Photoinduced Plasticization of Azobenzene-Containing Epoxy Glasses for Impact Mitigation

High-Tg, epoxy-amine networks are used ubiquitously across various industries as lightweight adhesives; however, a cross-cutting need is improved ductility in these glassy materials. Tailoring the location of photoactive azobenzene moieties (interstitial, pendent, or at the cross-link) within high-Tg networks elucidated several structural relationships to mechanical responses and improvements in ductility. Within the linear viscoelastic regime, probed using dynamic mechanical analysis, the mechanical changes during irradiation were strongly dependent on light intensity, with a glass-to-rubber transition realized as the intensity increased from 330 to 3000 mW/cm2. Using embedded thermocouples, a bulk thermal contribution to the material softening was estimated, helping to decouple the photothermal and photomechanical response in the activated networks. Nonuniform light penetration during quasi-static testing of thick (6 mm) sample specimens illustrated a trade-off in performance with azobenzene concentration. Improvements in high-rate impact performance relied on a change in failure mechanisms within the impacted plate caused by the local change in ductility with irradiation. Maintaining the rapid (∼10 s) mechanical response within these materials across several testing length-scales and deformation rates highlights a relatively unexplored avenue to manage an “on-demand” mechanical response.