Influence of the glass transition on the secondary relaxation of an epoxy resin

The dynamics of diglycidyl ether of bisphenol-A (DGEBA) was studied by employing broadband dielectric spectroscopy over a wide temperature range extending from below to above the glass-transition temperature. Dielectric spectra reveal the existence of two relaxation processes: the structural relaxation, slowing down for decreasing temperature and freezing at T-g, and a secondary process present in both liquid and glassy phase. Above the glass transition the temperature behavior of the structural relaxation time is properly described by a Vogel-Fulcher-Tamman equation with the singularity at T-o=234 K. The analysis of the variation of the relaxation strength versus temperature of both processes clearly shows the onset of the structural relaxation at T(on)approximate to 350 K. Below this temperature the two relaxations progressively separate their time scales and change their shapes. The occurrence of the glass-transition phenomenon is markedly evidenced by the changes of the relaxation strength and of the low-frequency slope of the secondary relaxation.