Dynamics of a glass-forming triepoxide studied by dielectric spectroscopy

Dielectric measurements of an epoxy resin, N, N-diglycidyl-4-glycidyloxyani line, have been carried out in the supercooled and glassy phase over a broa d frequency range (10(2)-6x10(9) Hz). The measurements reveal electrical tr ansport due to ionic impurities as well as three different dipolar relaxati ons-in addition to the alpha- and beta-relaxation, a slower alpha'-relaxati on 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 acros s Tg The major inference from the results concerns the existence of a trans ition in the dynamics, occurring some tens of degrees above Tg, in the vici nity of the temperature Ts where the peaks of the alpha- and beta-relaxatio ns merge. Evidence in favour of such a transition is given by: (i) the chan ge in the temperature dependence of the a-relaxation time; (ii) independent ly, the change in the temperature dependence of the de conductivity; (iii) the breakdown of the Debye-Stokes-Einsten model, replaced at lower temperat ures 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 no t involved in a splitting phenomenon with the beta-relaxation. Several hypo theses concerning the nature of the alpha'-process are explored.