Probing Beta Relaxation in Pharmaceutically Relevant Glasses by Using DSC |
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Authors: | Sergey Vyazovkin Ion Dranca |
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Affiliation: | (1) Department of Chemistry, University of Alabama at Birmingham, 901 S. 14th Street, Birmingham, Alabama 35294, USA |
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Abstract: | Purpose This study was conducted to demonstrate the use of differential scanning calorimetry (DSC) in detecting and measuring β-relaxation
processes in amorphous pharmaceutical systems.
Methods DSC was employed to study amorphous samples of poly(vinylpyrrolidone) (PVP), indomethacin (IM), and ursodeoxycholic acid (UDA)
that were annealed at temperatures (Ta) around 0.8 of their glass transition temperatures (Tg). Dynamic mechanical analysis (DMA) was used to measure β-relaxation in PVP.
Results Reheating the annealed samples gives rise to annealing peaks that occur below Tg. The peaks cannot be generated when annealing below the low temperature limit of β-relaxation. These limits are around 50°C
for PVP, −20°C for IM, and 30°C for UDA. The effective activation energy (E) of the sub-Tg relaxation has been estimated for each Ta and found to increase with Ta, reflecting increasing contribution of the α-process. Estimates of E for β-relaxation have been obtained from the lowest Ta data, and are as follows: 68 (PVP), 56 (IM), 67 (UDA) kJ mol−1.
Conclusions DSC can be used for detecting β-relaxation processes and estimating its low temperature limit, i.e., the temperature below
which amorphous drugs would remain stable. It can also provide comparative estimates of low temperature stability of amorphous
drugs in terms of the activation energies of the β-relaxation. |
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Keywords: | amorphous systems differential scanning calorimetry (DSC) kinetics molecular mobility |
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