Heterogeneity in epoxy nanocomposites initiates crazing: significant improvements in fatigue resistance and toughening.

Crazing is a failure mode of bulk polymers and occurs under predominant uniaxial tensile load when the bulk eventually forms denser ligaments (or fibrils) while preserving its continuity. [1‐2] The bridging of cracks by such fibrils is an importantmechanismforenergydissipationandtougheningin thermoplastic polymers. However, craze phenomena are not observed [3‐5] in thermosetting polymers such as epoxies due to the high crosslinking density of the epoxy chains, which limits molecular mobility and inhibits craze fibril formation. Such thermosetting epoxies typically display a brittle failure. [6‐7] We demonstrate here that thermosetting epoxies reinforced with amido-amine-functionalized multiwalled carbon nanotubes (A-MWNTs) exhibit crazing. We show order of magnitude reduction in fatigue crack growth rates as a result of the crazing. The fracture toughness and ductility of the brittle epoxy is also significantly enhanced by the crazing. Importantly these enhancements in fatigue resistance and toughness are achieved without any softening of the material. In fact, the Young’s modulus of the nanocomposite is � 30% greater and the average hardness of the nanocomposite is � 45% higher than the baseline (pristine) epoxy. We show that this effect is related to heterogeneous curing of the epoxy, which results in localized pockets of uncrosslinked epoxy that are trapped (or frozen) at the nanotube‐matrix interfaces. Under mechanical loading, these localized regions of high molecular mobility can evolve (or coalesce) to generate conditions that are favorable for crazing. Recently, in a very interesting study, [8] crazing has been reported for a poly(lactide