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

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Abstract
A detailed description of the morphology and the complex melting behavior of poly(ethylene terephthalate) crystallized isothermally in the range 100–245°C is presented. Double or single melting endotherms can deliberately be obtained by varying the heating rate in the differential scanning calorimeter (DSC). In the case of double melting peaks, the first endotherm (I) corresponds to the melting of the crystalline material formed at the crystallization temperature Tc. The second melting endotherm (II) originates from the melting of a fraction of the original crystalline material reorganized during the DSC scan. The appearance of one melting endotherm (I or II) relates to the melting of the original structure or the melting of a completely transformed structure. Taking the end of the melting endotherm I as the final melting temperature TM of the material, an equilibrium melting temperature of 290°C is obtained from a plot of TM vs. Tc. A detailed morphological study of the isothermally crystallized samples by electron microscopy and both small‐angle and wide‐angle x‐ray scattering raises questions about the use of the two‐phase concept in relating morphological parameters and melting behavior at low crystallization temperatures.