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

Relaxation times of the alpha -process in the fragile glass-forming liquid diglycidyl ether of bisphenol-A (EPON 828) were measured in a broad pressur e (1-1500 bar) and temperature (264-293 K) ranges by means of the depolariz ed 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 D-T-param eter from the Vogel-Fulcher-Tammann Law, and was studied as a function of p ressure. It was found, that while the steepness index depends on pressure, the D-T parameter is pressure independent. The pressure dependence of the g lass transition temperature T-g in EPON 828 was found to be nonlinear. Addi tionally, we established a relationship between the steepness index m(T), t he activation volume DeltaV(#), and the coefficient partial derivativeT(g)/ partial derivativeP(g). In this pressure dependent study we found that also for EPON 828 the nonexponentiality of the correlation function of the alph a -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. (C) 2001 American Institute of Physics.