SUMMARY Previous work, at foreign laboratories, essentially based on electron microscopy of longitudinal sections, has suggested a radial morphology for the aromatic poly‐amide high modulus fibres; the present paper gives direct evidence of such a morphology, thanks to a special preparation technique which allows a great improvement in the quality of the cross‐sections of these fibres. This is demonstrated for both a commercial ‘Du Pont de Nemours’ yarn sample ‘Kevlar 29’, and an experimental high modulus aramid RPT (Rhǒne Poulenc Textile) yarn. In the first case, Ag2S insertion technique was used and permitted one to see, on the cross‐sections, an alternation of dark and clear radial bands, again with a tendency towards tangential splitting. In the second case the fibres were included into an amorphous polymer, a sample preparation technique which greatly enhances the quality of the cross‐sections; optical microscopy showed the radial morphology fairly well; dark‐field transmission electron microscopy—using the equatorial doublet of the electron diffraction pattern—allowed satisfactory resolution: both the radial, and occasionally the tangential, splitting, and the size of the cross‐sectioned crystallites were easily revealed: these crystallites appear as isodiametric bright particles c. 15 nm in lateral size.
SynopsisStructural changes occurring during the fatigue failure of polyester fibers have been identified, and a comparison has been made with untested fibers and fibers which were subjected to cyclic loading conditions which did not produce fatigue. Fatigue failure was seen to result in a distinctive fracture morphology. Infrared spectrometry and X-ray diffraction revealed a lowering of crystallinity under fatigue conditions but not under other loading conditions. Transmission electron microscopy and electron diffraction revealed the creation of amorphous zones which are supposed as coalescing to form an amorphous band seen along and ahead of the fatigue crack. The zone just ahead of the fatigue crack tip is shown to contain voids. Crack propagation involves, therefore, the joining up of these voids and development along the amorphous band.
Résumé. 2014 Nous avons mesuré les longues périodes de plusieurs polymères trempés à partir de l'état fondu. Nous donnons la relation entre la longue période mesurée dans la phase semi-cristalline et la rigidité de la chaîne dans l'état liquide juste avant la cristallisation. On montre que la mesure des longues périodes d'échantillons préala-blement recuits à l'état fondu, à différentes températures, et pendant des temps différents, nous permet de remonter à un temps de relaxation des pelottes dans le fondu.Abstract. 2014 The long spacing L of several crystalline polydispersed polymers quenched from the liquid state at various temperatures has been measured. We correlate the variations of the long period L with the variations of the dimension r0 > of the ideal coils in the melt just before the crystallization. The relationship between L and the rigidity C of the liquid chain is given. The annealing of the melt at different temperatures and for different times, and the subsequent crystallization allow us to measure the relaxation time of the coils in the liquid state.
SynopsisCrystallinity 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 area when production speed increases. INTRODUCTIONThere is no doubt that the results of the first studies carried out by x-ray diffractometry on natural or artificial fibers contributed much to the development of the structural models which, apart from some slight changes, are still used for the understanding of the structure properties of modern man-made fibers.The choice made in relation to the idea of a two-phase fibrillar structure with a more or less accentuated separation between the crystalline and amorphous parts explains the interest of the research workers in their effort to determine methods of calculation for this crystalline fraction responsible for the diffraction of x-rays. The appearance of new investigation techniques and their application in the study of fibrous materials have led to other parameters linked with crystallinity (crystallinity by IR spectrography, NMR, neutron diffusion, DSC, etc.) and more or less correlated to that measured by x-ray diffraction. Such divergences linked with the specificity of each technique and of the relative structural level should be used for a...
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