Network structure in diamine-cured tetrafunctional epoxy by UV-visible and fluorescence spectroscopy

Network structure in a nearly stoichiometric mixture of diamine (diaminodiphenyl sulfone, DDS)-cured tetrafunctional epoxy (tetraglycidyl diaminodiphenylmethane, TGDDM) was investigated by p,p-diaminoazobenzene (DAA), which mimics the reactivities of the diamine curing agent by UV-visible, fluorescence, and IR spectroscopic techniques. UV-visible spectra show red shifts due to the reaction of DAA with epoxide to produce tertiary amines. The deconvolution of UV-visible spectra provides the cure composition including the amount of the tertiary amine species of DAA, which is at least equal to that in a stoichiometric mixture of diepoxide-diamine epoxy. Fluorescence intensity, which is mostly due to the tertiary amine, gives more evidence for the substantial amount of the tertiary amine in diamine-cured tetrafunctional epoxy. Even though a part of the tertiary amines can be a cyclic ring product, a significant amount is expected to contribute to the network structure either as a cross-link or a branch point. The fluorescence emission maxima of DDS curing agent also shows red shifts, indicating formation of tertiary amine species. IR spectra confirm the overall reaction of epoxide with diamine but indicate that ether formation becomes significant only late in the cure. Before ether formation becomes important, cure composition data are analyzed on the basis of independent epoxide reactivity in TGDDM, and a similar reactivity is found between the primary amine and the secondary amine with epoxide. The extent of DAA reaction is found to be fairly close to the extent of epoxide reaction. Graphs, Spectra. 20 ref.--AA