On the origin of the multiple melting behavior in poly(ethylene naphthalene-2,6-dicarboxylate): Microstructural study as revealed by differential scanning calorimetry and X-ray scattering

Abstract

In this article a study on the melting behavior and microstructure of semicrystalline poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) prepared by crystallization from the glass under different annealing conditions is presented. The influence of the annealing temperature (Ta), annealing time (ta), and the heating rate (Rh) at which Ta is reached on the endothermic behavior of the samples was investigated by means of differential scanning calorimetry (DSC). A dual melting behavior appeared for low Rh values (2 deg · min−1) within the range of 145 °C < Ta < 250 °C and 1 min ≤ ta. ≤ 16 h. Samples subjected to fast heating rates (Rh = 200 deg · min−1) to reach a Ta ≥ 230 °C showed DSC traces in which a transition is observed from three peaks to a single melting peak when ta increases in the 30–240 min range. On the basis of the DSC results, PEN samples were prepared displaying single or dual endothermic behavior. The microstructure of these samples was studied by wide (WAXS) and small-angle X-Ray scattering (SAXS) techniques. The SAXS data were analyzed using the correlation function and interface distribution function formalisms, respectively. In samples with a single melting behavior, microstructural parameters such as the long spacing, the amorphous and the crystalline phase thicknesses are consistent with a lamellar stacking model in which the thickness distributions of both phases are almost the same. For samples exhibiting two melting endotherms, a dual lamellar model, which is in agreement with the experimental results is proposed. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1167–1182, 2000