Molecular Dynamics (MD) Simulation of a Polymer Composite Matrix with Varying Degree of Moisture: Investigation of Secondary Bonding Interactions

摘要

Epoxy-based polymer composites are widely used in structural applications. Environmental degradation through moisture absorption considerably deteriorates the performance of these materials, particularly in the aviation and automotive industries. Absorbed moisture behaves differently in pristine and damaged composites. In a pristine structure with fewer diffusion pathways and voids, absorption is generally less and there is a higher degree of interaction between the resin and water molecules, which limits water mobility. Conversely, in a damaged structure, voids and micro-cracks allow accelerated diffusion and a greater proportion of “free” molecular water. The ratio of free to bound water states can be exploited to characterize damage via the dielectric properties. Structures with greater damage, and thus greater proportion of free water molecules, will exhibit a higher relative permittivity than a pristine structure, independent of water content. Without damage, water is more restricted from rotating under the influence of an applied electromagnetic field, leading to a lower relative permittivity. To simulate this phenomenon on a molecular scale, MD simulations are performed to characterize the moisture interaction in a polymer composite matrix system with the same epoxy (DGEBA) and hardener (DETA) constituents but with varying moisture concentrations. The system is constructed using the MAPS Amorphous Builder and the simulations were performed in LAMMPS using the AMBER-Cornell force field. Cross-linking is executed based on a reaction cut-off distance and the desired degree of cross-linking using the crosslinker module in MAPS. The simulation results are then analyzed and different properties like the diffusion coefficients and dipole moment fluctuations are evaluated to establish a primary understanding of how moisture interacts with the polymer matrix. To gain a deeper insight into the distinction between “free” and “bound” water, the hydrogen bonds between different polar species are counted at varying moisture concentrations to understand how secondary bonding mechanisms influences diffusion and dielectric properties.