Regenerated cellulose, crystal structure

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. 

TEMPO-mediated oxidation. With regenerated and mercerized celluloses, the oxidation leads to water-soluble p-l,4-linked poly glucuronic acid sodium salt (cellouronic acid, CUA) quantitatively [16]. In contrast, with native celluloses having the cellulose I crystal structure, the cellulose slurries maintain the slurry states even after TEMPO-mediated oxidation. These modified celluloses form water-insoluble fibers [17]. This has enabled modification of the surface of cellu-losic fibers. 

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. 

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. 

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. 

Cellulose is a polymer that meets these requirements as an adhesive. However, due to its semicrystalline structure, highly hydrogen-bonded cellulose cannot be dissolved easily in conventional solvents, and it cannot be melted before it burns. This is because the attractive forces and stability of crystal structures are greater than those that result from interaction between polymer and solvent. Hence, cellulose itself is not suitable for use as an adhesive. The same can be said of regenerated cellulose. In order to make cellulose soluble or meltable, the hydrogen bonds must be broken (i.e., cellulose molecules must be more flexible and possess high entropy, so that they can be separated easily). 

Stipanovic AJ, Sarko A (1976) Packing analysis of carbohydrates and polysaccharides, 6. Molecular and crystal structure of regenerated cellulose II. Macromolecules 9 851-857... 

Although it is sometimes referred to as a "white amorphous powder , there is a published record of x-ray diffraction from lichenan as early as 1930.( ) No crystal structures were proposed but at that time, it was recorded as a powder pattern and interpreted as similar to 3-cellulose, an early terminology for regenerated cellulose. 

The use of synchrotron X-ray data collected from ramie fibers after ad hoc treatment in NaOH provided a revised crystal-structure determination of mercerized cellulose II at 1 A resolution." The unit-cell dimensions of the P2i monochnic space group are a = 8.10 A, h = 9.04 k,c= 10.36 k,y= 117.1°. As with the regenerated cellulose, the chains are located on the 2i axes of the cell. This indicates that the different ways of preparing cellulose II result in similar crystal and molecular structures. The crystal structure consists of antiparallel chains having different conformations, but with the... 

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. 

Since the details of the crystal structure are of very little importance with regard to the structure and the behaviour of cellulose gels, we shall forbear from discussing them. Neither the lattice structure nor the amount of crystalline substance in cellulose has, as yet, been found to be affected by ordinary mechanical deformation. The orientation of the crystallites changes, but it would seem as though the crystallites themselves remain, as a rule, unaffected. In all regenerated cellulose fibres the percent e of crystalline substance is astonishingly constant, a fact which remains to be explained... 

The bulk properties of regenerated cellulose are the properties of Cellulose II, which is created from Cellulose I by alkaline expansion of the crystal structure (110,111) (see Cellulose). The key textile fiber properties for the most important current varieties of regenerated cellulose are shown in Table 1. Fiber densities vary between 1.53 and 1.50. 

In conclusion, the appearance of the micrometer-sized structures formed by the xylans seems to be related to the morphology of the cellulose substrate rather than the degree of crystallinity. The crystal structure of the cellulose substrate (cellulose I or II) is not unimportant but its influence is indirect, through the absence of fibril-like surface features that can induce the formation of xylan structures on the regenerated substrates (cellulose II). On a nanometer scale, the xylan layer looks similar on all of the cellulose substrates, supporting the conclusion that the cellulose surfaces studied are different on the micro scale but quite similar on the nano and molecular levels. 

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