Electrical properties of polyimides modified with metal salts

Abstract

Polyimides, due to their high thermal stability, excellent chemical stability, useful mechanical properties, and extremely high electrical resistivity, are utilized in aerospace, electronic, and specialty consumer markets. However, in some applications, lower electrical resistivity is preferable. Toward this goal, polyimide films have been modified with metal salts and metal complexes. Depending upon processing conditions, the films contain ionic species uniformly distributed throughout their bulk, or are highly anisotropic structures containing near-surface metal or metal oxide.

Evaluation of solvent cast films by a variety of analytical techniques has been used to develop structure-property-process correlations in cobalt chloride modified polyimides. To date, no interaction between the additive and the matrix has been indicated by ultraviolet, visible, or infrared spectroscopy, though by differential scanning calorimetry and a specialized thermogravimetry technique, some interaction is implied. Elemental analysis has verified that polyimides having a metal oxide surface also have residual bulk metal ion content. A major controller of the bulk resistivity of metal ion modified polyimide films was found to be the polymer glass transition temperature; a lower glass transition temperature resulted in lower electrical resistivity at a particular temperature.

Central to this research work was the design and construction of a sensitive and reliable electrical resistivity measurement system. The system was also found useful for probing polymer contamination and molecular motion. Further, the electrical measurements indicated that uniformity between samples was poor. Modification of an inert gas oven, allowing processing in controlled atmosphere, proved that film surface conductivity is critically influenced by humidity. A model has been proposed that supports both the d.c. electrical properties and surface spectroscopic data. The model and details in the ceramic literature were the primary factors in pursuing a specific codoped polyimide system. With the codoped system, the electrical resistivity and activation energy for conduction, compared with either of the singly doped polyimide films, were predictably and favorably reduced.