DSC analysis with a heating rate of 10C/min was employed to study the phase transition of crystalline indomethacin. The first DSC run shows melting of the crystal at 160°C. In the second run the sample was quickly cooled at -30°C and the DSC scan showed a glass transition at temperature range 45-49°C (which was the same with the glass transition of amorphous indomethacin observed when studying the amorphous indomethacin thermal behaviour).
What does the presence of the glass transition indicate? Does it indicate that some portion of crystalline sample is amorphous? Or, does it mean that upon fast cooling the melted crystal to -30°C not enough time was allowed for molecular rearrangement and the crystal converted into a glass-like state?
DSC scan from the second run attached below.
DSC scan from the first run showing a sharp endothermic peak at 160C.
The presence of the very distinct glass transition in your re-heating curve shows that there is at least some amorphous fraction in your sample. If you look at your cooling curve (don't know if you saved it or not) you should be able to see signs of crystallization from the melt, if it had occurred (exothermic peak). I unfortunately don't know anything about the crystallization kinetics of indomethacin, so I can't say for sure if your cooling rate was sufficient to completely suppress crystallization or not.
This is typical in quenched samples, whre there is not enough time for crystalllization. If may occur in polmyers, where the melt viscosity increases fast with decreasing temperature. In these cases it frequently occurs that in the second heating run you observe a glass transiton (usually at about 2/3 Tcr) than comes an exotherm (crystallization) followed immeditaely by an endotherm (melting). Annealing at or slightly below the mleting point may help in removing the amorphous fraction and yue get a lot of information in crystallization kinetics.
As a scientist working with oxide glasses, it is very common to run XRD in order to check crystalline peaks or the lack thereof.
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