The inflection point in the pressure dependence of viscosity under high pressure: A comprehensive study of the temperature and pressure dependence of the viscosity of propylene carbonate

The pressure dependence of the prototypical glass-former propylene carbonate has been investigated over a broad range of temperature and pressure that were inaccessible in previous investigations using dielectric spectroscopy. We find that the viscosity measurements validate the scaling relation, $η(T,V)=I(TVγ)$⁠, with a scaling parameter $γ$ close to that found from dielectric relaxation measurements. In the pressure dependence of the viscosity, we observe an inflection point in the $log(η)$ versus $P$ response, similar to that found previously for other materials. However, this inflection has never been observed in dielectric relaxation measurements. Using the scaling property above, it is possible to determine the behavior of the dielectric relaxation time in this otherwise inaccessible experimental range and compare it with the viscosity measurements. We find that the behaviors of $η$ and $τ$ are very similar, and a very good agreement between the function $ϕP$ calculated for these two quantities is found. Starting from the validity of the scaling properties, we show that the inflection point in the pressure dependence of the viscosity can be attributed to the convolution of the pressure dependences of the compressibility $κT$ and the apparent activation energy at constant volume $EV$⁠.