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 c arried out under isobaric conditions (atmospheric pressure) by changing the temperature down to the supercooled and glassy phases: One of these system s (diglycidyl ether of bisphenol A (DGEBA)) was also measured under isother mal conditions at 293 K by changing the pressure from 0.1 up to 235 MPa. Th e analysis of variable-temperature measurements gave evidence of a connecti on 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 tempe rature regimes. The variable-pressure measurements revealed that the pressu re dependence of the alpha-relaxation time in DGEBA is better described by a second order polynomial function rather than a Vogel-Fulcher-like functio n. 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 p ressure. From the comparison of the density dependence of isobaric and isot hermal relaxation times it turns out that the glass transition is controlle d not by only the volume but by both the temperature and the volume. The re lative influences of these two variables on the relaxation dynamics of DGEB A at 293 K and atmospheric pressure were also evaluated.