An adhesive study by electron tunnelling: Ethyl α-cyanoacrylate adsorbed on an oxidized aluminium surface

The interfacial properties of an adhesive system, ethyl a-cyanoacrylate/aluminium, are investigated by the adsorption of a thin adhesive layer upon the Al oxide insulator of an Al?Pb tunnel junction. Such junctions exhibit electrical properties determined in part by the physical and chemical nature of the adsorbed layer. Inelastic electron tunnelling spectra, and resistance and capacitance data are presented which indicate that exposure of the oxide to pure adhesive vapour produces an inhomogeneous adsorbed layer, which increases in overall thickness with increasing vapour exposure time. By retarding vapour polymerization, a more uniform layer is adsorbed, whose thickness is less strongly dependent on exposure time. Tunnelling spectra agree well with those obtained using bulk i.r. spectroscopy (which are also presented here), allowing IET vibrational mode assignments to be inferred. Shifts in C?O and C?O-stretching frequencies are observed in the tunnelling spectrum of the uniform layer which have been attributed by other workers using a reflectance i.r. technique to adhesive hydrogen bonding at the oxide surface. The absence of C?C and ?CH2 modes in the tunnelling spectra is indicative of cyanoacrylate polymerization at the monomolecular level. A chemical model of the interface based on this, and on peak intensity data, is presented.