Violation of the power-law dynamic mechanical behaviour at the gel point threshold in non-stoichiometric epoxide systems

The dynamic mechanical behaviour of fully cured off-stoichiometric epoxy systems prepared from poly(oxypropylene)diamine (Jeff D-400) or -triamine (Jeff T-403) and diglycidyl ether of Bisphenol A (DGEBA) with initial ratios of reactive amine (NH2) and epoxide (E) groups, r(H) = (2[NH2])/[E] ranging from 2.4 to 4.3 was investigated in the gelation threshold region. The evolution of the dynamic behaviour of two stoichiometric samples with r(H) = 1 with reaction time, t(r), was also studied. The critical ratios for gelation, r(H)(c), of fully cured samples were determined from extraction experiments (r(H)(c) = 2.54 for Jeff D-400/DGEBA and r(H)(c) = 4.15 for Jeff T-403/DGEBA systems, respectively). For both stoichiometric critical gel (CG) structures obtained by changing the curing time t(r), a power-law rheological behaviour (G' similar to G" similar to omega(n), G' and G" are the storage and loss moduli, respectively, omega is angular frequency and n is a critical exponent) with the loss tangent, tan delta = G"/G' independent of frequency, was found. On the other hand, both CG off-stoichiometric systems with r(H)(c) ratios show a small dependence of tan delta on omega, so that the critical power-law behaviour is not exactly obeyed. More complex CG structure in these samples, formed due to differences in the structure growth process, was suggested to account for violation of the power-law behaviour.