Fragility in liquids and polymers: New, simple quantifications and interpretations

In order to provide a firmer base for efforts to interpret the different fragilities of liquids (i.e., the different deviations from simple Arrhenius temperature dependence of relaxation processes), a new and simple one-scan method of quantifying fragility is described and verified. It is based on detecting, by simple differential thermal analysis (DTA), the temperature of maximum energy dissipation during temperature scanning under single-frequency excitation (electrical or mechanical) of the liquid. The frequency is chosen so that the DTA signal indicates directly not only T_g but also the temperature T_1/2 (where the relaxation time reaches 10^-6 s) of the new and unambiguous fragility metric F_1/2,¹ which is defined by F_1/2 = 1(T_g/T_1/2) - 1. The measurement is verified against dielectric data for liquids of widely varying fragility. The usefulness of such rapid scan methods in confirming or denying a vibrational density of states origin of fragility in liquids is considered.