Hello everyone,

I'd like to request your help regarding a strange phenomenon I noticed in some of my PLA samples. Between ~120-135 degrees C, a strange exothermic bump is seen in the second heating cycle. In my procedure, I heat the sample once to 200 degrees C, cool it down to -20 degrees C, then heat it back up to 200 degrees C. This has been affecting my area integration for the melting enthalpy, since the left side of the melting curve is higher than the right side. This is often seen in commercial grade PLA developed by Natureworks and Total Corbion.

FYI, I have calibrated the DSC following the associated TA Instruments manual and ASTM standard, so I do not think it is a calibration issue. I have attached the graph with the temperature labels for your information. It looks almost as if a peak is being suppressed, similar to suppressing a cold crystallization peak via annealing. Does anyone know what is causing this issue? If so, what would be the proper way to integrate this melting curve for accurate enthalpy value? I look forward to hearing from you all.