Crystal structure and banded spherulite of poly(trimethylene terephthalate)

Developments of single crystal ED patterns in melt-crystallized poly(trimethylene terephthalate) (PTT) have been successfully achieved. Five different zonal electron diffraction patterns containing a total of 14 independent reflections were obtained. The PTT structure under strain-free conditions was identified as a triclinic structure with a = 4.5 Angstrom, b = 6.3 Angstrom, c = 18.2 Angstrom, alpha = 97.51 degrees, beta = 91.4 degrees, and gamma = 111.7 degrees with a calculated density of 1.44 g/cm(3). On the basis of the corresponding reflections in ED patterns, each significant diffraction peak in wide-angle X-ray diffraction powder pattern was identified and indexed. A much more precise determination of the unit cell parameters has been thus achieved. They are a = 4.53 Angstrom, b = 6.20 Angstrom, c 18.70 Angstrom, alpha = 97.6 degrees, beta = 93.2 degrees, and gamma = 110.1 degrees. Unlike poly(butylene terephthalate), structure deformation has not been found in PTT fibers after removal of the extension. This specific feature may explain the better performance in resilience recovery for PTT as compared to PET and PET. Banded spherulite with negative birefringence has been observed in PTT under polarized light microscopy. The formation of banded spherulites is attributed to lamellar twisting. The twisting mechanism was evidenced by the observations of wavylike morphology from reflected light microscopy and transmission electron microscopy. The geometry of crystal lamellae has been identified according to lamellar morphology and its correlated single-crystal ED patterns. The PTT lamellae grow from the basal plane consisting of a and b axes. In connection with the observed morphology, we suggest that the lamellar twisting is attributed to the tilted chain stems which are nonorthogonal to fold surface. The nonorthogonal geometry results from the growth of PTT lamellae with triclinic structure where internal stress is gradually accumulated so as to drive the crystal twist along the radial direction of spherulite.