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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 Å, b = 6.3 Å, c = 18.2 Å, α = 97.5°, β = 91.4°, and γ = 111.7° with a calculated density of 1.44 g/cm³. 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 Å, b = 6.20 Å, c = 18.70 Å, α = 97.6°, β = 93.2°, and γ = 110.1°. Unlike poly(butylene terephthalate), structure deformation has not been found in PTT fibers after removal of the extension. This specific feature …