Cellulose crystalline structure

Cellulose pyrolysis has been studied in detail from a variety of points of view mainly related to chemical utilization of wood pyrolysis products or to fire related problems. Analytical pyrolysis of cellulose is not often used as a tool for cellulose detection, but it is a common procedure for studying the pyrolysis products. A variety of analytical procedures have been applied for this study, pyrolysis/gas chromatography/mass spectrometry (Py-GC/MS) being the most common [11-16]. Besides Py-GC/MS, other analytical procedures also have been utilized, such as Py-MS [17,18], Py-IR [19], and off-line Py followed by HPLC [20]. The Py-MS spectrum of cellulose was shown in Figure 5.4.1 (B). Some procedures applied GC/MS on derivatized pyrolysis products (off-line), the derivatization being done by silylation [21], permethylation, perbenzoylation [22], etc. Information about cellulose also has been obtained from the analysis of pyrolysis products of several cellulose derivatives, such as O-substituted cellulose [23]. Also the study of cellulose crystalline structure with X-ray during pyrolysis has been used [23a] to generate information about the pyrolysis mechanism. 

Several parameters can be used to determine the cellulose crystalline structure. The d-spacings between cellulose chains can be calculated using the Bragg equation. The crystalline index, proposed by Hermans et al. [7,30] is described as follows ... 

Kataoka, Y. and Kondo, T. (1996) Changing cellulose crystalline structure in forming wood cell walls. Macromolecules, 29 (19), 6356-6358. 

Kataoka, Y. and Kondo, X. (1998) FT-IR microscopic analysis of changing cellulose crystalline structure during wood cell wall formation. Macromolecrdes, 31... 

Cheng C, Varanasi P, Li CL, Liu HB, Menichenko YB, Simmons BA, Kent MS, Singh S (2011) Transition of cellulose crystalline structure and surface morphology of biomass as a function of ionic liquid pretreatment and its relation to enzymatic hydrolysis. Biomacromolecules 12(4) 933-941. doi 10.1021/bml01240z... 

In addition, cellulose undergoes changes in crystalline structure with relative ease. The most common modification is the conversion of cellulose I (i.e. la and 1/8) to cellulose II. This can be achieved by dissolution and regeneration or by simply treating cellulose I with sodium hydroxide. Cellulose II is usually considered to be more thermodynamically stable than biosynthesised cellulose I. However,... 

The X-ray diffraction spectrum in Figure 8 shows the crystalline structure of a normal cellulosic membrane. Diffraction peaks appeared around 10, 11, 16, and 21 degrees of 20. This spectrum... 

How modeling has been useful in the crystal structure analysis of polysaccharides—and how it could lead to a better understanding of other condensed j)hase states—can be illustrated with structural worK done on cellulose. It is one of the world s most important and widely used raw materials whose structure, properties, derivatives, and transformations remain under continuous study. Some of the results, problems, and indications of future directions resulting from the study of its crystalline structure—and the attendant roles for molecular modeling—are briefly described in the following. 

The most relevant property of stereoregular polymers is their ability to crystallize. This fact became evident through the work of Natta and his school, as the result of the simultaneous development of new synthetic methods and of extensive stractural investigations. Previously, the presence of crystalline order had been ascertained only in a few natural polymers (cellulose, natural rubber, bal-ata, etc.) and in synthetic polymers devoid of stereogenic centers (polyethylene, polytetrafluoroethylene, polyamids, polyesters, etc.). After the pioneering work of Meyer and Mark (70), important theoretical and experimental contributions to the study of crystalline polymers were made by Bunn (159-161), who predicted the most probable chain conformation of linear polymers and determined the crystalline structure of several macromolecular compounds. 

At the present moment it is difficult to decide which of the two hypotheses concerning the structure of cellulose is correct the idea of an amorpho-crystalline structure, or that postulating solely an amorphous texture. Nikitin assumes that the first hypothesis is the more probable, more especially as it is well in line with the most recent work of Zaydes and Sinitskaya [45] who conclude on the basis of electron diffraction investigations that in the natural cellulose fibre of Chinese nettle, there exist phases having a distinct microcrystalline structure. This suggests that structures shown in Figs. 78, 79 and 80 are the most probable. 

