Non-Stoichiometric Effects on Viscoelasticity in DGEBA-EDA Systems: Insights from Brillouin Light Scattering.

The initial and final stages of the isothermal curing reaction between diglycidyl ether of bisphenol A and ethylenediamine were investigated by using Brillouin light scattering spectroscopy. High-frequency acoustic parameters were measured for these stages as a function of the diamine molar fraction, ranging from the pure prepolymer to the pure cross-linker. Significant differences in the concentration dependencies of the mechanical parameters are interpreted in relation to changes in the system's viscoelastic properties. At the final stage of the reaction, the concentration dependencies exhibit notable changes at two characteristic nonstoichiometric compositions─one corresponding to amine excess and the other to epoxy excess. While these concentrations align well with Flory's critical gel points, experimental evidence suggests they should instead be interpreted as vitrification points.