Crystal structures, of cellulose

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,...

Segment of a crystal structure of cellulose 1 illustrating the relationship between unit cell and crystal structure. Hydrogen bonds (not shown) are in the direction of 0 to B creating sheets of aligned chains. (Adopted from Horii and Wada [39])... 

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-... 

The exact crystal structure of cellulose I, therefore, still leaves room for further investigation. 

This work has shown that the variations in crystal structures of cellulose are not likely to arise from the computer techniques used. When care was taken to make reasonable comparisons of results of the four programs studied, the results were essentially the same. The data is the main factor in determining which of the competing models is selected as best. 

Atalla and Van der Hart (11, 12) concluded, based on their Raman and NMR spectra, that the molecules in cellulose I and II have different conformations. Based on x-ray analyses, Sarko et al. (13i H) and Blackwell et al. (15, 16) both concluded that crystal structures of cellulose I and II were based on parallel and antiparallel packing, respectively, of chains that have similar backbone conformations. Sarko (17) concluded that the allomorphs in the I and II families were based on parallel and antiparallel chains, respectively. The irreversibility may arise from the increase in entropy when parallel packing is converted to antiparallel packing. 

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... 

Whereas the molecular and crystal structure of cellulose IIIn has not yet been solved, that of cellulose IIIj was established from synchrotron and neutron fiber diffraction. It consists of a one-chain unit cell in a monoclinic P2i space group, which implies a parallel arrangement of die cellulose chains. The chain conformation resembles the center chain of the cellulose II stracture, as established previously. The primary hydroxyl groups are in the gauche-trans conformation, and there is one bifurcated hydrogen bond 0-3...0-5 and 0-3...0-6. 

A. Sarko, J. Southwick, and J. Hayashi, Packing analysis of carbohydrates and polysaccharides 7. Crystal structure of cellulose HI, and its relationship to other cellulose polymorphs, Macromolecules, 9 (1976) 857-863. 

A. J. Stipanovieh and A. Sarko, Molecular and crystal structure of cellulose triacetate E A parallel chain structure. Polymer, 19 (1978) 3-8. 

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.)...

Figure 3.4 Crystal structure of cellulose II according to ref. [17], Top viewed along the chain axes, bottom perspective view perpendicular to the chain axes, one corner and one centre chain selected. (Reprinted from ref. 1181 with permission from Elsevier.)...

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... 

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