Building Constitutive Relationships in Responsive Thermosets: A Multi-scale Experimental and Simulation Effort

The aim of the proposed work is to incorporate dynamic bonds in epoxy based thermosets, yielding dynamic materials with high mechanical toughness. Applications include materials with superior ballistic impact resistance and matrices for fiber-reinforced composite structures that are able to recover a large fraction of their mechanical strength after an impact event. The dynamic bonds in this case are disulfide bonds incorporated into either a diamine or diepoxide monomer. While a variety of types of dynamic bonds exist, the disulfide (S-S) linkage is in our view the most appropriate choice for applications of relevance to the army, where a rapid materials response is required. Disulfide bonds will break at lower stresses than C-C, C-N, C-O, or C-S bonds present in the network. Thiyl (RS ⋅ ) radicals formed during the the rupture of the disulfide bonds can react with other disulfides in the system, resulting in a stress-induced reconfiguration of the network structure. This behavior is responsible for the excellent fatigue performance of sulfur-crosslinked rubber. Our hypothesis is that that this same mechanism will impart enhanced toughness to glassy systems by affecting the nature of the damage zone throughout which energy is dissipated during material failure. Furthermore, bond reformation subsequent to an impact event will enhance the structural integrity of the material and its ability to withstand subsequent impacts. A combined effort involving both experiment and molecular simulation is proposed to optimize the development of these new materials.