Time-resolved X-ray scattering and calorimetric studies on the crystallization behaviors of poly(ethylene terephthalate) (PET) and its copolymers containing isophthalate units

Time-resolved small-angle X-ray scattering (SAXS) measurements were carried out for PET and its copolymers undergoing isothermal crystallization. Wide-angle X-ray diffraction and differential scanning calorimetric measurements were also performed. Our data analysis of the SAXS results for PET and the copolymers clearly demonstrate that the one layer thickness l₁ (derived directly from the correlation functions of the measured SAXS profiles) is the lamellar crystal thickness d_c, not the amorphous layer thickness d_a. The observed d_c values are found to be always smaller than d_a, regardless of polymer composition. d_c is highly dependent on the crystallization temperature, showing that the degree of supercooling is the major factor determining the thickness of lamellar crystals. No thickening, however, occurs in isothermal crystallizations. The kinked isophthalate units in the copolymer are found to be mostly excluded from the lamellar crystals during the crystallization process, leading to an increase of the amorphous layer thickness. Moreover, the kinked, rigid nature of the isophthalate unit was found to restrict crystal growth along the chain axis of the copolymers and also to lower their crystallinity. Unlike d_c, d_a decreases with crystallization time, causing a reduction of the long period in the lamellar stack. This drop in d_a is interpreted in this paper by taking into account several factors that could influence crystallization behavior: the d_a distribution in the lamellar stacks and its variation with time, the number of lamellae in the lamellar stacks and their effect on the SAXS profile, and the relaxation of polymer chains in the amorphous layers.