Dear Sepideh Heydari, this a well common 'artifact' behavior observed also with other pilymers such as PLA. Though crystallinity type (isomers) and perfection are the main suspected reasons, but actually there there is no clear cut explanation. Please check the following RG thread and the links included. My Regards

https://www.researchgate.net/post/How_can_I_interpret_DSC_analysis_of_polyethylene_terphetalate_PET_fiber

This is an interesting RG free document.

https://www.researchgate.net/publication/290429725_Crystallization_Behavior_of_PET_Materials

Hi;

I think that other methods should be used to confirm the results and information obtained from the DSC curves. The ideal is the combination with DTA and SDTA or the combination between DSC and Raman spectroscopy. This last coupling allows to identify, in situ, the microstructural changes that the material undergoes during a thermal cycle.

My regards