Temperature dependence of the rigid amorphous fraction in poly(ethylene terephthalate)

• The crystalline and rigid amorphous fraction evolution of PET during fusion is determined.
• PET exhibits triple and double melting behaviour after crystallization at Tc’s below and above 215 °C, respectively.
• The discontinuity in the melting behaviour of PET is linked to RAF development at Tc’s below 215 °C.
• The critical temperature for the formation and the disappearance of the RAF in PET is approximately 215 °C.

The analysis of the cp melting curve of poly(ethylene terephthalate) (PET) after non-isothermal crystallization, according to a three-phase enthalpy-based procedure, has allowed to monitor the correct crystalline and rigid amorphous fractions evolution during fusion at low heating rate and to identify the temperature of complete mobilization of the rigid amorphous fraction, located in the proximity of 215 °C. This limit temperature for the disappearance of the rigid amorphous fraction in PET corresponds to the transition between the triple and the double melting behaviour displayed by PET after isothermal crystallization. The multiple melting behaviour of PET is usually attributed to the fusion of original crystals recrystallized during the heating at conventional scanning rate, and to the presence of dual lamellar stacks. In the present study, PET was isothermally crystallized at various temperatures (Tc) ranging from 185 °C to 225 °C, and the double and triple melting profile that is observed at conventional DSC heating rates, after isothermal crystallization at Tc higher and lower than 215 °C respectively, has been put in relation with the absence and the presence of RAF around the original crystal lamellae. All the experimental evidences here reported indicate that the critical temperature for the formation and the disappearance of the rigid amorphous fraction in the PET sample analyzed in the present study at conventional DSC scanning rates, is around 215 °C.

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