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Cellulose unit cell structures

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. [Pg.169]

Most of the conformations proposed for cellulose have a two-fold screw axis along the chain, but it has been suggested that this concept be abandoned and a new, monoclinic, unit-cell structure similar to that of the original Sponsler-Dore cell has been put forward (unit-cell dimensions a = 10.85, b = 10.3, c = 12.08 A. (8 = 93°14 ). [Pg.220]

Table VI. The Unit Cell Structures of the Various Crystal Forms of Cellulose... Table VI. The Unit Cell Structures of the Various Crystal Forms of Cellulose...
Early work on the fine structure of cellulose has been reviewed extensively by Sisson [255] and by Hermans [256]. The observations of Herzog and Jancke [257] who, in 1920, recognized that natural cellulosic material gave identical x-ray diagrams and led to the conclusion that these materials should have identical crystalline structure. The unit cell structure proposed by Meyer and coworkers [258-260] is still used to describe cellulose. The structure of the unit cell for cellulose is shown in Figure 10.82. [Pg.749]

Figure 4.17 Structure of cellulose unit cell (Cellulose I). Source. Reprinted with permission from Dyer J, Daul GC, Rayon Fibers, Lewin M and Pearce EM eds., Handbook of Fiber Chemistry, Marcel Dekker, New York, 775, 1998. Copyright 1998, CRC Press, Boca Raton, Florida. Figure 4.17 Structure of cellulose unit cell (Cellulose I). Source. Reprinted with permission from Dyer J, Daul GC, Rayon Fibers, Lewin M and Pearce EM eds., Handbook of Fiber Chemistry, Marcel Dekker, New York, 775, 1998. Copyright 1998, CRC Press, Boca Raton, Florida.
Less organized (amorphous) cellulose is also present along with the crystalline cellulose. The crystalline forms la and 1(3 differ by their crystalline unit cell structure and overall hydrogen bonding pattern, but the main intermolecular hydrogen bond is the same for both, i.e., 06-H 03 (Fig. 21.2). The intramolecular hydrogen bond of 03-H 05, which is partly responsible for the cellulose chain stiffness and contributes to load transfer along the chain, is also shown in Fig. 21.2. Other crystalline forms of cellulose include cellulose 11, cellulose 111, and cellulose... [Pg.541]

One of the first structures to be determined was the natural polysaccharide cellulose. In this case the repeat unit is cellobiose, composed of two glucoside rings. In the 1980s, NMR experiments established that native cellulose is actually a composite of a triclinic parallel-packed unit cell called cellulose / , and a monoclinic parallel-packed unit cell called cellulose I. Experimentally, the structures are only difficultly distinguishable via X-ray analysis (7a, 7b). Figure 6.1 (3) illustrates the general form of the cellulose unit cell. [Pg.241]

A few words have already been said about the crystalUne structure of cellulose (see Figure 6.1). The monocUnic unit cell structure illustrated for cellulose I was postulated many years ago by Meyer et al. (3-6) and has been confirmed many times. The b axis is the fiber direction, and the cell belongs to the space group P2, containing four glucose residues. [Pg.253]

Based on the unit cell structure for cellulose I, calculate its theoretical crystal density. (See Rgure 6.1.)... [Pg.320]

Figure 5.2. Unit cell structure of cellulose in cotton. Figure 5.2. Unit cell structure of cellulose in cotton.
It is interesting to note that in their first paper on cellulose (11) Meyer and Mark proposed a structural unit cell model which is classic and accepted, for the largest part, even today. They proposed a cellulose crystallite in which all... [Pg.63]

Using the two-chain unit-cell,3 with a = 0.817 nm, b = 0.785 nm, c = 1.034 nm, andy = 96.38°, the modified intensity-data of Mann and coworkers,37 and several residue-geometries, the structure of native ramie cellulose was refined. The resulting R factors were 15.8%, 18.5%, and 17.5% for, the antiparallel, parallel-up, and parallel-down models, respectively. A temperature factor of 0.23 nm2 was necessary in order to obtain a good fit with the observed data. It was suggested that the antiparallel packing of the chains cannot be discounted for cotton and ramie celluloses. [Pg.386]

Single crystals of the D-glucan from Acetobacter xijlinum were prepared in water-methanol solutions at room temperature. Using electron and X-ray diffraction, a hexagonal unit-cell, with a = h = 0.518 nm and c = 2.0 nm, was derived. It was concluded that the chain axes lie parallel to the surface of the lamellar crystals, that there are 2.5 water molecules per /3-D-Glcp residue, and that the structure is similar to that of cellulose hydrate II (Ref. 42). [Pg.388]

It is shown that ether oxygen atoms must participate in the hydrogen bonding in all three modifications. The symmetry of the cellulose chain and the relation of the two chains in the unit-cell are discussed with reference to the number of distinguishable 0—H groups. Structures consistent with the infrared data and stereochemical considerations are described. [Pg.308]


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See also in sourсe #XX -- [ Pg.178 ]




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