To erase the thermal history, and also it helps to evaporate residual solvent.

This is a well known phenomenon called cold crystallisation. By heating the polymer, you first observe the glass transition, above which the polymer chains are far more mobile. The heat provided by heating give them more energy to move and tend towards a more stable state, which is the crystalline state. As the crystalline state is more stable than the amorphous state, excess energy is produced, from where the exothermal peak of crystallisation. But as the temperature keeps on increasing, it finally reaches the melting point of the crystal, and you observe the melting (endothermal) peak. Just remember that a melting point is clearly identified only if the solid was formerly crystallised. Hence 2 peaks in opposite directions.

Alain

Dear Mohammad,

cold crystallization of PLA is a reason for rapid cooling of the melt in the mould cavity during the process.  Macromol. Chem. Phys., 2013, 214, 1434-1444. follow this refer.

Dear Jassim, Alain, Catherine and Sreenath

Thanks for your nice answers

Would you see my attached graph and then talk about Tr?

I find some references that called this phenomena as a "melt re-crystallization peak" what you think?

Would you introduce some references that have great discuss about this peak?

Thank you

More explanation!

We fabricate a medical PLLA interference screw through the micro-injection molding. So we were eager to know about the PLLA thermal behavior of injected polymers. The injected screw almost stay less than 2 minutes in the mold, after that put out and cooled at room temperature. The attached DSC is for fabricated screw. 

Thanks

Dear Catherine

Salam

Thank you for nice answers

Catherine Esnaashari

may you suggest any related article related to this suggested answer;

Melting temperature of PLA is far above the cold crystallization temperature (about 70 C) at a rate of 10 C/min (which I suppose you performed the test). So if you say you observed the exothermic peak exactly before the melting peak, it may be due to mrr.

mrr in which thickening of lamellae occurs usually leads to double (or multiple) melting peaks. I guess as a result of the superposition of the lower melting endotherm and the recrystallization exotherm, the lower endotherm is hardly detected and you can only see the exothermic peak before the melting peak. Is the exothermic peak very small compared to the melting peak? I wonder if it can be attributed to a small fraction of crystalls formed during cooling cycle.

Another possible reason may be the crystal polymorphism of PLA (alpha and alpha prime forms). Some researchers attribute the small exothermic peak prior to the main melting peak to phase transition from less stable alpha prime form crystal to alpha form one during heating scan

tq

Nicely said.....

Another approach is to use TA Instruments Modulated DSC (trademark) or Mettler-Toledo Instruments TOPEM (trademark) DSC analysis. This allows the heat flow to be separated into heat capacity related and kinetic change components.....and it is fascinating to see what is really going on within the samples. Heat set temperatures in fibers or films are readily viewed as is crystal rearrangement and crystal perfection. In addition, the small exothermic peak immediately prior to melting is seen in many slow crystallizing materials....especially those that are able to be influenced by hydrogen bonding in the chain....see Y. Khanna publications for additional details.