Probing the structure of the buried epoxy/coupling agent/glass interfacial region with an immobilized fluorescent probe

The goal of this thesis work was two fold. The first objective was to develop an in situ technique to study the buried polymer/substrate interfacial region. The second objective was to demonstrate the practical potential of using the technique as a sensor for composite cure monitoring. The buried resin/coupling agent/glass interface was studied by covalently grafting a fluorescent dye to the glass surface. This interfacial region is critical to the mechanical performance of fiber reinforced composites. Coupling agents are commonly applied to glass fibers to promote fiber/resin adhesion and enhance durability in composite parts. In this study, a coupling agent multi-layer on glass was doped with trace levels of the dimethylaminonitrostilbene (DMANS) fluorophore. The fluorophore was immobilized on the glass surface by tethering the molecule to a triethoxy silane coupling agent, creating a fluorescently labeled silane coupling agent molecule (FLSCA). FLSCA was then diluted with commonly used coupling agents and grafted to glass microscope cover slips. When the coated cover slips were immersed in epoxy resin, a blue shift in fluorescence from the immobilized FLSCA can be followed during cure of the resin over-layer, giving this technique potential to monitor the properties of the fiber/resin interface during composite processing. The fluorescence behavior of grafted FLSCA was used to detect differences in both the chemical and physical structure of the buried interfacial region relative to the bulk resin. The grafted FISCA could detect an apparent glass transition in the interfacial region. The temperature of the interfacial transition depended on the initial structure of the grafted silane coupling agent layer. To make the technique practical as a process control tool, FLSCA was grafted to a glass fiber optic, which can potentially be inserted into a composite mold to perform in-situ measurements.