Polymer matrix composites, which are materials consisting soft or ductile polymers filled with high stiffness nanoparticles, nanofibers, or fibers are of great interest to various engineering applications as these materials can enhance the matrix properties or impart new functionalities. Of specific interest to NIST, they present the opportunity to combine the strength of the stiff filler with the ductility of the matrix to realize tough durable composite materials.  However, fundamental structure-mechanical property relationships are needed to enable diverse applications of these materials. There is a need for quantitative measurement methods to study the interfacial properties of the filler particle-polymer interface that is critical for their enhanced performance.  This interphase is also highly susceptible to enhanced degradation through the combined effects of mechanical loading, temperature cycling, and absorbed interfacial moisture.  This project aims to develop in-situ mechanical tests, coupled with mechano-responsive fluorophores, moisture sensitive fluorophores, and advanced imaging methods such as super-resolution Fluorescence Lifetime Imaging (FLIM) and Förster Resonance Energy Transfer (FRET) to characterize the deformation mechanisms, fracture, and fatigue behavior of the polymer matrix composites. We are particularly interested in the role of particle filler fraction, size, and geometry of filler on the interfacial properties and their relationships to overall toughness and impact strength of the entire composite material as a function of deformation rate, temperature, and environmental conditions.