Double (or multiple) melting peaks may be due to melingt-recrystallization-remelting phenomenon in which thickening of lamellae occurs or due to the occurrence of at least two distinct crystal populations. Scanning at different heating rates is a powerful tool for distinguishing between those two explanations. if at fast heating rates the double peak transforms into a single peak then it can not originate from a melting-recrystallization-remelting process on heating.

I'm not familiar with PET, but if it can crystalize in two or more polymorphisms , then you may get double melting peak too if the cooling condition allows forming different crystalline forms.

you can consult this paper too:

DOI 10.1007/s00216-009-3169-y

THanks ,

I quenshed my sample them immedatily heat it up in DSC ( first hrat cycle 10 C/min )  until 270 then hold it for 5 min then cooled it dowen to RT by 10 C/min , then second heat run (10/min) in same go. (( heat-cool-heat mode)

@Al shammari...

For your series of questions on PET, answers may be in these series of very well written papers by Groeninckx...

1. Morphology and melting behavior of semicrystalline poly(ethylene terephthalate). I. Isothermally crystallized PET.

2. Morphology and melting behavior of semicrystalline poly(ethylene terephthalate). II. Annealed PET

3. Morphology and modulus–temperature behavior of semicrystalline poly(ethylene terephthalate) (PET)

Double (or multiple) melting peaks may be due to melingt-recrystallization-remelting phenomenon in which thickening of lamellae occurs or due to the occurrence of at least two distinct crystal populations. Scanning at different heating rates is a powerful tool for distinguishing between those two explanations. if at fast heating rates the double peak transforms into a single peak then it can not originate from a melting-recrystallization-remelting process on heating.

I'm not familiar with PET, but if it can crystalize in two or more polymorphisms , then you may get double melting peak too if the cooling condition allows forming different crystalline forms.

you can consult this paper too:

DOI 10.1007/s00216-009-3169-y

The thermal treatment you are giving the sample is almost sure to make twoi types of crystals (or two crystallite size populations (surface free energy etc.)) which will have two melting points.  If you have access to depolarized light intensity analysis with a hot stage you can probably follow the process optically.  Also, check to see that the sample has not separated into two blobs.  If so the differences in the thermal resistanxces between the double samples could produce apparent multiple melting points.