Thermal Behavior of Poly(ethylene terephthalate) Crystalline and Amorphous Phases by Wide Angle X‐ray Scattering

Abstract

Structural changes in the crystalline and amorphous phases of semicrystalline, compression molded, isotropic poly(ethylene terephthalate (PET) samples obtained from bottle grade resin, when it was heated from room temperature to temperatures near the crystalline melting temperature, were investigated by wide angle X‐ray scattering (WAXS). Semicrystalline PET samples were obtained by thermally treating initially amorphous samples at 120°C and 200°C for 24 h, and WAXS curves were taken in intervals of approximately 20°C up to a temperature near Tm. For each temperature, crystallographic data were obtained by using Winplotr and Celref programs to perform fittings of the WAXS curves and to calculate related parameters like unit cell dimensions, density, and volume, as well as crystallite sizes in directions, degree of crystallinity (X c), and distance between chains in the amorphous phase. The crystallite sizes in the and directions slightly increased, indicating a growth in the crystal lateral dimensions, while in the direction practically no variation was observed. The unit cell volume had a discrete increase due to thermal expansion, and its density decreased. The X c had a significant decrease, changing from 34% to 14% in the sample treated at 120°C, and from 34% to 21% in the sample treated at 200°C. The X c decrease with increasing temperature was accomplished by the increase in the intensity and symmetry of reflections and crystal rearrangement due to melting/recrystallization processes of crystals formed at lower annealing temperatures. The increase in temperature leads to variation in the two amorphous halos between 0 and 35°. While the distance between chains remains constant and the area increases for the first halo (located at approximately 17.5°), the distance between chains and the area of the second halo (which is located at about 24.5°) decreases. This second halo was associated with paracrystallinity.