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In DSC, heat flux difference between sample and reference is measured as function of temperature while temperature of sample and reference are maintained the same. DSC is often used for polymer analysis.

What I am not clear is how does DSC differentiate between exothermic and endothermic changes like for example crystalline and melting point of polymer since in both state changes temperature of sample remains the same? In both cases, to keep both sample and reference at the same temperature, heat flux for reference is going to decrease because if not temperature of reference would become bigger than of sample since we are giving latent heat to the sample.

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  • $\begingroup$ The sign of peak (negative vs. positive) tell us endothermic or exothermic changes. Se his graph researchgate.net/figure/… $\endgroup$
    – AChem
    Oct 11, 2021 at 13:49
  • $\begingroup$ In the case of melting, the reference temperature goes up and up but the sample temperature remains the same. $\endgroup$
    – AChem
    Oct 11, 2021 at 13:50
  • $\begingroup$ Yes, sign of peak tells us if transition is exothermic or endothermic. However, how can we determine if process is endo or exothermic if in the case of both, reference temperature goes up? $\endgroup$ Oct 11, 2021 at 14:04
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    $\begingroup$ The substance used as reference must absolutely have no exothermic or endothermic transformation when heated $\endgroup$
    – Maurice
    Oct 11, 2021 at 14:15
  • $\begingroup$ @Dario, It cannot be both exothermic and endothermic with positive or negative peak if the reference temperature is going up. $\endgroup$
    – AChem
    Oct 11, 2021 at 14:46

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In DSC, differential power (heat) is provided to keep the sample and the reference at the same temperature. The DSC plot has differential power on the y-axis and temperature on the x-axis. Also, as reference is chosen in such a way that it will not undergo any phase change or state change in the chosen temperature range. The instrumentation is not as simple as one might think. There are two separate heaters which can be independently controlled. I quote from O'Reilly's Instrumental Analysis book (slightly outdated but still a pretty good reference for concepts). It describes DSC nicely,

There are two separate heating circuits, the average-heating controller and the differential heating circuit. In the average-temperature controller, the temperatures of the sample and reference are measured and averaged and the heat output of the average heater is automatically adjusted so that the average temperature of the sample and reference increases at a linear rate. The differential-temperature controller monitors the difference in temperature between the sample and reference and automatically adjusts the power to either the reference or sample chambers to keep the temperatures equal. The temperature of the sample is put on the x-axis (time) of a strip-chart (read "computer" today) recorder and the difference in power supplied to the two differential heaters is displayed on the y-axis. The power difference is calibrated in terms of calories per unit time.

Now imagine that you are heating the polymer and the reference, their temperature is increasing but their difference is zero. You have a flat baseline, as shown in the figure (Taken from Google Images). Let us continue heating both, independently, a temperature is reached when the polymer is melting. There is a phase change and the sample temperature is not changing anymore, but it needs heat to stay at that temperature. The reference, when brought to the melting point, does not need further heating, so its temperature is at the melting point of the sample. The sample is consuming power, but the reference is not. You get a negative peak in the DSC due to an endothermic process.

Apply the same idea to an exothermic process, such as crystallization. The temperature of the sample is becoming higher than the reference. The reference needs power to catch up to the temperature of the sample. The sample heater is not consuming power but the reference's heater is, in order to be at the same temperature. This time you get a positive (exothermic) peak.

DSC

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  • $\begingroup$ Buttonwood, I think the OP wanted clarification about the principal/instrumentation due to a misconception that power is continuously being supplied to the reference. $\endgroup$
    – AChem
    Oct 11, 2021 at 18:32
  • $\begingroup$ Thanks for detailed answer. I do agree with everything you said about melting. Reference is stopped heating while we need to heat the sample to keep the same temperature and to go through phase shift. I am not clear about crystallization since you've written that temperature of sample is increased during crystallization and that reference needs to catch up. However, what about latent heat of transition? Shouldn't temperature of the sample stay the same during phase change (crystallization) like in melting and any other phase change? $\endgroup$ Oct 11, 2021 at 22:24
  • $\begingroup$ Since crystallization is exothermic we don't need to heat the sample until phase change happens, but if temperature stays the same during phase change we also don't need to and we shouldn't heat the reference to keep both at the same temperature. $\endgroup$ Oct 11, 2021 at 22:30
  • $\begingroup$ Lets say, crystallization is exothermic at a temperature T. We need to heat the sample until it reaches the crystallization process. Now, the temperature of the sample cell is spontaneously increasing due to an exothermic process. It does not need heat at this moment from an external source. The reference cell must be heated now in order to bring the temperature at par with the sample. $\endgroup$
    – AChem
    Oct 12, 2021 at 0:51
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    $\begingroup$ Don't bring cooling at this moment. Let us stick to an exothermic process. The temperature must change due to an exothermic process. Don't you think so? $\endgroup$
    – AChem
    Oct 12, 2021 at 19:41

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