Does fragility depend on pressure? A dynamic light scattering study of a fragile glass-former

Abstract

Relaxation times of the α-process in the fragile glass-forming liquid diglycidyl ether of bisphenol-A (EPON 828) were measured in a broad pressure (1–1500 bar) and temperature (264–293 K) ranges by means of the depolarized dynamic light scattering—photon correlation spectroscopy. Based on this experimental data the fragility of the supercooled liquid was calculated in two ways: as a steepness index m of the “Angell plot” and as the DT-parameter from the Vogel–Fulcher–Tammann Law, and was studied as a function of pressure. It was found, that while the steepness index depends on pressure, the DT parameter is pressure independent. The pressure dependence of the glass transition temperature Tg in EPON 828 was found to be nonlinear. Additionally, we established a relationship between the steepness index mT, the activation volume ΔV#, and the coefficient ∂Tg/∂Pg. In this pressure dependent study we found that also for EPON 828 the nonexponentiality of the correlation function of the α-process correlates well with the non-Arrhenius behavior (steepness index) of the relaxation times. An equation of state describing the temperature and pressure dependence of the structural relaxation time was proposed and verified using experimental data.