Variations of the effective activation energy (Eα) throughout the glass transition were determined for 1,2-propanediamine (12PDA), 1,2-propanediol (12PDO) and its solutions of LiBF4 by applying an isoconversional method to differential scanning calorimetry (DSC) data. For 12PDA, Eα is found to markedly decrease from 660 to 188 kJ mol—1 throughout the glass transition, whereas 12PDO does not show such drastic change in Eα, which remains at around 145 kJ mol—1. Although the two simple liquids are similar in molecular structure and size, the cooperative dynamics in the glass transition can be quite different because of differences in the strength or stability of hydrogen-bonding structure between them. We also have discovered that the addition of LiBF4 significantly affects Eα for 12PDO. The glass transition dynamics are affected more by adding LiBF4 at an early stage of the glass-to-liquid transition rather than at later stages. As the mole fraction x of LiBF4 increases, the value of Eα initially increases, but it decreases dramatically during the glass transition. The expansion of cooperativity and its fragmentation, which are reflected in the abrupt change in Eα, can be explained in terms of competition between the hydrogen-bond networks of the alcohol solvent and the ionic interactions due to the added salt. The variability of Eα with temperature is found to correlate strongly with the kinetic fragility.
Keywords:glass transition, activation energy, cooperativity, fragility, differential-scanning calorimetry (DSC)
Publication Date: 2018-07-25