SynopsisFor many aliphatic and wholly aromatic polymer fibers, meridional small-angle x-ray reflections (SAXR's) are not observed. The morphological features which give rise to this effect have received little attention in the literature. I t is sometimes speculated that this is evidence of a morphological homogeneity along the fiber axis (no fluctuation of electron density-no long periods), ignoring other possible reasons (morphology is heterogeneous but very irregular or the maximum intensity of SAXR ( I -) is low and SAXR cannot be observed against the background of diffuse scattering.) Fibers with homogeneous crystalline morphology should have an elastic modulus E, clase to that of the crystallites E,, along the polymer chains over a broad range of stresses. Rut, as was found in this work for the overwhelming majority of such nonreflection fibers (including Kevlar aramid fibers) there is a considerable difference between E,, and E, (for some nonreflection fibers the ratio E,/E, is on the order of 10). This means that their morphology is heterogeneous along the fiber axis and contains "weaker" intercrystalline regions. This heterogeneity appeared to be regular enough to give rise to SAXR, as determined from the appearance of SAXR during elastic extension and after the beginning of pyrolysis in some nonreflection fibers. Thus, it was concluded that the lack of observable SAXR for these fibers is a result of the weakness of I,,relative to the diffuse scattering. Support for this statement is seen in the analysis of the decrease of I,,,, to zero with increasing draw ratio for poly(viny1 alcohol) fibers.The key factors influencing the decrease of I,, in this case were found to be the decrease of fibril diameter (fibril splitting), an increase of the dispersion of the long periods and some possible decrease in the density difference between crystalline and amorphous regions. The particular combinations of such factors can be different for each particular case of missing SAXR and are discussed in this paper.

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X‐ray scattering from a series of poly(ethylene terephthalate) (PET) fibers spun at differet speeds is analyzed to probe the morphology in the direction transverse to the fiber axis. Both the apparent crystal modulus, determined from the change in wide‐angle X‐ray scattering angle with fiber stretching, and the transverse degree of crystallinity indicate there is a substantial interfiberillar amorphous content. In the PET fiber spun at conventional speeds, only roughly one‐quarter of the fiber cross‐section is actually occupied by fibrils. The transverse crystallinity increases for fibers spun at speeds sufficient to cause crystallization in the spin line. The X‐ray moduli and fibril diameters are correspondingly larger in these high speed spun fibers. © 1993 John Wiley & Sons, Inc.