Real-Time Evolution of the Lamellar Organization of Poly(ethylene terephthalate) during Crystallization from the Melt:  High-Temperature Atomic Force Microscopy Study

High-temperature atomic force microscopy (AFM) was used for in-situ monitoring of melt−crystallization of poly(ethylene terephthalate) (PET) at 233 °C. The observed evolution of the lamellar structure allowed identification of the stack thickening process at the secondary crystallization stage. This finding explains the puzzling decrease of the small-angle X-ray scattering (SAXS) long period observed during isothermal annealing of PET. The quantitative analysis of high-temperature AFM images provides statistically meaningful parameters for the semicrystalline structure and an accurate choice of a structural model for the interpretation of SAXS data.