The use of thermal methods for predicting glass-former fragility

Glass-former fragility describes the changing dynamics of a supercooled liq uid with temperature and so dictates the temperature of glass transition as well as the dynamics of the non-equilibrium glassy state. Fragility parame ters can be calculated from either experimental relaxation time or viscosit y data. Predictions of fragility can also be made using thermal methods. Th e objectives of this manuscript are to evaluate three thermal methods of fr agility prediction and, using these methods, to predict the fragility of a number of pharmaceutical glass-formers. Using differential scanning calorimetry, fragility predictions were perform ed by extrapolating configurational entropy to zero and by calculating an a ctivation enthalpy of structural relaxation at the glass transition (AET.) from the scanning rate dependency of the glass transition temperature, and glass transition width. On comparison with experimental Vogel-Tammann-Fulch er (VTF) fragility parameters for four glass-formers, all thermal methods w ere found to have reasonable predictive ability. Characterisation of pharma ceutical glass-formers by all thermal methods yielded predicted VTF D param eters in the range of 7-15. Predictions for a further 10 pharmaceutical gla ss-formers using only the configurational entropy method were within this r ange suggesting that moderately 'fragile' behaviour may be a common feature of such materials. (C) 2001 Elsevier Science B.V. All rights reserved.