The performance of synthetic fibers cannot be completely understood without any knowledge of the structure in the noncrystalline regions. X-ray diffraction measurements led to the conclusion that the noncrystalline regions in PET fibers can be divided into isotropic noncrystalline (amorphous) and oriented noncrystalline fractions, the amount of which is dependent on the thermal and mechanical history. But x-ray diffraction data, thermal analysis, and dyeing experiments left many effects unexplained: the dependence of glass-transition and dye uptake on thermal history, the real order-disorder situation within the noncrystalline regions, certain mechanical properties, and their time-dependence such as relaxation. The glass-transition temperature Tg of PET fibers, as determined by viscoelastic measurements, runs through a maximum depending on the heat-setting conditions. This can be explained by the assumption that the average size of crystallites reaches a minimum, and therefore the number of crystallites per unit volume shows maximum values at a heat-setting temperature of about 190°C. By raising the heat-setting temperature above 190°C, the crystallites become larger. Segregation of the crystallites in the noncrystalline matrix occurs.

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