Surface Modification by Ordered Monolayers: New Ways of Protecting Materials Against Corrosion

Abstract

Metal/polymer composites are used in numerous technical applications. For example, polymer coatings on metal surfaces are used for corrosion protection, metal films on polymers inhibit static buildup, and polymers between two metals can serve as a “glue” for connecting materials that cannot be welded. Polymer/metal composites also play an important role in modern electronics. In condensers, polymers serve as insulating layers between metallic leads and are used to encapsulate entire electronic circuits. In all circumstances, interfaces are formed between the two different materials, and since the chemistry and structure change abruptly, interfacial failure is frequently observed. The cause of failure may just be mechanical (e.g., shrinkage of the polymer during curing), or the interface stability may be degraded by attack of aggressive species, resulting in delamination. More specifically, loss of adhesion is directly caused by interfacial electrochemical reactions that nucleate at a defect and progress into intact regions of the interface. This occurs for encapsulated electronic parts in humid atmospheres as well as for lacquers on automotive parts. Thus the investigation of corrosion reactions at a buried interface is an important area of research, but it is made very difficult by the fact that most electrochemical methods do not give information on localized reaction kinetics at a buried (metal/polymer) interface. This situation has changed with the invention and development of the scanning Kelvin probe (SKP). This method allows, for the first time, local analysis of reactions occurring at a buried metal/polymer interface. Based on the results obtained with the SKP, a detailed reaction model for the delamination process has been developed. This understanding has led to the development of new approaches that protect the interface from delamination. The idea is to chemically modify the interface using Afunctional molecules that promote adhesion between metal and polymer surfaces.