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

Abstract

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.

Figure 1

Semilog plot of the Brillouin spectra recorded at the initial (t = 0) and the final (t → ∞) stage of polymerization reaction for the sample with x_A = 0.33. Solid lines are the results of fitting with eq 2.

Figure 2

Concentration (amine molar fraction) dependence of the product of hypersound velocity and refractive index, nv_B, registered at the initial (full symbols) and the final (open symbols) stages of polymerization. x_A,1 and x_A,2 are two critical gelation compositions. x_A,Stoichio corresponds to the stoichiometric composition. Lines are guides for the eye. Schematic representation of molecular structures is shown in insets. At the beginning of the reaction, the system is a homogeneous mixture. Formation of cross-links translates to the nontrivial mechanical behavior.

Figure 3

Concentration (amine molar fraction) dependence of hypersound attenuation, αλ_B, registered at the initial (full symbols) and the final (open symbols) stages of polymerization. Lines are guides for the eye. Frequency dependencies of sound attenuation, αλ_B(ω), for selected compositions are schematically illustrated within insets (scales are defined in the uppermost inset). The position of the curve, with respect to BLS frequency, ω_B (and so the value of αλ_B measured), depends on the actual dynamics (relaxation time, τ) of the system. It should be noted that ω_B is not strictly constant but also depends on composition and bond conversion.

Figure 4

Hypersonic velocity (nv_B—upper panel) and its time derivative (dnv_B/dt) as a function of polymerization reaction time (log scale) for two mixtures being within or outside of x_A,1 < x_A ≤ x_A,2 gelation range (x_A = 0.50 and x_A = 0.83, respectively). Notice how the reaction stops drastically for the x_A = 0.5 system.