Epoxy and polyurethane networks in thin films on metals - Formation, structure, properties

Thin adhesive films on metals are used as an appropriate model for the characterization of the interphase in polymer-metal compounds. This interphase is a result of the specific interactions between the adhesive partners. It is particularly relevant for the performance, and the long-term stability of adhesively bonded structures and coatings. The influence of the metal substrate affects the formation of the polymer network during the polymerization, as well as its resulting structure and properties significantly as compared to the bulk. As examples, a thermosetting epoxy ( EP) and a polyurethane ( PU) elastomer are chosen for the investigation of the morphology and the chemical structure of the thin films ( thickness between 20 nm and 3 mu m) on native surfaces of gold, aluminium, and copper. The results for the two types of polymers reveal characteristic differences with respect to reaction rate and final degree of cure as a function of the film thickness on the different metals. The final structure turns out to be a consequence of competing processes such as polyaddition and side reactions including their kinetics, catalysis or inhibition, complexation and dissolution of the complexes, preferential adsorption and phase separation, and the changes in molecular mobility.