Epoxy Homopolymerization as a Tool to Tune the Thermo-Mechanical Properties and Fracture Toughness of Vitrimers

Epoxy/dicarboxylic acid vitrimer was prepared by the solvent-free reaction of diglycidyl ether of bisphenol A (DGEBA) and 1,4-cyclohexane dicarboxylic acid (CHDA) with the addition of monobutyltin oxide (Sn) as a catalyst. By tailoring the catalyst content (≥5 mol %), an effective conversion of functional groups during cure demonstrated the network polymerization mechanisms and a sequence of the side reactions. Indeed, the manufactured vitrimers exhibit creep and full stress relaxation thanks to catalytic transesterifications. By changing the epoxy/diacid ratio, the thermo-mechanical properties and mechanical behavior of the epoxy/acid vitrimers can be tuned while keeping self-healing ability. At high epoxy excess, both glass-transition temperature (T_g) and solid–liquid viscoelastic transition temperature (T_v) shift to a higher temperature. At vitrimer formulations 1:0.6 and 1:0.5 (epoxy/acyl), a remarkable improvement of fracture toughness (K_Ic) is observed, indicating the transition from stiff to relatively ductile materials at 1:0.6. This is attributed to the altered network structures due to etherification and epoxy homopolymerization. The rough fracture surface suggests more energy dissipation during crack propagation in vitrimer with a high excess epoxy. After healing, welded vitrimers still exhibit good fracture toughness with only a slight reduction (<10%) in K_Ic. We believe that these vitrimer formulations are promising as matrices in the composite fields.