDTT) film and methyl viologen (MV+/2+) as the redox couple ranged between 10(-5) and 10(-1) cm s(-1), depending on p olymer thickness, substrate potential, and MV concentration. Under the proper experimental conditions, k(eff) could be directly correlated w ith the conductance of the polymer film, where poly(33 '' DTT) thin fi lms in the conducting state showed a metallic character.