Effects of Dynamic Disulfide Bonds on Mechanical Behavior in Glassy Epoxy Thermosets

Inclusion of dynamic covalent chemistries within polymer networks has been a topic of significant research due to the ability to endow these materials with reprocessability and recyclability; however, cleavage of dynamic bonds could also serve as a toughening mechanism in glassy thermosets. The influence on mechanical properties of including dynamic disulfide bonds within thermoset epoxies is investigated across broad temperatures and by utilizing several techniques including dynamic mechanical analysis, uniaxial tension and compression, microindentation, and fracture toughness testing. By analyzing the results in the context of a cohesive zone model, it was determined that the observed improvement in toughness in the disulfide-containing epoxies likely results from increased extensibility in the process zone due to preferential cleavage of disulfide linkages before macroscopic network failure. Observation of mechanochromic thiyl radicals in compression samples also provides evidence for increased plasticity in these dynamic glasses and suggests high strains are required for mechanical activation.