Surface characterization of polyimide films

Various kinds of poly(amide acid)s were prepared by the reaction of four kinds of acid dianhydride and three kinds of diamine in N,N-dimethylacetamide (DMAc). Polyimide films were prepared by casting the poly(amide acid) solution on soda glass substrates, followed by thermal imidization at various temperatures. Contact angles of polyimide films for the sides in contact both with air and with glass substrate (air side and glass side, respectively) were measured to evaluate the dispersive component (gamma(S)(D)) and the nondispersive component (gamma(S)(P)) Of surface free energy (gamma(S)) of polyimide films. It was shown that, for the air side, gamma(S)(P) value decreased greatly, and gamma(S)(D) values increased slightly with the development of imidization. Values of gamma(S)(P) for the glass side were much higher than those for the air side. Poly(amide acid) solution was also cast on quartz glass and silicone rubber, and was thermally imidized. The gamma(S)(P) for the quartz glass side was almost the same as that for the soda glass side. But the gamma(S)(P) for the silicone rubber side was as low as the gamma(S)(P) for the air side. Attenuated total reflection infrared spectroscopy of polyimide films showed that the degree of imidization for the glass side surface was not as high as that for the air side surface, and that the amount of polar groups for the glass side surface was higher than that for the air side surface. Among the various kinds of polyimides, there is a slight but clear difference in the values of gamma(S) and its components, which can be rationalized from the difference in the chemical structure. It was also found that thermal degradation and oxidation can be easily detected from the change of surface free energy and its components for the polyimide films after being treated at high temperatures in both air and vacuo.