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-butanediol (DGEBD) and 4,9-dioxa-1,12-dodecanediamine (4D) was studied between 40 and 70 °C by rheological and viscosimetric methods near the gel point. This temperature domain is located well above the maximum glass transition temperature of the fully cured network, for which T g ∞ = −12 °C. At the gel point, the theoretical extent 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 (equireactivity, n = 1). For the times corresponding to x gel, the rheological curves follow a classical behavior, i.e., (i) divergence of the viscosity in steady flow conditions, (ii) crossover of the tan δ curves measured as a function of time at several frequencies, and (iii) proportionality between G ‘ and G ‘‘ and the pulsation ω Δ ( G ‘(ω) and G ‘‘(ω) are proportional to ω Δ ). Above 50 °C, the exponent, Δ, 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 Δ observed at low temperatures (40 °C) can be explained assuming that, in such a case, the longer relaxation times become similar to the observation times. Close to the gel point, the power laws η ∝ to ε - k and G ‘ ∝ ε z (where ε = | x − x gel |/ x gel ), which govern the viscosity, η, and the elastic modulus, G ‘, are verified within a large domain. The exponent k is constant with the temperature and is equal to 1.44 ± 0.03. The log G ‘ vs f (log ε) curves display two linear domains, at least for low temperatures and high frequencies. In the second domain, the exponent z varies with frequency, but above 50 °C, its value of ω = 1 rad/s remains constant with the temperature ( z 0 = 2.65 ± 0.02). The values of exponents k, z 0, and Δ are in good agreement with those obtained from the percolation theory with molecular chains obeying the Rouse model.