Reactive Molecular Dynamics Study of Hygrothermal Degradation of Crosslinked Epoxy Polymers

Epoxy thermosets are often exposed to high humidity environments in various applications, undergoing reversible and irreversible degradation depending on the environment. This study presents a reactive molecular dynamics (MD) simulation framework to gain deeper insights into the hygrothermal aging process, which is essential to develop a targeted approach to combat water-assisted degradation in epoxy thermosets. By applying ReaxFF potential, an epoxy–amine network is created at low temperatures to avoid unwanted high-temperature side reactions, where the water molecules are added to achieve the desired degree of moisture contamination. The simulations show that in addition to the plasticization effect from the moisture ingress, the epoxy network shows recovery in mechanical properties and density due to the multi-site interaction of the water molecule with the electronegative sites within the network. Moreover, long-term exposure to humidity or direct exposure due to cracking can induce irreversible changes in the epoxy–amine network. The protonation of the water molecule and nucleophilic attack on the C–O bond of the ether linkages in the epoxy–amine networks are successfully simulated by applying reactive MD simulations. Remarkably, the simulations show that the selectivity of water molecules for the hydrolysis reaction in the epoxy network depends on the spatial arrangement and the steric hindrance of the network. This work provides molecular level insights into hygrothermal aging by elucidating the interplay between free volume and polarity of the network in the physical aging of the moist epoxy networks, paving a way for advanced design strategy toward better durability and performance of epoxy thermosets in humid environments.