Effect of Variations of Amine Content and Network Branching on Thermomechanical Properties of Epoxy Systems

In thermosetting epoxies, thermomechanical properties
can be enhanced
by conscious selection of curing agents. Full cross-linking leads
to a maximum in the glass- transition temperature. However, the relation
between the glass transition temperature and the epoxy matrix depends
on several factors beyond the cross-linking degree, such as the molecular
weight of the polymers, network organization, amount of branching,
and the presence of hydrogen bonds. In this study, we investigated
adding non- stoichiometric ratios of the epoxy resin Epikote 828 and
the curing agent Jeffamine D230. The investigations were done through
a combination of molecular dynamics simulations and experiments, primarily
differential scanning calorimetry and nanoindentation. Reorganization
of the network to fewer clusters with a higher degree of linearity
overcomes the effect of cross-linking and leads to a reduction of
glass-transition temperature with increasing concentrations of curing
agent to epoxy. The elastic, shear, and compressive moduli remained
constant. Hence, moderating the curing agent con- tent has the potential
to improve thermal properties while maintaining mechanical properties
for this epoxy system.