Vitrification, nucleation and crystallization in phenyl-2-hydroxybenzoate (salol) studied by Differential Scanning Calorimetry (DSC) and Thermally Stimulated Depolarisation Currents (TSDC)

Abstract

The behaviour of phenyl-2-hydroxybenzoate (salol) with respect to crystallization and vitrification was studied by Thermally Stimulated Depolarisation Currents (TSDC) and Differential Scanning Calorimetry (DSC). It was found that this compound can be easily supercooled, and vitrified by further cooling. The degree of supercooling can be high, even for low cooling rates. The fragility index was found to be m ≅ 60 by DSC and by TSDC, a value that is lower than the values previously reported. Furthermore, glassy salol crystallises on heating (cold crystallisation). Two different polymorphs were detected, and the conditions of formation of each of them have been clearly established. Finally, it was shown that the β-relaxation of salol, that is difficult to observe by Dielectric Relaxation Spectroscopy, was very clearly detected by TSDC. This mobility, that is believed to assist the occurrence of the nucleation process, was found to be narrowly distributed in activation energy, with a mean value around E_a = 46 kJ mol⁻¹. The mean characteristic time at the glass transition temperature of these molecular motions was found to be of the order of τ_β(T_g) ≅ 10⁻² s.