Native cellulose crystallites

Aqueous suspensions of cellulose microcrystalhtes obtained by acid hydrolysis of native cellulose fibers can also produce a cholesteric mesophase [ 194]. Sulfuric acid, usually employed for the hydrolysis, sulfates the surface of the micro crystallites and therefore they are actually negatively charged. Dong et al. performed some basic studies on the ordered-phase formation in colloidal suspensions of such charged rod-like cellulose crystallites (from cotton filter paper) to evaluate the effects of addition of electrolytes [195,196]. One of their findings was a decrease in the chiral nematic pitch P of the anisotropic phase, with an increase in concentration of the trace electrolyte (KC1, NaCl, or HC1 of < 2.5 mM) added. They assumed that the electric double layer on... 

To express the results in terms of a few distinctly linear stages is a simplification. We have reviewed a number of factors that can influence the ease of chain scissioning. A point that we have not discussed, however, is the fact that several researchers (31, 32) have suggested that high-molecular-weight native celluloses may have a significant crystallite or chain-dislocation unit of the order of DP = 500. The overall rate must certainly represent transitions between the predominating influence of one or another of the factors noted. 

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. 

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. 

X-ray diffraction studies have shown that native cellulose is a two phase system one is amorphous, with lower order and compactness, and is localized at the elemental fibril surface the other one is highly ordered and compact (crystallites), where the polymer chains are well organized (crystalline structure) and strongly bound by hydrogen bonds. 

In native cellulose, the structure develops under conditions of thermodynamic equilibrium and occurs very slowly. For regenerated cellulose, however, not only must the structure be formed rapidly, but also the organization of the macromolecules by crystallization is constrained by the extent of tangling present in the solution. It was suggested by Baker [261] that the structure of cellulose derivatives could be represented by a continuous range of states of local molecular order rather than by definite polymorphic forms of cellulose. This view is supported by the observation that the x-ray diffraction pattern of rayon often reveals both cellulose II and IV components to an extent, depending on the conditions used to make the fiber. Hindeleh and Johnson [262] have described an x-ray diffraction procedure to measure crystallinity and crystallite size in cellulose fibers by which the relative proportions of cellulose II and IV in rayon can be determined. 

Visible are the individual crystallites after removal of the amorphous areas by hydrolysis. The finegrained crystals are a few nanometer In size, with similar dimensions in all directions (isome-trio). The sample was a rayon fiber (compare to the native cellulose in shown in Fig. 5.73). 

The transformation of cellulose I to cellulose II during mercerization has been suggested to be the result of a progressive shift of the sheets of cellulose chains within the crystallites of a microfibril from the quarter-staggered relationship in cellulose I to the complete correspondence found in cellulose II. Observed changes in lateral discnrder, cell dimensions, swellii, and AT-ray diffraction reflections of cellulose fibres support this theory. Such a shift may occur in the transformation of native celluloses with antiparallel structures as well as those with parallel polarity. 

Figure 21.2 shows the X-ray difffactograms of the natural fibers studied. In order to examine the intensities of the diffraction bands, establish the crystalline and amorphous areas more exact and determine the crystallite sizes, the diffractograms were deconvoluted using Gaussian profiles. Crystallographic planes are labeled according to the native cellulose structure as described by Wada et al. [30]. 

The structure of cellulose has been studied since the 19th century, when Carl von Nageli proposed the idea that natural cellulose contains ciystalline micelles—small crystallites (Wilkie, 1961 Zugenmaier, 2009). Only 70 years later, this idea was confirmed by X-ray diffraction, and as a result, the first model of monoclinic unit cell for crystalline structure of native cellulose Cl was developed by Mayer and Mish (Mayer et al., 1937). The model of Mayer and Mish with antiparallel arrangement of chains existed 30 years, whereupon it was replaced by a more accurate model with parallel arrangement of cellulose chains within crystallites (Gardner et al., 1974). Later it was discovered that in addition to crystalline structure of native cellulose Cl, there are also other crystalline allomorphs, CII, CIII, and CIV (O Sullivan, 1997). 

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. 

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