Cellulose chain axis, orientation

Orientation. The orientation of the cellulose chain axis in a number of different fibers has been studied in detail (21-22). Much less is known about the cellulose orientation in the plane perpendicular to the chain axis. The orientation in this plane is determined by the lateral arrangement of the microfibrils relative to each other. In algal celluloses, the evidence from x-ray and electron diffraction indicates that the microfibrils are arranged nonrandomly in the plane perpendicular to the chain axis (21-29). Preston (22) proposed the model shown in Figure 1 to explain his x-ray data. There are two different orientations of the microfibrils. The 002 planes in one set of microfibrils are approximately perpendicular to the 002 planes in the second set. In both sets of micro-fibrils, the 101 planes are oriented parallel to the cell wall surface (refer to Figure 1). Preston s model has been confirmed in more recent studies (29). In the remainder of this report, the type of orientation shown in Figure 1 will be referred to as alternating orientation. 

Two classes of experiments were conducted. In both sets of experiments, fibers in which the cellulose chains are oriented parallel to the fiber axis were used. In the first class of experiments, the plane of polarization of the incident light was changed relative to the axis of the fibers by rotating the fibers around the optical axis of the microscope (see Figure 2a). The dependence of the band intensities on the polarization of the incident light was studied to determine the directional character of the vibrational motions. This information was used to advance the assignment of the Raman spectrum of cellulose. Spectra from Valonia, ramie, and mercerized ramie fibers, which have different allomorphic compositions, were compared to study the structural differences between the allo-morphs. 

Lamellar, single crystals of ivory-nut mannan were studied by electron diffraction. The base-plane dimensions of the unit cell are a = 0.722 nm and b = 0.892 nm. The systematic absences confirmed the space group P212121. The diffraction pattern did not change with the crystallization temperature. Oriented crystallization ofD-mannan with its chain axis parallel to the microfibril substrates, Valonia ventricosa and bacterial cellulose, was discovered ( hetero-shish-kebabs ). 

FIGURE 5.8 Projections of short cellulose chains. Leftmost along chain axis. Left-center along ribbon edge. Right-center Maximal width view. Rightmost Ball and stick model in the same orientation as the space-fiUing model to its left. 

Cellulose orientation in the plane perpendicular to the chain axis was studied by recording spectra of ramie cross sections with different polarizations of the incident light relative to the cell wall surface. 

In the second class of experiments, spectra were recorded from ramie fiber cross-sections. The plane of polarization of the incident light was changed relative to plane of the cell wall by rotating the cross-sections around the optical axis of the microscope axis (see Figure 2b). The information from these spectra was used to study the orientation of the cellulose in the plane perpendicular to the chain axis. 

The variation in the relative intensities of the 1095 and 1123 cm"l bands between the 0 and 45" spectra suggests anisotropy in the cellulose orientation. Table 1 shows that these peaks are skeletal stretching modes that are most incense when the electric vector of the incident light is parallel to the chain axis. Since the 1095 cm"l peak is very sensitive to the orientation of the incident electric vector relative to the chain axis, the intensity variation suggests that Che plane of sectioning was not exactly perpendicular to Che cellulose chain axes so chat Che chains are tilted relative to the plane of sectioning. 

If the cellulose is oriented randomly in the plane perpendicular to Che chain axis, then the band intensities would be the same regardless of whether the incident electric vector was parallel, perpendicular, or 45" to the cell wall surface. The cross-section spectra, therefore, are consistent with random cellulose orientation in the plane perpendicular to the chain axis. These results conflict with our earlier spectra of tension dried cotton fibers (34) that indicated the methines were oriented preferentially perpendicular to the cell wall surface. More recent spectra of cotton fibers have shown that if the fibers are not dried under tension, the methine orientation is random in the plane perpendicular to the chain axis. Therefore, it appears the cellulose orientation can be influenced by the sample preparation methods. Since microtoming exerts large forces on the fibers, it is also possible that the cellulose orientation could have been disrupted during the preparation of the cross-sections. Further experiments will be necessary to understand the factors which influence the cellulose orientation. 

Spectra recorded from ramie cross-sections suggest that the cellulose is oriented randomly in the plane perpendicular to the chain axis. It appears, however, that the sample preparation methods can influence the cellulose orientation. Therefore, further studies will be necessary to characterize the molecular orientation in cellulose fibers. 

As previously mentioned, natural fibres present a multi-level organization and consist of several cells formed out of semi-crystalline oriented cellulose micro fibrils. Each microfibril can be considered as a string of cellulose crystallites, linked along the chain axis by amorphous domains (Fig. 19.10) and having a modulus close to the theoretical limit for cellulose. They are biosynthesized by enzymes and deposited in a continuous fashion. A similar structure is reported for chitin, as discussed in Chapter 25. Nanoscale dimensions and impressive mechanical properties make polysaccharide nanocrystals, particularly when occurring as high aspect ratio rod-like nanoparticles, ideal candidates to improve the mechanical properties of the host material. These properties are profitably exploited by Mother Nature. 

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. 

In 1937, Mayer, Mark, and Misch conducted the X-ray experiment on cellulose powder, which resulted in determining a unit cell of cellulose 1 that is still regarded as an early reference. On the basis of X-ray diffraction pattern, researchers proposed a model with monoclinic unit cell (a = 8.35 A b = 7.0 A c = 10.3 A y = 96°) with chains oriented antiparallety [7]. One unit cell was situated in the corner, while the other at the center of the cell, parallel with the chain axis (Fig. 21.3). In this approach, the alternating glucose units of the chain were rotated through 180° so that the origin... 

Typically, this approach leads to a large number of possible structures, because Jones allowed both the 0-3 -0-5 and 0-2-0-6 type of intra molecular hydrogen bond. Comparison of the most plausible structures for cellulose I and II with observed equatorial and meridional intensity led Jones to conclude that several structures, including that of Meyer and Misch, are fair approximations to the actual structure, but none of them show especially good agreement with the x-ray intensities observed. As a possible solution, Jones suggested a statistical structure, in which there is randomness of chain polarity, but in which adjacent chains have one characteristic shift along the b axis when they have parallel orientation, and a different one for the antiparallel situation. 

The actual state of the xylans and glucomannans in the living tree is still largely unknown. Since they do not crystallize readily without some chemical modification, such as partial depolymerization or removal of side chains, or both, it would seem likely that they occur in the amorphous state in the wood. The possibility that they could form microfibrils, as does mannan B in vegetable ivory, cannot be entirely excluded.More probably, however, they occur as a powder between and around the cellulose microfibrils. Using a polarized infrared technique, Marchessault and coworkers have obtained indications that, not only the cellulose, but also the xylans and glucomannans, may be oriented in the direction of the fiber axis in wood. 

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