Time-Dependence of Molecular Mobility during Structural Relaxation and its Impact on Organic Amorphous Solids: An Investigation Based on a Calorimetric Approach

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Abstract

Methods

The time-dependent enthalpy relaxation times of amorphous sorbitol, indomethacin, trehalose and sucrose were estimated based on the nonlinear Adam-Gibbs equation. Fragility was determined from the scanning rate dependence of Tg. Time evolution of the fictive temperature was determined from Tg, the heat capacity of the amorphous and crystalline forms, and from the enthalpy relaxation data.

Results

Relaxation time changes significantly upon annealing for all compounds studied. The magnitude of the increase in relaxation time does not depend on any one parameter but on four parameters: Tg, fragility, and the crystal-liquid and glass-liquid heat capacity differences. The obtained mobility data for indomethacin and sucrose, both stored at Tg−16 K, correlated much better with their different crystallization tendencies than did the Kohlrausch-Williams-Watts (KWW) equation.

Conclusions

The observed changes in relaxation time help explain and address the limitations of the KWW approach. Due consideration of the time-dependence of molecular mobility upon storage is a key element for improving the understanding necessary for stabilizing amorphous formulations.

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