Modification of polyimide surface morphology: relationship between modification depth and adhesion strength

The morphology of a polyimide film surface is modified from a semicrystalline state to an amorphous state without altering the bulk properties. The outer layer (0.5-25 nm) of fully cured poly(pyromellitic anhydride-oxydianiline) (PMDA-ODA) and poly(biphenyl dianhydride-p-phenylene diamine) (BPDA-PDA) polyimides is chemically modified to polyamic acid, which is subsequently imidized at 230-250°C for 30 min to give a disordered polyimide surface. This disordered layer seems amorphous since it swells well in 1-methyl-2-pyrrolidinone (NMP) and the ions in the modified layer can be easily removed with a solvent such as water or ethanol. The modified surfaces are analyzed by surface-sensitive techniques such as contact angle measurement, X-ray photoelectron spectroscopy (XPS), ion scattering spectroscopy (ISS), secondary ion mass spectroscopy (SIMS), and external reflectance infrared spectroscopy (ERIR). The adhesion of polyimide layers onto the amorphous polyimides is greatly enhanced. Interdiffusion and subsequent mechanical interlocking are the major contributors to the polyimide-polyimide adhesion. The relationship between the depth of modification and the peel strength is studied. The deeper the modification depth, the greater is the peel strength.