| Characteristics of vapor-deposited conjugated polymer thin films such as chemical structure, film thickness, interfacial and electronic properties were analyzed utilizing high resolution electron energy loss spectroscopy (HREELS). Vapor deposition eliminates solvent effects, which allows for better control of film uniformity and thickness. Conjugated polymers studied in this research project were polyaniline, poly (p-phenylene vinylene) (PPV) and polypyrrole. Reactive monomers and oligomers are able to react and polymerize to form longer chains on the substrate surface during vapor deposition. Although the molecular weights are low, HCl-doped polyaniline films still exhibit metallic conductivity. These studies also demonstrated that the choices of starting material for vapor deposition and substrate can influence the chemical properties of the films. A minimum chain length (12–16 monomer units) is necessary for vapor-deposited polymer thin films to be highly conducting, allowing sufficient π-electron transport through the polymer backbone. In addition, polyaniline thin films appear to form strong covalent bonds with Cu surface but not with Ag or Au surfaces. This indicates that interfacial interactions with substrate can play an important role at the initial stage of film growth. Finally, thin films of poly(p-phenylene-1-chloroethane) (PPCE), the PPV precursor, can only be synthesized at a surface temperature above 305 K or else parylene thin films are formed. Thus, processing conditions such as surface temperature during film deposition can influence the chemical structures of the polymer thin films. |
