PRESSURE AND TEMPERATURE-DEPENDENT VISCOSITY OF 2 GLASS-FORMING LIQUIDS - GLYCEROL AND DIBUTYL PHTHALATE

The pressure and temperature dependent viscosities of two glass formin g liquids, glycerol and dibutyl phthalate (DBP), have been studied in the range P=0-3 GPa, T=0-125-degrees-C, and eta=10(1)-10(10) cP. These studies were made using a combination of a Tolling-ball and a centrif ugal-force diamond anvil cell viscometer. The majority of the results extend up to viscosities of 10(7) cP, with those at 22.5-degrees-C goi ng to 10(10) cP. The overall precision of the data are approximately 1 0% or better throughout. This level of precision allows us to define a viscosity surface which can then be extrapolated to the glass transit ion along both temperature and pressure cuts. The T-dependence of visc osity is larger for glycerol than DBP but the P-dependence smaller for glycerol than for DBP, whereas the T-dependence is much more pressure sensitive for DBP. These data provide an assessment of the T-dependen ce of an isothermal model (free volume), the P-dependence of an isobar ic model (Vogel-Tammann-Fulcher) and by extension that for isochoric c onditions. Fragility parameters are evaluated for these three isometri c conditions. For glycerol and (less conclusively) DBP under isobaric conditions, the fragility increases markedly at high pressure. Under i sochoric conditions, the fragility for both glycerol and DBP increases with increasing density. This is dramatic for DBP, which goes from a strong to an intermediate-strength liquid. For the isothermal model, w e derive a new measure of fragility. Using this, DBP shows a trend com mon to several liquids, a decrease in fragility with increasing temper ature. Glycerol, however, becomes more fragile over the same temperatu re range. For glycerol, the trends towards increased fragility at elev ated pressure and temperature are consistent with diminished hydrogen bonding under those conditions. The P-dependence of the glass transiti on is also determined over a wide range of T. The slope, dT(g)/dP, is positive with the pressure dependence for glycerol being considerably smaller than for DBP; both are nonlinear, tending to saturate in tempe rature at high pressures.