Characterization and atomistic modeling of the effect of water absorption on the mechanical properties of thermoset polymers

Abstract

An extensive experimental program and molecular dynamics (MD) simulations have been conducted to ascertain the effect of water absorption upon the mechanical degradation of thermoset polymers. Mechanical tests were conducted to determine the degradation of the mechanical properties at regular testing intervals for up to 300 days of water immersion. The outcome of our studies reveals the following: (i) a decrease in the elastic modulus and the tensile strength as a result of the plasticization of the considered epoxy caused by water-induced swelling, and (ii) a reduction of approximately 50% in the fracture strain due to water ingression. SEM micrographs of the fracture surface revealed an abundance of aggregates, crazes, and microcracks as a result of aging in water. Meanwhile, AFM images further depicted step and stacked lamellar topological features of the fracture surface. In addition to the plasticization effect, there is a partial recovery of the elastic modulus at certain water absorption content. Such anti-plasticization effect of water molecules on the epoxy was demonstrated by both experiments and MD simulations. The elastic properties of the epoxy with and without different water contents were predicted by MD simulations, and the results corresponded with the experimental aging trend. The MD predictions further revealed that the water diffusion coefficient decreases with the increase in the epoxy density.