The use of thermal methods for predicting glass-former fragility

Glass-former fragility describes the changing dynamics of a supercooled liquid with temperature and so dictates the temperature of glass transition as well as the dynamics of the non-equilibrium glassy state. Fragility parameters can be calculated from either experimental relaxation time or viscosity data. Predictions of fragility can also be made using thermal methods. The objectives of this manuscript are to evaluate three thermal methods of fragility prediction and, using these methods, to predict the fragility of a number of pharmaceutical glass-formers. Using differential scanning calorimetry, fragility predictions were performed by extrapolating configurational entropy to zero and by calculating an activation 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-Fulcher (VTF) fragility parameters for four glass-formers, all thermal methods were found to have reasonable predictive ability. Characterisation of pharmaceutical glass-formers by all thermal methods yielded predicted VTF D parameters in the range of 7-15. Predictions for a further 10 pharmaceutical glass-formers using only the configurational entropy method were within this range suggesting that moderately 'fragile' behaviour may be a common feature of such materials. (C) 2001 Elsevier Science B.V. All rights reserved.