Enthalpic, Entropic, and Square Gradient Contributions to the Surface Energetics of Amine-Cured Epoxy Systems

Abstract

The evolution of surface tension during polymerization of three amine-cured epoxy systems was investigated. Due to the chemical reaction of the epoxy groups with primary and secondary amines, the energetic status of an epoxy–amine system increased during polymerization. At the same time, the polymerization process induced entropic variations, also contributing to the evolution of surface energetics. A simple relation expressing the surface tension as a function of the bulk energy, the entropy of the system, and the square gradient of the polymer density was derived. The bulk and surface energetics were expressed in terms of solubility parameter and surface tension, respectively. The former was predicted using the Van Krevelen group contribution method, while the latter was directly measured using the Wilhelmy wetting method. Results indicated that, in all the three epoxy–amine systems under investigation, a unique relationship combining the surface tension, the bulk energy, the entropy, and the density square gradient of the system could be used. On the basis of the present study, and taking into account all contributory factors, it was concluded that the enthalpy component to the surface energetics is the dominant contribution.