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
Dear German,
Your second sample is recycled transparent pet. I assume it to be a bottle flake. ıf the first white sample is of the same family ie polymer made for bottle production I have an explanation to your double peaks. The make up of bottle pet contains some added isophthalic acid in order to render polymer difficult to crystallise during bottle blowing so to avoid possible splitting or breaking of thicker parts which will cool slower and has tendency to crystallise. This means we are facing a randomly polymerised co-polymer named poly ethylene terephthalate- co- isophthalate. This will inevitably have a range of crystal structures that will co exist. Depending on the thermal history you may see one or more peaks at thermodynamic phases.
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