The studies of Miles and Craik [13] merit special attention since they obtained the following X-ray diagrams of ramie (Chinese nettle) cellulose, which is notable for a distinctly crystalline structure (Fig. 83). 

To hydrolyze crystalline cellulose efficiently by enzymatic means, the inaccessibility of crystalline structures must be overcome. T. reesei and some other true cellulolytic microorganisms produce a cellulase complex that is capable of efficiently hydrolyzing crystalline cellulose. One explanation of this capability was first proposed by Mandels and Reese (7). In this model, two factors, Ci and C worked together to disrupt and hydrolyze cellulose. Ci first disrupted the crystalline structure of the cellulose while Cx attacked the available sites formed by Ci. In other words, Ci and C exhibit synergism in hydrolyzing cellulose. Since then, the combined action of cellobiohydrolase ( Ci ) and endoglucanase ( C ) has been identified as the source of the apparent synergism (6,26,55). 

Major obstacles in the hydrolysis of cellulose are the interference of lignin (which cements cellulosic fibers together) and the highly ordered crystalline structure of cellulose. These obstacles necessitate a costly pretreatment step in which elementary cellulosic fibrils are exposed and separated. 

Early on, before the existence of macromolecules had been recognized, the presence of highly crystalline structures had been suspected. Such structures were discovered when undercooling or when stretching cellulose and natural rubber. Later, it was found that a crystalline order also existed in synthetic macromolecular materials such as polyamides, polyethylenes, and polyvinyls. Because of the polymolecularity of macromolecular materials, a 100% degree of crystallization cannot be achieved. Hence, these polymers are referred to as semi-crystalline. It is common to assume that the semi-crystalline structures are formed by small regions of alignment or crystallites connected by random or amorphous polymer molecules. 

Corn stover, like lignocellulosic materials in general, is resistant to enzymatic hydrolysis, because of both the tight network in the lignocellulose complex and the crystalline structure of the native cellulose. These difficulties can be overcome by employing a suitable pretreatment (7). 

Hemicelluloses are constituted of different hexoses and pentoses glucose, mannose, xylose, etc. Since these heteropolysaccharides are often branched polymers, they cannot constitute crystalline structures. However, their function in the constitution of natural fibres is crucial. Together with lignin, they constitute the bonding matrix of the cellulose microfibres. 

The crystalline structure of cellulose has been characterized by X-ray diffraction analysis and by methods based on the absorption of polarized infrared radiation. The unit cell of native cellulose (cellulose I) consists of four glucose residues (Figs. 3-6 and 3-7). In the chain direction (c), the repeating unit is a cellobiose residue (1.03 nm), and every glucose residue is accordingly displaced 180° with respect to its neighbors, giving cellulose a... 

Gao, P. J., Chen, G. J., Wang, T. H., Zhang, Y. S., and Liu, J. 2001. Non-hydrolytic disruption of crystalline structure of cellulose by cellulose binding domain and linker sequence of cellobiohydrolase I from Penicillium janthinellum. Shengwu Huaxue Yu Shengwu Wuli Xuebao, 33,13-18. 

Anhydrous ammonia also is known to cause temporary platicization of wood. The ammonia swells and plasticizes both the lignin and the cellulose, and the crystalline structure of the cellulose is converted to a different form in the process. To shape the wood, it is immersed in liquid ammonia or treated with gaseous ammonia under pressure until the cell walls have been penetrated and the wood becomes pliable and flexible. In this condition it is easily shaped and formed by hand or mechanically. The ammonia readily vaporizes and evaporates from the wood, so that the wood regains its normal stiffness but retains the new form into which it has been shaped. With this process the wood can be distorted into quite complex shapes without springing back to its original form. Treating plants have been developed on a pilot-plant scale, but the process has not been widely adopted. 

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