Can a melted crystalline compound show a glass transition before an LC transition on a DSC? In a compound we observed a DSC trace with a large melting peak, then a drop in heat flow that could be attributed to a glass transition and then a small liquid crystal transition. This seems strange to me, as a glass should occur in a subcooled liquid, but the effect seems reproducible.
Hi Thomas,
thank you for your note. The DSC in question is a Calvet-type with no modulation (a Setaram uDSCIII evo device), so the effect might be different than decomposition. Moreover, the heating rates are really low, I measured at 0.2°C/min. I was thinking about decomposition, but the melting-LC-freezing transitions are reproducible within the repeated measurements of the same sample. The Cp difference in the baseline is a relatively small one, but corresponding in heat flow to the peak height of the LC transition. I'm currently running an experiment only in the incriminated temperature interval with different heating rates, hopefully this will shed some light.
The intriguing thing about this is that an independent measurement on different heat-flux TA DSC didn't show this baseline drop, just the melting and the LC peak, even though different heating rates were used, but then the heating rates in this type of device are orders of magnitude higher than for the former one.
As for the complementary experiments, these are a great idea and I will look for ways to obtain those, as I find this really interesting (BTW, I wasn't looking for a liquid crystal material, this is a by-product of a research on thermal storage... :-)).
Best,
Magdalena
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