Plasma polymerization and iodine doping of thiophene derivatives

Plasma polymerization of poly(2-methylthiophene) and poly(3-methylthiophene) thin films have been synthesized. Plasma polymerization parameters were studied. Microwave power in the range of 350-450 W was employed for 2 minutes. Plasma-polymerized poly(2-methylthiophene) and plasma-polymerized poly(3-methylthiophene) films were characterized by various spectrophotometric methods. Infrared analyses showed absorption frequencies of important functional groups mostly similarly observed in the case of monomer. Results from Energy-dispersive X-ray spectroscopy analysis were suggestive of partial fragmentation of the films. The C/S ratio of plasma-polymerized poly(2-methylthiophene) is lower than the theoretical value. For the plasma-polymerized poly(3-methylthiophene), the C/S ratio is equal to or higher than theoretical value. Electrical conductive measurements revealed that the plasma-polymerized poly(2-methylthiophene) and the plasma-polymerized poly(3-methylthiophene) films exhibit conductivities 1.04 x 10⁻⁸ to 1.20 x 10⁻⁷ and 1.05 × 10⁻⁷ to 2.01 × 10⁻⁷ S/cm, respectively. As for the doped plasma-polymerized poly(2-methylthiophene) and plasma-polymerized poly(3-methylthiophene) materials, initial conductivities (2.29 × 10⁻⁵ and 1.85 × 10⁻⁵ S/cm, respectively) were lower than the plasma-polymerized films which were doped with the conventional method. However, it was found that conductivity of the latter decreased more rapidly and reached an undoped value in a short time (24 hours). In contrary, the decaying rate of conductivity of in situ doped material could be sustained for a longer period of time (more than 160 hours).