The surface and interfacial chemistry of polyimide films

The surfaces of polyimide films and the structure just below the surfaces have been extensively studied as people have sought to improve and understand the key factors controlling adhesion. Treatments of all types from primers to plasma etching to sand blasting have been evaluated with varying efforts depending on the application. In recent years, the emphasis has been on understanding the chemical and morphological changes effected by these treatments and then correlating chemistry and morphology with adhesion. The picture that emerges is that surface energy alone, as is the case with most polymers, usually is insufficient to predict adhesion to polyimides. Instead, initial bond strength and bond durability, whether with adhesives or metals directly deposited on the film, depend on chemical bonding, diffusion between deposited layers and the polyimide, formation of a micro composite region controlled in part by topography and the viscoelastic properties of the polymer below the surface. Poor viscoelastic behavior frequently is characterized as a weak boundary layer. Recent work has shown that small amounts of organometallics that diffuse to the surface during the film forming process can significantly affect bendability both to adhesives and to vacuum deposited metals. The possible effect of these additives on bond formation, viscoelastic properties, diffusion and topography is under investigation and will be discussed along with an over view of the primary film forming steps that might affect surface chemistry and structure.