Effect of conversion on epoxy resin properties: Combined molecular dynamics simulation and experimental study

We investigated epoxy resin consisting of diglycidyl ether of bisphenol A (DGEBA) and bis-(p-aminocyclohexyl) methane (PACM) and found that the density increased and decreased in the low- and high-conversion regions, respectively, by using experiments and all-atom (AA) molecular dynamics (MD) simulations. To understand this feature qualitatively, we conducted course-grained (CG) MD simulations. For the flexible and rigid CG models, the calculated density increased and decreased monotonically, respectively, in contrast to the experimental density. To develop a more realistic CG model, which is denoted as CG-EP, we derived angular parameters based on AA-MD simulations. It was found that the CG-EP successfully reproduced the trend of the experimental density, suggesting the importance of molecular flexibility. In addition, the progress of the conversion monotonically increased the free volume hole size, which is consistent with the result of positron annihilation lifetime spectroscopy. Furthermore, we experimentally observed that the Young's modulus suddenly decreased at 50%, as the conversion progressed. The CG analysis indicated that this trend was also attributed to the molecular flexibility.