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 sy stems prepared from poly(oxypropylene)diamine (Jeff D-400) or -triamine (Je ff T-403) and diglycidyl ether of Bisphenol A (DGEBA) with initial ratios o f 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 evol ution of the dynamic behaviour of two stoichiometric samples with r(H) = 1 with reaction time, t(r), was also studied. The critical ratios for gelatio n, r(H)(c), of fully cured samples were determined from extraction experime nts (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) str uctures 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 critic al exponent) with the loss tangent, tan delta = G"/G' independent of freque ncy, 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 c ritical power-law behaviour is not exactly obeyed. More complex CG structur e in these samples, formed due to differences in the structure growth proce ss, was suggested to account for violation of the power-law behaviour.