Hyperbranched Polymer Toughened and Reinforced Self-healing Epoxy Vitrimer

Unlike conventional thermoset epoxy resins, epoxy vitrimers with excellent malleability can be recycled, remolded and reshaped. However, most epoxy vitrimers usually shows high fragility and low mechanical properties, which significantly limits their practical applications. To address this issue, we used a carboxyl terminated hyperbranched polymer, Hyper C102, to simultaneously toughen and reinforce a class of vitrimers based on glutaric acid crosslinked bisphenol F epoxy resin (BPF), in which 1-methylimidazole was used as catalyst to endow the system with dynamic exchange properties. Fourier transform (FTIR) and swelling experiments confirmed the formation of covalent crosslinking network in the epoxy vitrimers. DSC and DMA were used to study the dynamic mechanical properties and the rate of transesterification reaction of the materials. The result shows that the crosslink density of the epoxy vitrimers decreases first and then increases with the increasing content of Hyper C102. Such phenomenon can be well explained by the cavitation theory. More intriguingly, the Hyper C102 modified epoxy vitrimers still show high efficiency of transesterification reaction at 180 degrees C. Their modulus can relax to 1/e of the initial modulus within 30 min, and to 10% of the initial modulus within 1 h. Meanwhile, the tensile strength and strain at break can be simultaneously improved upon the introduction of Hyper C102. Compared with Hyper0 which contains no hyperbranched polymer, the tensile strength and fracture energy of Hyper7.5 that contains 7.5 wt% Hyper C102 is improved by 136% (from 28 MPa to 66 MPa) and 504% (from 280 kJ/m(3) to 1410 kJ/m(3)), respectively. Such significant and simultaneous improvement in both tensile strength and toughness has not been realized in previous studies. Moreover, the epoxy vitrimers manifest decent self-repairing and recyclable properties after mechanical damage. These results fully demonstrate that the addition of the carboxyl terminated hyperbranched polymer can not only maintain the dynamic transesterification, but also significantly improve the mechanical properties of epoxy vitrimers.