Surface characterization and adhesion of plasma-modified polyimides

LaRC-TPI, an aromatic thermoplastic polyimide, and Kapton®, a poly(pyromellitimide) were exposed to oxygen, argon and ammonia plasmas as pretreatments for adhesive bonding. Chemical changes which occurred in the surface as a result of the plasma treatments were investigated using x-ray photoelectron spectroscopy (XPS) and infrared reflection-absorption spectroscopy (IR-RAS). Water contact angle analysis was utilized to characterize the changes in surface wettability, and the ablative effects of the plasmas were monitored using ellipsometry and high resolution scanning electron microscopy (HR-SEM). Both XPS and IR-RAS results revealed the formation of polar functional groups at the surface. Contact angle analysis showed enhanced water wettability of the plasma-treated surfaces. As monitored by ellipsometry, oxygen and argon plasmas were seen to be highly ablative, whereas an ammonia plasma was only moderately so. HR-SEM micrographs revealed texturized surfaces in the case of oxygen and argon plasmas, but not in the case of ammonia plasma. Oxygen and argon plasmas appear to react with the polyimides via a fragmentation/oxidation mechanism, forming a loosely attached layer composed of low molecular weight polymer chains. The effect of ammonia plasma is postulated to be imide ring-opening resulting in the formation of amide functional groups. The 180°peel test was utilized to determine the receptability of the plasma-treated polyimide surfaces toward bonding with other polymeric materials. Adhesives used were a pressure sensitive acrylate and poly(ether sulfone). The pressure sensitive adhesive, although not representing a realistic bonding situation, does represent a system which presents the least disturbance to the plasma-modified layer, allowing the physical nature of the plasma-treated surface to be probed. The peel test values of the pressure sensitive adhesive/plasma-treated polyimide systems fell below the level of the non-treated controls, regardless of the plasma treatment used. Peel surface analysis ...