Molecular structure of interfaces formed with plasma-polymerized silica-like primer films

The molecular structures of the adhesive/a-SiO₂ and a-SiO ₂/metal interfaces were investigated for plasma polymerized a SiO ₂ films that were used as adhesive primers on metal substrates. Since these interfaces were buried and difficult to observe it was necessary to develop special sample preparation techniques. For the adhesive/primer interface, a low molecular weight molecule that modeled the chemical properties of epoxy adhesives was applied to the surface of the primer film for chemical interaction. The chemical interactions between the adsorbed model compound and the a-SiO ₂ film were monitored using surface analytical techniques. It was determined that chemical adsorption of the model adhesive occurred via amine protonation by acidic silanol groups on the a-SiO₂ primer surface. The interaction strength was tested by heating the adsorbate in ultra-high vacuum beyond the temperature necessary to desorb molecules that had adsorbed by hydrogen bonding. It was found that desorption did not occur, in support of chemisorption by forming the strongly bonded protonated amine. This indicated that reactions between the amine groups of the epoxy and the silanol groups of a-SiO ₂ primer film impeded destabilization of the interface by moisture.; In order to study the a-SiO₂/metal interface, a-SiO₂ films were deposited onto metal substrates by plasma polymerization using novel plasma reactors designed for in-situ characterization with X-ray photoelectron spectroscopy and reflection absorption infrared spectroscopy. These complimentary analytical techniques were used to analyze ultra-thin films without contamination from atmospheric exposure. By analyzing successively thinner films, the intensity of signals due to the bulk were reduced and the signals due to the interface were analyzed.; This work showed that during the creation of the a-SiO₂/metal-oxide interface, specific chemical bonds were formed, which promoted the overall mechanical strength and durability of the interface. In the case of the a-SiO ₂/metal substrate, the metal substrate was still oxidizing during the initial stages of a-SiO₂ deposition. This resulted in oxidation of the substrate, the formation of silicon suboxide, and the formation of primary bonds between the oxide of the substrate and the primer film. These reactions lead to an interface with high mechanical strength and excellent environmental durability.