Hierarchial Structure and Physical Properties of Natural Cellulosic Fibers
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HIERARCHIAL STRUCTURE ANDPHYSICAL PROPERTIES OF NATURAL CELLULOSIC FIBERS
LUDWIG REBENFELD TRI/Princeton, P.O. Box 625, Princeton, NJ 08542 USA Abstract Natural cellulosic fibers have in common the fact that cellulose is the key polymeric component in the structure, although the chemical composition varies widely depending on the specific source of the fibers. Cellulose is a long-chain linear condensation polymer of f-D-glucose with three free hydroxyl groups on each monomeric unit, resulting in strong inter- and intramolecular hydrogen bonds. Because of the hydrogen bond network, and also due to restricted rotation around the polymeric 1,4the chemical with a T. well above and stiff chain cellulose is a rigid i-linkage, decomposition temperature. Despite the high T. native cellulose is invariably highly crystalline as a result of the biosynthetic process. In naturally occurring cellulosic fibers, the cellulose crystallites are aggregated into fibrils which constitute the underlying building blocks of the fiber. In cotton, the fibrils are laid down during the development or growth of the fiber in the form of concentric layers. The fibrils are disposed at an angle of 23* with respect to the fiber axis and thus they describe a helical pattern. The sense of the helix reverses frequently along the length of the fiber. This morphology is unique to cotton; other cellulosic fibers such as ramie and jute have similar fibrillar structures, but fibrillar angles in the 5 to 10 degree range, with no reversals. Wood fibers, on the other hand, are structurally more heterogeneous and may be considered as composites. Introduction The complex hierarchial structure of natural cellulosic fibers provides an excellent example of the subtle dependence of material properties on chemical composition, molecular architecture, and supermolecular morphology. This paper will emphasize the structure and properties of cotton fibers, although there are many other fibers derived from plants that are of commercial interest. Table I. CHEMICAL COMPOSITION OF NATURAL CELLULOSIC FIBERS Fiber
Cellulose
HemiCellulose
Pectin
Lignin
Extractives
Cotton Kapok
94.0 43.2
2.0 32.4
2.0 6.6
15.1
Flax Hemp Ramie Jute
71.2 74.3 76.2 71.5
18.5 17.9 14.5 13.3
2.0 0.9 2.1 0.2
2.2 3.7 0.7 13.1
6.0 3.1 6.4 1.8
Abaca Sisal
70.0 73.2
21.8 13.3
0.5 0.9
5.7 11.0
1.8 1.6
-
2.0 -
The chemical composition of natural cellulosic fibers is shown in Table I. As can be seen, cellulose is the key component of these fibers and its chemical structure has been established as poly-1,4-p-D-anhydroglucopyranose. The generally accepted value of the degree of polymerization for native plant cellulose is 3,000. In the solid
Mat. Res. Soc. Symp. Proc. Vol. 255. 01992 Materials Research Society
400
state, cellulose is strongly hydrogen bonded, both inter- and intramolecularly, through its hydroxyl groups. Because of this hydrogen bonding and also in view of its high molecular weight, cellulose is insoluble in normal nonreactive solvents. The intermolecular hydrogen
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