Measurement of enthalpic relaxation by differential scanning calorimetry-effect of experimental conditions

Differential scanning calorimetry (DSC) is the most commonly used technique for studying enthalpic relaxation in amorphous systems. Our objective was to study the effect of experimental conditions, specifically heating and cooling rates, on the enthalpic relaxation measurement by differential scanning calorimetry. Amorphous trehalose was prepared by freeze-drying an aqueous solution of trehalose dihydrate. It was subjected to differential scanning calorimetry. The enthalpic recovery (ER) at the glass transition temperature (Tg), for identically aged samples, depended on the heating rate (HR). The Tg onset increased as a function of the heating rate and so did the completion of enthalpic recovery, i.e., the temperature at which the enthalpy curve meets the super-cooled liquid line after the glass transition. Therefore, the enthalpic recovery, and by extension the observed extent of relaxation, was influenced by the heating rate. As the aging time increased, there was an increase in the Tg onset as well as the completion of enthalpic recovery. Since the calculated enthalpic recovery value is strongly dependent on this endpoint, there is a potential for overestimation of the relaxation below Tg. This is particularly important for longer aging times and higher heating rates. It is generally believed that the enthalpic recovery can be minimized by keeping the cooling and heating rates identical. This observation can also be explained by the effect of heating rate on Tg onset and the endpoint of enthalpic recovery. The enthalpic recovery at Tg may not reflect the state of the sample, and may be strongly influenced by the experimental conditions. Thus, the effects of experimental conditions need to be carefully evaluated in order to obtain meaningful results.