Hi everyone,
I am working on synthesis thermoplastic polyurethane via pre-polymer polymerization. I usually use FTIR for quick checking chemical structure of my samples. After synthesizing and checking chemical structure as usual, I got different results. Instead of one peak of NH stretching, the NH stretching was split into two peaks (as you can see from the picture in attachment). This was the first time that I got this issue, even I did everything the same for the synthesis procedure. The only difference was I used argon gas instead of Nitrogen gas because the nitrogen gas was empty. I thought it should not affect the reaction since both are an inert gas.
Does anyone have an advice or a suggestion about this problem?
Your help will be really appreciated.
Thank you in advance.
I looked at your FTIR spectrum once again. In the region of N-H stretching you have 2 peaks and you express your surprise that you observe 2 peaks. BUT these 2 peaks are expected: the 3406 cm-1 - shows the presence of free N-H stretching;
while the broader peak which is observed at lower frequencies represents the H-bonded N-H groups. It should be exactly this way. I would be very much surprised to see a single (N-H str.) peak for your TPU. The doublet for N-H stretching is a correct result.
In order to answer your question you should give
1. A more detailed description of the chemical structure of your TPU
2. And present both the spectra: with and without splitting the NH peaks into doublet.
I looked at your FTIR spectrum once again. In the region of N-H stretching you have 2 peaks and you express your surprise that you observe 2 peaks. BUT these 2 peaks are expected: the 3406 cm-1 - shows the presence of free N-H stretching;
while the broader peak which is observed at lower frequencies represents the H-bonded N-H groups. It should be exactly this way. I would be very much surprised to see a single (N-H str.) peak for your TPU. The doublet for N-H stretching is a correct result.
Dear Dr. Leonid V Vladimirov
Thank you very much for you suggestion.
However, I have one more example for this strange circumstance.
According to attached FTIR results below, the two samples should have the same chemical structure because I used exactly the same chemical reactants and also synthesis procedure.
The chemical reactants were Methylene diphenyl diisocyanate, polycarbonate diol and 1,6 Hexane diol. I used DTBL as catalyst as well. As I wrote earlier, I am really surprised with the result. Although I did everything exactly the same, the chemical structure of final products is different.
Is it possible that the side reaction might occur during reaction or from some impurities?
If you have more comments about this result, please let me know
Thank you in advance.
Did you check for your water content of the HD and polycarbonate diol? The urea band at ca. 1650 cm-1 seems to be different and this maybe the reason for differences in properties.
I agree with Gerhard Behrendt ,do a moisture analysis of the diol/polyol you are using.
I agree with Leonid V Vladimirov on the point that broader peak is subjected due to H-bonding.
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