Crystal structure and banded spherulite of poly(trimethylene terephthalate)

Developments of single crystal ED patterns in melt-crystallized poly(trimet hylene terephthalate) (PTT) have been successfully achieved. Five different zonal electron diffraction patterns containing a total of 14 independent r eflections were obtained. The PTT structure under strain-free conditions wa s identified as a triclinic structure with a = 4.5 Angstrom, b = 6.3 Angstr om, c = 18.2 Angstrom, alpha = 97.51 degrees, beta = 91.4 degrees, and gamm a = 111.7 degrees with a calculated density of 1.44 g/cm(3). On the basis o f the corresponding reflections in ED patterns, each significant diffractio n peak in wide-angle X-ray diffraction powder pattern was identified and in dexed. A much more precise determination of the unit cell parameters has be en thus achieved. They are a = 4.53 Angstrom, b = 6.20 Angstrom, c 18.70 An gstrom, alpha = 97.6 degrees, beta = 93.2 degrees, and gamma = 110.1 degree s. Unlike poly(butylene terephthalate), structure deformation has not been found in PTT fibers after removal of the extension. This specific feature m ay explain the better performance in resilience recovery for PTT as compare d to PET and PET. Banded spherulite with negative birefringence has been ob served in PTT under polarized light microscopy. The formation of banded sph erulites is attributed to lamellar twisting. The twisting mechanism was evi denced by the observations of wavylike morphology from reflected light micr oscopy and transmission electron microscopy. The geometry of crystal lamell ae has been identified according to lamellar morphology and its correlated single-crystal ED patterns. The PTT lamellae grow from the basal plane cons isting of a and b axes. In connection with the observed morphology, we sugg est that the lamellar twisting is attributed to the tilted chain stems whic h are nonorthogonal to fold surface. The nonorthogonal geometry results fro m 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.