Structure of Celluloses

Cellulose can be regarded as a polymer of glucose formed by condensation through removal of water molecules. It may well be that cellulose, and also other polysaccharides, are synthesized from glucose in plants according to the equation  

This hypothesis is based mainly on the results of hydrolysis, in which glucose is the only product. In contact with dilute sulphuric acid cellulose is broken down as follows  

The yield of this reaction may approach the theoretical value (about 96%). 

D-glucose. This reaction demonstrates the basic difference between the structure of starch and that of cellulose. Starch, in contact with an enzyme amylase (diastase), is broken down to form a different biose maltose, which when hydrolysed in the presence of acid is also converted into D-glucose. 

The constitution of both of these bioses has been established by Haworth et al. [15]. It was found that the compounds differed from one another in the spacial configuration of the oxygen bond (glucosidic bond) this linkage joins the carbon atoms 1 and 4 which occupy the /3-position in cellobiose, and the a-position in maltose  

---

Study of the structure of cellulose (Figure 22.2) leads one to expect that the molecules would be essentially extended and linear and capable of existing in the crystalline state. This is confirmed by X-ray data which indicate that the cell repeating unit (10.25 A) corresponds to the cellobiose repeating unit of the molecule. 

FIGURE 7.27 The structure of cellulose, showing the hydrogen bonds (blue) between the sheets, which strengthen the structure. Intrachain hydrogen bonds are in red and interchain hydrogen bonds are in green. 

In the case of grinding, the cellulose fibers go over a state of fine fibrillation into a more or less powdery substance. This mechanical severance of cellulose may break main valence bonds and will, therefore, decrease its degree of polymerization. In addition, the crystal structure of cellulose fibers is nearly lost [32]. Grinding of the cellulose fibers also, appreciably increases its surface area. 

The molecular structure of cellulose, unlike that of starch, allows for strong hydrogen bonding between polymer chains. This results in the formation of strong water-resistant fibers such as those found in cotton, which is 98% cellulose. Cotton actually has a tensile strength greater than that of steel. The major industrial source of cellulose is wood ( 50% cellulose). 

The CP/MAS NMR spectra are an important source of information regarding the structure of cellulose and its polymorphos. A number of groups have investigated these spectra 11 15) and also reviews on the subject have been published 16 17>. For an orientation in the field Table 1 shows the most important features of the solid-state NMR spectra of cellulose I, II and IV and in Fig. 3 the numeration of the carbon atoms of the cellulose basic unit is given. It is evident that the polymorphs... 

The incorporation of water in the structure of cellulose influences. Upon the hydrogen bond structure of the macromolecule. A great deal of work has been done in this area. Calorimetric methods have been invaluable in helping to solve the problem 23 It is, however evident that solid-state NMR spectroscopy may also give valuable information. 

The structure of cellulose/solvent system complexes has been described by several schemes, differing essentially in the role played by the Ii+ and Cl ... 

The length-to diameter ratio strengthens the assumption that pectic acid is a fairly rigid, rod-hke molecule and comparable to the structure of cellulose... 

The application of water-repellent finishes to fabrics actually involves a chemical reaction between the material and the finish. Cellulose-based fibers such as cotton possess hydroxyl (-OH) groups that exist on the surface of fabrics spun and woven from the fiber. The basic structure of cellulose portrayed in Fig. 7.6.1 reveals... 

The structure of cellulose reveals the two key components for hydrogen bonding OH functional groups (the X-H unit) and additional oxygen atoms (the Y atom). Hence, water molecules may form intermolecular hydrogen bonds with cellulose in one of two ways ... 

In Geneva, he resumed with new energy his studies of macromolecules. He was able to obtain the cooperation of A. J. H. van der Wijk, who was one of his most devoted coworkers the latter s realistic criticisms were a valuable balance to Meyer s great enthusiasm. Studies on the thermodynamics of large molecules in solution, and on the structure of cellulose and chitin, were pursued with C. Boissonnas, W. Lothmar, and L. Misch. A theory of the elasticity of rubber evolved from his work with C. Ferri and his previous observations with Susich and Valk6. 

Incredibly, a review of Mark s publications also shows that he authored six papers on surface characteristics and dying, five on the X-ray structure of cellulose and polymers, four on X-ray diffraction (including one on the structures of CIi, and CBrij), and twelve review papers. In all Mark contributed to more than seventy papers. More impressive, he penned six books while at the University of Vienna. Two of his books, "Roentgenographic Untersuchung von Kristallen" coauthored by F. Halla (57) and "Hochpolymere Chemie"coauthored by K. H. Meyer (58), were particularly well received. A seventh book,... 

FIGURE 9.20 Chemical structure of cellulose (a), which is a glucose polymer, and xylan (b), a typical component of hemicellulose. 

Cellulose is a linear polymer (polysaccharide) made up of glucose monomers. Figure 46 shows the molecule structure of cellulose. It has a degree of polymerisation (DP) on average 10 000 glucose units, which corresponds to a length of 5 im. The chemical formula for cellulose is usually written as [(CgHjjO,) ]. [64,65]... 

This implies that the selective layer of reverse osmosis membranes may have a different origin from that of the micelles. Such a case is clearly identified by examination of the skin structure of cellulose acetate/poly(bromophenylene oxide phosphonate) alloy membranes (1 ), which exhibit a high flux and high salt separation (Figure 13). The skin rests on an assembly of giant spheres (up to 1 pm in diameter) and is certainly originated by a different coagulation mechanism than that of the spheres. 

As the above results show, the gross features of the cellulose I crystal structure predicted by various methods do not differ appreciably, but the accompanying deviations in the R -factors are significant. When these predictions are used to assess, for example, whether the cellulose I crystal structure is based on parallel- or antmarallel-chains, the range in the R"-factors seen for the parallel models (cf. Table II) is comparable to that between the two different polarity models. As shown in Fig. 5, the most probable parallel- and antiparallel-chain structures of cellulose I, refined by minimizing the function O, differ in R -factors by approximately the same extent as the three predictions for the parallel model shown in Fig. 4 and Table II. 

Figure 3. Isolated chain conformations of cellulose predicted by MM2(85) (left) and PS79 (middle). The conformation on the right is that of the crystal structure of cellulose I (3). Hydrogen bonds are shown by dashed lines.

---