Toughening, Reprocessing, and Shape Recovery of Epoxy Networks Enabled with Nanostructures and Dynamic Boronic Ester Bonds

In this contribution, we reported a strategy that simultaneously improves fracture toughness and achieves reprocessing of epoxy networks. To create the nanostructures, the epoxy was incorporated with a polystyrene-block-poly(ethylene oxide) diblock copolymer. To achieve the reprocessing behavior, an aromatic amine bearing dynamic boronic ester bonds was used as the hardener, which was synthesized through the esterification of 3-aminobenzeneboronic acid with a tetraol; the latter was readily synthesized with the reaction of 1,4-butanediol diglycidyl ether with 2,2-bis(hydroxymethyl)butyric acid. The generation of nanostructures was evidenced by small-angle X-ray scattering atomic force microscopy and transmission electron microscopy. Thanks to the generation of nanostructures, the epoxy networks were significantly toughened; the critical stress intensity factor can be enhanced by 2.8 times as the control epoxy. Benefiting from the dynamic exchange of boronic ester bonds, the nanostructured epoxy networks displayed excellent reprocessing properties. Cured with this hardener, the epoxy networks were formulated to display the glass transition temperatures in the range of 80 ∼ 120 °C, allowing regulation of the shape memory properties under the mild condition. More importantly, the shape memory behavior featured reconfigurability through the dynamic exchange of boronic ester bonds. It was found that the generation of nanostructures significantly impacted the rates of shape recovery, depending on the temperature of shape recovery.