The aim of this study was to investigate if amorphous indomethacin samples, prepared using different
preparation methods, exhibit different structural and kinetic characteristics and if these differences
can be correlated to their physical stability (time to crystallisation). Samples were prepared by melt
quenching, spray drying, ball milling, and cryo-milling. The resulting amorphous materials were characterised
using X-ray diffraction, Raman spectroscopy and differential scanning calorimetry. All freshly
prepared samples were completely X-ray amorphous (with a halo being the only feature in the diffractograms).
The shape of the halos in the diffractograms, however, varied depending on the preparation
method, suggesting structural variations in the near order of the molecules between the differently prepared
amorphous forms. Principal component analysis of the Raman spectra of the various amorphous
forms revealed that the samples clustered in the scores plot according to preparation method, again
suggesting structural differences due to preparation method. The range of vibrations associated with
the largest spectral differences in the loadings plot showed that these differences were due to a range
of molecular conformations and intermolecular interactions. The ranking of the samples with respect
to stability was: quench cooled amorphous samples > cryo-milled ( -form) > spray dried > ball milled
( -form) > ball milled ( -form) = cryo-milled ( -form). This ranking was not correlated with the diffractogram
shapes or sample distribution in the scores plot of the Raman spectra, suggesting that physical
stability was not directly affected by structural variation in the samples. However, ranking of stability
of the differently prepared amorphous forms of the drug could be predicted by determining the relaxation
time values, for all amorphous samples. The relaxation times, calculated by using the Adam Gibbs
and Kohlrausch–Williams–Watts equations, were in accordance with the experimentally determined
stability order.
This study showed that correlation of physical stability with calculated relaxation time is possible
for the same amorphous systems prepared by different methods. This could aid in selecting the most
appropriate preparation techniques in situations where there are a variety of suitable methods.
