Cellulose polymorphic forms

Recent studies of the Raman spectra of Celluloses I, II, and IV have indicated that the different polymorphic forms involve two basically different molecular conformations in addition to the differences in crystalline packing (7,8,9). The conformation variations suggested by the Raman spectra are such that they could play an important role in determining the susceptibility of glycosidic linkages to attack by hydrolytic agents. The questions raised by this possibility will be addressed in this chapter. 

Chitin is the 2-acetamido derivative of cellulose and serves as the fibrous component of skeletal tissues in many lower animals. At least two polymorphic forms of chitin have been recognized, (26) of which the a- and 0-forms are the best characterized. The unit cells and space groups of a- and 0-chitins are given in Table 1. Both have approximately the same fiber repeat as cellulose, and apparently have the same 2j helical conformation. 

Cellulose powders can be created by cutting fibers into small particles, perhaps with a Wiley mill (Arthur H. Thomas Company, Swedesboro, New Jersey). On a laboratory x-ray system, powder diffraction patterns take 30 min. The positions of the peaks indicate the polymorphic form (I-IV) the powder diffraction pattern is often used as a fingerprint for comparison with the known pattern for a given crystalline form [207]. The breadth of the peaks is related to the extent of crystallinity (Figure 5.17, bottom). Using the Scherrer formula [245,246] and assuming no other distortions, the crystallite size can be calculated. Values for cotton perpendicular to the molecular axis are around 40 A. That corresponds to a 6x6 array of... 

This is best illustrated by viewing the spectra of highly ordered samples of the three well-established polymorphic forms of cellulose, celluloses I, II, and IIIj, shown in Figure 7. The designation of cellulose III as III] is to indicate that it was prepared from cellulose I. Indeed the sample of the spectrum which is shown in Figure 8 was prepared from the sample that gave rise to the spectrum identified as I in the same figure. 

The discussion of cellulose dissolution must recognize that cellulose can exist in four polymorphic forms native cellulose known as cellulose I polymorph cellulose II obtained by regeneration of cellulose I cellulose III, which is derived from the liquid ammonia treatment of cellulose I or cellulose II and cellulose IV, which refers to the thermal treatment of cellulose I or cellulose 111 [2]. It is important to recognize these distinctions because the respective cellulose polymorphs can have different solubility characteristics in particular solvents, as will become evident further in this chapter. 

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. 

Polymorphism, which means existence of different crystalline forms, is a phenomenon observed in many organic and inorganic compounds. Also, natural (e.g., cellulose) and synthetic polymers show numerous polymorphic forms. Among synthetic semicrystaUine polymers, which can be a matrix in composites with lignocellulosic materials, the isotactic polypropylene is the one to take a close look on this phenomenon. 

Looking further at Figure 15-12, it would be useful to have some idea of other low-energy regions that are not populated by observed structures, and the barriers between a particular isolated structure and the majority conformation. Finally, computerized models are an important aide to thinking about why observed structures occur. For example, is the twofold conformation found in all of the pure cellulose polymorphs an intrinsically ideal form, or is it the result of intermolecular forces resulting from crystallization ... 

Going from the polymorphic forms of cellulose back to cellulose I is difficult but can be accomplished by partial hydrolysis. The subject of the polymorphic forms of cellulose has been reviewed (10,11). 

X-ray diffraction analysis indicates that the cellulose fibers formed from this solvent system exist in the cellulose III polymorph conformation. This polymorph structure is revealed by X-ray diffraction peaks at circa 20.8°, which correspond to both the (002) and (101) planes and another at circa 12.1° which corresponds to the (101) plane. The intensity of these X-ray diffraction peaks of the fiber suggests that it consists of a crystalline structure, in this case cellulose III crystals. As displayed in Table 12.2, the d(oo2) and d(ioi) are similar to those of cellulose III. These interplanar spacings are the average distance between the crystalline unit planes, and they are different from one cellulose polymorph to another. This suggests that the CH2OH moiety of the cellulose polymers are in the gg conformation and are free of... 

In the present work, we extend the method to compensate for the hydrogen bonds present in carbohydrates. The hydroxylated character of carbohydrate polymers influences between-chain interactions through networks of hydrogen bonds that occur during crystallization. Frequently, several possible attractive interactions exist that lead to different packing arrangements, and several allomorphic crystalline forms have been observed for polysaccharides such as cellulose, chitin, mannan and amylose. The situation is even more complex when water or other guest molecules are present in the crystalline domains. Another complication is that polysaccharide polymorphism includes different helix shapes as well. 

It now appears that cellulose I is not exclusively the native polymorph present in all organisms. The results reported originally by Sisson (61), which provided evidence that cellulose II was the native polymorph present in Halicystis (Ulvophyceae) cell walls, were recently reinvestigated and confirmed (62). Additionally, cellulose II producing mutants of Aceiobacier have been isolated and analyzed with x-ray and low-dose electron diffraction (63). When cellotetraose is induced to crystallize in solution it forms a structure which has been used as a model compound approximating the crystallographic nature of cellulose II based on x-ray diffraction, electron diffraction and CP-MAS 13C NMR evidence (64). Significantly, in all cases where Aceiobacier cellulose synthase in vitro activity has been reported,... 

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