Structure of Natural Polymer Fibers

The chemical structure of natural polymer fibers is more complex than that of synthetic fibers. Two most important building units for natural polymer fibers are cellulose and protein. Natnral cellulose fibers come from the stringy portions of plants, ranging from the fine seed fibers from the cotton plant to the coarse pineapple leaf fibers. Natnral protein fibers are hairs of animals like the sheep and the delicate filaments spun by silkworms and insects. In addition to these natural fibers, manufactured cellulose and protein fibers also are important fibers that are based on natural biopolymers, but are processed like synthetic polymer fibers. 

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In the course of his studies of the dyeing process, he became deeply interested in the structure of natural fibers, and most of his efforts were directed toward this new field of research, with the help of able associates, among them R. Brill, M. Dunkel, G. von Susich, and E. Valkd. His investigation of various aspects of the problem utilized physical means (for example, x-ray diffraction, optical properties, and viscosity) and the purely chemical approach. A young scientist, H. Mark, who later became an authority in the field of high polymers, was appointed head of the physical chemistry laboratory. 

Once dehydrated, the microfibrils are practically without functionality in ordinary food processing and preparation operations, because the inert microcrystallites are difficult for water to penetrate. The polymorphs, cellulose I and II (Blackwell, 1982 Coffey el al., 1995), are differentiated by their molecular orientation, hydrogen-bonding patterns, and unit-cell structure. Cellulose I is the natural orientation cellulose II results from NaOH treatment under tension of cellulose I with 18-45% alkali (mercerization). The I—II transition is irreversible. Mercerization strengthens the fibers and improves their lustre and affinity for dyes (Sisson, 1943). Sewing thread was relatively pure mercerized cotton until the advent of synthetic polymer fibers. 

In such a way, an ordered supermolecular structure could be spontaneously formed from polymers exhibiting a random coil structure. The fundamental cause of this transformation is naturally the first-order structure of the polymer, i.e. its chemical structure. If a polymer with the proper structural units was prepared, then fiber formation would be accelerated and would especially account for the formation of a highly ordered structure. 

X-ray diffraction techniques are the only way of determining the crystal structure of natural and synthetic polymers, although the x-ray data itself obtained from a crystalline polymeric fiber or film is not sufficient to allow complete refinement of the structure. Conformational analysis and electron diffraction represent complementary methods which will facilitate the determination of the structure. The necessary requirements for the x-ray approach are crystallinity and orientation. X-ray data cannot be Obtained from an amorphous sample which means that a noncrystalline polymeric material must be treated in order to induce or improve crystallinity. Some polymers, such as cellulose andchitin, are crystalline and oriented in the native state.(1 )... 

Cellulose, the most abundant renewable and biodegradable polymer, is the promising feedstock for the production of chemicals for their appUcatimis in various industries. Annual production of cellulose in nature is estimated to be lO"—10 t in two forms, partially in a pure form, for example seed hairs of the cotton plant, but mostly as hemicelluloses in cell wall of woody plants (Klenun et al. 1998). The versatility of cellulose has been reevaluated as a useful structural and functional material. The environmental benefits of ceUulosics have become even more apparent (Hon 1996a). Cellulose is revered as a constmction material, mainly in the form of intact wood but also in the form of natural textile fibers like cotton or flax, or in the form of paper and board. The value of cellulose is also recognized as a versatile starting material for subsequent chemical transformation in production of artificial ceUulose-based threads and films as well as of a variety of cellulose derivatives for their utilization in several industries such as food, printing, cosmetic, oil well drilling, textile, pharmaceutical, etc. and domestic life. 

Chapters 2-10 discuss in detail the different properties of natural lignocellulosic fibers, their processing and fabrication of polymer composites. Chapter 11 summarizes the structure, chemistry and properties of different agro-residual fibers such as wheat straw corn stalk, cob and husks okra stem banana stem, leaf, bxmch reed stalk nettle pineapple leaf sugarcane oil palm bunch and coconut husk along with their processing. 

Lignin is part of the composition of natural polymers in variable proportions. The aromatic structure of the lignin can be used as source of several phenolic products, which may substitute petroleum-based compounds. Bio-based composites have gained prominence over the past two decades owing to both environmental concerns and waste disposal problems. Lignin-based biomaterials include carbon fibers, polymer modifiers, resin/adhesives/binders and others. 

Of course, there are many kinds of natural polymers. Starch and bread are discussed in Section 14.3, and silk fibers in Section 14.4 both are semicrystalline materials. The hierarchical structure of polymers has been reviewed by... 

There have been several applications of IGC to the determination of sur ce interactions (15-24). In particular, IGC was applied to several studies of natural polymers. Among them are cellulose (2, wood (26), potato starch as Amylopectin (27) and lignocellosic surfaces (2S). In these studies, die surface diermodynamic characteristic of wood fiber and its relationship to the fiber s water vapor adsorption was determined by IGC (26) Also, the surface ener, surface acid-base flee energy, enthalpy of desorption of acid-base probes, surface acid-base acceptors, and donor parameters were determined by IGC (26). Cellulose was also found by IGC to be sensitive to the presence of adsorbed water which possibly disorders its surface structure. 

Some thermoplastics are prepared as thin filaments that can be spun into fibers similar to natural fibers. The length of the polymer molecule must be at least 500 nm, which corresponds to a minimum average molecular weight of lO. The structure of the polymer chains must also provide sufficiently strong intermolecular forces to give the fiber an adequate tensile strength. 

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