Polymorphs, cellulose

Table 1. 13-C chemical shifts (relative to TMS) from CP/MAS NMR spectra of solid cellulose polymorphs Ref.15>...

Finally, the question of the ability of the modeling methods to predict the crystal lattice (i.e., the unit cell) from the conformation of the chain should be addressed, despite the expected computational difficulties. Based on previous work, in which the prediction of the unit cells of all four cellulose polymorphs in both parallel and antiparallel chain packing polarities was... 

When crystalline cellulose I is treated with aqueous alkali solutions of sufficient strength, a process known as mercerization takes place. As a result of it, cellulose I is converted to cellulose II, the most stable or the four crystalline cellulose polymorphs. The conversion proceeds in the solid state, without apparent destruction or change in the fibrous morphology of the cellulose. As our diffraction analysis indicates, however, it is accompanied by a reversal of the chain packing polarity—from the parallel-chain cellulose I to the... 

Since its introduction several years ago, the virtual bond, constrained optimization method has proved very useful in studies of polysaccharide crystal structure. Notable among the successes that can be ascribed to it are the structural determinations of the double-helical amylose (.11), the cellulose polymorphs of different chain polarities (.12, 13), and of a number of other polysaccharides and their derivatives. As described in a review of amylose structures elsewhere in this volume, the use of this refinement method has produced structural detail that has previously been unavailable (ll). These results have provided much-needed... 

Moreover, studies on the structures of cellulose polymorphs are still being pursued extensively this is evident from the numerous entries on cellulose in this and the previous articles1,2 of this series. In addition to the conformation of the isolated, cellulose chain, the packing of the chains in the lattice presented formidable difficulties in the past. From the modest, theoretical attempts by D. W. Jones,3 Ramachandran,4 and Sundararajan5 to analyze the chain conformation and the... 

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. 

Unit cell dimensions for different cellulose polymorphs (in nm)... 

Figure 4.42 Cartoon of the view down the c axis of various cellulose polymorphs shaded rectangles represent glucan chains in the opposite sense and dotted lines hydrogen bonds.

A. D. French, D. P. Miller, and A. Aabloo, Int. J. Biol. MacromoL, IS, 30 (1993). Miniature Crystal Models of Cellulose Polymorphs and Other Carbohydrates. 

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. 

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. 

Nishino, T., Takano, K., Nakamae, K., 1995. Elastic modulus of the crystalline regions of cellulose polymorphs. J. Polym. Sci. Polym. Phys. 33 (11), 1647—1651. 

Nishino et al. [56] have studied the elastic modulus of the crystalline regions of cellulose polymorphs in the direction parallel to the chain axis, which was measured by X-ray diffraction. The values of cellulose... 

Kim NH, Imai T, Wada M, Sugiyama J. (2006). Molecular directionality in cellulose polymorphs. Biomacromolecules, 7, 274-280. 

TABLE 3.1 The dimensions of cellulose polymorph unit cells (Krassig, 1996)... 

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 ... 

Sternberg U., Koch F.-T., PrieB W, and Witter R. 2003. Crystal structure refinements of cellulose polymorphs using solid state 13C chemical shifts. Cellulose 10 189-199. 

Isogai A., Usuda M., Kato T., Uryu T., and Atalla R.H. 1989. Solid-state CP/MAS carbon-13 NMR study of cellulose polymorphs. Macromolecules 22 3168-3173. 

Figure 15-1. The six different chain shapes from the crystal structures of the cellulose polymorphs I, II, and IIIj, superimposed at their Cl, 04, and C4 atoms to show the differences in the molecular shapes. Indicated for the five-residue segments are the linkage torsion angles, N and P. There are two unique chains in both the ip and II structures (with 06 tg) and one each from la and IIIj (with 06gt). The single-chain la structure has two sets of N and P values because of its lower symmetry. Atomic munbering is indicated the reducing end is to the right and the nonreducing end is on the left...

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... 

Table 12.2 The diffraction planes of cellulose polymorphs measured by WAXS...

Figure 3.Z Equatorial X-ray diffraction profiles of cellulose polymorphs.

Figure 3.8 and Figure 3.9 show the equatorial and meridional X-ray diffraction profiles of cellulose polymorphs [22]. These modifications are said to have the same skeletal conformation as cellulose I that is, a fairly extended zigzag conformation. However, the chain packing, chain stacking, chain direction, and intra/inter hydrogen bonds are different from one another, which is reflected in the diffraction profiles. 

Ultimate Properties of Natural Fibers 1117 Table 3.2 Unit cell parameters of cellulose polymorphs. 

Table 3.2 surmnarizes the unit cell parameters of cellulose polymorphs. These parameters are well defined for the cellulose I series [20, 23]. However, those of the cellulose II series vary largely depending on the origins and treatment conditions. 

Figure 3.11 Relationship between the/-value and the FIP for a series of cellulose polymorphs (open circles) and cellulose trimesters (filled circles), chitin, and chitosan.

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