For glycerol, the trends towards increased fragility at elevated pressure and temperature are consistent with diminished hydrogen bonding under those conditions. Glycerol, however, becomes more fragile over the same temperature range. Using this, DBP shows a trend common to several liquids, a decrease in fragility with increasing temperature. For the isothermal model, we derive a new measure of fragility. This is dramatic for DBP, which goes from a strong to an intermediate‐strength liquid. Under isochoric conditions, the fragility for both glycerol and DBP increases with increasing density. For glycerol and (less conclusively) DBP under isobaric conditions, the fragility increases markedly at high pressure. Fragility parameters are evaluated for these three isometric conditions. These data provide an assessment of the T‐dependence of an isothermal model (free volume), the P‐dependence of an isobaric model (Vogel–Tammann–Fulcher) and by extension that for isochoric conditions. The T‐dependence of viscosity 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. This level of precision allows us to define a viscosity surface which can then be extrapolated to the glass transition along both temperature and pressure cuts. The overall precision of the data are approximately 10% or better throughout. The majority of the results extend up to viscosities of 10 7 cP, with those at 22.5 ☌ going to 10 10 cP. These studies were made using a combination of a rolling‐ball and a centrifugal‐force diamond anvil cell viscometer. The pressure dependence of glass transition temperature and fragility index of glycerol were calculated from model equations being the results in good agreement with data selected from the literature.The pressure and temperature dependent viscosities of two glass forming liquids, glycerol and dibutyl phthalate (DBP), have been studied in the range P=0–3 GPa, T=0–125 ☌, and η=10 1–10 10 cP.
#VISCOSITY OF GLYCEROL FREE#
The free volume proved to be more accurate in the calculation of viscosity in wide T and p ranges with average absolute percentual deviations ( AADs) ranging from 4% to 14% for data from different authors. High-pressure viscosity data allowed studying the temperature, pressure and density dependences of this property using the free volume theory and the thermodynamic scaling of viscosity. Stickel derivatives were calculated for the correlation equations and they were compared with values found from viscosity data using numerical techniques. Some of the tested equations give overall absolute deviations less than 6% in the range (190–440) K, a value which is close the experimental uncertainty. From BSCNF the structural effects taking place near the glass transition were discussed in light of results obtained by recent experimental techniques. The physically sound equations of Mauro (MYEGA) and the Bond Strength-Coordination Number Fluctuation (BSCNF) model were used to correlate values selected from the database. The main purpose of database construction was the development and evaluation of reliable correlation models of viscosity valid in wide ranges of temperature and pressure. An extensive viscosity database for this substance was developed combining the values of this work with those published in literature covering a wide range of temperatures at atmospheric pressure from the calculated glass transition temperature ( T g = 188 K), and measurements of viscosity reported over the temperature range (263–398) K at pressures from (10 −4 to 3) GPa. The presented results are in good agreement with most values from the literature. The combined expanded uncertainty of reported viscosity is better than 3.0% with a level of confidence 0.95 (k = 2). The dynamic viscosities of glycerol were measured over the temperature range (293–394) K and atmospheric pressure using a Brookfield thermosel system.
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