The reaction takes place routinely at 550C with no change in energy. Has anyone experienced this before?
Strange! A mass loss of 60% is caused of either evaporation of e.g. solvents (endothermic event!) or reaction of some kind (exothermic event!). In my opinion the heat flow should change in every case. If there is no thermal event, the mass should not change either! Did you weight the pans befor and after the experiment, is there really a mass change? May be there is a fault in either the balance or the DSC unit. Physically a mass loss without heat exchange is impossible. Regards Günther Höhne.
an exo or endo reaction related to the physical transition, I think in this case there is not a transition.
So then a a chemical re-arrangement? I would have thought even that would have displayed a difference in energy?
Strange! A mass loss of 60% is caused of either evaporation of e.g. solvents (endothermic event!) or reaction of some kind (exothermic event!). In my opinion the heat flow should change in every case. If there is no thermal event, the mass should not change either! Did you weight the pans befor and after the experiment, is there really a mass change? May be there is a fault in either the balance or the DSC unit. Physically a mass loss without heat exchange is impossible. Regards Günther Höhne.
The mass of the pan was subtracted from the calculations and the system was solvent free! This phenomena was witness in 4 tests total from two suppliers and occurred 100% of the time. Perhaps the 'DSC type' technology is not as sensitive as required. It is technically C-DTA not DSC.
See below of the resulting plots, I cannot reveal what the exact polymers are, but non the less it is still interesting to discuss
Hi Thomas, I have chacked your plot and found endothermic heat events which fit to the weight-change processes. (see attached drawing). Obviously you used a very small amaount f substance (some milligrams only?) for your experiment leading to a very small temperature change and consequently low heat flow signal. Nevertheless the aproximated areas are proportional to the heat absorbed and seme reasonabel. Regards Günther Höhne.
Thank you for your assistance in this Gunther. What I have been basing the heat flow signal was a calibration test with the phase transition of Indium (see below). Here, there is a distinct and abrupt endothemic process due to melting. On the above chart, the endotherm seems to be a more drawn out process. It must be quite subjective as the where the heatflow baseline is, as to what was determined to be endo or exothermic.
The Indium run is OK, the thermal signal is rather "sharp" because the phase transition is a "sudden" event taking place at one temperature. In the case of your "polymer" ther is a time and temperature deppending processover a broad temperature region (as you see in the weight change curve) as a consequence the heat flow is distributes over a wide temperaure region as well. The heat of reaction is proportional to the area between the "baseline" (which I assumed where I plotted it) and the DTA signal. In your case endothermic signals are below the "baseline" and exothermic signals are above the "baseline" (see right upper corner of the diagram : EXO arrow upward). For the "1st process" the thermal signal is very clear visible. For the following processes the wight change as well as the thermal signal is "smeared" over a broad temperature region, comparable with the weigght hange. To get better result you should repeat the experiment with a larger sample amount. Good luck Günther Höhne.c
The lack of transition in the DSC traces is indeed puzzling. The TGA traces are typical. It is not common, and in fact, not recommended, to run DSC to high temperatures where polymer decomposition can happen because it could actually ruin the instrument sensors. In fact, what I prefer is to run a TGA first and set an upper limit for a DSC run to the onset of descomposition, ca. 300°C in your case. At that point you could see in the DSC the onset of a decomposition by a mechanism consistent with the gas you are using. What gas is used? From the graph provided it appears to be air (although nitrogen can also show this, depending on the polymer).
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