Vitrification and Devitrification of Rigid Amorphous Fraction of PET during Quasi-Isothermal Cooling and Heating

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Poly(ethylene terephthalate), PET, was studied by quasi-isothermal (QI) temperature-modulated differential scanning calorimetry (TMDSC), in the limit of extremely slow scanning rates. For the first time, both the temperature-dependent crystalline fraction and rigid amorphous fraction (RAF) were quantitatively analyzed during quasi-isothermal cooling and reheating. Specific reversing heat capacity measurements show that most RAF vitrifies step by step during quasi-isothermal cooling after completion of crystallization. Upon subsequent QI reheating, the RAF devitrifies also step by step, and only a small RAF of 0.04 remains at 470 K, while melting starts above 473 K. To obtain the exact temperature of the start of melting, heat capacity measurements were made using subsequent standard DSC heating, after QI cooling. The temperature-dependent crystalline fraction was determined from Euler’s equation. By combining this method with the QI results, the temperature-dependent phase fractions were obtained during standard DSC heating. We conclude that RAF completely devitrifies before the temperature reaches the crystal melting endotherm under the conditions used in this work. Therefore, bulk properties of PET that are dependent upon the fraction of the mobile amorphous phase will change within the temperature range from above T g to the start of observable melting, even though the crystalline fraction is constant in this range.