The Crystal Structure of Cellulose

Relative Reactivities of the Hydroxyl Groups Within the D-Glucose 

This article will deal mainly with recent aspects of the determination of the structure and reactivity of cellulose. Some mention is also made of the chemical treatments which are used to modify the physical properties of cellulose, although as yet relatively little is known concerning their exact nature. 

Two previous reviews on the structure of cellulose have appeared in this Series, and a different publication also contains much relevant material. 

Cellulose I is natural cellulose as found in cotton, wood, and Valonia. Although the Meyer and Misch structure (unit-cell dimensions a = 8.35, 

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 ). 

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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. 

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.

The 13C-CP-MAS spectrum has also been used to complement the diffraction data relating to the crystal structure of cellulose, but hitherto the results have been inconclusive [242]. A significant amount of H single crystal spectroscopy has been reported and has been used as a basis for correlation with neutron diffraction data and with theoretical ab-initio molecular orbital calculations [243, 244]. 

The crystal structure of cellulose I trinitrate (CTN I), prepared from cellulose I, differed from that of CTN II prepared from cellulose II. Recrystallized CTN I and CTN II were both regenerated to give cellulose II. The unit cell of CTN II is monoclinic, with a = 1.23 nm, b (fiber axis) = 2.54 nm, c = 0.855 nm, and /3 = 91°. The CTN I has a bent chain structure, and CTN II has a bent-twisted type of structure. The relationships of cellulose polymorphs to those of CTN were examined. 

Figure 2. Axial projection (top) and planar projection (bottom) of the crystal structure of cellulose I. The planar projection shows the hydrogen-bonding network within the layers. (Reproduced with permission from Ref. 8. Copyright 1974, Elsevier Scientific Publishing Company,...

In keeping with its important position as the world s most abundant, renewable, raw material, efforts at understanding the crystal structure of cellulose have proceeded apace over the past fifteen years, since authori-... 

Figure 4 shows the chain conformations of cellulose I ("bent") and of cellulose II ("bent and twisted") proposed in our previous work (28) on the crystal structure of cellulose II. These models of cellulose I and II have one and two kinds of Intrachain (03 -05) hydrogen bond, respectively. The number of 0-H stretching peaks in... 

Two interesting points are the number of cellobiose units per cell for cellulose triacetates I and II is 4, versus 2 for celluloses I and II and the measured density for cellulose triacetate II was 1.315 g/ cc, which is less than the calculated density of 1.348 g/cc as expected because cellulose triacetate is not 100% crystalline. The above studies on the crystalline structure of cellulose triacetate lead to the conclusion that commercial heat-treated cellulose triacetate is expected to have the cellulose triacetate II crystalline structure. Analysis of the crystal structure of cellulose triacetate continues [55]. 

Figure 3.3 Comparison of the crystal structures of Cellulose / (left) and /p (right). Top viewed along the chain axes, middle perpendicular to the chain axes and in the plane of the hydrogen bonded sheets, bottom perpendicular to the chain axis and the hydrogen bonded sheets. (Reprinted with permission from ref. [14], Copyright 2003 American Chemical Society.)...

The most recent investigations on the crystal structure of cellulose are those by Meyer and Misch, Gross and Clark, Kiessig, Kubo, Schiebold and Peirce. ... 

See also the recently appeared papers by F. T. Peirce, Trans. Faraday Soc.y 42 (1946) 545, 560 where also a general improvement of the picture of the crystall structure of cellulose accounting for some thus far unexplained features in the X-ray pattern is given. 

Ford Z.M., Stevens E.D., Johnson G.P., and French A.D. 2005. Determining the crystal structure of cellulose IIIj by modeling. Carbohyr Res, 2005. Carbohydr Res 340 827-833. 

Cellulose EAPap material is composed of molecular chains with a dipolar nature. In particular, the crystal structure of cellulose II is monoclinic, which is noncentro-symmetric and exhibits piezoelectric and pyroelectric properties. To investigate the dipole effects in EAPap, thermally stimulated current (TSC) measurement was conducted (Hongo et al. 1996). The classical procedure in TSC includes (1) heating the sample to a given temperature (200°C) (2) applying... 

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. 

Natural cellulose is now referred to as cellulose I. Cellulose 11 is obtained from cellulose I by the mercerization process, i.e., soaking and wash out with aqueous NaOH. The crystal structure of cellulose is shown in Fig. 1.20. Later researches have proposed variant crystal structures of cellulose (cellulose I [118], cellulose II [119, 120], and indeed a cellulose III [121], and cellulose IV [122]) different from the Meyer-Misch model. 

Figure 21.8 The crystal structures of cellulose la (on left side] and...

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