Hello everyone
I have done a DSC test on the poly-lactic acid by Mettler DSC set-up. As the result shows, there are three endothermic peaks in the red curve that I marked by the black color. It is obvious that peak number one belongs to the glass transition temperature, and peak number three is the melting temperature, but I don't know exactly what is the type of peak number two. I have this concern that the second peak might be the effect of impurity or a metastable modification in the polymer tested.
Any guidance, suggestion, or opinion would be helpful.
Best regards.
For a polymer thermal analysis of the type of sample you have, you can first examine the TGA thermogram and identify the temperature range your polymer is stable. T hen, use DSC to heat to the temperature you are certain that your polymer is not degrading. Then, stop heating and cool the sample slowly in the DSC instrument, and start the heating cycle again. Any of those phenomena that are not reproducible, such as polymerization exotherm, impurity evaporation, anthalpic relaxation, etc will disappear in the second heating curve. Although impurity, including residual water, is a possibility for the second peak, what I am concerned is the slight exothermic event right before endothermic event that followed. Impurities usually do not show that. Have you made sure that you do not have residual lactic acid that did not polymerize?
Dear Hatsuo Ishida
Thank you so much for your contribution. I'm not sure about the residual lactic acid part that you mentioned before. would it be possible to explain more about the effect of polymerization of the residual lactic acid on the DSC curve?
Again, thanks a lot.
For poly (lactic acid) (PLA), which I've studied for my Ph.D. long time ago :) -- there is a metastable crystal polymorph that Strobl calls the alpha-prime form, which transforms into the alpha-form which is thought to have a higher melting point (I'm not sure anyone has directly observed the melting point of alpha-prime, since I have never seen a direct transition of alpha-prime ==> melt, only what seemed to be an alpha-prime ==> alpha i.e. a solid-to-solid transformation).
When molten PLA is cooled below 100 degC, most of the crystals you form is of the alpha-prime type.
Upon reheating, the alpha-form can start to form above 100 degC, although it'll be a mixture of alpha and alpha-prime forms.
==> So the exotherm around 100 degC is likely the crystallization exotherm of a mixture of alpha-prime and alpha forms of PLA.
Professor Ishida also pointed out the possibility of impurity evaporation for the endotherm around 110 degC - I think that's a great insight and a possibility (it might be delayed water evaporation for example). It may also be due to melting of unreacted L-lactide (the monomer that is polymerized to make semicrystalline PLA).
The alpha-prime form does transform into the alpha form but not at these (110 degC) low temperatures, it would happen at around 150-160 degC. You can actually see distinct differences in the X-ray diffraction patterns well below vs. at those temperatures (i.e. below the melting of alpha form = >165 degC for your sample) -- you can prepare such sample for the high temperature by leaving your sample at say 160 degC for 1 hour, and then taking out of the open and dunking it into liquid nitrogen for example (and do that safely, obviously). As you pointed out, the glass transition temperature of the PLA is around 50-60 degC, so it's quite easy to freeze the structure nearly at what it was around 160 degC. And PLA doesn't crystallize easily -- it's more like PET (slow crystallization) than PE (very rapid crystallization).
The paper where we contributed to estimation of the enthalpy of melting of the alpha and alpha-prime form via DSC, is here:
https://www.sciencedirect.com/science/article/abs/pii/S0032386110011110?via%3Dihub
Dear Kaoru Aou
Thank you so much for your opinion and guidance. It is really helped to understand the thermal behavior of this test.
My regards.
Dear Vadim S. Gorshkov
The experiment was performed in the N2 gas environment which is the standard purge gas of the DSC set-up.
Thank you in advance.
Vadim S. Gorshkov
, you are right, and I'm sorry, there was mistyping. the gas environment was O2. by excluding the chemical processes, what do you mean, would it the possibility to explain more?best regards.
Thank your Dear, Vadim S. Gorshkov
After retesting with the same parameter on a different gas environment, what aspects should I look for?
Best regards.
Thank you so much Dear Vadim S. Gorshkov
Your suggestion is really helpful. I'll keep those in mind.
Best regards.
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