I have two DSC curves, attached below. One of them shows an endothermic peak at 90 and another one at 71⁰C. Are these two peak shapes glass transition temp (considering enthalpy relaxation process)?
Moreover, in the high temp region if 314 and 157⁰C are considered to be melting temp of the compds what could be the possible processes involved for the other two peaks at 337 and 231⁰C?
Any suggestions are appreciated.
-Bidyut
Hi
Above DSC curve shows that either your compound is a mixture of two or three components or degraded on high temperature, for confirmation you must have done TGA and FTIR.
Mayuri Gupta Thank you. But I have purified the compounds prior to DSC and as per NMR there are no such impurities in the NMR spectra.
Hi Bidyut Das
My initial thought was just like Mayuri Gupta : that you have a problem with the purity of the meta isomer. But on second thought, the peak at 71C doesn't look like a glass transition. It is more likely to be a kinetically inhibited phase transition in the solid, as is (less so) the transition at 157C. The better candidate for a glass transition (presumably in a plastic-crystal-like phase because so much entropy has been gained already at lower temperatures) is the short plateau around 180C.
The molecules in the solid para isomer seem to be more mobile, hence the sharper transition at 90C (compared to the meta one at 71C) and the very sharp solid-state transition at 314C.
If you can get access to an XRD instrument with a temperature-controlled stage you may be able to learn more about these transitions and the structural changes in these solids.
Prof Emanuel Cooper thank you so much. As per NMR measurements the compounds are pure. Can this be due to a kind of bond rotation within the compound, like cis trans isomerism? Moreover, you have mentioned about phase transition in solid, what could be plausible phase transitions? If possible could you provide me some literature? In some cases of literature I have got that such peak arises in glass tranisitions (rather than as plateaus) due to enthalpy relaxations/ structural relaxations. We do not have Variable temp XRD here. Moreover, am a bit new to this thermochemistry part so I am in a process of learning.
Hi
Than only Thermogravimatric analyzer (TGA) can confirm that peaks at 314C and 157C are melting point or thermal degradation/ decomposition of material.
I never recommended directly for DSC Analysis. First, do the TGA study and then decide your range for the DSC study. In DSC analysis, go for a slow heating rate of up to 10C/min and do at least two DSC runs, so your sample is free from any previous history and gives a proper thermal analysis.
Jogender Rathore thank you so much. I have submitted the samples for TGA, actually, I am a bitnew towards this thermal analyses and ourselves we do not have an access to a DSC-TGA so we need to require to submit to another institute where they do it by their procedures. The heating rate was however 10⁰/min as in the data details mentioned.
Bidyut Das you mentioned that "In some cases of literature I have got that such peak arises in glass transitions (rather than as plateaus) due to enthalpy relaxations/ structural relaxations."
Could you kindly provide such literature references?
Thank you in advance
Dear, Errj Sansonetti , here are three such documentations mentioning such cases. The 3rd one is a book I think
Hope it helps.
https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://www.eng.uc.edu/~beaucag/Classes/Characterization/DSCParts/Artifacts%2520in%2520DSC%2520Usercom_11.pdf&ved=2ahUKEwirqKK60tqCAxXtUWwGHQvgCOgQFnoECBAQAQ&usg=AOvVaw2qkQ7QBk3YGnuuusT7hWEK
https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://www.tainstruments.com/pdf/literature/TA039.pdf&ved=2ahUKEwirqKK60tqCAxXtUWwGHQvgCOgQFnoECBEQAQ&usg=AOvVaw0n5YTVdHHxJn8UAwHVCOPn
https://www.sciencedirect.com/topics/chemistry/relaxation-enthalpy
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