HI, I'm trying to find an answer why we need cooling process in DSC analysis? Can I perform only heating process without the cooling? Or is it cooling here just to reconfirm sample temperature when in heating process?
Note: my sample is a new compound.
And how to decide one cycle or two cycles of analysis?
It depens on the thermal hystory of the material.
for instance, in the case of polymers, some heating and cooling are required to get more insight about the behavior that hapen, but for a pure material like a simple molecule (e.g salt) it is not necessary.
** THis case for DSC but not necesary at all for TGA.
Different processes occur when sample is cooled or heated, various effects might happen. Moreover, the thermal history of the sample is very significant, cooling/heating in the first cycles will affect the structure of your material. This could be a very long answer but you can find all you need and more in the following book:
http://www.springer.com/la/book/9783662033029
This depends on what you are trying to learn about your sample.
If every effect can be clearly identified in a single heating run and you are just interested in the material as it is, than only heating might be sufficient.
But if you are interested in the thermal history of your sample and how it affected the material it is very helpful to perform several heating cooling cycles.
In the first heating you get the information of the material as it is with all influences from thermal history (which is often unknown) which is then removed by heating to a big extent. Then during controlled cooling you give the sample a new thermal history and with the cooling rate you have an influence on the possible crystallization processes and so on.
In the second heating you get the information of the new controlled thermal history and can compare whethter the thermogram shows any differences to the first heating. 2. cooling with the same parameters as the first and than 3. heating are used determine whether second and third heating are the same (as they had the same thermal history) which shows that the sample was stable over the procedure (2. and 3. cycle).
For polymers (especially thermoplastics) it is quite common to use the three cycles.
As you always have some temperature lag (and possibly also a typical process kinetic), it is not expected that the processes appear at the same temperature in heating and cooling. With increasing heating rate the differences become bigger.
I hope this helps.
Hi Matthias ,
Thank you for you valuable information.
But I would like to know, if at first heating, there is a peak below 100 degree, is it I can ignore it as one of solvent or impurities trace? Then I continue with cooling and heating again, another peak shown above 200 degree. Then, can I consider my melting point is at temperature above 200 degree?
Thank you
It is not that easy.
Why solvents? why impurities? exothermal or endothermal? Possibly a curing reaction?
If you are sure that it could only be solvents and it is an endothermal effect slightly higher as the evaporation temperature, then its sounds reasonable. But there might be a lot of different possibilities.
Overall i think you should provide a figure to allow for interpretation.
As Matthias Jaunich mentioned, whether you perform heating and cooling or heating only, depends on what kind of information you are looking for. If I have samples of known materials prepared with different process parameters (which I assume may influence material structure) than I do heating, because I am looking for differences (crystallinity level, crystal size, position of Tg and glass transition intensity) caused by different process parameters. You do not need to perform cooling unless you suspect that processing with different condition might lead for different degradation level. In this case you can learn something from differences in crystallization temperature and crystallinity level achieved in cooling process in DSC. If you have pellets of different commercial grades of some polymer, then probably you will more interested to perform cooling than heating, or to be more precise first heating is not so interesting as cooling and second heating. From cooling you can get information about crystallization temperature, and overall crystallinity level. Those number may give you some insight into presence of nucleating agents and molecular weight, which may have impact on materials processability and properties of product. Second heating may reveal some additional information. These are example from my work. You may encounter other cases.
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