Morphology of Secondary Wall Fibrils in Cotton

Abstract

The secondary wall of cotton fiber contains well aligned helical macrofibrils of about 1000 Å diameter, changing sign in irregular intervals. The sign reversals are in some respect cross sections of enhanced mechanical strength, particularly in respect to swelling. The macrofibrils are composed of a great many microfibrils of varying width, which, as a rule, seem to be narrow, ribbon-like lamellae consisting of one single layer of elementary fibrils. The former have lateral dimensions of about 200 Å, the latter of about 50 Å, as may be derived from electron micrographs of stained samples. The mechan ical deformation of such a system of helices involves a relatively large amount of sliding motion of every single helix down to the smallest independent element which may be the microfibril or some larger agglomerate, e.g., the macrofibril. Surface forces between adjacent helices reduce the capability for sliding and, therefore, enhance the rigidity, elastic mod ulus, shape retention, and brittleness of the cotton fiber.

From the absence of an equatorial small-angle, x-ray scattering maximum, one concludes that the lateral dimensions of elementary fibrils must appreciably vary in the cotton fiber, in spite of the fact that on single micrographs their thickness and packing seem rather uniform. In the longitudinal direction, the crystalline core exhibits some interruptions and the quasiamorphous boundary layers between adjacent elementary fibrils exhibit a great many interruptions at irregular intervals. If this is not an artifact, it can be interpreted as an occasional lateral extension of the less ordered material beyond the boundary layer, thus interrupting the crystal lattice of the fibril, and as crystalline bridges by which the fibrils laterally coalesce. The coalescence is favored in the (101) surfaces containing more hydrogen bridges per unit area than the (101) planes, thus producing single-layered lamellae of the microfibrils. Dark-field electron microscopy further re veals a finite extension of equally oriented crystal lattice blocks. The subsequent blocks in the fibril axis seem to have the same orientation of molecular chain axis, but are mismatched in the perpendicular direction. The corresponding screw dislocation planes between consecutive blocks seem to be less permeable to swelling agents than the quasiamor phous layers between the fibrils.