Effect of thermal curing on molecular orientation in polyimide films having rodlike molecular skeleton formed on substrates

Films of a single polyimide having a rodlike molecular skeleton of uniform thickness were prepared on silicon and quartz substrates under several different heating rates during the thermal curing cycle. The molecular orientation in the cured polyimide film was evaluated by polarizing infrared and visible light investigation. The molecular orientation in the film was very sensitive to the heating rate during the curing cycle at the interface with the substrate, but less sensitive to that at the surface. The rodlike molecules in the film which were slowly heated were highly oriented and nearly parallel to the film plane, whereas those which were quickly heated were randomly oriented at the interface. It was shown that the increase in thermal stress in the polyimide film with increasing heating rate reflected the degree of decrease of the in-plane orientation.