The Role of Fast Relaxations in Cross-Linked Polymer Networks for Impact Mitigation

Composites for ballistic impact resistance integrate high strength reinforcements, such as high strength fibers or ceramic, with a polymeric binder that imparts ductility to the system. Together these components provide the strength plus ductility to realize tough ballistic composites. The binder is often a cross-linked polymer networks where the reactive liquid monomers can be impregnated into the composite structure and then be cross-linked into a rigid component. To this end, there is a significant interest in the molecular origins of toughness in cross-linked networks - why some systems lead to tough resins whereas others lead to a brittle response. Here we explore a series of model epoxy networks with moderate toughness to a series dicyclopentadiene networks which display incredible toughness deep into the glassy state. Quasielastic neutron scattering (QENS) is used to quantify molecular motions on the time scale of ps to ns in these networks. This reveals a strong correlation between these fast polymer relaxations and toughness. Collective many atom vibrations are important for toughness, but not enough. The many atom vibrations must lead to dissipative, many atom relaxations in order to dissipate energy and enhance toughness under ballistic impact conditions.