Exploring Whether a Buried Nanoscale Interphase Exists within Epoxy–Amine Coatings: Implications for Adhesion, Fracture Toughness, and Corrosion Resistance

Epoxy resins remain some of the most widely used industrial materials, comprising the matrix component of many paints, adhesives, and structural composites. Extensive interphase structures within epoxies have often been proposed on the basis of thin film studies, where thickness-dependent thermal and chemical properties are commonly observed. The nature of these regions, thought to extend up to hundreds of micrometers into resins, is therefore widely considered to control properties such as the fracture toughness of composites and the adhesion and corrosion resistance of epoxy coatings formed on metal oxides. Nonetheless, little is understood about the formation mechanism and extent of interphase regions. Here, we first confirm the thickness-dependent curing properties of diglycidyl ether of bisphenol A (DGEBA) cross-linked with triethylenetetramine (TETA). Increased levels of residual epoxy are consistently measured for thinner films, but this is found to be substrate-independent (on gold or carbon steel). Furthermore, transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) analyses rule out organometallic formation by amine complexation by the absence of iron in nanoscale regions around the interface. Instead, it is shown that the excess epoxy in thin films develops as a result of amine consumption at the air–polymer interface. Finally, nanoscale functional group mapping of cross sections is achieved using atomic force microscopy–infrared (AFM-IR) analysis, and this demonstrates that no chemical gradient exists within films. We therefore conclude that no buried nanoscale chemical interphase is formed within the epoxy–amine coatings.