Prediction of the onset of crystallization of amorphous sucrose below the calorimetric glass transition temperature from correlations with mobility.

The objective of the present work is to determine if crystallization onset observed for an amorphous solid correlate with relaxation time at temperatures above and below the calorimetric glass transition (T(g)). Crystallization onset of spray-dried and freeze-dried amorphous sucrose were measured calorimetrically. Relaxation times measured in two temperature ranges by different techniques (isothermal calorimetry, dielectric spectroscopy) followed the expected modified Vogel-Tammann-Fulcher (VTF) behavior when extrapolated to a temperature near T(g). However, the change in slope was more conspicuous for freeze-dried sucrose, indicating that amorphous materials generated using different techniques differ in their mobilities for temperatures below T(g). Dielectric relaxation time values obtained above T(g) were well correlated to onset of crystallization. The model predicted 21 days for crystallization onset for spray-dried samples stored 7 K below T(g), compared to the experimentally observed crystallization onset of 17 days. Onset times versus temperature for freeze-dried sucrose, however, show a change in slope on approaching T(g), with the onsets somewhat decoupling from measured mobility for temperatures below T(g). Molecular mobility in amorphous materials at temperatures both above and below T(g) can be correlated to macroscopic physical change such as crystallization, but prediction of crystallization onset from relaxation time is only qualitatively correct at temperatures well below T(g).