Elucidating atomistic mechanisms underlying water diffusion in amorphous polymers: An autonomous basin climbing-based simulation method

Conventional molecular simulations of diffusive molecules in amorphous polymers lead to discrepancies in understanding diffusion mechanisms at low temperatures due to the short timescales of the simulations. This work proposes a combined scheme of the autonomous basin climbing (ABC) method accompanied with kinetic Monte Carlo and reactive molecular dynamics (MD) simulations to investigate water molecules' diffusion pathways in amorphous epoxy resins across a wide range of temperatures. The ABC method, along with a reactive force field, provides an accurate potential energy surface sampling approach to elucidate diffusion mechanisms affected by hydrogen bonding interactions between water molecules and an epoxy network. The simulation framework allows the capture of thermal effects on the diffusion coefficients and offers a predictive simulation tool for predicting diffusion coefficients of water molecules in amorphous polymers. The simulation results show that the activation energy of the diffusion coefficient is in agreement with experimental data. The proposed simulation framework not only estimates kinetic properties of water diffusion in epoxy resins that are consistent with experimental observations, but also provides a predictive tool for studying the diffusion of small molecules in other amorphous polymers.