Polymer matrix composites are traditionally valued for use in lightweight mechanical applications (aerospace, automotive, etc.) due to their attractive combination of mechanical properties (toughness, strength/stiffness) and low density. For use in extreme environments, these materials must possess slow degradation rates when exposed to O₂, H₂O, and salts. Moreover,  maintaining structural integrity over a wide temperature range requires a low thermal expansion coefficient mismatch between matrix/reinforcement and a high glass transition temperature (T_g). Composites must also demonstrate excellent resistance to fracture, determined in part by the mechanical properties (described above) and by the preferred failure mode, which is a function of the reinforcement architecture and fiber/matrix interface. The polymer matrix is prepared by combining epoxy, crosslinker, and filler (as-needed), which controls the crosslinking density and heterogeneity within the matrix structure. The choice and preparation of reinforcement sets the morphology and microstructure of the reinforcement phase.  The architecture of the reinforcement (and therefore, composite) is set during layup and curing and is defined by the dispersion, aggregation, and orientation of the reinforcement. Post-layup annealing and aging can improve the matrix/reinforcement interface, and is critical to the mechanical properties, particularly the failure mode and toughness.