A polyurethane without any melting peak just means that it either doesn't have a well-ordered hard domain (which is normal), or else your polyol soft segment itself does not crystallize (which is normal if you're using PPG for example).

If you have a polyurethane rubber then normally you would see a "Tg" transition with an easily measured heat capacity (C_p) change (or heat flow change, depending on what variable you're graphing with)  around -30 to -60 degC.  If you have a polyurethane 'glass' then that's a different story -- the C_p change would be small that it might not be easily discerned.  In those cases, it's better to use Dynamic Mechanical Analysis (it's known as DMA, DMTA or DMS depending on the literature you read), where you're not looking at heat flow but at change in actual mechanical response of the material.

Fabio,

I agree with the answer given by Kaoru.  The exotherm at 125C, I think, indicates that your compound is crystallizing.  Can you provide more information on the polyurethane?  What are you using as a polyol and are you using a chain extender?  I have done a fair amount of work with polyurethanes, both thermoplastic and thermosets, and with more information may be able to provide a better answer to your question.

Dear all,

thank you for all the answers. My purpose is to synthesize test samples of polyurethane elastomers to see their behavior in DSC experiments. I used poly(tetrahydrofuran) (Mn = 1000) as the polyol and 1,4-butandiol as the chain extender. I tried also with different NCO/OH ratio and annealing in oven a polyurethane sample (prepolymer NCO/OH = 2) but the resulting graph does not change. Is possible that the lenght of the pTHF is the cause of this behavior?

Yes, is possible random "cristallization". Have you a peak only in cooling but not in second heating? this is possible if the "matrix" change structure.

Hi Fabio,

I am getting similar results in my experiments of modifying PCL with isosorbide. Have you got any clues after your work?? 

Dear Rachana,

unfortunately we've not get any clue about this strange behavior. Recently I tried to perform the DSC experiment with a major heating rate (20 K/min) in order to point out transitions or peaks, but nothing appear again. I will try with DMA as suggested by Kaoru, and so (if you are interested) I'll share the result of the experiment.

That's could be a good Idea, show us her a graph of your measurements. The Problems are is to guess how the DSC diagram looks like. If you don't want to change the diagram with the complete world you can send me a graph.