Temperature and volume effect on the molecular dynamics of supercooled ibuprofen at ambient and elevated pressure

In this paper we have studied the relative importance of thermal effects and volume in controlling structural relaxation of supercooled ibuprofen at ambient and elevated pressure. The relative contribution of both parameters on the structural relaxation times was estimated by means of the ratio E(V)/E(P) (i.e., the activation energy at constant volume to enthalpy of activation at constant pressure), which can be simply estimated from dielectric relaxation and pressure-volume-temperature (PVT) measurements. We found out that at ambient pressure the effect of thermal energy and molecular packing on structural relaxation dynamics is practically equaled. However, with increasing pressure the role of thermal effects in governing molecular dynamics becomes more prominent, leading to its complete domination in the pressure region of around 1 GPa. These results are discussed in the context of remarkably different behavior of supercooled ibuprofen crystallized at various thermodynamic conditions, as reported in our previous paper. The implication is that, when molecular mobility of supercooled ibuprofen is governed primarily by the thermal energy, significant slowing down of crystallization progress at isostructural relaxation conditions is observed.