A ReaxFF MD based effect investigation of diamino curing agents in the initial thermo-oxidative pyrolysis of epoxy resins

[Display omitted] •The oligomer-diamine-oligomer type cured epoxy chains with varied diamino curing agents were established.•The un-xlinked epoxy rings at the end of chains are most likely to be attacked in the high-temperature oxygen environment.•CH2O is the dominant product generated from the thermo-oxidative pyrolysis of epoxy resins.•In the initial stage of 1300 K pyrolysis, the crosslinked epoxy matrixes would be destroyed, with carbon backbones stable.•4,4-SDA curing agents encouraged the thermo-oxidation resistance by sacrificing themselves. To explore the influence of curing agents on the oxygen participated thermal decomposition mechanisms of the epoxy resin, the epoxy-O2 models, cured by 3,3-DDS, 3,3-SDA, 3,3-SSDDS, and 4,4-DDS, respectively, were first established. The ReaxFF-based MD simulation is applied to track the initial reactions of the pyrolysis. The results show that the O2 is much easier to transfer in the crosslinked epoxy matrixes at the spark temperature. The epoxy rings at the end of epoxy chains are likely to be attacked by the oxygen under high temperature, releasing the tens of CH2O, as the dominant product type. More than 65 wt% large-size molecules remained in the systems, indicating a nice thermal resistance. Based on the DFT calculation, the 4,4-DDS-type epoxy chain shows the highest reactivity with oxygen, which the lowest HOMO energy. 4,4-DDS is going to protect the carbon backbones of epoxy by scarifying, at the initial pyrolysis.