| Abstrakt: |
A high-Tg epoxy−amine system based on diglycidyl ether of bisphenol A (DGEBA) and 4,4-methylenebis[3-chloro-2,6-diethylaniline] (MCDEA) was studied near the gel point in isothermal conditions from 80 to 180 °C by means of a rheological method, thus, below and above the glass transition temperature of the fully cured network, Tg∞ (Tg∞ = 177 °C). The lower limit of this temperature range is close to gelTg (gelTg = 50 °C), the temperature at which gelation and vitrification occur simultaneously. Close to the gel point, the power laws relating viscosity, η, to ε-k and the storage shear modulus, G, to εz (ε = |x − xgel|/xgel) are verified above 150 °C. The scaling law ∂(log G*)/∂t proportional to ω-κ is verified only at 170 °C (κ = 0.25) and 180 °C (κ = 0.18). The exponents k and Δ are constant above 150 °C (k = 1.43 ± 0.03, Δ = 0.69 ± 0.01) and are very close to those found in the Rouse percolation model. Below 150 °C, these exponents diminish as the curing temperature decreases. The exponent z is frequency dependent at a given temperature, and its value z0 for ω = 1 rad/s decreases with temperature. z0 and κ are found to be more sensitive to the vitrification phenomenon than the parameters k and Δ. At 180 °C, thus above Tg∞, the values of exponents k, z0, Δ, and κ are in good agreement with those derived from the percolation theory with macromolecular chains obeying the Rouse model. Below 150 °C, this behavior is no longer observed. These results are compared to those obtained for a low-Tg epoxy−amine system for which only the gelation phenomenon occurs. |
