Reconciling calorimetric and kinetic fragilities of glass-forming liquids

The liquid fragility index (m(vis)) describes the rate of viscosity change of a glass-forming liquid with temperature at the glass transition temperature (T-g), which is very important for understanding liquid dynamics and the glass transition itself. Fragility can be directly determined using viscosity measurements. However, due to various technical complications with determining viscosity, alternative methods to obtain fragility are needed. One simple method is based on measurement of the calorimetric fragility index (m(DSC)), i.e., the changing rate of fictive temperature (T-f) with heating (cooling) rate in a small T-f range around T-g. The crucial question is how most is quantitatively related to m(vis). Here, we establish this relation by performing both dynamic and calorimetric measurements on some selected glass compositions covering a wide range of liquid fragilities. The results show that m(DSC) deviates systematically from m(vis). The deviation is attributed to the Arrhenian approximation of the log(1/q(c)) similar to T-g/T-f relationship in the glass transition range. We have developed an empirical model to quantify the deviation, by which m(vis) can be well predicted from m(DSC) across a large range of fragilities. Combined with the high-T viscosity limit (10(-2.93) Pa.s), we are able to obtain the entire viscosity curve of a glass-forming liquid by only performing DSC measurements. (C) 2016 Elsevier B.V. All rights reserved.