Critical dependency of properties of conducting polymer thin films on the hydrophobicity /hydrophilicity of substrate surfaces

The basic purpose of this research was to optimize conditions for chemically preparing high quality thin films of the conducting polymers, (doped) polypyrrole and (doped) polyaniline, by a novel “in-situ” deposition method and to evaluate their conductivity and optical properties. The most significant results were: (1) Cohesive, semi-transparent films of polypyrrole and polyaniline could be controllably deposited on substrates from ∼1 wt.% aqueous solutions of the polymerizing monomers during 3 to 45 minutes in the presence of oxidants (FeCl3, (NH4) 2S2O8, etc.) and selected dopant anions. (2) Films deposited on hydrophobic (glass) substrates were ∼2 to ∼4 times thicker than those deposited on hydrophilic (glass) substrates during the same time but had a conductivity ∼104 times greater than those deposited on the hydrophilic substrates. The large, unexpected difference in conductivities is consistent with the SEM observation that films on hydrophobic substrates consisted of a continuous granular matte whereas films on hydrophilic substrates consisted of discrete particles having little (electrical) contact between them. (3) The electronic structure and probably the molecular conformation of polypyrrole were significantly dependent on the nature of the substrate (hydrophobic vs. hydrophilic) as shown by UV-VIS-NIR spectroscopic studies. (4) Polypyrrole films (anthraquinone-2-sulfonate, AQSQ –, dopant) deposited on a hydrophilic substrate showed a “memory effect” with respect to the original substrate which lessened with increasing film thickness, but still persisted even when the film thickness was increased 8-fold. (5) The nature of the substrate (hydrophobic vs. hydrophilic) had a much larger, more clearly defined effect on the nature of deposited polypyrrole films than the dopant anion used. (6) By utilizing the large difference in conductivity between polypyrrole films deposited on hydrophobic vs. hydrophilic substrates, patterned polypyrrole (AQSA– dopant) films with a resolution ∼8 μm could be achieved by combining a non-lithographic, micro-contact printing technique with the above in-situ deposition method. (7) By adjusting the thickness of the in-situ-deposited polypyrrole films, semi-transparent conducting polymer electrodes were prepared and used in liquid crystal display devices. (8) All-polymer electrostrictive systems could be fabricated using flexible, elastic polyppyrrole (AQSA– dopant) thin films as electrodes to replace conventional rigid gold electrodes