Native cellulose crystal structure

TEMPO-mediated oxidation. With regenerated and mercerized celluloses, the oxidation leads to water-soluble p-l,4-linked poly glucuronic acid sodium salt (cellouronic acid, CUA) quantitatively [16]. In contrast, with native celluloses having the cellulose I crystal structure, the cellulose slurries maintain the slurry states even after TEMPO-mediated oxidation. These modified celluloses form water-insoluble fibers [17]. This has enabled modification of the surface of cellu-losic fibers. 

The Unit Cell Dimensions of the Crystallites Present. Cellulose occurs in four recognized crystal structures designated Cellulose I, II, III, and IV (27). These can be distinguished by their characteristic x-ray diffraction patterns. Cellulose I is the crystal form in native cellulosic materials. Cellulose II is found in regenerated materials such as viscose filaments, cellophane, and mercerized cotton. Cellulose III and IV are formed by treatment with anhydrous ethylamine and certain high temperatures, respectively. These four crystal forms differ in unit cell dimensions—i.e., the repeating three-dimensional unit within the crystalline regions. These dimensions are shown in Table VI for the four crystal forms. 

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. 

Polymers with rigid, cyclic structures in the polymer chain, as in cellulose and poly(ethy-leneterephthalate), are difficult to crystallize. Moderate crystallization does occur in these cases, as a result of the polar polymer chains. Additional crystallization can be induced by mechanical stretching. Cellulose is interesting in that native cellulose in the form of cotton is much more crystalline than cellulose that is obtained by precipitation of cellulose from... 

Saito, T. and Isogai, A., TEMPO-mediated oxidation of native cellulose. The effect of oxidation conditions on chemical and crystal structures of the water-insoluble fractions. Biomacromolecules 2004, 5 (5), 1983-1989. 

Woodcock C, Sarko A (1980) Packing analysis of carbohydrates and polysaccharides, II. Molecular and crystal structure of native ramie cellulose. Macromolecules 13 1183-1187... 

Cellulosic materials usually form crystal structures in part, and water cannot penetrate the inside of crystalline domains at room temperature. Native celluloses form crystalline microfibrils or bundles of cellulose chains 2-5 nm in width for higher plant celluloses and 15-30 nm for algal celluloses, which are observable by electron microscope. Almost all native celluloses have X-ray diffraction patterns of cellulose I with crystallinity indexes (Cl) 13] of about 40-95 %. 

Recently, solid state - C-NMR analysis of native celluloses has revealed the presence of two different crystal structures in native cellulose I, celluloses and I, on the basis of the resonance patterns of Cl singlet for and... 

Single crystals of cellulose II have been obtained by controlled saponification of cellulose triacetate. They have a lamellar structure and give rise to the x-ray diagram of cellulose II. The chain molecules are oriented at 90° to the plane of the lamellae, and, since their length considerably exceeds the thickness of the lamellae, it is thought that chain folding must take place. Similar folding has been proposed in the case of synthetic polymers and native cellulose. ... 

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 )... 

Current fiber x-ray diffraction studies, including new calculations by the authors, are reviewed. Because of different conventions used to describe the crystal structure of native cellulose, the preferred parallel structure described as "up by Gardner and Blackwell for Valonia corresponds to the "down" structure that was strongly rejected for ramie by Woodcock and Sarko. 

It was observed In earlier studies of controlled alkall-mercerlzatlon of ramie cellulose that the crystal structure of native cellulose Is transformed to cellulose II through a series of crystalline alkali-cellulose complexes (1,2). The relationships between these "Na-celluloses" and their pathways of transformation are Illustrated In Fig. 1. It has further been observed that all of the transformations are crystal-to-crystal phase changes, not Involving Intermediate amorphous phases. All of the experimental evidence has suggested... 

At this point the major features of the structures of cellulose I and II can be considered to be solved. Nevertheless, questions remain, especially as regards the differences between the Infrared, Raman, and solid state NMR spectra obtained for different native celluloses, all of which appear to have the crystal structures shown in Figure 1. This has been a further motivation for our analyses of the structures of cellulose solvent complexes, which are described below. 

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