The nature of secondary crystallization in poly(ethylene terephthalate)

The nature of secondary crystallization in poly(ethylene terephthalate) (PE T) was examined during isothermal crystallization and subsequent melting by time-resolved synchrotron small-angle X-ray scattering (SAXS), differentia l scanning calorimetry (DSC) and temperature modulated DSC (MDSC) technique s. In one experiment, the process of isothermal crystallization was sustain ed over 72 h to induce a relatively large crystallinity (46%, by weight). T he purpose of this experiment was to resolve the issue of controversial ass ignment for the crystal lamellar thickness (I-c) by the correlation functio n analysis of the SAXS data. Results suggest that a two-stage decrease mech anism exists in both long period (L) and I(c )during isothermal crystalliza tion: (1) a significant decrease in the initial stage (primary crystallizat ion dominant), and (2) a much slower decrease in the later stage (secondary crystallization dominant) that is nearly linear with log time. We attribut e this behavior to the formation of thinner separate stacks of lamellae bet ween the primary stacks by secondary crystallization. Both secondary and pr imary stacks can undergo a great deal of crystal perfection and rearrangeme nt with time. From DSC measurements, a triple-melting behavior was observed in the samples crystallized at 205 and 215 degrees C for 1 h, and a double -melting behavior at higher temperatures of 225 and 231 degrees C for 2 h. Temperature scanning SAXS and MDSC directly characterize aspects of crystal perfection and melting. Consistent with some of the literature, we confirm that for short annealing ( similar to hour) at 200-220 degrees C, the firs t (low) endotherm is related to melting of secondary crystals, the middle e ndotherm is due to melting of primary crystals, and the third endotherm is due to melting of crystals reorganized during heating. With prolonged cryst allization at 231 degrees C for 24 and 72 h, a single higher melting endoth erm was observed even though SAXS experiments indicate a slight decrease in average lamellar thickness. In PET, eater exchange reactions contribute to unusual high mobility, allowing chains to avoid topological constraints su ch as entanglements and tie chains. The results suggest that the change in population of tie molecules in the non-crystalline phase reduces the entrop y of melting causing an increase in T-m,, and that this overwhelms the cont ribution of the decrease in I,. (C) 1999 Elsevier Science Ltd. All rights r eserved.