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 ωΔ (G‘(ω) and
G‘‘(ω) are proportional to ωΔ).
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.
