Infrared Spectroscopic Characterization of Epoxy Matrix Composites

Infrared spectroscopic characterization of composites is possible by many sampling techniques, including transmission, specular reflection, diffuse reflection, internal reflection, photoacoustic, and emission. The choice of technique depends on the physical form of the specimen and, in particular, on the scattering characteristics of the reinforcement. Examination of the interphase between the reinforcement and the matrix, in isolation from the rest of the composite, requires some sort of a model system involving either (a) an increased interfacial volume (e.g., use of a high-surface-area analog of the reinforcement), (b) spectroscopic labeling of the interphase, or (c) the use of a model for the reinforcement surface (e.g., a coated internal-reflection spectroscopy element). Several examples of category (a) and (c) are described here. It is demonstrated that the surfaces of commonly used reinforcements (e.g., metal oxides, aramids, carbon) modify the local crosslinking chemistry of epoxy matrices. In particular, the desorption of water from the reinforcement surface and preferential adsorption of polar components of the matrix precursor reduce the local crosslink density and modify the chemical properties of the polymer in the interphase.