An atomistic simulation study investigating the effect of varying network structure and polarity in a moisture contaminated epoxy network

Absorbed moisture is a frequent contributor to performance loss in polymer-based composite structures operating in nearly all environments. However, the fundamental mechanisms that govern water-polymer interaction remain poorly understood. In this molecular dynamics study, the polarity and internal structure of an epoxy-based composite matrix was varied through manipulation of crosslink density. A commonly used epoxy-hardener combination (DGEBA- DETA) was chosen and four different models were created with crosslinking density of 20%, 51%, 65% and 81% respectively. The results indicate that the increase in crosslinking leads to a greater availability of polar sites with a concomitant rise in available free volume. The rise in network polarity aids the hydrogen bonding interactions between the absorbed water and the composite matrix, but the greater availability of free volume also allows water molecules to cluster together through mutual hydrogen bonding activity. This results in a subsequent decrease in moisture interaction with polar sites at very high crosslink densities. It was also found that the diffusivity and average dipole moment of the absorbed moisture are correlated with the state of water molecules and that a greater percentage of network-bonded molecules tends to lower both of these quantities. These results are consistent with previously published experimental results which have contemplated the dual nature of water molecules in an epoxy network. The results also highlight the potential of leveraging this phenomenon for non-destructive inspection of the physical and chemical state of a polymer network.