Understanding interaction and dynamics of water molecules in the epoxy via molecular dynamics simulation

The robust structural integrity of the epoxy plays an important role in ensuring the long-term service life of its applications, which is affected by the absorbed moisture. In order to understand the mechanism of the moisture effect, the knowledge of the interaction and dynamics of the water molecules inside the epoxy is of great interest. Molecular dynamics simulation is used in this work to investigate the structure and bonding behaviour of the water molecules in the highly cross-linked epoxy network. When the moisture concentration is low, the water molecules are well dispersed in the cross-linked structure and located in the vicinity of the epoxy functional groups, which predominantly form the hydrogen bond (H-bond) with the epoxy network, resulting in the low water mobility in the epoxy. At the high concentration, the water favourably forms the large cluster due to the predominant water–water H-bond interaction, and the water molecules diffuse primarily inside the cluster, which leads to the high water mobility and the accelerated H-bond dynamics. The variation of the bonding behaviour and dynamics of the water molecules reported here could be exploited to understand the material change and predict the long-term performance of the epoxy-based products during the intended service life.