Molecular dynamics simulation study of motions and interactions of water in a polymer network

Molecular dynamics simulation was conducted of the interactions and dynamics of water in a fully cured stoichiometric epoxy-amine network with moisture content ranging from 0 to 4.75 mol/L (0 to 8 wt %). Three types of hydrogen bonds (HBs) were identified in the dry network. Each of those three HBs involves hydrogen of the network hydroxyl group (HO) and one of the three acceptors: oxygen of the network hydroxyl group (OH), oxygen of the network ether group (OC), or nitrogen of the network tertiary amine (NC). The majority of the absorbed water molecules are located in the vicinity of the polar groups on the polymer network. The absorbed water breaks up some of the initially present HBs and forms new ones, both with the network groups and with other water molecules. The total number of HBs per one water molecule reaches 1.6 at 3.80 mol/L (6 wt %) moisture content, well below the value of 3.26 for bulk liquid water. The orientational motion of the absorbed water is anisotropic: the rotation of the vector mu, which lies in the direction of the dipole moment, is considerably slower (more than two decades) than the rotation of the vector that connects two hydrogen atoms and/or the vector perpendicular to the plane of the water molecule. The calculation of the time autocorrelation function for various HB pairs confirms that hydrogen bonding between the water oxygen and the hydrogen of the network hydroxyl group is at the heart of the observed slowdown in the dynamics.