Dear Maryam Kheyrandish

The peak which was appeared in about 115 C can be attributed to HDT of the polymer and also the other exotherm can be due to the crystalization of uncrystalized part of the polymer.

According to my experience, at first, you should heat the sample to remove any previous thermal history, then cool the sample and repeat the test on the sample. This helps you to have more useful DSC diagram.

It could possible ,, but first i suggest you to check your equation you used and second if crystallinity comes in negative, that means that your sample was not in proper fibrous Architecture and one more factor here to consider is the gas purged while doing dsc and another major factor could be presence of different phase while melt spinning of pet.

Maryam

It could also be possible that not all of crystals that crystallized melt at the same temperature. Might be you have some polymorphs of different density that melts at different temperature. Alternatively you can also measure relative crystallinity (1-amorphicity) based on the change of heat capacity during glass transition.

I agree with Masoud Tayefi, you have to delete the thermal history of the polymer before making hypotheses.

Record DSC for at last two heating/cooling cycles, possibly by quench cooling, and by comparison between the first and second cycle you will get information about your sample crystallinity. Melt spinning implies a cooling stage that can impart a different degree of crystallinity to the final product: this piece of information is embedded in your first cycle DSC.

However, I suggest performing a parallel TGA experiment to exclude that the second exothermic peak is due to a decomposition reaction.