POLYTHIOPHENES VIA THIOPHENE, BITHIOPHENE AND TERTHIOPHENE IN PROPYLENE CARBONATE - AN ELECTROCHEMICAL AND IN-SITU FTIR STUDY

Polythiophene films obtained by the galvanostatic polymerization of thiophene (Th), bithiophene (BTh) and terthiophene (TTh) in 0.5 M LiClO4 + propylene carbonate have been investigated by cyclic voltammetry and in-situ Fourier transform IR (FTIR) spectroscopy. The polymerization potential in the propylene carbonate electrolyte decreases in the order Th > BTh > TTh. Both the charge capacity and the doping level of the resulting polymers increase in the order polythiophene (PTh) < polybithiophene (PBTh) much less than polyterthiophene (PTTh). For PTTh, a doping level of 37% is obtained during cycle 10 and 31% during cycle 1000. In-situ FTIR spectra of the neutral PBTh and PTTh films show a single band at 3063 cm-1 (corresponding to aromatic beta-(C-H)-groups) which suggests a regular alpha,alpha'-linking of the monomeric units. For thin PTh films the intensity of this band is very weak, indicating a disordered cross-linked polymer structure. During the electrochemical oxidation of the three polythiophenes investigated from 3.3 V up to 4.4 V vs. Li/Li+ the intensity of the electronic absorption band (above 2000 cm-1) increases with increasing potential, but it strongly decreases at potentials above about 4.4 V, indicating a strong irreversible oxidation of the polymer films. Furthermore, strong CO2 evolution is observed at potentials above 4.2 V.