| The polymerization of the bifunctional epoxy resin, diglycidyl ether of bisphenol A (DGEBA) with 4,4'-diaminodiphenyl sulfone (DDS), a fluorescing curing agent, is investigated using dielectric spectroscopy, fluorescence spectroscopy, differential scanning calorimetry (DSC) and gel permeation chromatography (GPC). The isothermal reaction was studied at five different DGEBA: DDS ratios---2:0.5, 2:0.75, 2:1, 2:1.5 and 2:2. Dielectric properties were measured in the time domain, and characteristic loss peaks resulted for each of the ratios for a range of frequencies from 0.1 kHz--100 kHz. Fluorescence emission and excitation spectra were obtained from samples cured for different lengths of time. Spectral red shifts were observed for both solutions and the neat mixture. Among the different ratios, similar shifts (approximately 17 nm) were observed for the solutions, whereas for the neat 2:1 cure mixture, a spectral red shift of 24 nm was observed. DSC was used to determine the glass transition temperature (Tg) at different times during the reaction as well as the extent of reaction. It was found that for polymerization at different reactant ratios, the isothermal glass transition temperature increased almost linearly with time for most of the reaction. A correlation is presented between the Tg and dielectric loss peaks for three different DGEBA/DDS molar ratios. Molecular weights and molecular weight distributions were determined using GPC. Results indicated the transient presence of distinct low molecular weight oligomers during the reaction. The results justify the development of a unique real-time cure monitoring technique that utilizes simultaneous dielectric and fluorescence spectroscopy. This technique may be applied to the processing of nanocomposites. |
