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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.

Some possibilities of using ultramicrotomy in an electron-microscope study of cellulosic fibers are considered.Interest is focused on the importance of preparative techniques, especially "fixation" and "inclusion." The well-known fact of expansion of the secondary wall (separation into lamellas of various thicknesses) is considered to be the consequence of polymerization of the methacrylate within the accessible regions of the swollen fibers after inclusion. Fixation, i.e., cross-linking, is considered to occur either in formaldehyde treatment or in periodic acid oxidation; in such cases, no polymerization takes place inside the fibers and, accordingly, no burstings are observed on thin sections. In the case of alkali treatments of sufhcient concentration, homogeneous swelling is observed and no fixation is necessary to prevent the sections from swelling when they come in contact with water in the ultramicrotome bowl.Some histochemical techniques, designed to enhance contrast in the electron image and reveal fine structure, are discussed. Silver ions can be used after periodic acid oxidation, followed by thiosemicarbazide blocking; uranyl and lead give good results, especially when applied to fibers rich in free carboxylic groups; and caesium can be introduced through an exchange reaction by using alcoholates so that swollen structures can, thus, be studied. _