Splitting Between Main and Secondary Relaxations in Mono-, Di-, and Tri-Epoxy Compounds

Wideband dielectric spectroscopy (10^2 - 2•10^10 Hz) was used to study the dynamics of mono-, di-, and tri-epoxide compounds from below to above the glass transition temperature, Tg. Dielectric spectra above Tg revealed the existence of two relaxations, a structural and a secondary process, which merge at the splitting temperature Ts, located some tenths of degrees above Tg. A d.c. conductivity contribution is also present. The glass transition phenomenon markedly affects the temperature dependence of both the dielectric strength and the low frequency slope of the secondary process. The prediction of the Stokes-Einstein-Debye (SED) model was verified for mono- and di-epoxide, while a fractional power law (FSED) replaces the SED relation in tri-epoxide for T<Ts. Moreover, a transition temperature TB~Ts between two different Vogel-Fulcher regimes was recognised in all systems. The overall picture of the dynamics of the systems is enriched and very recent ideas on the splitting between main and secondary relaxations are confirmed. Finally, the triepoxy compound shows an additional relaxation, which is masked by the conductivity contribution and it is slower than the structural one. Such relaxation seems to be related to the conductivity and it is found to follow the FSED law.