Influences of the molecular structures of curing agents on the inelastic-deformation mechanisms in highly-crosslinked epoxy polymers

The nature of the inelastic-deformation characteristics of highly-crosslinked epoxy polymers has been understood at the microscopic level and in consideration of the structural network-topology differences. The structural differences that arise from different types of curing agents (aliphatic and aromatic) have been estimated using the compressive loadingeunloading responses in terms of the energy, stress, and geometric characteristics. The energy and stress distributions at 300 K revealed that the nonbonded interactions of the polymer chains and the local dihedral-angle behaviors are key internal-potential components that accommodate the applied levels of the deformation energy and stress. In particular, a residual dihedral-angle stress was observed in the monomers of aromatic curing agents after the unloading, while the aliphatic-cured system displayed a spring-like elastic response. The plastic response of the aromatic-cured epoxy is attributed to the plastic folding of a local dihedral angle that is owing to the mobility discrepancy of a benzene ring and the flexible chain segments that are linked to the benzene ring. From the energy perspective, plastic dihedral-angle transitions were observed in the 1-K deformation simulations. The plastic-folding behaviors of the dihedral angles are evident near the yield point, which is coincident with the molecular-kink behaviors of the classical yielding theory. (C) 2017 Published by Elsevier Ltd.