Dynamics of a glass-forming triepoxide studied by dielectric spectroscopy

Dielectric measurements of an epoxy resin, N, N-diglycidyl-4-glycidyloxyaniline, have been carried out in the supercooled and glassy phase over a broad frequency range (10(2)-6x10(9) Hz). The measurements reveal electrical transport due to ionic impurities as well as three different dipolar relaxations-in addition to the alpha- and beta-relaxation, a slower alpha'-relaxation is recognized, whose loss peak is disclosed after subtraction of the de conductivity contribution. The glass transition is found to affect markedly the secondary relaxation, whose strength and shape parameters change across Tg The major inference from the results concerns the existence of a transition in the dynamics, occurring some tens of degrees above Tg, in the vicinity of the temperature Ts where the peaks of the alpha- and beta-relaxations merge. Evidence in favour of such a transition is given by: (i) the change in the temperature dependence of the a-relaxation time; (ii) independently, the change in the temperature dependence of the de conductivity; (iii) the breakdown of the Debye-Stokes-Einsten model, replaced at lower temperatures by a fractional regime. Concerning the alpha'-process, it shows a Vogel-Fulcher behaviour with the same temperature To as the alpha-relaxation but, unlike this last, it is not involved in a splitting phenomenon with the beta-relaxation. Several hypotheses concerning the nature of the alpha'-process are explored.