Natural fibers native cellulose

Native cellulose are commonly modified by physical, chemical, enzymic, or genetic means in order to obtain specific functional properties, and to improve some of the inherent properties that limit their utility in certain application. Physical/surface modification of cellulose are performed in order to clean the fiber surface, chemically modify the surface, stop the moisture absorption process, and increase the surface roughness. " Among the various pretreatment techniques, silylation, mercerization, peroxide, benzoylation, graft copolymerization, and bacterial cellulose treatment are the best methods for surface modification of natural fibers. 

Nishikawa and Ono recorded the crystaUine nature of cellulose using the X-ray diffraction patterns from fiber bundles from various plants. Cellulose is known to exist in at least four polymorphic crystalline forms, of which the structure and properties of cellulose 1 (native cellulose) and ceUulose II (regenerated cellulose and mercerized cellulose) have been most extensively studied. As a first approximation, the crystal structure of cellulose I determined by X-ray diffraction can be described by monoclinic unit cell which contains two cellulose chains in a parallel orientation with a twofold screw axis (Klemm et al. 2005). Cellulose I has two polymorphs, a triclinic stmcture (la) and a monoclinic structure (IP), which coexist in various proportions depending on the cellulose source (Azizi Samir et al. 2005) (Nishiyama 2009). The la structure is the dominate polymorph for most algae (Yamamoto and Horii 1993) and bacteria (Yamamoto and Horn 1994), whereas ip is the dominant polymorph for higher plant cell wall cellulose and in tunicates. 

Figure 21.2 shows the X-ray difffactograms of the natural fibers studied. In order to examine the intensities of the diffraction bands, establish the crystalline and amorphous areas more exact and determine the crystallite sizes, the diffractograms were deconvoluted using Gaussian profiles. Crystallographic planes are labeled according to the native cellulose structure as described by Wada et al. [30]. 

By adding a non-dissolution solvent such as water, ethanol or acetone, cellulose miscible in ILs can be precipitated from solution and then separated from the mixture of IL and nondissolution solvent either by filtration or centrifugation. Due to non-volatile nature of IL, it can be recovered by distillation of the mixture, thus eliminating the non-dissolution solvent. The precipitated cellulose can appear in different forms such as monoliths, fibers and films and can have the same degree of polymerization and polydispersity as native cellulose but it... 

Ott was sensitive to the full range of industrial issues, as well as all the scientific ones. The nature of a cellulose sample was a strong function of its source and method of purification. It was also a strong function of the processing conditions and history. The crystal structure of mercerized cellulose is different than native fiber cellulose. Cellulose is often used as fibers. The morphology of cellulose fibers was one of Ott s specialties. 

Manufactured cellulose fibers have a very different physical structure from natural cellulose fibers. The crystal lattice formed by native cellulose chains in natural cellulose fibers is called Cellulose 1 (Figure 5.2). When cellulose is dissolved and spun into fibers, the crystal lattice changes to Cellirlose 11. Table 5.3 compares the imit cell parameters and crystal densities of Cellirlose 1 and Cellulose 11. The... 

Cellulose layers are produced from native, fibrous or microcrystalline cellulose (Avicel ). The separation behaviors of these naturally vary, because particle size (fiber length), surface, degree of polycondensation and, hence, swelling behavior are all different. 

Acetylation rates have also been studied by Centola37 who treated natural and mercerized ramie fibers for varying times with acetic anhydride and sodium acetate and examined the reaction products chemically and by X-ray diffraction. The reagent was considered to penetrate into the interior of fibers. A heterogeneous micellar reaction was believed to occur that converted a semi-permeable elastic membrane around the micelles into the triacetate. The rate of acetylation of mercerized ramie was observed to be faster than that of unmercerized fiber. Centola concluded that about 40 % of the cellulose in native ramie is amorphous and acetylates rapidly. 

Work with electron microscopes showed that there is preferential enzymatic activity at only one end of the native microfibrils. This indicates that the reducing ends are all at one end of the microfibril and thus the chains are parallel, not antiparallel [240]. Electron microscopy and diffraction work on algal and bacterial cellulose confirmed the parallel-up nature of the chain orientation in the unit cell and the addition of new glucose residues to the cellulose chain at the nonreducing end [241]. Similar attempts with ramie fibers were not successful. 

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 )... 

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