Use of Surface Analysis Methods to Probe the Interfacial Chemistry of Adhesion

Abstract

This chapter explores the manner in which the surface analysis methods of X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) can be used to extract information regarding the interfacial chemistry of adhesion from polymer/metal systems such as adhesive joints. It will be shown that the analysis of a failure interface is an uncertain method to extracting interface chemistry but in certain situations, where a very thin layer of polymer remains on the metal oxide surface, this provides spectra characteristic of the interphase. In most situations, some form of chemical or mechanical sectioning is necessary, and microtomy and dissolution methods are described as ways in which chemical information at high depth resolution can be extracted from the interphase zone.

An alternative manner in which interphase chemistry can be examined is by the use of model specimens consisting of thin layers of polymer deposited on the metal substrate. The construction of an adsorption isotherm from the liquid phase is a useful precursor to such studies as it provides an indication of the solution concentration at which monolayer coverage occurs. The shape of the isotherm can provide invaluable information regarding the adsorption of various components from a formulation on the substrate, indicating which component(s) has(have) the greatest affinity with the metal oxide. Specimens from the plateau region of the isotherm will have a monolayer of polymer deposited on them, and XPS and ToF-SIMS can be used to probe the interphase region through this very thin layer. This provides a powerful method for the elucidation of the specific interactions, such as covalent bonds, that are responsible for the forces of adhesion. Examples are given in this chapter of a range of systems featuring iron, zinc, aluminum, and ceramic substrates and polymeric systems as diverse as radiation-cured coatings and adhesives, through structural adhesives to an isocyanate-based systems turning finally to the fracture and analysis of a polyester-based nanocomposite.