Temperature dependence of the behavior of a reactive epoxy-amine system by means of dynamic rheology .2. High-T-g epoxy-amine system

A high-T-g 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 degrees C by means of a rheological method, thus, below and above the glass transition temperature of the fully cured network, T-g infinity (T-g infinity = 177 degrees C). The lower limit of this temperature range is close to T-gel(g) (T-gel(g) = 50 degrees C), the temperature at which gelation and vitrification occur simultaneously. Close to the gel point, the power laws relating viscosity, eta, to epsilon(-k) and the storage shear modulus, G', to epsilon(z) (epsilon = \x - x(gel)\/x(gel)) are verified above 150 degrees C. The scaling law partial derivative(log G*)/partial derivative t proportional to omega(-k) is verified only at 170 degrees C (kappa = 0.25) and 180 degrees C (kappa = 0.18). The exponents k and Delta are constant above 150 degrees C (k = 1.43 +/- 0.03, Delta = 0.69 +/- 0.01) and are very close to those found in the Rouse percolation model. Below 150 degrees C, these exponents diminish as the curing temperature decreases. The exponent z is frequency dependent at a given temperature, and its value z(0) for omega = 1 rad/s decreases with temperature. z(0) and k are found to be more sensitive to the vitrification phenomenon than the parameters k and Delta. At 180 degrees C, thus above T-g infinity, the values of exponents k, z(0), Delta, and kappa are in good agreement with those derived from the percolation theory with macromolecular chains obeying the Rouse model. Below 150 degrees C, this behavior is no longer observed. These results are compared to those obtained for a low-T-g epoxy-amine system for which only the gelation phenomenon occurs.