Adsorption of Model Adhesive Compounds onto Plasma-Polymerized Silica-Like Films

Plasma-polymerized silica-like primer films on aluminum substrates were prepared and exposed to diethanolamine (DEA), a model adhesive compound, from dilute solution. Reflection-absorption infrared (RAIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to investigate the chemical interactions between the adsorbed compound and the substrate. RAIR spectroscopy showed significant changes in the structural chemistry of the silica-like films after exposure to ethanol with and without DEA. Although no evidence of adsorption was observed in the infrared spectra, XPS indicated that a small amount of model compound did adsorb onto the surface of the silica-like film. The N(1s) photoelectron spectra of the adsorbate showed two components; one was typical of the bulk amine, and the other indicated that the amine group of the adsorbed model adhesive had chemically interacted with the substrate. The adsorbate/substrate interaction strength was also examined by heating the sample in ultra-high vacuum (UHV) and observing the temperature at which desorption occurred. The sample was heated to 250°C for several hours and no desorption was detected. It was concluded that protonation of the amine group had occurred forming a quaternary ammonium ion. It was also likely that bonding between the alcohol functionality and the silica surface added to the adsorption strength.