Crystallites, in cellulose

The mean cross sectional area (A) of the crystallites in cellulose [32] is obtained as follows ... 

Sisson has traced the evolution of current concepts of the crystalline part of cellulose structures. The fiber diagram obtained by X-ray diffraction is now known to be produced by a series of elementary crystals, called crystallites, which have a definite arrangement with respect to the fiber axis. It is also known that the crystallites in regenerated cellulose may be oriented to varying degrees with respect to the fiber axis and that the crystallites in regenerated cellulose and mercerized cotton differ from those in native fibers. These hydrate type crystallites appear to be more reactive chemically than the native type. 

From these considerations there evolved the concept of "primary valence chains" in cellulose, held together in bundles, or micelles (crystallites) by secondary forces, as propounded by Meyer and Mark (5). This view was then extended to encompass other high polymers as well. It should be noted however, that Freudenberg had already proposed a chain structure for cellulose, based on degradation experiments (6). If the micelles were to... 

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... 

FIGURE 5.17 Cellulose diffraction patterns. Top left synchrotron radiation x-ray diffraction pattern for cotton fiber bundle. The fiber was vertical and the white circle and line correspond to a shadow from the main beam catcher and its support. (Credit to Zakhia Ford.) Top right electron diffraction pattern of fragments of cotton secondary wall. The much shorter arcs in the top right figure are due to the good alignment and small number of crystallites in the electron beam. (Credit to Richard J. Schmidt.) Bottom a synthesized powder pattern for cellulose, based on the unit cell dimensions and crystalline coordinates of Nishiyama et al. [209]. (Credit to Zakhia Ford.) Also shown are the hkl values for the Miller indices. The 2-theta values are for molybdenum radiation instead of the more commonly used copper radiation. 

Water vapor at room temperature will not penetrate well-defined crystallites but will be adsorbed in the amorphous regions. Consequently, moisture sorption measured gravimetrically at a given relative vapor pressure and temperature has been used to determine order in cellulosic materials. In the case of Valentine [252] and Jeffries [253], the fraction of ordered material was obtained by correlating moisture sorption with values obtained by the deuterium... 

The surface of crystallites also represents a portion of the cellulose component readily accessible to chemical agents. Thus, the nature of crystallite surface along with crystallinity influences the apparent accessibility of cellulose, especially when measured by chemical reactions. Among major polymorphs, cellulose I and II are most important in cellulose reactions. 

Cellulose is found not to be uniformly crystalline. However, the ordered regions are extensively distributed throughout the material and these regions are called crystallites [27]. Cellulose exists in the plant cell wall in the form of thin threads with an indefinite length. Such threads are cellulose micro-fibrils, playing an important role in the chemical, physical, and mechanical properties of plant fibers and wood. 

It apparently depends on the amount of the fillers (hence, amount of the plastic) and ability of the fillers to interfere with crystallization of the plastic. The less the crystallites in the filled plastic, the less the shrinkage. The less the plastic in the filler composite, the less the shrinkage. At the same filler loading, fillers with nucleation effects lead to lesser mold shrinkage. For example, when polypropylene, showing mold shrinkage of 1.91%, was fllled with some mineral and cellulosic fillers, its mold shrinkage was as follows [2] ... 

The Unit Cell Dimensions of the Crystallites Present. Cellulose occurs in four recognized crystal structures designated Cellulose I, II, III, and IV (27). These can be distinguished by their characteristic x-ray diffraction patterns. Cellulose I is the crystal form in native cellulosic materials. Cellulose II is found in regenerated materials such as viscose filaments, cellophane, and mercerized cotton. Cellulose III and IV are formed by treatment with anhydrous ethylamine and certain high temperatures, respectively. These four crystal forms differ in unit cell dimensions—i.e., the repeating three-dimensional unit within the crystalline regions. These dimensions are shown in Table VI for the four crystal forms. 

We wish to express our profound thanks to Dr. J.-F. Revol of the Pulp and Paper Research Institute of Canada for his interest, discussions, and his willingness to characterize the lateral dimensions of crystallites in certain of the algal cellulose preparations. 

In the low frequency region (Figure 7), there are only minor differences between the spectra of native ramie and Valonia. The peaks in the Valonia spectra are narrower and better resolved. The reason for this is probably the larger size of the crystallites in Valonia cellulose (38-39). When the crystallites are larger, the environment of the molecules is more homogeneous. Therefore, the vibrational energy of the molecules is more uniform, resulting in narrower bands. 

The differences between the spectra of ramie and Valonia are quite small compared to the differences between native cellulose and cellulose II (see Figure 7). In the spectra of ramie and Valonia, the different peak widths and relative intensities can be attributed to the difference in the crystallite sizes. In the spectrum of cellulose II, however, the frequency and number of peaks is significantly different. In previous publications, the differences between the spectra of celluloses I and II have been interpreted as evidence for different conformations in celluloses I and II (40-41). The spectral differences which are indicative of conformational change are not observed in the spectra of ramie and Valonia. Since ramie and Valonia have different I to Ip ratios, it would appear that celluloses 1 and Ip must have similar molecular conformations. 

Moisture Regain. The advantage of this method is that water vc r normally does not penetrate well-defined crystallites of cellulose (26). Thus the amount of moisture adsorbed a sample after conditioning at a i >ecific relative ve r pressure and tenf)erature can be used to estimate its accessibility and degree of order. In one method using water sorption (27), the fraction of amorf ous material (F ) has been calculated from the sorption ratio (SR) of the sanple using the equation... 

J. F. Revol and D. A. I. Goring, Directionality of the fiber c-axis of cellulose crystallites in microfibrils of Valonia ventricosa. Polymer, 24 (1983) 1547-1550. 

Crystallites of cellulose have been isolated from wood pulp in this way by treatment with acid to hydrolyze and remove the amorphous regions. Typical dimensions of the remaining crystallites were 46 nm long by 7.3 mn wide, corresponding to bundles of about 100 to 150 chains in each crystallite. 

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