Synthesis and structure-property relationship of epoxy vitrimers containing different acetal structures

In this work, three amine compounds containing different acetal dynamic covalent bonds (formal, acetal and ketal bonds) were synthesized and used to cure epoxy resins. The three acetal-containing amines (ACAs) show comparable curing reactivity toward epoxy resins. Owing to the dynamic features of the three acetal bonds, all the cured epoxy resins exhibit stress relaxation at elevated temperature and can be reprocessed by hot pressing. Interestingly, the three cured epoxy resins show significantly different structural dynamic behaviors owing to the different structures of incorporated acetal bonds. Among them, the epoxy resin containing the ketal bond exhibits the shortest stress relaxation time at the same test temperature and the highest retention rate of tensile per-formance after reprocessing. Theoretical calculations of C-O bond lengths and energies in three acetal bonds provide evidence about relationship between the dynamic behaviors of cured epoxy resins and acetal structures. The mechanical and thermal characteristics of all the original and reprocessed cured epoxy resins were inves-tigated and compared. Finally, the chemical resistance of cured epoxy resins prepared by three ACAs was evaluated in various acid solutions. We believe the findings in this study would contribute to the molecular design for recyclable thermosetting polymers based on acetal bonds.