Temperature and pressure dependences of the relaxation dynamics of supercooled systems explored by dielectric spectroscopy

A wide-band (10(2)-2 x 10(10) Hz) dielectric study of epoxy compounds was carried out under isobaric conditions (atmospheric pressure) by changing the temperature down to the supercooled and glassy phases: One of these systems (diglycidyl ether of bisphenol A (DGEBA)) was also measured under isothermal conditions at 293 K by changing the pressure from 0.1 up to 235 MPa. The analysis of variable-temperature measurements gave evidence of a connection between the alpha, beta-splitting phenomenon, the breakdown of the Debye-Stokes-Einstein (DSE) relation which turns into a fractional DSE law, and the transition of the alpha-relaxation dynamics between two different temperature regimes. The variable-pressure measurements revealed that the pressure dependence of the alpha-relaxation time in DGEBA is better described by a second order polynomial function rather than a Vogel-Fulcher-like function. The perfect scaling observed between couples of isobaric and isothermal spectra with the same value of the alpha-relaxation time suggests that the dielectric response is controlled in the same way by both temperature and pressure. From the comparison of the density dependence of isobaric and isothermal relaxation times it turns out that the glass transition is controlled not by only the volume but by both the temperature and the volume. The relative influences of these two variables on the relaxation dynamics of DGEBA at 293 K and atmospheric pressure were also evaluated.