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

In this article a study on the melting behavior and microstructure of semic rystalline poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) prepared by c rystallization from the glass under different annealing conditions is prese nted. The influence of the annealing temperature (T-alpha), annealing time (t(alpha)), and the heating rate (R-h) at which T-alpha is reached on the e ndothermic behavior of the samples was investigated by means of differentia l scanning calorimetry (DSC). A dual melting behavior appeared for low R-h values (2 deg min(-1)) within the range of 145 degrees C < T-alpha < 250 de grees C and 1 min less than or equal to t(alpha). less than or equal to 16 h. Samples subjected to fast heating rates (R-h = 200 deg min(-1)) to reach a T-a greater than or equal to 230 degrees C showed DSC traces in which a transition is observed from three peaks to a single melting peak when t(alp ha) 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 c orrelation function and interface distribution function formalisms, respect ively. In samples with a single melting behavior, microstructural parameter s such as the long spacing, the amorphous and the crystalline phase thickne sses are consistent with a lamellar stacking model in which the thickness d istributions 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. (C) 2000 John Wiley & Sons, Inc.