Compositional variation within the epoxy/adherend interphase

At a polymer-solid interface, an inorganic adherend may influence the structure and/or chemistry of the polymer in the near-interface region. This region between the adherend and the homogeneous polymer is referred to as the interphase. While there is general agreement that interphases exist, extensive debate revolves around the characteristic length scale of the interphase and its composition and structure. The present study examines the size and composition of the epoxy-aluminum interphase using spatially resolved electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). Aliphatic bis(p-aminocyclohexyl)methane (PACM20) curing agent and aromatic diglycidyl ether of bisphenol-A (DGEBA) epoxy resin were selected as a model system. Spatially resolved pi - pi*, carbon, and thickness profiles were measured using EELS in the epoxy region immediately adjacent to the interface between the bulk epoxy and the nanoporous oxide on the aluminum adherend Surface. These profiles systematically show deviations in the spectral intensities characteristic of carbon and aromaticity over distances extending 90 +/- 15 nm from the oxide surface. Simulations of such data indicate that these fluctuations cannot be accounted for by variations in specimen thickness but rather result from changes in epoxy composition near the oxide surface. The results show that this epoxy-aluminum interphase is enriched in curing agent, as indicated by a gradual compositional change from 25 +/- 5 vol% PACM20 in the bulk epoxy to 80 +/- 15 vol% PACM20 at the epoxy/oxide interface. This chemical segregation may have important implications on the properties and performance of epoxy-aluminum adhesive joints.