Molecular Weight and Crystallization Temperature Effects on Poly(ethylene terephthalate) (PET) Homopolymers, an Isothermal Crystallization Analysis

The isothermal crystallization of poly(ethylene terephthalate) (PET) homopolymers with different molecular weight was studied in a wide temperature range (140-230 degrees C) using different experimental techniques. Three different morphological regions, labeled r(1), r(2) and r(3), were distinguished as a function of crystallization temperature (T-c). In r(1) (low T-c) crystallized samples were characterized by a low crystalline degree with a small spherulite texture containing thin crystals. In r(2) (intermediate T-c) samples showed medium size spherulites composed of two distinct crystalline families (thin and thick crystals). In this temperature range, the crystallization exhibited a maximum value and it was associated with a high content of secondary crystals. In r(3) (high T-c), samples presented considerable amorphous zones and regions consisting of oversized spherulites containing only thick crystals. Time-resolved wide-angle X-ray diffraction measurements, using synchrotron radiation, indicated a rapid evolution of the crystalline degree within the second region, in contrast with the quite slow evolution observed in the third region. On the other hand, by small-angle X-ray scattering (SAXS) and time-resolved SAXS experiment, it was found that the long period (L) as well as the lamellar thickness (lc) increase as a function of T-c, corroborating the formation of the thickest crystals in the third region. From all these observations, a morphological model was proposed for each region.