In situ Near-IR Cure Monitoring of a Model Epoxy Matrix Composite

Abstract

To improve the performance of composites it is imperative that the interphase region between the inorganic reinforcement and the polymer matrix be more completely understood. It is in this region that the stress transfer between the matrix and the reinforcement occurs. To this end, the curing of epoxy adjacent to an embedded silica optical fiber has been monitored in situ by evanescent wave spectroscopy. The epoxy studied is partially fluorinated and has a lower refractive index than the silica optical fiber. This combination of epoxy/silica served as a model composite system. The lower refractive index of the partially fluorinated epoxy allowed the silica optical fiber to be used as a waveguide for the internal reflection of near-infrared light. The epoxy curing was determined as a function of time and temperature by analysis of the near-infrared spectrum from the epoxy adjacent to the fiber obtained by the interaction of the evanescent wave that occurs at each internal reflection with the low-refractive-index epoxy. The results obtained from the examination of the near-infrared spectrum, particularly the disappearance of the NH2 stretching/bending combination band at ∼4925 cm−1 and the concomitant increase of the C–N overtone band at ∼4725 cm−1, showed that epoxy ring-opening and cross-linking reactions could be followed in real time. Finally, treatment of the fiber with a silane coupling agent had no observable effect on the curing reaction of the epoxy.