I'm studying two samples of PET, the first is a "virgin" PET (with a white color) and the second is a recycled PET (transparent).
In the first run of the first sample there is a double melting peak ( heating rate of 10ºC/min from 25ºC to 290ºC , then a cooling at -10ºC/min from 290ºC to 25ºC), in the second run there's a glass transition with an endothermic hysteresis peak, a cold crystallization and a single melting peak.
In the first run of the second sample there's a glass transition, a cold crystallization and a single melting peak (identical temperature program) and in the second run there's a glass transition and a semi-double melting peak.
I'd like to know if this double peak behaviour can be explained by a melt-recrystallization process, and in the case of first sample the result could indicate that material were crystallized at a smaller cooling rate , so at -10ºC/min it become more amorphous, and just the opposite in the second sample.
The multiple melting behaviour is typical of PET and was extensively studied by Kong et al..
https://doi.org/10.1016/S0032-3861(02)00814-5
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