AbstractCrystallinity measurements were carried out on PET fibers following Ruland's method, taking into account an isotropic disorder parameter, and the “relative method” which is used for routine controls. Two extreme kinds of samples were studied: relatively crystallized industrial yarns (tire cords) with different abilities of shrinkage and annealed without tension, and preoriented yarns (POY) of very low crystallinity, produced at speeds of 1000–4000 m/min. Special care was taken during the experimental work—for sample preparation, monochromatization of the x‐ray beam, corrections of intensity values, drawing of the background. In this way it was possible to find an excellent correlation between crystallinities measured by Ruland's calculations and indexes of crystallinity obtained by the relative method. There is a large increase of crystallinity in heat‐treated samples when a strong shrinkage is allowed. In this case the disorder parameter which is specific of the crystalline phase is also increased. There is only a slight increase in the crystalline fraction in stabilized samples (little or no ability to shrink) when annealed, but an improvement of the order of the crystalline areas. Nevertheless it appears impossible to improve the order of the crystallites in shrunk fiber above a “critical value,” even if extended heat treatments are carried out. There is a small crystalline fraction in POY produced at 3000–3300 m/min; these “crystalline nuclei” are characterized by their very low disorder parameter (thermal disorder). In order to estimate the mesophase in POY x‐ray patterns, an attempt was made to draw the boundary between peaks and background, following two references: first, an “amorphous fiber” x‐ray diagram, then a “calculated true amorphous” one. It was shown that the estimated mesophase fraction in low‐oriented yarns (spinning speed 1000–2500 m/min) progressively decreases at the benefit of a crystalline
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