NONDESTRUCTIVE CHARACTERIZATION OF EPOXY DICYANDIAMIDE INTERPHASES USING SURFACE-ENHANCED RAMAN-SCATTERING

Surface-enhanced Raman scattering (SERS) is a process in which the Raman scattering cross-section of molecules adsorbed onto the surfaces of metals such as silver, copper and gold is increased by as much as six orders of magnitude compared with the cross-section for normal Raman scattering from the bulk. However, the enhancement is short range and is restricted to the first few layers of molecules adjacent to the surface. As a result, SERS can be used to characterize adsorption of organic compounds onto metals and can be used for non-destructive characterization of interphases between polymer films and metal substrates as long as the polymer films are not so thick that normal Raman scattering from the bulk of the film is more intense than SERS from the interphase. SERS spectra obtained from thin films of dicyandiamide (dicy) adsorbed from dilute solutions onto silver island films were characterized by very strong bands assigned to in-plane vibrations and very weak vibrations related to out-of-plane vibrations, indicating that dicy was adsorbed with a vertical configuration involving coordination through the nitrilo nitrogen atom. SERS spectra obtained from model adhesives consisting of 8:1 mixtures of diglycidyl ether of bisphenol-A (DGEBA) and dicy were identical with SERS spectra of dicy, indicating that dicy was preferentially adsorbed from the adhesive onto the silver surface and that the interphase in epoxy-dicy adhesives was likely to have composition that was enriched in dicy when compared with the bulk adhesive. Some evidence for adsorption of both DGEBA and dicy was observed in SERS spectra obtained from 500:1 mixtures of DGEBA and dicy spin coated onto silver island films from dilute solutions, indicating that in those cases there was insufficient dicy available to occupy all the SERS-active surface sites. © 1990.