Longitudinal structural relaxation of glass-forming epoxy oligomer

The glass transition process of a common epoxy oligomer (diglycidyl ether of bisphenol A) has been characterized by Brillouin scattering (GHz range) and ultrasonic pulse spectroscopy (MHz range). As the temperature decreases, longitudinal wave velocity rapidly increased. It showed a large frequency dispersion at temperatures higher than the thermodynamical glass transition temperature (Tg). At Tg, velocity changes showed a clear kink. Wave attenuation also showed a peak of the main (alfa) relaxation at temperatures higher than Tg. The peak temperature shifted to lower temperatures in the MHz range, following the VFT type relaxation behavior, which is very common in the main relaxation. The velocities, however, still showed a clear frequency dispersion at temperatures lower than Tg. Because the main relaxation usually results from a large-scale conformational rearrangement of the polymer chain backbone, the effect of this relaxation seems to be neglected at these temperatures. Taking into account the frequency changes of attenuation peak widths, the effect of other subrelaxations at lower temperatures is discussed. These results are not in contradiction with the acoust-optic dispersion measurements of this oligomer [Kruger et al., Phase Transition (to be published)].