Evaluation of Molecular Orientation in Aromatic Polyimide Films by FT-IR Reflection Absorption Spectroscopy

Abstract

A method for the quantitative evaluation of molecular orientation in organic film containing rigid parts has been developed. In the method, mean square of cosine of angle between normal to the substrate and transition dipole moment vectors due to molecular vibration was evaluated by means of comparing infrared reflection-absorption (RA) spectrum of a film with the transmission spectrum of the same molecule in the unoriented bulk state such as in K Br pellets. Application of the method to the polyimide films prepared by vapor deposition polymerization showed that both the molecular chain axis and zigzag plane of the chain were nearly parallel to the substrate, and that the pyromellitimide and the phenylene ring were inclined to the substrate. As the heating temperature for the imidization rose, both the chain axis and the zigzag plane showed a tendency to be parallel to the substrate, the phenylene ring showed a tendency to cline, but the orientation of the pyromellitimide did not vary. The various factors that influence the accuracy of the orientation analysis were also discussed.