Excitation spectroscopy of a reactive label for characterization of the cure process in an epoxy network

Fluorescence excitation spectra of a reactive label, p,p'-diaminoazobenzene (DAA), were investigated to characterize the cure process in an epoxy network composed of a stoichiometric mixture of diglycidyl ether of bisphenol A and diaminodiphenyl sulfone. Under ideal conditions, the corrected excitation spectra should resemble UV-visible absorption spectra with an intensity proportional to the fluorescence quantum yield. Therefore, corrected excitation spectra of the model cure products as well as a function of various composition of cure products were simulated. Due to the sharp increase in the fluorescence quantum yields of the later cure products, such simulated excitation spectra are well separated from each other. Experimentally obtained excitation spectra of the model cure products and of the curing epoxy showed some distortions in the spectra that are probably due to instrumental factors. Nevertheless, the excitation maximum was observed near 470 nm, corresponding to the absorption maximum of the tertiary--tertiary amine product of DAA. The intensity of the experimental excitation spectra after calibration with UV-visible absorption spectra was found to show sharp increase as a function of cure time, followed by leveling off after vitrification of the epoxy network. This profile was very similar to that of the intensity of fluorescence emission reported previously. Under the assumptions that the reactions between the diamines and the epoxide are the main cure reactions and the primary amine and the secondary amine react at similar rates, the cure product composition has been estimated, on the basis of the excitation intensity. The results show a reasonable trend of the formation and the disappearance of the cure species. Therefore, it is demonstrated that excitation spectra can be used for cure characterization of epoxy network. , . 9 ref.--AA