Poly(trimethylene teraphthalate) crystal structure and morphology in different length scales

Poly(trimethlene teraphthalate) (PTT) is one of the terephthalic polyester family members which can crystallize between its glass transition and melting temperatures. The crystal structure has been determined using both electron diffraction (ED) on single crystals and wide angle X-ray diffraction on powder and oriented fibers. The unit cell is triclinic with dimensions of a=0.46 (3) nm, b=0.61 (2) nm, c=1.86 (1) nm, α=97.5°, β=92.1°, and γ=110°. The bulk PTT samples usually crystallize to form spherulites in the crystallization temperature (Tc) region studied. Between Tc=135 and 165°C, the spherulites show a banded texture, and the band spacing increases with increasing Tc. The radial direction in the spherulites has been determined to be the crystal a-axis. Observations of successive ED patterns taken along the radial direction of a spherulite within one band reveal twist of lamellar crystals along the spherulite radial direction. The chain-folding direction, determined using a polyethylene decoration method, is along the a-axis direction in the lamellar crystals and parallel to the radial direction in the spherulites. Linear growth rates of the spherulites have also been measured and the maximum growth temperature is located at 165°C. This temperature is also the upper-limit temperature for PTT banded texture spherulite formation.