TEMPERATURE-DEPENDENCE OF THE BEHAVIOR OF AN EPOXY-AMINE SYSTEM NEAR THE GEL POINT THROUGH VISCOELASTIC STUDY .1. LOW-T-G EPOXY-AMINE SYSTEM

A flexible epoxy-amine system based on the diglycidyl ether of 1,4-but anediol (DGEBD) and 4,9-dioxa-1,12-dodecanediamine (4D) was studied be tween 40 and 70 degrees C by theological and viscosimetric methods nea r the gel point. This temperature domain is located well above the max imum glass transition temperature of the fully cured network, for whic h T-g infinity = -12 degrees C. At the gel point, the theoretical exte nt of reaction, x(gel), is equal to 0.5745, considering the reactivity ratio, n, of the secondary amines to the primary ones equal to 1.1 (e quireactivity, n = 1). For the times corresponding to x(gel), the theo logical curves follow a classical behavior, i.e., (i) divergence of th e viscosity in steady flow conditions, (ii) crossover of the tan delta curves measured as a function of time at several frequencies, and (ii i) proportionality between G' and G '' and the pulsation omega(Delta)( G ''(omega) and G ''(omega) are proportional to omega(Delta)). Above 5 0 degrees C, the exponent, Delta, is constant and equal to the average value of 0.70 +/- 0.02. The width of the relaxation time spectrum is evaluated by studying the fully cured network. The highest value of De lta observed at low temperatures (40 degrees C) can be explained assum ing that, in such a case, the longer relaxation times become similar t o the observation times. Close to the gel point, the power laws eta pr oportional to to epsilon(-k) and G' proportional to epsilon(z) (where epsilon = \x - x(gel)\/x(gel)), which govern the viscosity, eta, and t he elastic modulus, G', are verified within a large domain. The expone nt k is constant with the temperature and is equal to 1.44 +/- 0.03. T he log G '' vs f(log epsilon) curves display two linear domains, at le ast for low temperatures and high frequencies. In the second domain, t he exponent z varies with frequency, but above 50 degrees C, its value of omega = 1 rad/s remains constant with the temperature (z(0) = 2.65 +/- 0.02). The values of exponents k, to z(0) and Delta are in good a greement with those obtained from the percolation theory with molecula r chains obeying the Rouse model.