Recyclable Epoxy Vitrimer with High Strength and Rapid Stress Relaxation under Mild Temperatures

Epoxy (EP) vitrimers with excellent mechanical properties that could be efficiently healed under mild temperatures (<150 °C) are of great importance to practical applications but are difficult to accomplish yet up to now. In this work, we used l-cystine dimethyl ester (CDE), a disulfide-containing diamine, as the hardener to synthesize a type of EP vitrimer, EPV-CDE. The tensile strength of EPV-CDE reached 81.5 MPa mainly due to the relatively short chains of the CDE hardener and resulting high cross-linking density, and at the same time, EPV-CDE was able to be recycled under a mild temperature at 120 °C, with a 93.4% recovery ratio after the first recycling cycle, enabled by the relatively flexible chains and low steric hindrance of the CDE hardener. Compared to most of disulfide-containing EP vitrimers reported in the literature, EPV-CDE demonstrated higher tensile strength and lower activation energy and topology freezing transition temperature. Moreover, the liquid nature of CDE allows the incorporation of up to 25 wt % carbon fiber powder into the EPV-CDE matrix to prepare the EPV-CDE/CF composite, which achieved the tensile strength of 112.5 MPa and maintained excellent recyclability. Even though our EP vitrimer exhibited strong resistance performance to most solvents, it could also be chemically degraded by thiol-containing solvents such as dithiothreitol, offering environmental-friendly substitutes for unsustainable thermoset resins.